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Accession Number:
ADA235168
Title:
Cyclic Deformation, Damage, and Effects of Environment in the Ni3Al Ordered Alloy at Elevated Temperature
Descriptive Note:
Final technical rept. 1 Dec 1987-1 Dec 1990
Corporate Author:
GEORGIA INST OF TECH ATLANTA SCHOOL OF MATERIALS ENGINEERING
Report Date:
1991-03-29
Pagination or Media Count:
106.0
Abstract:
The objective of this study is an experimental determination of the effect of composition, temperature, and environment of the fatigue crack propagation resistance of Ni3A1 polycrystalline alloys. An analytical model for yield strength is introduced. The model utilities a thermodynamic framework which is consistent with presently observed characteristics of yielding in Ni3A1, including strain rate independence and thermal reversibility. Two alloy compositions were evaluated with the same equivalent stoichiometry and processing history. Both alloys were single phase, substoichiometric B-doped compositions, one binary Ni-Al, one ternary Ni-Al-Cr. The mean linear intercept grain size of the ternary alloy was 3 times smaller than that of the binary alloy. An additional heat treatment was used to grow identical grain sizes in the binary and ternary alloys. These alloys were then used to evaluate the Chromium effect on the elevated temperature deformation properties. The materials were subjected to tensile cyclic loads and fatigue crack propagation testing at ambient and elevated temperature in both air and vacuum environments. Results from static and cyclic testing indicated that the grain boundary cohesive strength of the Ni-Al-Cr ternary alloys was lower than that of the Ni- Al alloys at ambient temperatures. At elevated temperatures stress-assisted oxygen diffusion along grain boundaries significantly reduces the grain boundary strength of the Ni-Al alloys and results in degradation of the mechanical properties.
Distribution Statement:
APPROVED FOR PUBLIC RELEASE
