Effect of prior α grain size on the strength of ordered Ni4Mo

ScriDta METALLURGICA

Vol. 20, pp. IS61-1563, 1986 Printed in the U.S.A.

Pergamon Journals Ltd. All rights reserved

EFFECT OF PRIOR ct GRAIN SIZE ON THE STRENGTH OF ORDERED Ni4Mo H. P. Kao and C. R. Brooks Materials Science and Engineering Department The University of Tennessee Knoxville, TN 37996

(Received July 18, 1986) (Revised August 18, 1986) Introduction The alloy Ni-20 at. % Mo has a short-range ordered, FCC structure (~) above 868oc, and an ordered, tetragonal structure (B) below this temperature. The a phase can be retained by quenching, and then ordered by heat treatment below 868oc. The ordering reaction occurs by the rearrangement of the atoms on the FCC lattice, so that the B retains a close-packed structure and a crystallographic relation to the ct from which it formed ]. The domains form with six crystallographic variants relative to the ~, leading to three types of domain boundaries. Thus, isothermal aging can develop a structure of fine domains inside the former ~ grain boundaries, in this condition, the only high angle boundaries between B crystals (domains) are those which were inherited from the former a crystals. (The physical metallurgy and mechanical properties of Ni-Mo alloys, including Ni4Mo, have been reviewed in a recent paper (I).) Upon aging (in the range 600-868oc) to form 13, the strength and hardness increase, the magnitude depending upon the aging temperature and time; for example, after 30 rain at 775°C, the yield strength has approximately doubled from the as-quenched (a) value. Simultaneously, there is a decrease in the ductility, ~nd the alloy becomes very brittle (e.g., < 5% elongation at fracture). In the ordered condition, the alloy fractures along the high angle boundaries (the former (~ grain boundaries) (I). Since these boundaries are important in fracture, we have examined the influence of the prior ~ grain size on the yield strength, and we report here the results. Experimental Procedure The alloy was prepared by induction r e m e l t i n g under vacuum (0.1 Pa) a N i - M o master alloy with Ni correction, and casting into a w a t e r - c o o l e d Cu mold to produce ingots I9 mm in d i a m e t e r and about 60 mm long. Chemical analysis showed t h a t the alloy contained (wt. %) 70.50% Ni and, by difference, 29.50% Mo. Other elements present were quite low (for example, S < 2 ppm, P 1 ppm, Fe 10 ppm, Cr l0 ppm, etc.). The ingots were homogenized at 1150oc in a dry hydrogen-argon atmosphere for 100 h, then furnace cooled. They were reheated to 1000oc for 1 h, then w a t e r quenched to retain the c¢ phase. Then the ingots were rolled at 25°C, with i n t e r m e d i a t e anneals at 950-1000oc in argon followed by w a t e r quenching, to produce sheet 0.51 mm thick. From the sheets, tensile samples conforming to ASTM E8-66 s p e c i f i c a t i o n were punched; these had a gage length of 12.7 ram. The tensile tests were conducted at 25oc using a screw-driven Instron machine at a constant cross-head speed, giving a strain rate of 0.2 rain -1. The y i e l d strength was taken from the loadstrain curves at 0.2% strain. This stress was close t o the apparent p r o p o r t i o n a l l i m i t . Where the sample fractured at a strain less than 0.2%, the f r a c t u r e stress was taken as a measure of the yield strength. The e r r o r in the yield strength values is 10% or less. For heat t r e a t m e n t , the tensile samples were sealed in silica tubes under rough vacuum, held for d i f f e r e n t times (from 30 rain t o 2 h) at d i f f e r e n t temperatures (from 950 to 1200°C), to obtain a range of c~ grain sizes, then w a t e r quenched. This t r e a t m e n t was found to r e t a i n c o m p l e t e l y the short-range ordered, ct phase. The surface was then cleaned by eleetropolishing. The samples were then all aged at 725°C for 100 rain, giving a structure w i t h a domain size of 10 nm and a long-range order p a r a m e t e r of 0.7. in this condition, the d u c t i l i t y was low (<5%). The samples were again electropolished before tensile testing. The prior ¢¢ grain size was d e t e r m i n e d q u a n t i t a t i v e l y by measurements on metallographic samples taken from the tensile samples a f t e r testing.

1561 0036-9748/86 $3.00 + .00 CoDyri~ht (c] 1986 Pergamon Journals

Ltd.

1562

STRENGTH OP Ni4Ho

Vol. 20, No. 11

Results and Diseussion The results are tabulated in Table 1, and the yield strength is plotted in Fig. 1. Note that the elongation at fracture decreases with increasing grain size, being only 0.1% for a grain size of 113 ~m. The yield strength is plotted versus d -1/2 in Fig. 2. The non-linearity shows that the Hall-Peteh relation (2) is not followed. Instead, Fig. 3 shows that the yield strength is proportional to d-3/4. Shulson (3) has pointed out that the yield strength-grain size relation of many ordered alloys follows the Hall-Peteh relation. The only exception at that time was Ni3A1. Using data from tensile and eompresion tests, and on material made by different methods, $hulson reported that Ni3AI follows the relationship o y = 93

+ 2080 d-0"80

Eq. 1

Here the grain size is in pro, and the strength in MPa. The Ni3AI alloy yields discontinuously, and Shulson (3) argues that the yield strength is proportional to the square root of the disloeation density generated during the Luders strain. This dislocation density is taken as inversely proportional to the average distance that the dislocations move, and this distance is taken as the grain size. This then gives an expression for the yield strength being inversely proportional to the grain size to the 3/4 power, in good agreement with the findings (Eq. 1 above). The Ni4Mo ordered alloy follows the relationship Oy = 545 + 2799 d -0"75

Eq. 2

similar to that for Ni3A1. However, this alloy does not show any discontinuous yielding. It is not elear what the mechanism is that gives this particular grain size dependence. It may be related to the residual dislocation structure. The aged samples do have dislocations in both the domain interfaces and inside the domains. Aeknowledffements Appreciation is expressed to Prof. B. F. Oliver for assisting in the alloy preparation, and Dr. C. T. Liu of Oak Ridge National Laboratory for obtaining the chemical analysis and arranging the use of equipment. References 1. 2. 3.

C.R. Brooks, J. E. Spruiell and E. E. Stansbury, Int. Metals Rev. 29, 210 (1984). J . P . Hirth and J. Lothe, Theory of Disloeations, 2rid edition, Wiley, New York (1982). E.M. Sehulson, Mat. Res. Soe. Syrup. Proe., Vol. 29, Ed. by C. C. Koch, C. T. Liu and N. S. Stoloff, Materials Research Society, Pittsburgh (1985).

Vol.

20, No.

ii

STI~.ENC~TH 0 n N i 4 H o

Annealing T~Cr ~

TABLE 1 Yield Strength (Y.S), Fracture Stress (T.S.) and Elongation at Fracture of Ordered Ni#Mo for Various Prior a Grain Sizes. The Original Solution Heat T r e a t m e n t s to O b t a i n t h e Grain Sizes Are Given.

1563

Annealing Time (rr.n.)

Average

ASTM NO.

Grain S*ze (t~m)

Y.S. (MPa)

PS0

30

12.0

P.4

1000

30

18.0

8.2

1100

30

33.1

6.5

1100

120

4&.6

5.4

I

1200

120

$P.2

4.8

1;~00

1440

I

•

T. $. (MPa)

Elongation

gP4.0

1076.0

2,6%

" 072.0

1009.0

4'.4%

751.5

722.0

0.7%

695.0

706.5

0.2%

660.5

660.$

0.5%

634.0

1834,0

0.1%

m

11 ]

2,0

Heat Treatment Procedure: Step 1 AnneaNn 9 at separate temperatures (PS0*C. 1000"C. 1100"C. and 1Z00*C) for 30,120. or 1440 rain. 1o get different prain size, then wa te r quenched. Step 2 Aped at 72S'C for 100 rain., then wa te r quen¢hed ,to g*ve same LNO parameter (0.7) and same

d o m ~ n size (10 nm). ttoo

I

I

1 L LR0:0 7

--1000

I

[ 150

ZE: to nm

9oo

t30

SO0

.o g

700 i -

o 600

m

go ~ ta

Fig. 1 The effect of prior cc grain size on the yield strength of ordered Ni4Mo.

7- so0 t ZO

400

1 40

I I I 60 80 t00 GRAIN SIZE ~/~m)

t 420

t100

140

~too

t60

I

t40

120

~ 800

I

I

I

150 130 ~=

-= 900 :c

~ z

800 700

~ 700

600

~ 600 w 500 4OO

I

1000

t000

t~ 80 ~" I 0.05

I

I

I

O.t0 O.t5 0.20 (GRAIN $tZE) " t / 2

I

I

O.Z5 0.30 (~m} - t / 2

I O,35

Fig. 2 The yield strength versus the reciprocal of the square root of the prior a grain size. The non-linearity shows that the Hall-Petch relationship is not followed.

s

~t0 ~

t72

S

90 o

500 400

70 I

I

,I

I. ,,

0.04 O.O8 0,12 O.16 (GRAIN SIZE) -O'75 (/~m)-0"75

0.20

Fig, 3 The yield s t r e n g t h versus t h e r e c i p r o c a l of the prior ~ grain size to t h e 314 power) showing t h a t a linear relationship is followed,