Basic and Appl ied Research: S e c t i o n I

The Cr-Ni-V S y s t e m S.K. Singh* and K.P. Gupta**

Department of Metallurgical Engineering Indian Institute of Technology

Kanpur-208016, India

(Submitted August 2, 1993)

The Cr-Ni-V system was investigated at 1100 ~ using XRD, metallography, and energy dispersive chemical analysis of phases using scanning electron microscopy (SEM). The bcc c~ phase extends from the Cr-Ni binary to the V-Ni binary and has a maximum solubility of Ni in ternary cc phase al- loys of ~12 at. %. Awide fcc T phase field exists at the Ni corner. A ternary ~ phase (if2) exists; it differs from the binary Ni-V ~ phase (~l). The ffl phase has a tetragonal unit cell with parameters almost double those of a normal 6 phase (~2)- Results indicate difficulty in achieving equilibrium states in alloys containing more than 30 at. % V and less than 15 at. % Cr even after 480 h annealing. The phase analysis of alloys annealed for 170 to 480 h suggests <~2 first forms through a ternary peritectic reac- tion to subsequently develop the ffl phase by subsidiary reactions in those ternary alloys in the con- centration range where ~1 is the stable phase. A tentative reaction scheme, consistent with the phase relations established at 1100 ~ is proposed.

Introduct ion

Prior to 1987 during the evaluation of Ni base ternary systems by the Indian Alloy Phase Diagram Program, it was observed that no experimental data exist for the Cr-Ni-V system. An in- itial investigation of the Cr-Ni-V system was therefore under- taken, and the ], phase boundary was determined at 1100 ~ [89Mal]. During this first investigation, there were metal- lographic indications that the r phase from the Ni-V binary may not extend far into the ternary. Kodentzov et al. [87Kod] reported phase equilibria in the Cr-Ni-V system at various tem- peratures as established by electron probe analysis of phases produced in diffusion couples formed with Cr-Ni alloys and vanadium. The report indicated that a wide ~ phase field ex- tends from the Ni-V binary far into the ternary. This is contrary to the observation made by Malhotra et al. [89Mal]. The Cr-Ni- V system was reinvestigated at 1100 ~ and those results are reported here. The accepted binary diagrams for this investiga- tion are Cr-Ni [90Gup], Cr-V [90Gup], and Ni-V [9 IGup].

E x p e r i m e n t a l Procedure

This investigation used 99.9% pure Ni (supplied by Gallard Schelsinger Mfg. Co., USA), Cr (supplied by Union Carbide, USA), and V granules (supplied by Semi Elements Inc., USA). The alloys were prepared by arc melting under high purity ar- gon atmosphere. Each alloy was melted three times with inver- sion between each melting to produce a homogeneous alloy. The melting losses were <0.5 wt.%; hence no chemical analy- sis of the alloys was done. A total of 46 alloys were used to es- tablish the 1100 ~ isothermal section of the Cr-Ni-V system. The nominal compositions of the alloys are given in Table 1. Most alloys were sealed in evacuated fused silica tubes, an- nealed at 1100+ 1 ~ for 170 h, and then quenched in water though some were annealed for longer periods ranging from 192 to 480 h. Characterization of annealed alloys was done by XRD and metallography. Chemical analyses of phases in se-

lected samples were done by energy dispersive spectroscopy with a JEOL JSM-840A scanning electron microscope (JEOL Ltd., Tokyo, Japan) (SEM-EDS). XRD patterns were taken with a Seifert 2002 ISO-Debyeflex diffractometer (Rich. Seif- ert & Co., Gmbh & Co. KG, Ahrensburg, Germany) using Cr Kct radiation obtained with a graphite focusing crystal mono- chromator. High resolution diffraction patterns, practically free of 13-reflections and other background, were obtained at a slow scanning speed of 1.2 ~ in 20/min. For those alloys that were not brittle, powder was prepared by filing with sub- sequent annealing at 1100 ~ for 20 min in evacuated fused sil- ica capsules. Powder from brittle alloys was used directly for XRD work. The etching reagent found most suitable for metal- lography of all alloys was 10% aqueous solution of oxalic acid; etching was done electrolytically.

Res u l t s and D i s c u s s i o n

Results from the chemical analyses of and phases present in the Cr-Ni-V alloys are given in Tables 1 and 2; data are based on XRD, metallography, and SEM-EDS analyses. Interpreta- tion indicates that the bcc ~ phase extends from the Cr-Ni to the Ni-V binary. The ~ phase boundary ofa Cr:V ratio of 40:60 was obtained from the lattice parameters of a series of alloys having this ratio with varying Ni content. Lattice parameters of these alloys are plotted against Ni content in Fig. 1 and show the boundary composition near 12 at.% Ni. In the regions of the Cr-Ni and V-Ni binary limits, the metaliographic method lo- cated the ct phase boundaries. Lattice parameters of ~ phase al- loys containing 10 at.% Ni with varying Cr:V ratio when plotted against V content, as in Fig. 2, show a linear increase with rising V content.

*Present Address: Vikrarn Sarabhai Space Centre, Trivandrum, Ker- ala, India ** Present Address: Flat No. C-201,93/2 Kankulia Road, Calcutta, W. Bengal, India Pin Code - 70029.

Journal of Phase Equilibria Vol. 16 No. 2 1995 129

S e c t i o n I: B a s i c a n d A p p l i e d R e s e a r c h

As reported by Kodentzov et al. [87Kod], a o phase was ob- served near the middle of the ternary system. However, in con- trast 1o their report of a wide o phase field extending from the Ni-V binary into the ternary, the o phase field was found to be a narrow, lens-shaped region, only ~5 at.% wide across the Cr:V ratio of 40:60 and extending from - 17 at.% V to -55 at.%

V. On its Cr-V side, this o phase is in equilibrium with the bcc cz phase. SEM-EDS analyses (Table 2) of the et phase in the o phase matrices found ot phase compositions in reasonable ac- cord with the ct phase boundaries determined through metal- lography and XRD analyses. On the Ni-V side, the o phase at lower V contents was found in equilibrium with the fcc 7

2.98

[

2.96

V : C r = 6 0 : 4 0

2.94

I 2.92 I I I I I

0 5 10 15 20 2 5 At. Pct. Ni

F i g . 1 V a r i a t i o n o f l a t t i c e p a r a m e t e r o f ct p h a s e a l l o y s h a v i n g a

V : C r r a t i o o f 6 0 : 4 0 a s a f u n c t i o n o f N i c o n t e n t .

3.05

10 A'L Pct. Ni

o< 3.00

._c 2.95 o

2.90

2.85[ I I I I 0 20 40 60 80 100

At. Pct.V

Fig. 2 Variation of lattice parameter ofct phase alloys containing 10 at.% Ni as a function of V content.

Table 1 Phase Analyses of C r-Ni-V Alloys Annealed at 1100 ~

Alloy Intended composition, at. % No. Cr Ni

Annealing Phase analysis V time, h XRD Metallography SEM-EDS

A32 ........................................................ 83 A20 ........................................................ 80

A4 .......................................................... 50 A38 ........................................................ 38

A28 ........................................................ 36

A37 ........................................................ 34.8 A40 ........................................................ 33.8

$4 .......................................................... 31

S12 ......................................................... 29.2

S15 ......................................................... 28

A l l ........................................................ 10

AI2 ........................................................ 10

A13 ........................................................ 16

A I 4 ........................................................ 18 A42 ........................................................ 26

S l l ......................................................... 24.7

$3 .......................................................... 23 A I 8 ........................................................ 30 A19 ........................................................ 35

S13 ......................................................... 44

$8 .......................................................... 40 S17 ......................................................... 34 S16 ......................................................... 39

$20 ......................................................... 44

S1 .......................................................... 17

$2 .......................................................... 21 SI0 ......................................................... 17

$21 ......................................................... 20 A7 .......................................................... 10

A8 .......................................................... 10 A30 ........................................................ l l

A36 ........................................................ 11

7 10 170 10 10 170

40 10 170 a + 7 5 57 170

10 54 170

13 52.2 170

16.2 50 170 ~ + o 2

23 46 170 ~ + o 2

27 43.8 170 ~ + O 2

30 42 170 ~ + o 2 61 29 170 7

56 34 170 7

60 24 170 7

55 27 170 7

35 39 170 0 2

38 37.3 170 02 43 34 240 o2 + Y

60 10 170 7 55 10 170 Y

31 25 170 02 +

35 25 170 02 +

41 25 170 02 + y 41 20 170 02 + y

40 16 170 02 + Y

35 48 170 02

35 44 170 02

28 55 170 02 + 0[

25 55 170 0 2 + 0[ 40 50 240 + 240(a) o + ~ b )

30 60 240 ,480 01

33.5 55.5 240 o(b)

24 65 240, 480 o 2 +

1 ~hase 2 ~hase

2 ~hase 1 ~hase

1 ~hase

2 ~hase 2 ~hase

2 ~hase

2 ~hase

2 ~hase

1 ~hase

3 ~hase

1 )hase

2 ~hase

1 ~hase

2 phase 2 phase

1 phase

2 phase

2 phase

2 phase

2 phase 2 phase

3 phase

2 phase

2 phase 2 phase

2 phase 2 phase 3 phase

2 phase 2 phase

(a) 240 h anneal ing at 1125 ~ fol lowed by 240 h annealing at 1100 ~ (b) o represents o I or o 2 phase, which could not be identified by XR D.

(~+ 0 2

Y+o n +0 2

7 + 0 2

01 + 0 2

0 2 + 7

o 2 +Y 7+02+0[ 01 4- 0 2

01 + 0 2

O 2 + 0 [

O! + O 2 + 0 [

O 1 4- 0 2

0 + c~(b) (continued)

130 Journal of Phase Equilibria Vol. 16 No. 2 1995

B a s i c a n d A p p l i e d R e s e a r c h : S e c t i o n I

Table I Phase Analyses of Cr-Ni-V Alloys Annealed at II00 ~ (continued)

Alloy Intended composition, at. % Annealing No. Cr Ni V time, h

Phase analysis XRD Metallography SEM-EDS

A39 ........................................................ 12.5 A21 ........................................................ 10 5 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

$5 ........................................................... 5 S 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$9 ........................................................... 5 S14 ......................................................... 8 S18 ......................................................... 2.8 A31 ........................................................ 10 A34 ........................................................ 5 S19 ......................................................... 5 A33 ........................................................ 5 $22 ......................................................... l0 $23 ......................................................... 7

22.5 65 240 U 2 + ~ 2 phase o + ct(b) 11 79 170 t~ 1 phase ... 35 57 170 01 2 phase o I + o(b) 35 60 170 01 2 phase ... 35 65 192 01 1 phase ... 30 65 170 U 1 + U 2 2 phase ... 27 65 240 o 2 + o I 2 phase a + o(b) 32.2 65 170 f f l 2 phase ... 13 77 170 a 2 phase a + o(b) 22 73 240 o + a(b) 2 phase ~ + o(b) 40 55 170 01 + y 2 phase T + o(b) 45 50 240 01 + y 2 phase T + o(b) 38.2 51.8 240 o 1 2 phase ... 40 53 240 o 1 + 5' 2 phase 01 + o(b)

(a) 240 h annealing at 1125 ~ followed by 240 h annealing at 1100 ~ Co) o represents o I o r 0 2 phase, which could not be identified by XRD.

T a b l e 2 SEM-EDS Analyses of Phases i n C r - N i - V A l l o y s

Elements in different phases, at. % Alloy ct T ol 02 No. Cr Ni V C r Ni V Cr Ni V Cr Ni V

A8 .............. 7.8 3.7 88.5 . . . . . . . . . 2.2 31.9 65.9 11.3 30.7 57.9 A8(a) .......... 7.4 5.0 87.6 . . . . . . . . . 2.8 32.2 65.0 11.3 30.8 57.9 AI2 . . . . . . . . . . . . . . . . . . . . . 9.5 56.9 33.6 4.7 33.2 62.1 16.7 42.4 40.9 A14 . . . . . . . . . . . . . . . . . . . . . 17.1 55.6 27.3 . . . . . . . . . 29.1 39.9 31.0 A30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 32.2 65.8 11.0 33.4 55.6 A31 ............ 10.4 12.4 77.2 . . . . . . . . . 6.8 26.2 67.0(b) . . . . . . . . . A34 ............ 6.5 9.7 83.8 . . . . . . . . . 5.1 28.1 66.8(b) . . . . . . . . . A36(a) ........ 15.9 12.0 72.1 . . . . . . . . . 10.4 28.8 60.7(b) . . . . . . . . . A39 ............ 8.9 13.5 77.7 . . . . . . . . . 11.0 25.2 63.7(b) . . . . . . . . . SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 32.1 64.3 16.5 35.4 48.2(b) $7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34.3 65.7 . . . . . . . . . SI0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 31.7 65.5 15.5 29.8 54.7(b) S l l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 32.6 61.1(b) 25.5 37.7 36.8 S15 ............. 46.8 11.6 41.6 . . . . . . . . . . . . . . . . . . 25.7 32.0 42.3 S16 . . . . . . . . . . . . . . . . . . . . . . 15.1 55.1 29.8 . . . . . . . . . 24.6 40.5 34.9 $20 ............. 74.8 14.1 11.1 32.7 53.8 13.5 . . . . . . . . . 50.4 35.4 14.2 $21 ............. 28.3 14.1 57.6 . . . . . . . . . . . . . . . . . . 16.3 29.1 54.6 $23 ............. ... . . . . . . . . . . . . . . . 1.7 32.4 65.9 7.4 39.6 53.0(b)

(a) Alloys annealed at 1100 ~ for 48 h. (b) The 0 2 phase composition lies between the o I and ~2 phase regions.

Fig. 3 Microst ructure o f $ 2 0 al loy s h o w i n g three phases . A and D are fcc ] ,phase. B is bcc ~ phase . C is ~2 phase .

p h a s e . A t h i g h e r v a n a d i u m c o n c e n t r a t i o n s , t h e s e c o n d p h a s e in

t h e t~ m a t r i c e s o f s e v e r a l a l l o y s h a d c o m p o s i t i o n s c h a r a c -

t e r i s t i c o f t h e N i - V t~ p h a s e , - 2 to 3 a t . % Cr. T h u s at 1 1 1 0 ~

e v i d e n c e i n d i c a t e s t h e e x i s t e n c e o f t w o c p h a s e s in t h e C r - N i -

V s y s t e m . T h e c p h a s e c l o s e to t h e N i - V b i n a r y s y s t e m is d e s -

i g n a t e d h e r e a s t h e ~ l p h a s e , a n d t h e o t h e r ~ p h a s e o c c u r r i n g

n e a r t h e c e n t e r o f t h e t e r n a r y s y s t e m is d e s i g n a t e d a s t h e ~2

p h a s e . S E M - E D S a n a l y s e s o f p h a s e s in a t h r e e - p h a s e a l l o y

(F ig . 3) a t t h e C r - N i e n d o f t he t~ 2 p h a s e r e g i o n w e r e u s e d to f ix

t h e p h a s e b o u n d a r i e s o f t h e t~ 2 + ) ' + ct t h r e e - p h a s e r e g i o n .

T h e S E M - E D S a n a l y s e s o f s e v e r a l t w o - p h a s e a n d t h r e e - p h a s e

a l l o y s n e a r t h e ) ' a n d c 2 p h a s e r e g i o n s ( T a b l e 2) i n d i c a t e t h a t t h e

) ' p h a s e in e q u i l i b r i u m w i t h t he ~2 p h a s e h a s s l i g h t l y h i g h e r C r

a n d V c o n t e n t s t h a n t h o s e r e p o r t e d by M a l h o t r a e t al. [ 8 9 M a l ]

f r o m a n a l y s e s o f a l l o y s a n n e a l e d a t 1100 ~ f o r 4 8 h. S i n c e t h e

p r e s e n t w o r k s h o w e d t h e n e c e s s i t y o f l o n g e r a n n e a l , t h r e e o f

J o u r n a l o f P h a s e E q u i l i b r i a Vol. 16 No . 2 1995 131

S e c t i o n I: B a s i c a n d A p p l i e d R e s e a r c h

T a b l e 3 X R D P a t t e r n s o f t h e G 1 a n d (Y2 P h a s e s

For if2, a = 0.8934 nm, c = 0.4613 nm, c]a = 0.516; For if1, a = 1.7116 nm, c = 0.9353 nm, c la = 0.546 t~ 2 phase

Relative Observed Calculated Relative intensity distance, nm hkl distance, nm intensity

f f lphase

Observed distance, nm hld

Calculated distance, nm

21.0 ........................ 0.4101 101 0.4099

3.2 .......................... 0.3998 210 0.3996

5.1 .......................... 0.3726 111 0.3725

27.9 ........................ 0.2307 002 0.2306

97.1 ........................ 0.2165 112 0.2165

32.0 ........................ 0.2106 330 0.2106

42.8 ........................ 0.2049

73.9 ........................ 0.2000

202 0.2049

212 0.1998

420 0.1998

100.0 ...................... 0.1964 411 0.1961

61.6 ........................ 0.1917 331 0.1916

13.8 ........................ 0.1862 222 0.1863

17.4 ........................ 0.1785 312 0.1787

500,430 0.1787

6.5 .......................... 0.1690 322 0.1688

6.0 .......................... 0.1668 501,431 0.1666

0.1639 520 0.1659

3.0 .......................... 0.1630 511 0.1638

3.0 .......................... 0.1604 402 0.1605

2.3 .......................... 0.1580 412 0.1579

1.5 .......................... 0.1349 313 0.1351

522 0.1347

621 0.1351

22 .......................... 0.4142

2.8 ......................... 0.4035

1.4 ......................... 0.3852

5.6 ......................... 0.3766

30 .......................... 0.2333

5 ............................ 0.2264

5 ............................ 0.2226

89 .......................... 0.2188

22.5 ....................... 0.2125

24 .......................... 0.2100

23 .......................... 0.2072

59 .......................... 0.2022

100 ........................ 0.1983

46.5 ....................... 0.1936

11.3 ....................... 0.1885

1.4 ......................... 0.1843

4.2 ......................... 0.1818

12 .......................... 0.1804

1.4 ......................... 0.1767

4.2 ......................... 0.1706

0.1682

7 ............................ 0.1653

1.4 ......................... 0.1624

2.1 ......................... 0.1594

2.8 ......................... 0.1575

3.5 ......................... 0.1526

1.4 ......................... 0.1363

410

330 420

411 004

204

523

650

731

810, 740

324

722

603

820

811,741

334

732

660

652

812, 742

524

643

832

215

921, 76t

850

930

604

624

315

803

842

415

941

10, 1,0

425

554,714

932

10,0, 1

10,2,1

903

772 445

763,923

355

804

814, 744

216

664

616

10 ,2 ,4

12 ,0 ,2

755

883 11 ,6 ,0

982; 12, 1, 2

0.4151

0.4034

0.3827

0.3794

0.2339

0.2255

0.2226

0.2191

0.2185

0.2123

0.2098

0.2101

0.2105

0.2076

0.2070 0.2023

0.2026

0.2017

0.1984 0.1933

0.1883

0.1889 0.1841

0.1817

0.1821

0.1814 0.1804

0.1808

0.1769

0.1768

0.1764

0.1771

0.1705

0.1709

0.1703

0.1681

0.1682

0.1683

0.1684

0.1652

0.1624

0.1622

0.1591

0.1595

0.1577

0.1578

0.1572

0.1527

0.1527

0.1364

0.1364

0.1364

0.1363

0.1361

0.1366

0.1360

(conunued)

132 J o u r n a l o f P h a s e E q u i l i b r i a Vol . 16 N o . 2 1 9 9 5

B a s i c a n d A p p l i e d R e s e a r c h : S e c t i o n I

T a b l e 3 X R D P a t t e r n s o f t h e 6 1 a n d ~ 2 P h a s e s ( c o n t i n u e d )

For 02, a = 0.8934 nm, c = 0.4613 nm, c l a = 0.516; For t~l, a = 1.7116 nm, c = 0.9353 nm, c ] a = 0.546 0 2 phase

Relative Observed Calculated Relative intensity distance, nm hkl distance, nm intensity

t~ 1 phase

Observed distance, nm

Calculated hkl distance, nm

2.8 .......................... 0.1307 323 0.1307 442 0.1303

17.8 ........................ 0.1276 532 0.1276 631 0.1279 700 0.1276

5.1 .......................... 0.1264 550,710 0.1264

38.4 ........................ 0.1254 413 0.1254 602 0.1251

15.2 ........................ 0.1242 333 0.1242 612 0.1239

11.6 ........................ 0.1227 701 0.1230

15.9 ........................ 0.1219 711,581 0.1219 423 0.1217

5.1 .......................... 0.1205 622 0.1205

18.3 ....................... 0.1288

4.2 ......................... 0.1276

22.5 ....................... 0.1267

19.7 ....................... 0.1254

10.6 ....................... 0.1239

7.7 ......................... 0.1231

5 ............................ 0.1217

5.6 ......................... O. 1206

11,5,3 0.1290 13, 2, 1 0.1289

237 o. 1286 12,6,0 0.1276

407 o. 1275 945 o. 1273 337 o. 1268

12,5,2; 13,0,2 0.1267 12, 3, 3 o. 1265 12, 6, 1 o. 1264 13,2,2 0.1254 11,6,3 0.1251

796, 816 0.1257 11,4,4 0.1240 13, 3, 2 0.1237 12, 4, 3 0.1241

517 0.1241 12, 7, 0 0.1232

527 0.1232 12,6,2 0.1231

836 o. 1230 13, 5, 0 0.1229 13,5, 1 0.1218 12,0,4 0.1218

537 0.1216 617 0.1207 906 o. 1206

12,2,4 0.1206 965 o. 1208

11,9,0 0.1204

Malhotra's yphase boundary alloys (A11, A13, and A18) were reannealed at 1100 ~ for 122 h (a total of 170 h anneal). After this long anneal, these metastable two-phase alloys became single phase, indicating that the equilibrium T phase boundary at 1100 ~ has higher Cr and V contents than reported by Mal- hotra et al. [89Mal].

The existence of two t~ phases in the Cr-Ni-V ternary system is somewhat unusual. On the basis of thermal analyses of Ni-V t~ phase alloys, the existence of a high-temperature (above 700 ~ and a low-temperature form of t~ phase was proposed [52Pea] even though XRD revealed no crystallographic differ- ence between the two forms. The ternary ~ phase of the Cr-Ni- V system could be an extension of the low-temperature form of binary t~ phase, stabilized to higher temperatures by the addi- tion of Cr. To see the difference between the two r phases ex- isting at 1100 ~ and to check whether or not the ~2 phase is the low-temperature form of the Ni-V t~ phase, a binary alloy ($7) containing 35 at.% Ni and 65 at.% V (composition at the center of the binary 6 phase region) was prepared. After 192 h anneal- ing at 1100 ~ the alloy was found to be single phase. The XRD patterns of the t~ 1 ($7) and ~2 (A42) alloys annealed at 1100 ~ show (Table 3 and Fig. 4) that the t~ 1 phase has several minor diffraction lines that are not present in the diffraction

pattern of the 62 phase. The 6 2 phase diffraction pattern could be indexed completely on the basis of the normal 6 phase structure. The lattice parameters were c = 0.4613 nm and a = 0.8934 nm, and c/a = 0.516. The ol phase diffraction pattern could not be indexed on the same basis. All the diffraction lines of the ~1 phase could be indexed on the basis of a tetragonal cell with lattice parameters c = 0.9353 nm and a = 1.7116 nm, and c/a = 0.546. The cell parameters are almost double that of the 62 phase. The binary c 1 phase alloys annealed at 1100 ~ was further annealed at 650 ~ for 720 h in order to bring about a possible structural change in the t~ l phase. The diffraction pattern for the binary Ni-V t~ phase annealed at low tempera- ture was exactly the same as that of the 61 phase. Unless the long low-temperature annealing treatment given to the alloy was insufficient to bring about the desired structural change in the ~1 phase, then the 62 phase is not an extension of the binary t~ phase and may be treated as a true ternary t~ phase.

There are difficulties in establishing phase equilibria in the re- gion between the 62 phase and the Ni-V binary line for V con- centrations higher than 30 at.% and Cr concentrations less than 15 at.%. The main causes of these difficulties follow.

�9 The two t~ phases have very little difference between their XRD patterns. Only one medium intensity diffraction line

Journal of Phase Equilibria Vol. 16 No. 2 1995 133

S e c t i o n I: Bas ic and Appl i ed Research

A42

T c- 0J E

A7

$23

$19

,-Oq

$7

80 75 70 65 60 55 2e ~

Fig. 4 XRD patterns ofcy 1 and (~2 phases of a series of alloys con- taining <20 at.% Cr and40 at.% Ni.

Fig. 5 Microstructure of S1 alloy showing two phases. A is ~Y2 phase, and B is (~l phase.

(line number 10 in Table 3) of the 131 phase could be used ef- fectively for proper identification of the 131 phase in multi- phase alloys.

>.,, r E

bcc

~J ~ $10

$14

S9

$18

$7

80 75 70 65 60 55 50 20 ~

Fig. 6 XRD patterns of a series of alloys containing <20 at.% Cr and 65 at.% V.

�9 In the expected three-phase regions, the precipitated phases were usually quite small in size (even after 480 h anneal) es- pecially at higher V contents. The contrast difference be- tween some of the phase was also quite small and caused dif- ficulty in identification of the phases under an optical microscope.

�9 The highest intensity diffraction lines of the (z phase (110) and the 7 phase (111) superimposed on the two diffraction lines common to the two 13 phases. Moreover, the charac- teristic diffraction line for the identification of the 131 phase was between the two strong diffraction lines of the o~ and the I, phases, and presence of any one of these phases (c(or 7) ob- scured the 131 phase diffraction line.

�9 In two-phase alloys containing 131 and 132 phases, the 131 phase could be clearly seen under a microscope (Fig. 5), but the relative proportion of the phases did not seem to change in a systematic way with change in alloy composition. More- over, the SEM-EDS analysis of chemical composition of the matrix and the 131 phases always showed the precipitated par-

134 Journal of Phase Equilibria Vol. 16 No. 2 1995

Bas ic a n d Appl i ed R e s e a r c h : S e c t i o n I

Fig. 7 Microstructure of A8 alloy showing three phases. Matrix is o 2 phase. Ais Ol phase. B is ct phase. Magnification is 500x.

Fig. 8 Microstructure of A 12 alloy showing three phases. A is y phase. B is Ol phase. C is o2 phase.

90

20Z \ A~I X80

Fig. 9

3C

"" 40/ / / "~sxg~a / \

f: . o,o v / ,, \szl ~ s~2 o

0 0~13 ' 80 A13

O $15

90 o o A18 A~

Y V Y V V V

Ni 10 20 30 40 50 60 At. Pct. Cr

The expected 1100 ~ isothermal section of the Cr-Ni-V system.

70

,0

0 ~-

40

30

20

10

V V I V \ 70 80 90 Cr

ticles in the matrix to have a composition close to the 131 phase, and the composition of the matrix only slightly shifted from the gross composition of the alloy. The diffraction pat- terns, however, showed the presence of all the characteristic lines of the I32 phase, and if the alloy composition was closer to the 131 phase region, the characteristic 131 phase diffraction line was also present. All these observations clearly indicate that it is difficult to achieve equilibrium in the ratio of the phases as well as in the phase compositions, even when the alloys were annealed for 480 h at l 100 ~

Because of the experimental difficulties mentioned above, it was not possible to determine accurate phase equilibria in the composition region above -30 at.% V and below -15 at.% Cr. However, on the basis of analyses of XRD patterns, it is possi- ble to arrive at the expected phase equilibria. Through XRD, metallography, and SEM-EDS analyses of the observed phases in alloys in this region, the presence o f a c I + 132 phase region could be observed around a line of Ni content of 35 at.% Ni. At the higher V side of this two-phase region, a ~ + 131 + 132 phase region is expected. XRD patterns of a series of alloys

Journal of Phase Equilibria Vol. 16 No. 2 1995 135

Sec t ion I: Basic and Applied Research

V

N i ~ ~ Cr Cr- Ni Cr- Ni-V Ni-V Cr-V

13t,5~

f T2

T~

Fig. 10 The probable liquidus projection and reaction scheme for the Cr-Ni-V system.

containing the same amount of V but increasing Cr contents (in the expected three-phase region) are shown in Fig. 6. Figure 6 shows that as the Cr content increases, the intensity of the char- acteristic diffraction line due to the o I phase decreases. The other diffraction lines of the G] phase, however, do not change their intensities appreciably because of the presence of the G 2 phase. Only with a sufficient amount of ct phase does a strong t~ phase line appear in the diffraction pattern. Even though it was not possible to identify the c~ phase regions in a metal- lographic specimen of alloy S10, the XRD pattern of this alloy clearly shows the presence of a weak ct phase line. These data, together with the photomicrograph of the three-phase alloy A8 (Fig. 7) clearly show the existence of a G] + G 2 + Ct phase re- gion.

At the lower V side of the ~l + G2 phase region, a 7+ G~ + G2 phase region is expected. The diffraction patterns of a series of alloys (with constant Ni content) in the expected y + G 1 + G 2 phase region are shown in Fig. 4. Unlike the diffraction pat- terns of alloys in the t~ + G 1 + G2 phase region, Fig. 4 shows a strong superimposition of the 7 phase diffraction line on a dif- fraction line common to the two G phases (G 1 and G2) and on the characteristic diffraction line of the G l phase. For the two low Cr containing alloys, S19 and $23, the diffraction line of the G] phase is visible, but it is not visible in the diffraction pat- tern of alloy A7. The absence of the characteristic diffraction line of the G i phase in the diffraction pattern of alloy A7 is pos- sible due to the strong y phase diffraction peak completely masking the weak t~ 1 phase diffraction line. For the higher V containing alloys, the G 1 and G2 phases could be identified through the XRD patterns and SEM-EDS analyses. The chemical composition of the matrix of these alloys was shifted only slightly from the gross alloy composition and not near the G 2 phase region. The A 12 alloy, which has lower V content and is close to the y phase region, however, showed three phases (Fig. 8). SEM-EDS analysis (Table 2) showed the presence of ~'phase (matrix composition close to the gross alloy composi-

tion), G 1 phase, and c2 phase. The composition of the G 2 phase was again found several atomic percent off from the G2 phase boundary. In spite of a small chemical composition disparity of one of the phases, the A12 alloy clearly shows the presence of a three-phase region, y + G~ + G2. On the basis of these obser- vations, the expected 1100 ~ isothermal section of the Cr-Ni- V system is represented as given in Fig. 9.

In all alloys of the GI + G2 phase region, the G 1 phase appeared as small islands, and the matrix chemical composition was close to the gross alloy composition. The XRD patterns of these alloys, however, showed strong reflections of the G 2 phase indicating that the matrix is G 2 phase. These observa- tions suggest that at higher temperatures, the G2 phase region is possibly much wider than at 1100 ~ At temperatures higher than 1100 ~ whether the c 2 phase extends up to the Ni-V bi- nary is not known because this will require the G 2 phase to exist in the Ni-V binary, and at present no such experimental evi- dence exists. If G2 does not exist in the binary, then this phase must form by a ternary peritectic reaction, L + tz + ), ~ (Y2,

and precipitation of G l phase from the 62 phase may occur by subsequent reactions. A probable reaction scheme on this ba- sis, consistent with the phase relations observed in Fig. 9, is given in Fig. 10.

The diffusion couples used by Kodentzov et al. [87Kod] to es- tablish phase equilibria in the Cr-Ni-V system at 1150 and 1000 ~ were given diffusion anneal of 240 h only. Since the present results indicate difficulty in achieving equilibrium states in homogeneous high V and low Cr alloys even after an- nealing for 480 h at 1100 ~ it is doubtful that Kodentzov et al. could have achieved equilibrium states in their diffusion cou- ples. Since SEM-EDS analyses of 240 h or 480 h annealed al- loys- -A 7, As, A14 , etc.--show the matrix phase composition to be very close to the respective gross alloy compositions due to difficulty in achieving equilibrium, nonidentification of the G1 phase in binary and ternary alloys will give an apparently wide G phase field extending from the Ni-V binary. This may ex- plain why a wide t~ phase field was found at all temperatures by Kodentzov et al. [87Kod].

A c k n o w l e d g m e n t

The authors wish to acknowledge help of Mr. L. Vijayraghavan and Mr. S.K. Saha in some experimental work and Mr. V.P. Gupta for arc melting several alloys. The work was sponsored by the Aeronautical Research and Development Board, Gov- ernment of India, New Delhi.

Cited References

52Pea: W.B. Pearson and W. Hume-Rothery, "The Constitution and Structure of Ni-V Alloy s in the Region 0-50 at. pct. V,"J. Inst. Met., 80, 641-652 (1952).

87Kod: A.A. Kodentzov, S.F. Dunaev, and E.M. Slusarenko, "Determi- nation of the Phase Diagram of the V-Ni-Cr System Using Diffusion Couples and Equilibrated Alloys," J. Less-Common Met., 135, 15-24 (1987).

89Mal: A.K. Malhotra and K.P. Gupta, "Phase Equilibria in Cr-Ni-V System at 1100 ~ J. Alloy Phase Diagrams, 5, 132-135 (1989).

90Gup: K.P. Gupta, "Phase Diagrams of Ternary NickelAlloys, Part 1 ," Indian Institute of Metals, Calcultta, India, 1990.

91Gup: K.P. Gupta, "Phase Diagrams of Ternary Nickel Alloys, Part 2," Indian Institute of Metals, Calcultta, India, 1991.

136 Journal of Phase Equilibria Vol. 16 No. 2 1995