The Role of Dispersoids in Mechanically Alloyed NiAl
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THE ROLE OF DISPERSOIDS IN MECHANICALLY ALLOYED NiAI S. DYMEK, M. DOLLAR, Sj. HWANG AND P. NASH Department of Metallurgical and Materials Engineering, Illinois Institute of Technology, Chicago, I 60616
ABSTRACT Mechanical alloying followed by hot extrusion has been used to produce fully dense, crack free, very fine grained NiAl-based alloys containing a bimodal distribution of aluminum oxide dispersoids. The unique microstructure provides the materials with high strength and good compressive ductility at ambient and elevated temperatures. The emphasis of the paper is on the importance of the dispersion phase in controlling grain size, texture, deformation mechanisms and ultimately mechanical properties of the mechanically alloyed NiAl-based materials. 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. Among others, the NiAI 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 [II. However, before this material can be of practical use a number of problems must be overcome, including lack of ductility at room temperature and poor strength at high temperatures [2]. Our approach has been to use mechanical alloying followed by hot extrusion to produce several very fine grained materials containing oxide dispersoids, to address both the ambient temperature brittleness and high temperature strength problems. In this paper, the results of our studies on microstructure, texture, deformation mechanisms and temperaturedependent mechanical properties of a selected, mechanically alloyed (MA), near-stoichiometric NiAl-based material are presented and contrasted with the analogous observations of its cast counterpart. The comparison is aimed at emphasizing the importance of the unique microstructure developed during mechanical alloying and hot extrusion. In the course of our studies, we have become increasingly convinced of the importance of the dispersoid phase in controlling grain size, texture, deformation mechanisms and ultimately mechanical properties of the MA NiAI materials. Thus, the discussion focusses on the roles of dispersoids in the MA materials. The present paper is likely the first attempt to comprehend the significance of dispersion phase in MA NiAI-based materials. EXPERIMENTAL DETAILS AND RESULTS An NiAl-based alloy, with the chemical composition shown in Table I, was produced for the present study. The alloy was obtained from elemental Ni and Al powders. mechanically alloyed in an attritor mill, sieved, degassed and encapsulated under vacuum in a stainless steel can, and hot extruded at 1400 K at a ratio of 16:1. A cast NiAI ingot, with the chemical composition given in Table I, hot extruded under the same conditions, was also investigated for comparison with the MA NiAl. Mat. Res. Soc. Symp
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