The effect of predeformation on the creep and stress rupture of an oxide dispersion strengthened mechanical alloy
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THE effect of predeformation at either ambient or elevated temperature on the creep and stress rupture behavior is of engineering importance since predeformation may either improve or deteriorate subsequent mechanical behavior of heat resistant alloys. It has been shown recently that large deformation of ,/ strengthened superalloy causes void formation at grain boundaries and results in a degradation of creep and stress rupture properties? ,2 For a smaller amount of deformation for which voids are not formed, the results show in some cases improvement of the mechanical properties, 3,5while in another study degradation in properties was reported.4 In view of these results for the ~/strengthened superalloys it would be of interest to investigate the effect of predeformation on an oxide dispersion strengthened heat resistant alloy. Some improvements in the creep and stress rupture properties of the mechanically alloyed oxide dispersion strengthened (ODS) nickel-base Iconel* alloy MA 754 * Trademark of the INCO Family of Companies.
have been achieved by subjecting the material to pretreatments prior to creep testing. These pretreatments included hot isostatic pressing and uniaxial compression2,6 In this paper, the effects of small, controlled amounts of relatively high strain rate tensile creep deformation on the subsequent creep and stress rupture properties of MA 754 are reported and discussed. EXPERIMENTAL PROCEDURE Inconel MA 754 is a nickel-base solid solution alloy strengthened by a small volume fraction of yttrium oxide dispersoids. The nominal chemical composition of MA 754 in weight percent is 76.8 pct Ni, 20.1 pct Cr, 1.4 pct Fe, 0.5 pct Ti, 0.25 pct A1, 0.06 pct C, and 0.6 pct yttrium oxide particles. There is also approximately 0.3 pct free oxygen in the alloy. R. T. MARLIN, formerly a Graduate Student at Columbia University, is now at 323 Flying Training WG (ATC), PSC Box 55365, Mather AFB, CA 95655. F. COSANDEY and J. K. TIEN are Research Associate and Professor, respectively, Henry Krumb School of Mines, Columbia University, New York, NY 10027. Manuscript submitted December 18, 1979.
The material is mechanically alloyed and consolidated by hot extrusion. It also received a recrystallization heat treatment, which developed an elongated grain structure. The resulting grains have wide and short transverse grain sizes of about 60 and 45/~m, respectively, and a grain aspect ratio of approximately 10 to 1. The longitudinal grain direction was always parallel to the test specimen axis. The microstructure of MA 754 shown in Fig. 1 consists of a dispersion of yttrium oxides, and coarser inclusions which have been analyzed by combined X-ray and energy loss spectroscopy as A1203 and titanium carbonitride phases. 7 The coarser inclusions are randomly distributed throughout the matrix and at grain boundaries. This alloy possesses also a relatively high twin density. Further information concerning materials preparation and characterization are presented elsewhere.8 All creep tests were done on an Amsler type STFM 74
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