The effects of grain boundaries and precipitates on positron lifetime in inert gas atomized 304 stainless steel

Scripta METALLURGICA et MATERIALIA

Vol.

26, pp. 1199-1203, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

THE EFFECTS OF GRAIN BOUNDARIES AND PRECIPITATES ON POSITRON LIFETIME IN INERT GAS ATOMIZED 304 STAINLESS STEEL Jhiyong Kim and J. G. Byrne Department of Metallurgical Engineering University of Utah Salt Lake City UT 84112 ( R e c e i v e d F e b r u a r y 4, ( R e v i s e d F e b r u a r y 13,

1992) 1992)

Introduction Inert gas atomized 304 stainless steel powder is of interest after consolidation because its microstructure has the capability to remain stable during high-temperature annealing. In order to better understand this material, considerable work on mierostructural determination was done (1) in the form of transmission electron microscopy. To further resolve details of the microstructure, the Idaho National Engineering Laboratory undertook to explore the utility of positron lifetime measurements to follow changes in microstructural parameters. In particular, this paper explores this utility to examine grain growth and the growth of carbides. Both grain boundaries (2-4) and precipitates (5-8) have been successfully examined with positron annihilation techniques earlier. The carbides present in the current material are of the M23C6 type. Their average particle diameters and volume fractions as well as the average grain sizes of the samples (1) were made available for the studies of J. Kim (9). Ultimately, positron lifetime and intensity values correlated well with the known microstructural information on grain size and M23C6 carbide size and volume fraction as a function of annealing temperature. Table 1 shows the grain size behavior as a function of annealing temperature (1) and Table 2, although it shows only the behavior of carbides in the 30- to 50-#m range powder, is representative of the temperature response of the carbides in all of the powder size ranges mentioned in Table 1. One notices in Table 1 that the grain size generally begins to increase at about 1200°C and then very rapidly at 1300°C. Table 1. Average grain size (#m) of inert gas atomized 304 stainless steel samples after 1 hour annealing. After J. E. Flinn et al. (1).

10-30 ~m 30-50 #m 50-75/~m 100-150/zm 10-150 ~m

900°C

IO00°C

1100°C

1200°C

1300°C

7 7 7 7 7

7 8 8 8 8

7 8 8 8 8

19 22 21 23 22

228 208 208 200 194

1199 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press

Ltd.

1200

POSITRON LIFETIME

Vol.

26, No.

8

Table 2. Carbide precipitates (M23C6) behavior. After J. E. Flinn et al. (1).

Temp. (°C)

Particle Size 0zm)

Average Diameter (rim)

Volume Fraction (10 .4)

1000 1100 1200 1300

30-50 30-50 30-50 30-50

61 44 76 212

1.6 0.8 1.6 4.6

Exoerimental Procedures The inert gas atomized 304 stainless steel powders consisted of particle sizes in several ranges such as 10 to 30 /zm, 30 to 50 ~m, 50 to 75/~m, 100 to 150 ~m, and a "grab sample ~ consisting of 10 to 150 #m with a median equal to 50 zm. These powders were consolidated into bars by hot extrusion (heated to 900°C for 30 minutes and extruded to an 8:1 reduction in area). A series of one-bour anneals at intervals of I00°C from 900°C to 13000C was performed on these consolidated powders. TEM/STEM observations (1) of the samples established grain size and carbide size and volume fraction as a function of heat treatment. Positron annihilation samples were in the form of 1.5-mm thick sheet. These samples were mechanically polished with a 0.05-pm alumina slurry and then electropolished at 20 V in a solution of 94% ethanol, 5.4% perchloric acid and 0.6% deionized water. The positron source was prepared by placing a droplet (a few PtCi) of aqueous 22NaCI on one sample, evaporating the water and then covering the positron source with a second identical sample ("sandwich ~ configuration). The source correction was made by studying well-annealed A1. The measured lifetime spectrum was decomposed into two free exponential components (r I = 162 psec, ~'2 = 534 psec, and I2" = 3.4%). The longer lifetime component was attributed to the source component, and the short lifetime component was attributed to the perfect lattice. The source correction was made by subtracting a source correction spectrum from each measured spectrum. Exoerimental Results In order to be sure to attribute positron parameter changes only to grain growth, one must select a situation in which the carbides do not change, and vice versa. For example, in Table 2 for the 30- to 50-~m powder, both carbide parameters decrease in going from 1000 to 1100°C. If we now look at Fig. 1, we see that, for that powder size range (as well as for all the others), the mean lifetime, ~, falls off between 1000 and 1100°C. This change then can safely be ascribed to the reduction in volume fraction of carbide (see Table 2) and hence the amount of incoherent particle matrix interface available to act as a trapping site for positrons. (Note that in Table 1 the grain size does not change between 1000 and 1100°C). The further decrease o f f of the samples annealed at 1200°C was due to the factor of three increase of grain size as seen in Table 1. The increased ~ of the samples annealed at 1300°C will be shown to be due to growth of the carbide size and volume fraction for samples annealed at 1300°C. The latter can be explained by the increase of positron annihilations at the new trapping site of the incoherent matrix-precipitate interface (10, 11). Figure 2 shows the ¢2 values of samples consolidated from the various powder particle sizes. A two-term deconvolution produced the best results. The short lifetime component, ¢1, of all the samples, which was determined to be due to positron annihilations in the perfect lattice, converged to 115 ps with a standard deviation of a few ps. The second lifetime component, ~'2, which was determined mainly to be due to positron annihilation at grain boundaries and at incoherent matrix-precipitates interfaces, converged to about 250 ps.

*I2 is the intensity in percent of the second lifetime ¢2.

Vol.

26, No.

8

POSITRON LIFETIME

1201

A constraint was imposed on r 2 since r 2 scattered, independent of annealing temperatures, around mean value of 250 ps (11). The principal problem with using a constraint-parameter analysis is the question of how to extract the grain size and precipitate properties of interest from the intensity, 12, of the fixed second lifetime component, r 2. With r 2 constrained at 250 ps, the program produced the best overall statistics. In order to study the effect of grain size, one would want changes in 12 to only originate from grain boundaries. Therefore the 12 of the two 30- to 50-~,m samples, heat-treated at 1000*C and 1200"C, were compared, since their carbide average particle diameter and volume fraction values were essentially the same (see Table 2). Thus changes in I2 should be more closely related to the grain size. 12 decreases with increasing grain size in a way similar to that in which the mean lifetime decreases with increasing grain size. One sees in Fig. 2 that the fraction of positron trapping, 12, decreased by a factor of one-half for all particle sizes as the temperature increased from 1000 to 1200"C (or the grain size increased from about 8 ~m to about 21 t~m as in Table 3.) Table 3. Intensities 12 (%) of the lifetime component fixed at 250 ps.

Temp. (°C)

10-30 #m

30-50 t~m

50-75 #m

100-150 #m

10-150 #m

1000 1100 1200 1300

32.41 22.94 17.04 13.04

32.56 19.79 12.59 12.07

25.90 18.96 12.21 13.13

27.03 20.17 13.78 13.26

28.42 20.30 14.51 16.88

For incoherent particles, it is usually believed that the positron trapping is due to the incoherent precipitatematrix interface which has a low atomic density (10-12). Our results show that 12 is associated with the precipitate size and' volume fraction. It can be said that the positron lifetime at grain boundaries and precipitates is approximately the same and is the constrained value, 250 ps. The reason can be verified in the following manner. In the free exponential fitting without any constraint (Figure 2), the r 2 for the samples annealed at 1300°C converged to about 250 ps. The r 2 for the samples annealed at 1300°C cannot be thought of as due to positron trapping at grain boundaries, since the large grain size ( - 2 0 0 to -300/~m) of the sample precludes such trapping being a major contribution. This result indicates that r 2 is mostly a contribution from the precipitates. The substantially increased I2 of the sample annealed at 1300°C suggests an increased fraction of positron trapping at the incoherent carbidematrix interfaces produced by much carbide growth at this temperature. An additional point of interest in the study of the effects of carbides may be noted from two samples annealed at 1000°C and ll00°C (Table 1) which showed the same grain size and different precipitate data. Thus if in Table 3 we again compare the 30- to 50-/~m powder (also true for all sizes) between 1000 and ll00°C, we find that the fraction of positron trapping, I2, with the lifetime ~2, decreased by one-third corresponding to a decrease of the carbide volume fraction of one half with a decrease in carbide size of about 28%. Acknowledgement The authors wish to thank the U.S. Bureau of Mines and the Idaho National Engineering Laboratory/EGG for the financial support of this work. Dr. K. G. Lynn's assistance with using the computer program was invaluable, as were the many insights provided by Dr. John E. Flinn on the materials studied. References 1. J.E. Flinn, J. C. Bae, arid T. F. Kelly, "High Temperature Microstructural Stability in Iron and Nickel-Base Alloys from Rapid Solidification Processing," in Heat Resistant Materials, eds. K. Natesian and D. J. TiUack, ASM International, Metals Park, Ohio, 35 (1991). 2. K . G . Lynn, R. Ure and J. G. Byrne, Acta Met. 22, 1075 (1974). 3. I . K . Mackenzie, Phys. Rev. B16, 4075 (1977).

1202

4. 5. 6. 7. 8. 9. 10. 11. 12.

POSITRON LIFETIME

Vol.

26, No. 8

B . T . A . McKee, G. J. C. Carpenter, I. F. Watters, and L R. Schultz, Philos. Mag. A41, 65 (1980). S. Panchanadceswaren and L G. Byme, Philos. Mag. 43, 921 (1985). R. Krause, G. Dlubek, O. Brummer, S. Sidadnikiewitz, and H. Daut, Crys. Res. and Technol. 20, 267 (1985). A. Baharthi, C. S. Sundar, and K. P. Gopinathan, Philos. Mag. A.58, No. 5, 705 (1988). D. Tian, Z. X. Yu, C. G. Zhan, and S. L. Wang, Phys. Stat. Sol. A114, K135 (1989). L Kim, Ph.D. Thesis, University of Utah, 1991. N. de Diego and C. Hidalgo, Philos. Mag. A53, No. 1, 123 (1985). C. Hidalgo, N. de Diego, and F. Plazaola, Phys. Rev. B31, 6941 (1985). C. Hidalgo and N. de Diego, Appl. Phys. A27, 149 (1982).

170 10-30gm 30-50pro 50-751Jm

160 D----

100-150~m 10-150~m

150

140

130

120 800

I

I

I

I

!

900

1000

1100

1200

1300

Annealing

temperature

1400

(C)

Figure 1. Positron mean lifetimes of inert gas atomized 304 stainless steel samples as a function of annealing temperature.

Vol.

26, No.

8

POSITRON

LIFETIME

1203

300 30-50~m 280

•

10-150pm

•

50-75m

B

26O r~

cl m U

240

220

200 800

i

i

!

i

i

900

1000

1100

1200

1300

1400

40 o----

30

30-50pm

•

10-150gm

=

50-75gm

dP ~,

H

20

10

0 800

i 900

! 1000

',

! 11 O0

! 1200

i 1300

1400

Annealing temperature (C)

Figure 2. ~2 and 12 as functions of annealing temperature.