Effect of grain size on yield strength of Ni 3 Al and other alloys
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I. INTRODUCTION Nickel aluminides can be utilized for high-temperature structural components because of their anomalous temperature dependence of yield stress and their highdensity-compensated ultimate strength.' The number of research and development activities involving nickel aluminides has been increasing sharply since 1979, when Aoki and Izumi first reported a dramatic increase in room-temperature ductility with small additions of boron to Ni3Al.2>3 Liu etal.4 have systematically investigated the beneficial effect of boron on ductility and fracture behavior of Ni3Al alloys and found that boron tends to segregate to grain boundaries. The concentration of boron at grain boundaries decreases with increasing bulk aluminum concentration from 24—25 at. %, with a corresponding reduction of ductility from 50% to 6%. The mechanical properties of Ni3Al alloys are also strongly affected by the grain size, and extensive studies have been made on this subject.4"12 In general, the grain size is required to be finer from the viewpoints of fabricability and low-temperature mechanical properties, whereas it is coarser in considering high-temperature strength. Therefore, for high-temperature structural materials, the two criteria mentioned above are conflicting. In order to determine the optimum grain size to satisfy both, a compromise is required. Recent studies revealed that the powder extrusion of NijAl alloys produced fine grain size, causing an increased yield strength at ambient temperatures.56 However, the temperature of maximum yield stress (the peak strength temperature) was also found to decrease as the grain size becomesfiner.7'8In particular, attention J. Mater. Res. 3 (4), Jul/Aug 1988
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has also been given to the grain size dependence of yield stress in Ni3Al alloys.4"12 It is well known that the grain size dependence of yield stress for disordered materials at ambient temperatures obeys the Hall-Petch relation, .
i —1/2
/ 1\
where a0 and ky are material constants. However, Schulson et al. have recently reported that the yield stress in Ni3Al alloys, with and without boron, cannot be correlated well with the Hall-Petch relation; instead it obeys the following equation5: ,
T
i — 0.8 ± 0.05
C?\
where & is a material constant. This equation was obtained by means of regression analysis of the data for Ni3Al alloys on the basis of the equation ay = ao + kd~l, where / is a grain size exponent. The alloys with grain sizes from 2-1100 fxm in this analysis were prepared by several different processes (i.e., by powder extrusion, recrystallized single crystal, and homogenized ingot). In addition, they also observed that the slope k for undoped Ni3Al is larger than that for boron-doped Ni3Al. The theoretical interpretation5 of Eq. (2) has been made on the basis of the grain size dependence of Luders strain and the "strain hardening" effect. They obtained the empirical relationship between the Luders strain eL and grain size d as e^=Ad-p,
(3)
where A is a material constant and p is the gr
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