Effect of lithium on the mechanical properties and microstructure of sic whisker reinforced aluminum alloys

Effect of Lithium on the Mechanical Properties and Microstructure of SiC Whisker Reinforced Aluminum Alloys

DONALD WEBSTER

Aluminum-silicon carbide whisker composites containing nominally 3 to 5 pet Li in the matrix alloys have been fabricated and tested. Tensile and compression tests have been conducted at room temperature, and compression creep tests have been conducted at elevated temperatures. Lithium additions were found to increase the strengthening effect of silicon carbide whiskers at room and elevated temperatures. Lithium also reduced the density of the composites and increased the elastic modulus. Transmission electron microscopy showed no obvious chemical reaction between the whiskers and the aluminum-lithium alloy matrix.

I. INTRODUCTION

A L T H O U G H whisker reinforced metal matrix composites are not a recent introduction as evidenced by the early work of Sutton and Chorne ~ and Divecha e t a l , 2 there has been a revival of interest in the last five years due to the introduction of silicon carbide whiskers derived from rice hulls. These whiskers are potentially inexpensive and are currently being produced by Exxon Enterprises Materials Division (formerly, Silag Inc.) in quantities suitable for many aero- space applications.

Many early whisker materials used in aluminum alloys produced an increase in elastic modulus and an increase in the strength of pure aluminum and dilute aluminum alloys but little or no strength increase compared to commercially available high strength aluminum alloys.

Some reasons for this are:

1. An apparently weak bond at the whisker-matrix interface which allows the whiskers to pull out of the matrix before their full strength can be utilized) 2. The aspect ratios of the whiskers are markedly reduced during extrusion. Data from Exxon Enterprises Materials Division 4 indicates that a four-fold reduction in aspect ratio occurs during extrusion. 3. Early SiC reinforcement from Exxon contained a substantial proportion of particulate material which on theoretical grounds would be expected to produce a lower strength composite.

Recent (after 1979) improvements in whisker quality and composite fabrication techniques have resulted in composites which show increased strengths compared to com- mercial aluminum alloys such as A1 2024 and A1 7075. 4

The present program was designed to improve further the mechanical and physical properties of A1-SiC Whisker composites by the use of an aluminum-lithium alloy matrix. The anticipated advantages of this approach were a reduction in composite density and an improvement in the bond between the whiskers and the matrix due to the reactivity of the lithium. Lithium serves similar purposes in Du Pont's A1-Li-A1203 (FP) composites where the reinforcement is in the form of continuous fibers.

An increase in the matrix-whisker bond strength would reduce the amount of whisker pullout and might be expected

DONALD WEBSTER is Staff Engineer with Lockheed Missiles and Space Company, P. O. Box 504, Sunnyvale, CA 94088-3504.

Manuscript submitted July 27, 1981.

METALLURGICAL TRANSACTIONS A

to allow the stress in the whisker to reach a higher level and hence increase the strength of the composite.

II. MATERIALS

Four A1-Li-SiC composites were made by powder metallurgical techniques by Exxon Enterprises Materials Division. The composites had nominally 25 wt pct SiC Whiskers and were made by blending the whiskers with four aluminum-lithium alloys made by Homogeneous Met- als Inc. After blending the composites were liquid phase hot pressed to full density as 7.6 cm billets and extruded to 1.6 cm rods. Since the dissemination of information regard- ing metal matrix composite manufacturing technology is specifically restricted by the provisions of the United States Munitions Control List (1979), no further information on the techniques used to manufacture the composites used in this work can be given. The target and actual compositions of the composites and the aluminum-lithium matrix are given in Tables I and II, respectively.

I I l . EXPERIMENTAL T E C H N I Q U E

Tensile tests were conducted on round tensile specimens 9 cm long with a 3.5 cm long reduced section, 0.61 cm in diameter. Modulus determinations were determined by the average output of two strain gages bonded to opposite sides of the specimen to eliminate errors due to bending. The testing speed was 0.005 m / m per second to the yield stress and then 0.05 m / m per second until failure.

Compression tests were conducted on 1.6 cm diameter cylinders 5 cm long. Testing procedure was the same as that described above for tension tests. Testing was continued until the specimens either buckled or fractured.

Creep testing was conducted in compression on 1.6 cm diameter specimens 5 cm long. The change in specimen length was measured by a quartz rod extensometer capable of detecting creep rates down to 0.0002 pet per second. Heating was accomplished by passing an electric current through the specimen to produce rapid heating rates repre- sentative of those expected in a missile structure. A load is not applied until the specimen reaches and stabilizes at the desired temperature. Other experimental details of this test are described elsewhere. 5

Typical heating rates between room temperature and 900 K were about 60 seconds. Creep curves consisted of a

ISSN 0360-2133/82/0811-1511 $00.75/0 �9 1982 AMERICAN SOCIETY FOR METALS AND VOLUME 13A. AUGUST 1982--1511

THE METALLURGICAL SOCIETY OF A1ME

Table I. Target Compositions of AI-Li-SiC Whisker Composites

Target Composition

Wt Pct

Vol Pct Composite A1-Li Matrix

Material SiC SiC Li Mg Cu Li Mg Cu A1

1103 20.7 25 2.25 - - - - 3 - - - - Balance 1105 19.9 25 3.75 - - - - 5 - - - - Balance 6003 20.7 25 2.25 0.75 0.19 3 1 0.25 Balance 6005 19.9 25 3.75 0.75 0.19 5 1 0.25 Balance

Table II. Actual Compositions of AI-Li-SIC Whisker Composites

Actual Composition

Wt Pct

Vol Pct Composite AI-Li Matrix

Material SiC SiC Li Mg Cu Li Mg Cu A1

1103 22.8 27.5 2.3 - - - - 3.17 - - - - Balance 1105 20.9 26.2 3.5 - - - - 4.7 - - - - Balance 6003 26.1 31 2.1 0.5 0.14 3.04 0.72 0.20 Balance 6005 25.1 31 3.5 0.54 0.43 5.07 0.78 0.62 Balance

short region of primary creep followed by a region of steady state creep similar to the results obtained previously by the same technique on beryllium. 5 Creep data presented in this work is taken from the steady state region of the curves.

Mechanical test results given below are single tests only. Although composites in general show a wide scatter in mechanical properties, this is less apparent in whisker composites where variations in whisker strength and bonding are already averaged over millions of whiskers in each test specimen. The properties most likely to vary from specimen to specimen are strain to failure and tensile ultimate strength. Yield strength which is a more reliable parameter has therefore been used to compare materials and analyze strengthening mechanisms in this work.

Foils for transmission electron micrographs were pre- pared by electropolishing in a solution of one third nitric acid and two thirds methanol at a temperature of 245 K and a voltage of 10 V.

IV. RESULTS

A. Microstructure

As-pressed A1-SiC whisker composites have isotropic properties and a random whisker distribution. Large amounts of hot working by extrusion or rolling produce an extremely good alignment of the whiskers along the direction of metal flow. 6 A similar structure of aligned SiC whiskers exists in the AI-Li-SiC composites (Figure 1) although in this case large (1 to 12/xm) intermetaUic compounds of A1 Li are present.

Transmission electron micrographs of conventional aluminum SiC whisker composites show no observable reaction between the matrix and the whiskers. 3 Since one of the reasons for adding lithium was to improve the whisker- matrix bond, transmission electron micrographs were made to examine the matrix-whisker interface. The micrographs showed no obvious matrix reaction with the whiskers, al-

Fig. 1--Optical micrograph of composite 1103 showing SiC whiskers aligned in the direction of extrusion and large particles of A1 Li. Mag- nification 2000 times.

though as will be discussed below, the mechanical property changes suggest the lithium addition produced an improvement in the whisker-matrix bond strength. Many silicon carbide whiskers possess a fine scale internal structure

1512--VOLUME 13A, AUGUST 1982 METALLURGICAL TRANSACTIONS A

of lines approximately 10 nm apart, normal to the long axis of the whisker. 3 This structure which can also be seen inside whiskers in the A1-Li-SiC materials (Figure 2) is visible to the edge of the whisker and shows no evidence of a change in structure which would be expected if reaction with the matrix had occurred. Figure 3 shows the hardening precipitate A13Li (30 to 60 nm diameter) in the matrix between the silicon carbide whiskers. Extensive pullout of silicon carbide whiskers has been observed in previous aluminum- silicon carbide composites) It was hoped that an aluminum lithium matrix would completely prevent whisker pullout; however, a substantial amount of pullout is still present as shown in Figure 4, which also shows the excellent alignment of the whiskers after extrusion. No attempt was made to evaluate statistically the effect of lithium on pullout length.

Another factor which limits the strength of whisker composites is the fracture of the whiskers during processing. This effect was examined for the A1-Li-SiC materials by dissolving away the matrix of the extruded rod and examin- ing the residue by scanning electron microscopy. The residue from the 6005 extrusion is shown in Figure 5. There is a marked reduction in average whisker length compared to the original whisker reinforcement (Figure 6). Data developed by the Materials Division of Exxon Enterprises 4 indicate that a reduction in mean-whisker length occurs at each stage of processing. Whiskers that originally had a mean length of 4 9 / x m had this length reduced to 33/xm after compacting and 9/.~m after extrusion (16:1).

Fig. 2- -Transmiss ion electron micrograph of composite 1103 in the T6 condition. A SiC whisker 0.85 p.m in diameter with an internal structure of fine parallel lines (parallel to direction of arrow) in an AI-Li matrix (white area). SiC particulates are visible at P. Whiske r s which are not transparent to the electron beam are present at W. Magnification 27, 360 times.

Fig. 3- -Transmiss ion electron micrograph of composite 1103 (T6 condition) showing SiC whiskers (W) in an aluminum matrix containing AI3Li precipitates (at A) 30 to 60 nm in diameter. White areas are aluminum matrix. Magnification 21,600 times.

Fig. 4 - -Scann ing electron micrograph of fracture surface of composite 1105 showing SiC whiskers which have been pulled out of the matrix during fracture. Magnification 4,850 times.

No quantitative measurements were made on the residue in Figure 6 although the mean whisker length appears to be in general agreement with Exxon's data.

B. Physical Properties

The density of the A1-Li-SiC composites as determined by the liquid displacement method (acetonitrile - - an organic liquid used to avoid possible reaction with the lithium) is given in Table III with the density of an A16061 - - 25 wt pct SiC whisker composite given for comparison.

METALLURGICAL TRANSACTIONS A VOLUME 13A, AUGUST 1982--1513

Fig. 5 - - Scanning electron micrograph of the extracted SiC whiskers from composite 6005. Magnification 4t50 times.

Fig. 6 - - Scanning electron micrograph of SiC whiskers (grade F9) before composite manufacture. Magnification 4000 times.

Table IV. Tensile Results of AI-Li-SiC Whisker Composites in the T4 Condition

Material

0.2 Pct Yield

Proportional Strength Modulus Limit (MPa) (MPa) UTS (MPa) (GPa)

1103 189 417 571 141 1105 177 479 575 133 6003 235 473 544 146 6005 293 609 609 149

600

5O0

2OO

1s r ii o I

0,2

I II

,' / /

~ 1/11 # ii II

O 0.2 0 0,2 0 0.2

STRAIN PERCENT

Fig. 7--Stress-strain curves to the 0.2 pct yield stress for AI-Li-SiC whisker composites.

Table V. Compression Results for AI-Li-SiC Whisker Composites in the T4 Condition

0.2 Pct Yield Material Strength (MPa) CUS (MPa) Failure Mode

1103 363 780 Buckling 1105 446 765 Fracture 6003 481 846 Fracture 6005 573 586 Fracture

Table III. Density of AI-Li-SiC Whisker Composites

Material Wt Pct Li In Matrix Density (g/cc)

1103 3.17 2.63 1105 4.7 2.55 6003 3.04 2.66 6005 5.07 2.57

A16061 + 25 wt pct SiC 0 2.82

C. Mechanical Properties

1. Room Temperature

Tensile properties determined on the composites in the T4 condition (solution treated 733 K aged one week at room temperature) are given in Table IV. The tensile stress strain curves (T4 condition) up to 0.2 pct are shown in Figure 7.

Compression results for the material in the T4 condition are given in Table V. The compression yield strengths are slightly below the values obtained in tension.

Tensile and compression values for the composites in the T6 condition (solution treated 733 K, aged 16 hours, 464 K)

are given in Tables VI and VII, respectively. A16061 + 20 wt pct SiC is included in Table VI for comparison.

In the T6 condition the proportional limits and yield strengths are higher in compression than in tension, which is the reverse of the observations made in the T4 condition.

The composites other than 1103 are brittle in the T6 condition and fail before the tensile 0.2 pct yield strength is reached.

2. Elevated Temperature

Compression creep tests were conducted on A1-Li-SiC whisker composites 1103, 6003, and 6005 together with A16061, A1 2024, A1-3Li, A1 2024-SIC (28 wt pct)-HIP, 7 and a particulate A1-SiC (21.8 wt pct) composite made by DWA Composite Specialities. Creep rates were measured between 505 K and 755 K for all materials and in the case of the higher melting point matrices, testing was conducted up to 811 or 866 K. A wide range of creep rates was observed at each temperature by rapidly changing the stress after steady state creep was obtained. Typical data for 1103 is shown in Figure 8 and for A1-3 Li in Figure 9. In order


Table VI. Tensile Results for AI-Li-SiC Whisker Composites in the T6 Condition

Proportional 0.1 Pct Yield 0.2 Pct Yield Material Limit (MPa) Strength (MPa) Strength (MPa) UTS (MPa) Modulus (GPa)

1103 272 503 558 633 139 1105 311 528 (576) 551 139 6003 363 (571) (656) 552* 150 6005 369 (571) - - 473* 148

A16061 + 20wtpc t SiC - - - - 368 465 103

*Broke in grip region ( ) Extrapolated

Table VII. Compression Results for AI-Li-SiC Whisker Composites in the T6 Condition

Material

0.2 Pct Yield

Proportional Strength Limit (MPa) (MPa) CUS (MPa)

Modulus (GPa)

132 135 143 144

1103 297 584 807 1105 311 661 828 6003 386 704 869 6005 404 752 752

593

. A - - ~ - -~-E

755

-0--'-'-"----

_...a.

0.1 1 10 CREEP RATE (PCT PER SECOND)

Fig, 8 - - C r e e p strain v s stress data for composi te 1103.

to facilitate material comparisons, the stress to produce a strain rate of 2 pct per second at each temperature was extracted from the type of data shown in Figures 8 and 9.

The stress required to produce a 2 pct per second creep rate for composite 1103 at temperatures between 477 K and 866 K is shown in Figure 10. Data for an unreinforced AI-Li alloy of similar composition to the 1103 matrix is shown in Figure 10 for comparison. At the two extremes of the temperature range evaluated the creep strength of the matrix is not increased by the presence of SiC whiskers. However, at all temperatures in between 477 K and 866 K the whiskers produce a marked increase in creep strength.

The stress required to produce 2 pct creep per second at various temperatures in 6003 and 6005 is shown in Fig- ure 11. Data for AI 6061 and A1 6061 + 20 vol pct SiC whiskers are shown for comparison. The addition of lithium to an A1 6061-SIC whisker composite increases the creep

500

i0[

5C J

0.1 1 I0

CREEP RATE (PCT PER SECOND)

Fig. 9--Creep strain v s stress data for AI-3Li in the T6 condition.

500

A

60O 7OO 80O

TEMPERATURE (K)

Fig. 10--Stress to produce a creep rate of 2 pct per s for composite 1103-T6 and unreinforced AI-3Li-T6.

strength at all temperatures investigated. The increase in creep strength is greater for the composite with the highest lithium content (6005). With the exception of the tests run

METALLURGICAL TRANSACTIONS A VOLUME 13A. AUGUST 1982-- 1515

B

500

101 rd)05 (5.1 PCT, LI 11t ~11~]X)

6003 (3 PCT LI IN MATRIX)

, \ A16061 \

A]6OEI-20 WO SlC~

i r

TEMPERATURE (K)

Fig. 11 - - Stress to produce a creep rate of 2 pet per s for composites 6003, 6005, A1 6061-SIC, and unreinforced AI 6061, All materials are in the T6 condition.

at 700 K there appears to be no increase in creep strength produced by the addition of silicon carbide whiskers to an AI 6061 base unless lithium is present.

A comparison of the creep properties of SiC whisker and large SiC particulate composites in an A1 2024 base is

.500

AI 2024-TB51

~ 100

- (HIP)

50

c : l

5

do ' ' 7O0 800

TEMPERATURE (K)

Fig. 12--Stress to produce a creep rate of 2 pet per s for At 2024 + 25 wt pct SiC whiskers (HIP), AI 2024 + 21.8 wt pet SiC large particulate, and unreinforced A12024. The composites are in the T6 condition, the AI 2024 is in the T851 condition.

shown in Figure 12 with unreinforced AI 2024 shown for comparison. Both composites are unworked, the particulate composite is in the as-pressed and heat treated condition, and the whisker composite is hot isostatically pressed and heat treated to the same condition (T6). The whisker composite shows a small increase in creep strength above about 550 K compared to the A1 2024, while the particulate composite is slightly weaker than the AI 2024 at all temperatures tested.

V. DISCUSSION

The addition of lithium to aluminum-silicon carbide whisker composites appears to have several advantages. Pre- dictably the density of the composite is decreased, but in addition there are strength increases at room and elevated temperature. The difference between the room temperature strength of SiC whisker reinforced composites with and without lithium is illustrated in Figure 13. The effectiveness of the reinforcement is significantly greater in the lithium containing materials.

The part played by lithium in hardening the matrix alone as well as the composite is shown in Figure 14. Lithium hardens the aluminum by forming AI3Li precipitate. 8'9

The T6 condition produces a higher volume fraction of these precipitates and hence a higher strength than the T4 condition. Since the shear strength of the matrix affects the efficiency of load transfer in a discontinuously reinforced composite, the information presented in Figures 13 and 14 is used to isolate this factor from the effect of lithium. In Figure 15 the increment in yield strength due to 20 vol pet SiC whiskers is plotted against the yield strength

TENSION COMPRESSION

T6 T4 T6 T 4

I t �9 El []

l �9 ~ 0

1103

1105

6003

6005

f~ v T6 CONDITION

U~LLOYED ~ T4 CONDITION ALUMINUM

200

0 0 . i C,2 0,3

VOLUME FRACTION SiC

Fig. 13--The strengthening effect of SiC on an aluminum matrix with and without lithium.

1516--VOLUME I3A, AUGUST 1982 METALLURGICAL TRANSACTIONS A

�9 ~ T6 CONDITION

S Aged 16h 464K

500 / A A T4 CONDITION ~ Aged :> 16h 293/(

A 21 V/o SlC

"~ V

300

0 ^" ^ ~ AI-Li ALLOYS ONLY

200 - NO REINFORCEMENT

100

0

0 1 2 3 4 5 6

WEIGHT PERCENT LITHIUM

Fig. 1 4 - - T h e effect of lithium and aging temperature on the strength of AI-Li alloys and AI-Li-SiC whisker composites.

700

600

500

100

ESTIMATED INCREMENT OF STRENGTH FOR 20 v/o

OF WELL BONDED SiC WHISKERS OF ~ = 20

AI 6061-T6 �9 A1 2024-T6

UNALLOYED ALUMINUM

I i i i i

I00 200 300 400 500

O.2PCTTENSILE YIELD STRENGlltOFMATRIx (MPa)

Fig. 1 5 - - T h e increase in strength due to 20 vol pct SiC whiskers as a function of matrix strength for lithium-free and lithium containing alloys.

of the matrix for four lithium-free matrices and two alloys which contain 2.3 and 3.5 pct lithium. The strengths of all the alloys are corrected down to the expected values for

20 vol pct SiC from the data in Figure 13. The data for AI 7075, A1 2024, and AI 6061 were produced by Exxon Enterprises Materials Division. 4 Also shown in Figure 15 is an estimated increment of strength expected for a whisker reinforced composite using the equation of Kelly and Davies m based on a typical whisker diameter of 0 .5 /xm, a typical mean whisker length after extrusion of about 10/zm, 4 a whisker strength of 7 million MPa, '] and suf- ficient shear strength in the matrix or the interface to transfer the load at an L/D of 20. The required shear strength would be of the order of 70 MPa '2 which allowing for work hardening would be obtained in a matrix of ultimate tensile strength approximately twice the shear strength or 140 MPa. This matrix strength level is reached in all the composites shown in Figure 15 with the exception of the unalloyed aluminum matrix which has an ultimate tensile strength of 53 MPa.

When the effect of matrix strength and the effect of lithium are separated as in Figure 15, there is a substantial strengthening effect of lithium on the composite. This strengthening effect is greatest at the highest lithium level, where the difference between the actual composite strength and the calculated composite strength is 43 pct. The most feasible mechanism by which lithium acts to improve the room temperature strength is by increasing the bond strength at the whisker-metal interface so that pullout is reduced. However, since whisker pullout is not eliminated by lithium additions, it can only be stated that the above results are consistent with this mechanism, although until quantitative fractography involving whisker pullout lengths has been conducted, other mechanisms cannot be ruled out. Figure 15 indicates that the potential increase in strength due to SiC whiskers is most closely approached at low matrix strengths. With higher strength matrices the whiskers become increasingly ineffective strengtheners, possibly due to metal- whisker debonding at high stress levels. Further work in this area may therefore produce whisker reinforced composites with higher strengths.

Improvements in elastic modulus for a given volume fraction are, however, likely to be small since hot worked A1-SiC whisker composites already have about the same longtitudinal modulus values obtained from continuous silicon carbide composites in aluminum 3,6 (Figure 16). In the range of reinforcement investigated, 1 pct by volume of SiC (whiskers or fibers) increases the elastic modulus of aluminum by 3.3 GPa.

The strength advantages demonstrated at room temperature by the lithium containing composites are also evident at temperatures above 500 K when compared to unreinforced A1 2024 (Figure 17). It appears that the creep strength of the composite below 500 K is controlled pre- dominantly by the strength of the matrix so that unreinforced AI 2024 is stronger at low temperatures than whisker reinforced alloys with inherently weak matrices such as A1-3.2- Li (matrix of 1103), A1 6061, AI 6061 + 3Li (matrix of 6003), and AI 6061 + 5.1 Li (matrix of 6005). Above 500 K where the strength difference between A1 2024 and the other matrices decreases, the whisker contribution becomes the major strengthening factor in the lithium containing composites. As described earlier, the lithium-free composites show a much smaller increase in strength compared to their matrix material, so that the elevated temperature properties of A1 6061 + 20 pct SiC can be matched

METALLURGICAL TRANSACTIONS A VOLUME 13A, AUGUST 1982--1517

5O0

L , J

2OO

100

A cONTINUOUS FIBERS

�9 WI~IS~RS

0 WHISKERS IN AI-Li A

/ /

/ RULE OF MIXTURES /

/~~PERIMENTAL

' I I I 0 2O ~ ~ ~

VOLUME PERCENT SiC

Fig. 1 6 - - T h e effect of volume fraction of SiC whiskers and SiC fibers on the elastic modulus of AI-SiC composites.

L!QO

AIZO24 SOLIDUS

3O0

;IC-.HIP

- . . o

AI2024-21.8 W/O SiC (PARTICULATE)

�9 I -100 500 E,00 7 ~

TEMPERATURE [R)

Fig. 17 - -Pe t change in creep strength with temperature of various SiC reinforced composites compared to unreinforced AI 2024 T851.

by replacing the composite with a stronger aluminum alloy such as AI 2024.

The elevated temperature properties of the whisker strengthened materials with an aluminum-lithium base can be represented schematically by three regions (Figure 18). In region I there is little difference in properties between the matrix alloy and the composite. The inference is that the properties in this region are controlled by the matrix strength, since the composite strength is increased by

REGION I ' REGION II REGION III SOLIDUS I

~'~'~"*',,~ " STRENGTU BECOMING o z 10o �9 ~. ~ MATRIX CONTROLLED

A

) . : \ SrRE,Gr, C0~R0UED, \ : k / - 8v~,x : \ : \ , "

', \ ~ \ ,i t

ca I0" * *

~ : : . . . . \ \ ,

, I Q ' i i ! m

TEMPERATURE (lO

Fig. 18--Schematic representation of the effect of whisker reinforcement on the creep properties of an AI-3Li matrix at various temperatures.

increasing the strength of the matrix, e.g., changing from At 6061 to A1 2024. In region II the drop in strength with increase in temperature is small. Since region I1 does not occur in either the unreinforced alloy or the whisker reinforced composites with a lithium-free matrix, this region appears to be a result of strengthening by well bonded whiskers. In region II1 the difference between the strength of the matrix alloy and the composite decreases until, in the case of composite 1103 the difference is zero about 45 K below the solidus. In region Ili the decreasing influence of the whiskers may be due to the fact that the shear strength of the matrix is rapidly weakening and it is becoming increasingly unable to transfer the load to the whiskers. The matrix thus becomes the controlling factor again. These three regions are not apparent in the whisker reinforced aluminum composites which do not contain lithium because of the reduced strengthening effect of the whiskers in these materials.

In order to clarify the question of the influence of bond strength and whisker pullout, a magnesium alloy (AZ31) reinforced with 20 vol pet of SiC whiskers was tested in tension at room temperature and in compression creep at elevated temperatures. The significant point about this composite is that whisker pullout after fracture was not observed, indicating an excellent bond strength. The tensile properties were 566 MPa ultimate and 538 MPa yield strength. The strength increment produced by the SiC whiskers was therefore 434 MPa above the yield strength of the annealed matrix (104 MPa). Referring to Figure 15 this would place this composite above the values for composites with matrices of A1 2.3 Li and A1-3.5Li. Considering the reduced load transfer capability of the lower modulus matrix, l~ this composite may be strengthened to a value ap- proaching its theoretical limit. The creep properties of this composite are summarized in Figure 19 and show a very pronounced region II where the creep strength decreases very little with increase in temperature. This supports the above hypothesis that this region is a result of strengthening by well-bonded whiskers.

The creep tests on the particulate Al 2024-SIC composite are particularly significant at this time since these materials are being evaluated as possible replacements for whisker composites. The creep properties of the particulate composite (Figure 12) are lower than those of a whisker reinforced composite in the same condition (i.e., unworked)


A

a .

500

!00

10

Mg(AZ31} + 20 Vo SiC WHISRER$

h,

5 0 6 0 700 ~NO

TEMPERATURE (K)

Fig. 19--Stress required to produce a creep rate of 2 pct per s for a magnesium alloy reinforced with 20 pot SiC whiskers.

and even lower than the strength of the matrix alone. In its present form, therefore, this type of composite would not be selected where optimum high temperature performance is required. Particulate composites can be made with good high temperature strength by making use of the dispersion strengthening effect of fine stable dispersoids as used, for example, in Du Pont's thoria dispersed nickel 20 pct chro- mium alloy.13 However, to use this mechanism efficiently the particle size should preferably be less than 0.1 /x m so that the interparticle spacing is sufficiently small to act as an effective barrier to dislocation movement. The A1 2024-SIC particulate composite tested in this work has SiC particles that are typically 10 tzm long with a low aspect ratio (Figure 20) and therefore produce a negligible dispersion strengthening effect.

The anticipated role of lithium on the elevated temperature creep properties would be to increase whisker-matrix bond strength in the manner suggested for the improvement in room temperature tensile properties. However, since the creep tests in this work are in compression, an increase in bond strength will manifest itself as a reduction in the degree of whisker debonding and void formation at regions of tensile stress rather than a change in the amount of whisker pullout. A description of matrix-particle debonding under compressive loading for spheroidal high modulus particles has been given by Webster 14 for thoria dispersed nickel.

VI. CONCLUSIONS

1. The addition of 3 to 5 pct lithium to aluminum-silicon carbide whisker composites produces the following bene- ficial effects: a. A reduction in density of 5.7 to 9.6 pct. b. An increase in elastic modulus of 26 to 33 pct.

METALLURGICAL TRANSACTIONS A

Fig. 20--Optical micrograph of SiC particulate composite (DWA) showing aluminum grain structure and particle distribution. Magnification 235 times.

.

c. An increase in elevated temperature creep strength in the range 500 to 800 K.

The strengthening effects of adding lithium to aluminum- silicon carbide whisker composites are consistent with an increase in the bond strength between the whiskers and the matrix although no chemical reaction was observed between the silicon carbide whiskers and the aluminum- lithium matrix.

R E F E R E N C E S

1. W.H. Sutton and J. Chome: Met. Eng. Quart., 1967, vol. 3, p. 1. 2. A.P. Divecha, P. Lare, and H. Hahn: AFML-TR-67-321, October

1967. 3. D. Webster: Advances in Composite Materials, Proceedings of Third

International Conference on Composites, Paris, 1980, Pergamon Press, 1980, vol. 2, p. 1165.

4. Unpublished data, J. Cook Exxon Enterprises Materials Division, P.O. Drawer H, Old Buncombe at Poplar, Greer, SC, 1980.

5. D. Webster and D.D. Crooks: Metall. Trans. A, 1975, vol. 6A, pp. 204%54.

6. D. Webster: Enigma of the Eighties: Environment, Economics, Ener- gy, Proceedings of 24th National SAMPE Symposium and Exhibition, San Francisco, CA, 1979, published by SAMPE, Azusa, CA, 1980, vol. 24, book 2, pp. 1165-76.

7. G.B. Pinkerton: Lockheed Missiles & Space Co., P.O. Box 504, Sunnyvale, CA, Final Report on Contract N60921-78-C-0167, March 1980.

8. B. Noble and G. E. Thompson: Metal Science Journal, 1971, vol. 5, pp. 114-20.

9. D. Webster, G. Wald, and W. S. Cremens: Metall. Trans. A, 1981, vol. 12A, pp. 1495-1502.

10. A. Kelly and G.J. Davies: Met. Rev., 1965, vol. 10, pp. 1-77. 11. R.L. Mehan and J.A. Herzog: Whisker Technology, Wiley-

Interscience, New York, NY, 1970, pp. 157-95. 12. J.V. Mitewski: Plastics Compounding, 1979, Nov/Dec, pp. 17-37. 13. D. Webster: Trans. ASM, 1969, vol. 62, pp. 936-48. 14. D. Webster: Trans. TMS-AIME, 1968, vol. 242, pp. 640-48.

VOLUME 13A, AUGUST 1982--1519

Documents

Effect of lithium on the mechanical properties and microstructure of sic whisker reinforced aluminum alloys