The Effect of Boron on Microhardness in Ni 3 Al Polycrystals

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THE EFFECT OF BORON ON MICROHARDNESS IN Ni3 Al POLYCRYSTALS

X. R. QIAN AND Y. T. CHOU Department of Materials Science and Engineering and Materials Research Center Lehigh University Bethlehem, PA 18015 U.S.A.

ABSTRACT In a recent MRS symposium, the effect of boron on grain boundary hardness (GBH) in Ni3 Al was reported. Addition of 0.2 at.% of boron decreased the GBH in all three sets of alloys tested (24, 25, and 26 at.% Al). The present paper summarizes the study of the effect of boron on hardness within grains. Boron-induced softening was again observed in grain interiors. However, it occurred only in alloys with 24 and 25 at.%Al, but not in the alloy with 26 at.% Al. An explanation for this behavior is presented.

1.

INTRODUCTION

In the course of understanding the effect of boron on the mechanical behavior of Ni 3 Al, Liu et al. [1] and Huang et al. [2] reported that the presence of boron increased the yield strength of Ni Al polycrystals by 242 MPa to 270 MPa per atom percent. Baker et al. [31, based on their lattice measurements, found that boron has a large lattice-strengthening effect of 387 MPa per at.%. In contrast, Schulson et al. [4] illustrated that B would increase the mobility of grainboundary dislocations and thus reduce the grain-boundary strength. More recently, the present authors [5] reported that the microhardness in stoichiometric Ni alloy was decreased by the addition of 0.2 at.% boron. The decrease was observed both in grain interiors and on grain boundaries. A subsequent study on boron-induced grain-boundary softening in Ni3 Al polycrystals was later presented in the April MRS meeting in Reno [6]. Grain-boundary softening was observed in all three sets of Ni3 Al alloys with Al contents ranging from 24 to 26 at.%. The degree of softening was found to decrease with increasing Al content. The purposes of the present investigation were to continue to examine the boron effect in grain interiors by microhardness indentation and to determine the influence of alloy stoichiometry.

2.

EXPERIMENTAL The compositions of Ni

alloys used in this study are listed in

Table 1. The cast rods were heat treated at 1200'C(±1O'C) for 72 hours followed by furnace cooling to room temperature.

Mat.Ras. Soc. Symp. Proc. Vol.133. 11989Materials Research Society

The homogenized alloys

530

Table 1. Compositions of Ni3 Al Alloys Alloy

Composition, at.%

#1 #2 #3 #4 #5 #6

Al

B

24 25 26 24 25 26

0 0 0 0.2 0.2 0.2

Ni Bal. Bal. Bal. Bal. Bal. Bal.

had an average grain size of 0.9mm. The samples were mounted, ground, polished, and etched. Vickers hardness (DPH) was measured on a TUKON microhardness tester, with a load of 500 gm and a loading time of 30 seconds. An average of five indentation readings was taken to represent a datum point. 3.

RESULTS AND DISCUSSION

Figs. 1 to 3 show the frequency distributions of 30 averaged DPH in the grain interiors for stoichiometric and off-stoichiometric Ni Al alloys with and without boron. The 95% confidence intervals [5] for the population means (P) and for standard