The role of microstructure on strength and ductility of hot-extruded mechanically alloyed NiAl
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I.
INTRODUCTION
INTERMETALLIC compounds, such as nickel, iron, and titanium aluminides, have recently emerged as a new class of potential structural materials for high-temperature applications, such as gas turbine engines. Among others, the NiA1 compound is a possible high-temperature structural material, either in monolithic form or as a matrix phase in a composite, because of its low density, high melting temperature, good thermal conductivity, and excellent oxidation resistance. ~ll However, before this material can be of practical use, a number of technical problems must be overcome, including lack of ductility at room temperature and poor strength at high temperatures. Several attempts to resolve the problem of roomtemperature brittleness through modification of slip systems, tz'31 grain refinement, j4"5"~jgrain boundary elimination, 1vjand microalloying with boron I~l have been made. The successes were, at best, incomplete. 18-91 The approaches used to improve high-temperature strength included addition of dispersoids and/or precipitates lm,l~l and production of composites, i~2"~3~ Our approach has been to use mechanical alloying followed by hot extrusion to produce several very finegrained materials containing oxide dispersoids to address both the ambient temperature brittleness and hightemperature strength problems, jH'JSJ In this article, results of our studies on microstructure, texture, deformation mechanisms, and temperature-dependent mechanical properties of two selected mechanically alloyed (MA) NiAl-based materials are presented and contrasted with the analogous observations of their cast counterpart. The following discussion is focused on how the unique microstructure developed in NiAI during mechanical alloying and hot extrusion controls deformation M. DOLLAR, Associate Professor, and P. NASH, Professor, are with the Department of Metallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, 1L 60616. S. DYMEK, Assistant Professor, is with the Department of Metallurgy, Academy of Mining and Metallurgy, Krak6w, Poland. S.J. HWANG, Research Associate, is with the Institute of Materials Science, University of Tsukuba, Tsukuba, Japan. Manuscript submitted December 30, 1992. METALLURGICAL TRANSACTIONS A
mechanisms and improves mechanical properties. The discussion of present results is supplemented by an analysis of slip systems in MA NiA! based on our studies published recently.I~6]
II.
EXPERIMENTAL DETAILS AND RESULTS
A. Materials
In the present investigation, two MA NiAl-based alloys, with the chemical compositions shown in Table I, have been studied. One of the alloys, MA1, was obtained from elemental Ni and AI powders, while the other, MA2, was obtained from prealloyed NiAI powders. Both were mechanically alloyed in an attritor mill. Mechanically alloyed powders, collected after milling, were sieved to - 3 2 5 # , degassed at 973 K for 1.5 hours in a vacuum furnace, encapsulated under vacuum in a stainless steel can, and hot extruded at 1400 K at a ratio of 16: 1. For c
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