Characterization of Vacuum Plasma Sprayed Mechanofused NiAl
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CHARACTERIZATION OF VACUUM PLASMA SPRAYED MECHANOFUSED NiAl Z.J. Chen and H. Herman, Dept. of Materials Science and Engineering, State University of New York, Stony Brook, NY 11794-2275, USA; C.C. Huang and R. Cohen, Micron Powder Systems, 10 Chatham Road, Summit, NJ 07901, USA.
ABSTRACT Vacuum plasma spray is a melt spray process capable of producing coatings or near-net shaped deposits with ultra-fine grained structures. Mechanofusion is a novel technique for processing powdered materials. In this study, mixtures of elemental Ni and Al powders, either blended or mechanofused, were vacuum plasma sprayed, yielding a series of Ni-Al intermetallic phases. Mechanofusion resulted in significant effects on the phase distribution in subsequently as-sprayed deposits. After heat treatment at 1100 *0, single phase NiAl was obtained for both the mechanofused and blended samples. The microhardness of the NiAl is comparable to that produced by other methods. These preliminary experiments indicate that mechanofusion and vacuum plasma spraying can be employed to produce various compounds using mixed elemental powders.
INTRODUCTION Intermetallic compounds are candidates for high temperature structural materials. Of particular interest is NiAl, which has a high melting temperature, low density, and excellent oxidation resistance. The wide single-phase region of NiAl allows deviations from stoichiometry and alloying for improvements of the mechanical properties without entering a two-phase field (1). Furthermore, NiAl is a promising matrix for composite materials. There are various methods for producing Ni-aluminides, such as reactive sintering of mixed elemental powders (2), high-pressure shock loading (3), and mechanical alloying (4). Ni-aluminides produced by plasma spray have been reported by a number of researchers using different types of spray processing under various conditions (5-7). In this paper is described an alternative means for producing intermetallic NiAh: vacuum plasma spray (VPS) of mechanofusion (MF) processed mixtures of elemental Ni and Al powders. In addition, simply blended powders have been VPS processed, and these deposits are compared with those formed from MF processed samples.
PROCESSES DESCRIPTION 1). Vacuum Plasma S pray operates in a reduced pressure (e.g., 6x10' Pa) inert gas environment, Fig. l(a) (8,9). A distinct attribute of the VPS process is that it combines melting, quenching and consolidation in one operation. The high solidification rate (107 K/s) of the process can result in ultra-fine grained microstructures (10). Moreover, the structure of sprayed deposits is homogeneous without the large scale macrosegregation resulted from conventional casting. While VPS has generally been used to produce protective coatings, it also has the advantages of being capable of fabricating thick, dense and oxide-free bulk forms and near-net shapes. It has been shown that VPS-aluminides (11) have properties which can be superior to such materials formed using other techniques. In this paper VPS processing
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