Creep mechanisms in beryllium

Creep Mechanisms in Beryllium DONALD WEBSTER AND DONALD D. CROOKS

Observa t ions of be ry l l i um samples which have been creep tes ted between 922 K and 1422 K indicate that creep behavior is cont ro l led by the r e l a t ive s t rengths of the g ra in boundar ies and the mat r ix . Since c reep deformat ion can occur p redominant ly by g ra in boundary s l id ing or en t i r e ly by de format ion within the gra ins , the creep s t rength was found to be cont ro l led by the weaker of the two fea tures . Low mel t ing phases conta in ing a luminum and s i l i con which formed along the gra in boundar ies acted as s t r e s s concen- t r a t ions which favored local ized g ra in boundary deformat ion, and r ec rys t a l l i z a t i on . Creep r e s i s t a n c e was found to drop m a r k e d l y when the BeO content was reduced sub- s tan t ia l ly below 1 pct.

THE fac tors cont ro l l ing the creep ra te of b e r y l l i u m such as pur i ty gra in s ize and d is locat ion dens i ty have been prev ious ly desc r ibed . 1 In this i nves t iga - t ion the observable m e c h a n i s m s of creep a re de- s c r ibed . One of these m e c h a n i s m s , i .e. local ized de- fo rma t ion around a liquid g ra in boundary s t r e s s concen t r a t ion may be unique to be ry l l i um.

1.0 MATERIALS

The m a t e r i a l s used in this inves t igat ion a re l i s ted in TabIe I. RR243, BSP9 and BTP5 a re hot i so s t a t i - ca l ly p r e s s e d (HIP) powders . T30 and EF1 a re cas t m a t e r i a l s , and were tes ted in the a s - c a s t condi t ion with the s t r e s s axis pe rpend i cu l a r to the long d i m e n - s ions of the gra ins . EF1 was also tes ted as a fully r e c r y s t a l l i z e d upset forg ing having a negl igible tex- tu re and an equiaxed g ra in s ize of approximate ly 200 g m . The r e m a i n i n g s amp le s in Table I were p r o - duced by vacuum hot p r e s s i n g of powders .

2.0 EXPERIMENTAL TECHNIQUE

Compres s ion creep t e s t s were pe r fo rmed in an a r - gon a tmosphere on 1.27 cm diam spec imens at t e m - p e r a t u r e s between 922 K and 1366 K. The load was ma in ta ined constant at l eve ls chosen to give c reep r a t e s of 10 ~ to 10 "~ pct per s . S t r e s se s between 0.5 and 200 mN//m 2 were used. Length changes were m e a s u r e d by a quar tz ex t ensome te r of 2.54 cm gage length.

The deta i led creep r e s u l t s a r e desc r ibed e l s ewhere . 1 In this work the r e s u l t s a r e used only where they r e - late to m i c r o s t r u c t u r a l and m e c h a n i s m i c changes . M i c r o s t r u c t u r a l compar i sons were made on spec i - m e n s which had been tes ted under s i m i l a r condi t ions of s t r e s s and t e m p e r a t u r e and which pos se s sed s i m i l a r g r a in s i zes . However changes in g ra in s ize, oxide content , s t r e s s and t e m p e r a t u r e between 1000 and 1366 K did not appear to ove r r i de the e s sen t i a l m e c h a n i s m i c changes due to low mel t ing point i m p u r i - t i es .

Spec imens on which the sur face effects of d e f o r m a - t ion were to be s tudied were encapsula ted in vacuum in a g lass chosen to be p la s t i c at the t e m p e r a t u r e of t e s t ing . In this way the b e r y l l i u m could be p l a s t i ca l ly

DONALD WEBSTER and DONALD D. CROOKS are Research Sci- entists in the Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304.

Manuscript submitted October 2, 1975.

deformed without sur face oxidation. The extent of g ra in boundary s l id ing was d e t e r m i n e d in a conven- t ional m a n n e r by sc r ib ing a gr id on the spec imen surface before t e s t ing and examin ing the d i sp lacement when the l ines c rossed a g ra in boundary .

Photoe las t i c models were made to observe the lo- cation of s t r e s s concent ra t ions under condit ions which s imula ted those p r e se n t at high t e m p e r a t u r e s in be ry l - l ium. Above 1100 K be ry l l i um can be cons idered to consis t of ha rd c rys t a l l i ne g ra ins conta in ing a gra in boundary d i s p e r s i o n of l en t i cu la r l iquid globules . It can the re fo re be modeled for photoelas t ic obse rva t ions by a low me l t i ng point meta l globule (Wood's Metal , mel t ing point 348 K) in a c lear p las t ic epoxy. A model of this type was deformed at 373 K (i.e. above the mel t ing point of the metal) so that the liquid globule produced a s t r e s s concent ra t ion effect r e s e m b l i n g that in a b e r y l l i u m sample above 1050 K. In a la ter tes t the l iquid was removed f rom the sample and the tes t was repea ted to de te rmine the effect of the liquid on the s t r e s s f ield. In one tes t with the l iquid presen t , the loading was continued unti l f r a c t u r e occur red . For ease of ana lys i s , some photoelast ic m e a s u r e m e n t s were made on disks subjected to a concen t ra ted d iame- t r a l load. Another model s imi l a t ed a growing gra in boundary c r a c k by having a rounded notch which was deformed in Mode III shea r to produce f r inges at the points of m a x i m u m s t r e s s concen t ra t ion . These f r inges which a re l ines of equal shea r s t r e s s , a re developed because a ray of light in a s t r e s s e d t r a n s - pa ren t body wil l t r ave l f as te r along ce r t a in p r inc ipa l p lanes than o the r s . The number of f r inges or the f r inge o rde r is d i rec t ly p ropor t iona l to the applied s t r e s s below the propor t iona l l imi t of the photoelast ic ma t e r i a l . Models containing epoxy ' g r a i n bounda r i e s ' were made by sequent ia l cas t ing and sol idifying of epoxy blocks . Second phase pa r t i c l e s were placed on the in te r face while one gra in was still liquid.

3.0 RESULTS

3.1 M e c h a n i s m s in Ma t e r i a l s of Low Pur i ty

3.1.1 Locat ion of Impur i ty P h a s e s . It has been shown p rev ious ly ~ that the creep r e s i s t a n c e of b e r y l - l ium is s igni f icant ly lowered by the p r e s e n c e of impur i ty e l emen t s . In most be r y l l i um samples the s ign i - f icant impur i t y e lements a re a luminum, s i l i con and magnes ium. When p re sen t in s m a l l quant i t ies these impur i t i e s fo rm gra in boundary phases which a r e t r i - angular at t r ip le junct ions (Fig. 1) but l en t i cu la r on

METALLURGICAL TRANSACTIONS A VOLUME 7A, SEPTEMBER 1976-1307

Table I. Chemical Composition of Materials Used

Alloy Condition BeO Al Mg Si Fe C Grain Size, prn

RR243 As HIP 1.56 0.0016 0.0030 0.0036 0.0550 0.0200 3.5 RR243 Annealed 1477 K 4.5 RR243 Extruded + 125 K 4.25 T30 0.44 0.0965 0.0050 0.0710 0.7000 O. 1620 3,000 X 30,000 EF 1 Upset Forged <0.02 0.0030 0.0007 0.0050 0.0050 0.0100 200 EFI Cast 1,000 X 10,000 1707 Hot Pressed 3.52 0.0095 0.0175 0.0550 0.0870 0.0450 6.2 8084 Hot Pressed 3.57 0.0080 0.0020 0.0290 0.134 0.0850 5.4 1721 Hot Pressed 2.95 0.0065 0.0020 0.0350 0.0770 0.0420 5.7 BTP5 HIP 2.64 0.0033 0.0200 0.0150 0.0975 0.0560 N.D. BSP9 HIP 1.06 0.0154 0.0042 0,0159 0.0250 0.0520 10 SNO152 Hot Pressed 0.50 0.0080 0,0020 0.0160 0.0690 0.0150 10.3 9776 Hot Pressed 1.80 0.0800 0,0510 0.0200 0.1870 O. 1200 8.0 9713 Hot Pressed 1.75 0.0660 0.0400 0,0190 O. 1600 O. 1250 8.0 472-137 Hot Pressed 1.50 0.0180 0.0060 0.0630 0.0350 0.0340 N.D.

Fig. 1- -Transmiss ion electron micrograph of sil icon-doped, p lasma-sprayed P l powder. Two tr iple junctions contain t r i - angular liquid phases. Magnification 10,000 t imes.

the g r a i n f a c e s (F ig . 2). The s i l i c o n - r i c h (g r ay p h a s e in F i g . 2) and a l u m i n u m - r i c h ( l ight p h a s e in F i g . 2) a r e a s a r e o f ten s e e n to be s e g r e g a t e d to o p p o s i t e ends of the l e n t i c u l a r p a r t i c l e . A t y p i c a l m i c r o p r o b e a n a l y - s i s of a two p h a s e i n c l u s i o n in an i m p u r e c a s t m a t e - r i a l , T30, i s g i v e n in T a b l e II.

3.1.2 E f f e c t of S t r e s s on I m p u r i t y P h a s e s . When a c o m p r e s s i v e s t r e s s is app l i ed at t e m p e r a t u r e s above 1100 K to b e r y l l i u m con t a in ing l iqu id g r a i n b o u n d a r y p h a s e s , s e v e r e g r a i n b o u n d a r y c r a c k i n g o c c u r s (F ig . 3) wh ich a f t e r h igh s t r a i n i s e a s i l y v i s i b l e to the naked eye (F ig . 4). T h e m e c h a n i s m of f a i l u r e i s m a r k e d l y d i f f e r e n t f r o m tha t o c c u r r i n g in the s a m e m a t e r i a l a t 811 K ( F i g . 5) w h e r e the a m o u n t of l iqu id f o r m e d i s i n s u f f i c i e n t to i n f l u e n c e c r e e p s t r e n g t h ? F o r c o m p a r i s o n a c r e e p s p e c i m e n of a h igh p u r i t y

Fig. 2--Optical micrograph of lenticular grain boundary phase in T30. The silicon and aluminum have segregated to opposite ends of the particle during solidification. The white area is aluminum-rich and the gray area silicon-rich. Magnification 1,550 times.

m a t e r i a l (RR243) d e f o r m e d a t 1366 K (F ig . 6) s h o w s no v i s i b l e c r a c k s in the gage l eng th .

T h e c r a c k s tha t a r e o b s e r v e d in i m p u r e m a t e r i a l s c o m p r e s s i v e l y s t r e s s e d a b o v e 1100 K a r e p r e d o m i - n a n t l y e i t h e r p a r a l l e l to o r a t 45 deg to the a x i s of c o m p r e s s i o n . E x a m i n a t i o n s of s a m p l e s s t r e s s e d v a r i o u s a m o u n t s s u g g e s t tha t t he i n i t i a l c r a c k s g r o w p a r a l l e l to the c o m p r e s s i o n a x i s and a r e then r o t a t e d t o w a r d s an ang le of 45 deg . D u r i n g f u r t h e r s t r a i n i n g t h e g r a i n s a s s u m e a t r a p e z o i d a l shape wi th g r a i n s i d e s e i t h e r a t 45 deg o r 90 deg to the c o m p r e s s i o n a x i s w i t h c r a c k s ly ing on ly on t h o s e b o u n d a r i e s at 45 deg . O p t i c a l e x a m i n a t i o n of a c o a r s e g r a i n e d i m p u r e c a s t m a t e r i a l (T30) r e v e a l e d tha t the c r a c k s n u c l e a t e d a t t he t i p s of the l e n t i c u l a r g r a i n b o u n d a r y p h a s e s c o n t a i n i n g a l u m i n u m and s i l i c o n (F ig . 7). At a l a t e r

1308-VOLUME 7A, SEPTEMBER 1976 METALLURGICAL TRANSACTIONS A

s tage in the f r ac tu re p r o c e s s the c racks became much l a r g e r than the in i t i a t ing l iquid phases and g r a i n boundary undulat ions could be seen at the t ips of each c rack (Fig. 8). These undula t ions were r evea l ed .unde r po la r ized light to be a s soc ia t ed with an a r e a conta in- ing e i ther new r e c r y s t a l l i z e d gra ins or subg ra in s emana t ing f rom the c rack tip and s t re tch ing a long the g ra in boundary (Fig. 9). Th i s region, although it f o rms at e levated t e m p e r a t u r e s and is on a much f ine r scale , is analogous to the p las t ic zones that form at notches and fatigue c racks in ducti le me ta l s loaded in tens ion, at room t e m p e r a t u r e . In some a r e a s b ranched c racks were v i s ib le with subgra in fo rma t ion a long each crack (Fig. 10). An examina t ion of the s u r - face of an impure cas t m a t e r i a l (T30) deformed at 1311 K r evea l s cons ide rab le movement of indiv idual g ra ins along the i r bounda r i e s with l i t t le de format ion ins ide the g ra ins (Fig. 11). In some a r ea s comple te sepa ra t ion of the g ra ins has occu r red (Fig. 12) while

in other a r e a s where separa t ion is s t i l l occu r r ing (Fig. 13) the deformat ion appears to be by Mode III shear (i.e., the shear is pa r a l l e l to the leading edge of the c rack 2 along the gra in boundar i e s . The gra in boundary sepa ra t ion begins at an ea r ly stage in the deformat ion p r oc e s s and was found to be qui te marked af ter only 3 pct compress ion at 1255 K. The same type of g ra in boundary movement but on a much f iner scale was v i s ib le on the sur face of impure powder

Table II. Composition (Wt Pct) of Lenticular Grain Boundary Phase in T30

Area Fe AI Si Mg

Light 0.45 84.9 2.7 0 Gray 0.49 56.0 41.0 Not determined Matrix 0.59 9 9 0

Fig. 4--Commercial purity creep specimen (left) deformed 50 pct at 1366 K. Untested specimen is on the right. Grain boundary cracks are both parallel to and at 45 deg to the compression axis. Magnification 1 1/2 times.

(a)

(b) Fig. 3--Optical mierograph of commercial purity beryllium deformed 53 pet at 1366 K; (a) normal iilnmination; (b) polarized light, Magnification 275 times,

Fig. 5--Commercial purity creep specimen deformed to fracture at 811 K. Fracture occurs by brittle transgranular cleavage. Magnification 3 times.


Fig. 6--High purity, HIP specimen RR243 deformed approxi- mately 50 pct at 1366 K. No cracking has occurred in the gage length. Magnification 3 times.

Fig. 8--Optical micrograph of T30 deformed 10 pct at 1366 K showing a large crack growing along the grain boundary para l le l to the compress ion axis. Grain boundary undulations can be seen in the two grain boundaries adjacent to the large crack. Magnification 360 t imes.

Fig. 7--Optical micrograph of impure cast ing (T30) deformed 10 pet at 1366 K showing crack initiating at grain boundary part icle r ich in aluminum and silicon. Magnification 1850 t imes.

1310-VOLUME 7A, SEPTEMBER 1976

Fig. 9--Grain boundary crack and associa ted plastic zone in medium purity ingot beryll ium deformed 10 pct at 1366 K. Magnification 590 t imes.

METALLURGICAL TRANSACTIONS A

p r e s s i n g s indicat ing that the gra in boundary s l id ing was not prevented by the oxide pa r t i c l e s .

3.2 Mechan i sms in Ma te r i a l s of High P u r i t y

In very high pur i ty m a t e r i a l s such as RR243 block and the EF1 casting, g ra in boundary separa t ion dur ing e levated t e m p e r a t u r e deformat ion was r a r e l y obse rved . Deformat ions of 50 pct at 1366 K could be undergone without g ra in boundary cracking; the de-

format ion be ing en t i re ly within the g ra ins (Fig. 14). Scr ibed l ines on the s u r f a c e of the EF1 cas t ing showed no g ra in boundary s l id ing in mos t a r e a s (Fig. 15). The deformat ion me c ha n i sm at e levated t e m p e r a t u r e s is p redominan t ly by sl ip within the g ra in s as evidenced both by the change in gra in shape (Fig. 14) and by surface s l ip l ines (Fig. 15).

3.3 Effect of BeO Content on Creep Strength

The b e r y l l i u m creep data r epor ted p rev ious ly 1 which showed a close co r re l a t ion with impur i ty level was with one exception (T30) for m a t e r i a l with oxide levels between 1 and 4 pct. More r ecen t t es t s on m a - t e r i a l s conta in ing 0 to 0.5 pct BeO has r evea led creep s t r eng ths lower than would be expected on the bas is of pur i ty a lone. SN0152, a medium puri ty , hot p r e s sed block des igned for high room t e m p e r a t u r e duct i l i ty has at 1255 K, a creep ra te 2000 t i me s fas te r and a creep s t r eng th only one tenth that of s i m i l a r pur i ty m a t e r i a l s of higher oxide levels (Fig. 16). S imi l a r ly the high pur i ty (< 0.02 pct BeO) EF1 cas t ing which has

Fig. 10--Branched crack formed in T30 deformed 10 pct at 1366 K. Recrystallized grains are visible along each crack. Magnification 520 times.

Fig. 12--Surface of T30 deformed 18 pet at 1311 K. Complete separation of individual grains has occurred in this region. Magnification 135 times.

Fig. 11--Scanning electron micrograph of surface of T30 deformed 18 pct at 1311 K. Specimen tilted at 45 deg. Magnifi- cation 16 times.

METALLURGICAL TRANSACTIONS A

Fig. 13--Surface of T30 deformed 18 pct at 1311 K. Partial cracking of grain boundaries has occurred and cracks appear to be growing in Mode III shear. Magnification 80 times.

VOLUME 7A, SEPTEMBER 1976-1311

been upset forged and r e c r y s t a l l i z e d to give an equiaxed gra in s ize of about 200 ~zm has a c reep s t reng th (cor rec ted to a 10 ~m gra in s ize by 4 ~ 1/d (Ref. 1)) 24 t imes lower than that of a s i m i l a r pur i ty m a t e r i a l (RR243 annealed 1477 K) with a higher oxide content . The re is a 35 t imes d i f fe rence in creep s t r eng th between the EF1 and RR243 in the ' a s HIP ' condit ion where a high in i t i a l d i s loca t ion densi ty has been shown to exist . 1'3-4 The type of loading does not s eem to be an impor tan t factor in de t e rmin ing the c reep ra te s ince the tens i le c reep ra te de te rmined by Botch and Hauber 5 on cast m a t e r i a l of s i m i l a r pur i ty to EF1 produced creep r a t e s ve ry close to those d e t e r m i n e d in this work under c o m p r e s s i v e loading.

The effect of oxide level on c reep s t reng th is l e s s marked for the m a t e r i a l s with a high impur i ty leve l but the T30 with 0.44 pct BeO has a somewhat lower creep s t r eng th than c o m m e r c i a l l y pure powder p ro - ducts when co r rec t ion is made for i ts ve ry large gra in s ize (30 mm x 3 mm x 3 mm) . The creep s t r eng th of T30 would be expected to be lower s t i l l were it not for i t s abno rma l ly high i ron content (0.7 pct) which may provide both p rec ip i t a t ion harden ing and sol id solut ion s t reng then ing .

An obse rva t ion on the i m p o r t a n c e of g ra in boundary oxide r a t h e r than total oxide can be made f rom the creep s t r eng th of the extruded RR243. This m a t e r i a l has 1.56 pct BeO by weight o r 0.95 pct by volume. In the HIP or HIP and annealed condi t ions the BeO is confined p redominan t ly to the g r a i n boundar ies . M e a s - u r e m e n t s by t r a n s m i s s i o n e l ec t ron mic roscopy in a r e a s where the g ra in boundary is a lmos t pa r a l l e l to the foil indica te a volume f r ac t ion of 20.3 pct. After e x t r u s i on when the d i s t r i bu t i on of oxide can be cons ide red to be uniform, the a r e a f rac t ion of BeO cut by the boundary wil l be equal to the average volume f rac t ion , i.e. 0.95 pct. This is 21.3 t imes less oxide on the g ra in boundary than is p r e s e n t in the unworked condit ion. The RR243 ex t rus ion i s the re fore equivalent

Fig. 14--High purity, HIP powder block (RR243) deformed 54 pct at 1366 K; polarized light. Magnification 280 times.

d _ C0MMERC!AL P21LLLY

1707 rAl§

3 ~0-1510 p0m.

z

MEDIUM PURITY

�9 3084

1t21 AI+Si+M9 II! 260"420 ppm

g BTP5 2 (ANNEALED1505K) HIGH P UR LLY

�9 9713 9776 RR243

�9 472-137 (ANNEALED 1477K~/

1 �9 BSP 9 / ~5N0152 J(ANNEALED 1422K) / AIt,',SJ~

RR243 EXTRUS tON ~ |0Oppm. 0 a EF], i i t ~ i i _ ~ t

2 4 ~ ]0 12 14 16 18 20 CREEP STRENGTH (MNIm21 AI 10 -2 PERCENT /SECOND

Fig. 16--The effect of ]BeO content on the creep strength of beryllium of various purities.

Fig. 15--Surface of high purity ingot beryllium (EF1) deformed 20 pct at 1255 K. A straight line scribed on the surface before testing shows plastic flow within the grains but ao grain boundary offsets. Slip lines are visible inside each grain. Magnification 22 1/2 times.

Fig. 17--Fringe patterns formed in epoxy model containing an elliptical stress concentration from which liquid metal has been drained. The major axis of the stress concentration is at 90 deg to the compression axis.


in t e r m s of obs t ac l e s to g r a in boundary mot ion to a hot p r e s s e d product (where a l l the BeO is on the g ra in boundary) containing 1.$6 = 0.07 pct BeO. The c r e e p s t r eng th of the RR243 ex t ru s ion has been included in F ig . 16 at this oxide l eve l .

The poss ib le effect of t ex tu r e on c reep s t r eng th has been neg lec ted here in v iew of the above ment ioned s i m i l a r i t y in the r e s u l t s on the EF 1 used h e r e which has a negl ig ib le t ex tu re and the heavi ly t ex tu red ex t ruded m a t e r i a l of s i m i l a r pur i ty used by B o t c h and Hauber.5

3.4 P h o t o - E l a s t i c Studies of C r a c k s Under S t r e s s

In o r d e r to obtain m o r e in format ion on the s t r e s s d i s t r ibu t ion around c o m p r e s s i v e l y loaded l iquid c r a c k s mode l s of e l l ip t i ca l s t r e s s concen t ra t ions in c l e a r p l a s t i c d i scs were made . The s t r e s s concen t ra t ions w e r e ac tual ly l en t i cu la r p a r t i c l e s of Wood's M e t a l so that the appl ica t ion of a s m a l l amount of heat p roduced a c r a c k f i l led with a l iquid me ta l . This mode l t he reby s i m u l a t e d the m i c r o s t r u c t u r e of be ry l l i um above 1050 K where l en t icu la r c r a c k s f i l led with a l iquid m e t a l r i c h in a luminum and s i l i con ex is t ins ide the so l id b e r y l l i u m . When the p l a s t i c disc containing the l iquid c r a c k was sub jec ted to a concen t ra ted d i a m e t r a l load the d i s tu rbance of the n o r m a l f r inge d i s t r ibu t ion was smal l , sugges t ing that s o m e load is t r a n s f e r r e d through the liquid m e t a l . When the me ta l was d ra ined f r o m the mode l leav ing only the e l l ip t i ca l cavi ty ( i . e . a

condi t ion that s i m u l a t e s a l iquid f i l led c r a c k a f t e r c r a c k growth has s ta r t ed) the f r inge d i s t r ibu t ion indica ted a h igher s t r e s s concen t ra t ion around the cav i ty (Fig . 17). A s i m i l a r s t r e s s pa t t e rn was main ta ined for a l l o r i en ta t ions of the axis of the cavi ty with r e s p e c t to the c o m p r e s s i o n ax is . Acco rd ing to Bombolak is , 6 an e l l i p t i ca l cavi ty at a c r i t i c a l o r ien ta t ion to the axis of a c o m p r e s s i v e s t r e s s would have a t ens i l e s t r e s s concen t ra t ion

(a + b) a T - 4 a b

w h e r e a = s e m i m a j o r axis b = s e m i m i n o r axis

and a c o m p r e s s i v e s t r e s s concen t ra t ion

~C = - 3CrT �9

T h e s e concent ra t ions would occur near , but not qu i te at, the ends of the cavi ty . It can be seen f r o m the fo rm of the above equat ions that when the cavi ty is highly e l l ip t ica l , subs tan t ia l concent ra t ions of both t e n s i l e and c o m p r e s s i v e s t r e s s can occu r under a uniaxia l c o m p r e s s i v e s t r e s s . In b r i t t l e m a t e r i a l s under compre s s ion , c r a c k s w e r e found by Bomba lak i s to grow f r o m the point of max imum tens i l e s t r e s s concen t ra t ion at a gradual ly d e c r e a s i n g angle to the axis of c o m p r e s s i o n . Th is mode of c r ack growth is d e m o n s t r a t e d by a l en t i cu l a r globule of mol ten Wood 's m e t a l lying along an epoxy gra in boundary (Fig . 18). When a c o m p r e s s i v e load is appl ied at r ight ang les to the g ra in boundary a sharp c r a c k develops at the top of the globule and grows p a r a l l e l to the c o m p r e s - s ion ax is . At the s a m e t ime liquid me ta l is e j ec t ed f r o m the globule into the c r ack .

As ment ioned above, s u r f a c e m i c r o g r a p h s (Fig. 13)

indica ted that de fo rma t ion was by Mode III shea r . Ac - cord ingly the pho toe las t i c s t r e s s pa t t e rn s developed in a p l a s t i c mode l de fo rmed in Mode III shea r w e r e examined (F ig . 19). T h r e e a r e a s of high s t r e s s concen t ra t ion w e r e obse rved . The c e n t r a l and s m a l l e s t s t r e s s concen t ra t ion in the mode l i s n o r m a l l y bi- sec ted by the g ra in boundary in the b e r y l l i u m al loys under cons ide ra t i on and is usual ly the s i te of the in i t ia l c r ack . However as shown in F i g . 10, b ranched c r acks can occu r in b e r y l l i u m p r e s u m a b l y along the two outer and s t r o n g e r s t r e s s concen t ra t ions shown in Fig . 19.

Fig. 18--Crack formed during compression in an epoxy model containing an elliptical metallic stress concentration that was liquid at the deformation temperature. The crack has grown parallel to the compression axis and is filled with liquid metal ejected from the initial stress concentration.

Fig. 19--Fringe patterns obtained in epoxy model deformed in Mode III shear showing three areas of high s tress concentration.

METALLURGICAL TRANSACTIONSA VOLUME 7A, SEPTEMBER 1976-1313

4.0 DISCUSSION

It has been shown p r e v i o u s l y 1 that the c r e e p r a t e of b e r y l l i u m can be s ign i f i can t ly changed by s m a l l concen t ra t ions of a luminum, m a g n e s i u m and s i l i con . These e l e m e n t s which a r e a l m o s t i n so lub le in b e r y l - l ium have been shown 7 to occupy g r a i n bounda ry s i t e s , a l though the m e c h a n i s m by which they in f luence c r e e p s t r eng th has not p r e v i o u s l y been c l a r i f i e d .

When a s t r e s s i s app l i ed to an i m p u r e b e r y l l i u m a l - loy above 1050 K, p l a s t i c flow begins at the r e g ions of h ighes t s t r e s s which a r e the e l l i p t i c a l l iquid g r a i n boundary s t r e s s concen t r a t i ons . L o c a l i z e d p l a s t i c flow o c c u r s a long a n a r r o w g ra in bounda ry r eg ion a l - lowing the r e l a t i v e l y unde fo rmed g r a i n i n t e r i o r s to r o t a t e and s l i de . At some combina t ion of t e m p e r a t u r e and s t r a in , r e c o v e r y o r r e c r y s t a l l i z a t i o n o c c u r s along a g ra in boundary and the reby a l lows a l oca l t r a n s i e n t i n c r e a s e in c r e e p r a t e . The m e a s u r e d c r e e p r a t e is the i n t eg ra t ed va lue for a l l t hese m i c r o r e g i o n s .

I t i s appa ren t f rom the m i c r o s t r u c t u r e s that p l a s t i c flow is not confined to the p lane of the bounda ry as i s c l a s s i c a l g ra in bounda ry s l id ing but in c o a r s e g ra ined m a t e r i a l s may be o c c u r r i n g in a r eg ion 10 /~m thick. F o r th is r e a s o n the r e l a t i v e l y s m a l l (<0.5 /~m) BeO p a r t i c l e s that occupy the bounda r i e s in hot p r e s s e d b e r y l l i u m a r e not expec ted to p r e s e n t the s e v e r e ob- s t a c l e to g r a i n bounda ry s l id ing env i s ioned by Ashby 8 fo r m a t e r i a l s w h e r e d e f o r m a t i o n i s m o r e r e s t r i c t e d to the boundary p l a n e . In the p r e s e n t w o r k the 2 pct of g ra in boundary oxide p a r t i c l e s found in b e r y l l i u m of c o m m e r c i a l p u r i t y did not p reven t ex t ens ive g ra in boundary s l id ing . The g r a i n bounda r i e s in b e r y l l i u m do not however a p p e a r to be s o u r c e s of w e a k n e s s in m a t e r i a l s of high pu r i t y s ince EF1 shows v e r y l i t t l e g ra in boundary s l i d ing at 1366 K. Mos t d e f o r m a t i o n in high pur i ty m a t e r i a l s i s o b s e r v e d to o c c u r by s l ip within the g r a i n s and heavy s l ip bands can be s een within each g r a i n (Fig . 15). In effect p u r i t y changes the r e l a t i v e c r e e p s t r e n g t h s of the g r a i n boundary and the m a t r i x with the p r e d o m i n a n t d e f o r m a t i o n o c c u r r i n g in the weake r of the two r e g i o n s . An unde r s t and ing of th is 'weakes t l ink ' e f fec t i s n e c e s s a r y for the des ign of c r eep r e s i s t a n t b e r y l l i u m a l loys . F o r e x a m p l e , in a m a t e r i a l of c o m m e r c i a l pu r i ty t h e r e would be no benef i t f rom i n c r e a s i n g the m a t r i x s t r e n g t h by so l id so lu t ion ha rden ing s ince the weak g ra in b o u n d a r i e s a r e ensu r ing tha t v e r y l i t t l e s l ip i s o c c u r r i n g in the m a t r i x . S i m i l a r l y in a high pur i ty m a t e r i a l whe re c r e e p is o c c u r r i n g as a r e s u l t of m a t r i x s l i p t h e r e i s no point in i m p r o v i n g pur i ty , and t h e r e b y g r a i n bound- a r y s t rength , s t i l l f u r t h e r unti l the m a t r i x s t r e n g t h has been i n c r e a s e d .

The f a c t o r s con t ro l l i ng m a t r i x s t r e n g t h need fu r the r d i s c u s s i o n . I t has been shown p r e v i o u s l y 1 that the m a t r i x can be s t r e n g t h e n e d s ign i f i can t ly by the high in i t i a l d i s l o c a t i o n dens i t y p r e s e n t in high pur i ty , hot i s o s t a t i c a l l y p r e s s e d m a t e r i a l s such as RR243 and BSP9. The m a t r i x s t r e n g t h a f t e r p r e s s i n g depends on the p r e s s i n g t e m p e r a t u r e (normal ly 1186 K) and the r e s i s t a n c e to r e c r y s t a l l i z a t i o n . The l a t t e r f a c to r is convenient ly m e a s u r e d as the r e c r y s t a l l i z a t i o n t e m - p e r a t u r e (i.e. the f i r s t s ign of r e c r y s t a l l i z e d m a t e - r i a l as de t ec t ed by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y ) a f t e r 10 pct d e f o r m a t i o n . When the c r e e p t e s t s a r e run at t e m p e r a t u r e s above the p r e s s i n g t e m p e r a t u r e

the i n i t i a l d i s loca t ion de ns i t y can be c o n s i d e r e d to be r e l a t e d only to the r e c r y s t a l l i z a t i o n t e m p e r a t u r e of the m a t e r i a l . S i m i l a r l y if a m a t e r i a l i s annea led b e - f o r e t e s t i n g a new d i s loca t i on s u b s t r u c t u r e wi l l qu ick ly bu i ld up dur ing the c r eep t e s t and the c r e e p r a t e wi l l d e c r e a s e to a s t eady s t a t e . A s the d i s loca t ion sub- s t r u c t u r e p roduced dur ing c r e e p i s commonly a s - s u m e d to be a ba lance be tween s t r a i n ha rden ing and r e c o v e r y , then if r e c o v e r y i s made m o r e di f f icul t i t s e e m s r e a s o n a b l e to a s s u m e that m o r e s t r a i n h a r d e n - ing (i.e. a h igher d i s loca t ion dens i ty ) can be a t t a ined and s t a b i l i z e d dur ing s t eady s t a t e c r e e p . In b e r y l l i u m , i t a p p e a r s that d i s l oc a t i ons i n t roduced by m o d e r a t e amoun t s of s t r a i n fo rm s u b s t r u c t u r e s which a r e s t a - b le in the r ange 800 to 1400 K un les s r e m o v e d by r e - c r y s t a l l i z a t i o n . 4 In th is s i t ua t ion i t i s the r e c r y s t a l l i - za t ion r e s i s t a n c e r a t h e r than the r e s i s t a n c e to conven t iona l r e c o v e r y m e c h a n i s m s that d e t e r m i n e s the s t a b i l i t y of the s u b s t r u c t u r e . In p r a c t i c e th is m e a n s that a m a t e r i a l with a high r e s i s t a n c e to r e c r y s t a l l i - za t ion wi l l , dur ing a c r e e p t e s t , be in equ i l i b r i um with a s t r o n g e r m a t r i x (i.e. h ighe r d i s loca t ion dens i ty ) than a m a t e r i a l whe re r e c r y s t a l l i z a t i o n i s e a s i e r . In o the r w o r d s for o the rwi se equ iva len t m i c r o s t r u c t u r e s c r e e p r e s i s t a n c e should be i n c r e a s e d by the s a m e f a c t o r s tha t i n c r e a s e r e c r y s t a l U z a t i o n t e m p e r a t u r e . In b e r y l l i u m i t has been shown 4 that r e c r y s t a l l i z a t i o n t e m p e r a t u r e can be i n c r e a s e d fo r a given vo lume f r a c - t ion of oxide if the oxide p a r t i c l e s i ze is d e c r e a s e d . F a c t o r s govern ing the p a r t i c l e s i z e were l a t e r shown 8 to be p u r i t y and hot p r e s s i n g cyc le . It fol lows tha t for a g iven BeO p a r t i c l e s i ze and t e m p e r a t u r e t h e r e wi l l be a m i n i m u m volume f r a c t i o n that is capable of p r e - ven t ing r e c r y s t a l l i z a t i o n and ma in ta in ing a high c r e e p s t r eng th . I t a p p e a r s f rom F ig . 16 that at 1255 K the c r i t i c a l vo lume f r ac t ion i s be tween 0.5 and 1 pct .

A s c h e m a t i c i l l u s t r a t i o n of the r e l a t i onsh ip be tween c r e e p s t r e n g t h and r e c r y s t a l l i z a t i o n t e m p e r a t u r e for v a r i o u s types of m i c r o s t r u c t u r e is shown in F ig . 20. The sequence s t a r t s with an i m p u r e ca s t i ng where the s t r e n g t h of both the b o u n d a r i e s and the m a t r i x is low. The b o u n d a r i e s a r e weak b e c a u s e they contain l iquid p h a s e s and the m a t r i x i s weak b e c a u s e the l ack of g r a i n boundary p a r t i c l e s r e s u l t s in a low r e s i s t a n c e to r e c r y s t a l l i z a t i o n . D e f o r m a t i o n can occu r both by

/ X ~ k 30 RR243 AS HIP

~ 20 ~ RR243 ANNEALED

-

/

4 COMMERCIAL PURITY HOT PRESSED BLOCK

~ ] { ~ / ~ = I:~] H .GH PUR .TY CASTING(EF])

800 ?00 ]000 ]100 ]200 ]300 ]400 1500

RECRYSTALLIZATION IEMPER,'~,TURE ~ (]0% REDUCTION) Fig. 20- -Schemat ic i l l u s t r a t ion of m i e r o s t r u c t u r a l deve lop- ment leading to be ry l l ium of high c r e e p r e s i s t a n c e .

HIGH DISLOCATION DENSITY


gra in boundary s l id ing and by p las t ic flow ins ide the g ra ins . The creep s t r eng th can be improved by pur i f i - ca t ion so that no g ra in boundary liquid phases a r e p r e s e n t and p las t ic flow then occurs en t i r e ly in the ma t r i x . This type of s t r u c t u r e occurs in the high pu r i t y cast m a t e r i a l (EF1). A fur ther i n c r e a s e in c reep r e s i s t a n c e is obtained in samples where the r e c r y s t a l l i z a t i o n t e m p e r a t u r e of an impure m a t e r i a l is r a i s e d by gra in boundary oxide pa r t i c l e s . This is a typica l s t r uc tu r e of a c o m m e r c i a l puri ty, hot p r e s s e d m a t e r i a l where the m a t r i x is s t ronge r than the bound- a r i e s so that the dominant creep mechan i sm is g ra in boundary s l id ing. As the g ra in boundar ies in this ma - t e r i a l a r e the weakest link, s t rengthening of the m a - t r i x would se rve l i t t le purpose and the degree to which this could be done by i n c r e a s i n g the d i s loca t ion dens i ty would be l imi ted in view of the m a t e r i a l ' s low r e c r y s t a l l i z a t i o n t e m p e r a t u r e . As repor ted p r e - v ious ly 4 the 10w r e c r y s t a l l i z a t i o n t e m p e r a t u r e of i m - pure be ry l l i um is a r e s u l t of liquid phase enhanced oxide coarsen ing which reduces the eff iciency of the oxide d i spe r s ion . Cont inuing the sequence shown in F ig . 20 a fur ther i n c r e a s e in c reep r e s i s t a n c e is ob- ta ined if the pur i ty is enhanced to r emove the g ra in boundary l iquid phases so that g ra in boundary s l id ing is e l imina ted and p las t i c flow is l imi ted to the g ra in i n t e r i o r s . This s t r uc tu r e is typical of a high pur i ty m a t e r i a l , e i ther hot p r e s s e d or hot i so s t a t i ca l ly p r e s s e d and annealed. Since deformat ion in this m a - t e r i a l is occu r r ing p redominan t ly ins ide the g ra ins , the use of sol id solut ion hardening, p rec ip i t a t ion ha rd- ening or r e s idua l cold work should be effective in in - c r e a s i n g creep s t rength . No in format ion is ava i lab le on the f i r s t two m e c h a n i s m s in be ry l l i um but the high r e s idua l cold work of high pur i ty 'as HIP ' m a t e -

r i a l s 1,~ produces a fur ther 50 pct i n c r e a s e in creep s t rength .

5.0 CONCLUSIONS

1) Low mel t ing point phases r i ch in a luminum and s i l icon concen t ra te in the g ra in boundar ies of be ry l - l ium and above 1050 K they act as s t r e s s concen t ra - t ions which produce extens ive g ra in boundary s l iding and loss of creep s t rength.

2) Inso luble pa r t i c l e s such as BeO contr ibute to the creep s t r eng th of be ry l l ium by i n c r e a s i n g i ts r e s i s t - ance to r e c r y s t a l l i z a t i o n and hence p e r m i t t i n g a higher equ i l i b r ium dis locat ion dens i ty to be main ta ined dur ing s teady state creep.

ACKNOWLEDGM ENTS

The au thors would like to thank A. R. Hunter for guidance in the photoelas t ic s tud ies . This work was funded by the U.S. Surface Weapons R e s e a r c h Center under Cont rac t No. N60921-C-0285 of the REVMAT p r o g r a m . Contrac t technical moni to r was Wil l iam J. Beuhler and REVMAT P r o g r a m Coord ina tor was Leo F. Gowen.

REFERENCES

1. D. Webster and D. D. Crooks: Met. Trans. A, 1975, vol. 6A, pp. 2049-47. 2. F. A. McC|intock and G. R. Irwin: STP381, p. 84, ASTM Philadelphia, 1965. 3. D. Webster, R. L Greene, and R. W. Lawley: Met. Trans, 1974, vol. 5, pp.

91-96. 4. D. Webster, D. D Crooks, and A. E. Vidoz: Met. Trans., 1973, vol. 4, pp.

2841-47. 5. N. R. Borch and J. R. Hauber: Trans. TMS-AIME, 1968, vol. 242, pp. 1933-36. 6. E. G. Bombalakis: Fracture, vol. 7, pp. 93-154, Academic Press, 1972. 7. D. Webster: Met. Trans. A, 1975, vol. 6A, pp. 803-08. 8. M. F. Ashby: Surface Sci., 1972, vol. 3 l, pp. 498-542.


Documents

Creep mechanisms in beryllium