Structural and Chemical Composition of Ni-Al Powders
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STRUCTURAL AND CHEMICAL COMPOSITION OF Ni-Al POWDERS R.Maurer, G.Galinski, R.Laag, and W.A.Kaysser, Max-Planck-Institut fOr Metallforschung, Institut fur Werkstoffwissenschaften, SeestraBe 92, 7000 Stuttgart 1, FRG. ABSTRACT Ni 3A1 and NiAl pre-alloyed powders produced by argon gas atomization are investigated by light and transmission electron microscopy concerning their structure, chemical and phase composition. For example, Ni 3Al powder consists of a L12 matrix (7' -phase) with precipitates whose Al content is 6% higher than in the matrix. Selected single powder particles were isothermally deformed in a modified high resolution dilatometer to investigate the creep behavior. From these experiments HIP parameters are predicted and compared with HIP experiments at different pressures and temperatures. HIP diagrams were calculated according to Ashby's model. 1. INTRODUCTION Fine grained intermetallic compounds offer properties which allow forming of parts for technological applications. By Ar-atomization NiAl and Ni 3Al pre-alloyed powders of a grain size of only some micrometers can be produced. Consolidation of the powders yields a bulk material, which often reflects the problems of imperfect processing more than the inherent properties of the microstructure of the powders. The deformation and creep behavior of the initial powder microstructure may be determined by HIP experiments. These experiments are, however, tedious, time consuming and expensive. Therefore, a method for the direct determination of the creep parameters from the deformation of a single powder particle (e.g. for Ni aluminide) was developed, using a modified high resolution dilatometer [1,2]. To understand the relationship between microstructure and creep behavior, transmission electron microscopy (TEM), supported by optical metallography, was used to characterize the initial powders. 2. EXPERIMENTS Powders of the exact compositions Ni 75Al25 and Ni50 Al50 are produced by high pressure Ar gas atomization to obtain cooling rates > 104 K/s (fig. la). The microstructures of the atomized powders were investigated using etching with molybdic acid [3] which is generally applicable to Fe- and Ni-aluminides with Al contents < 50 at.9ai The molybdic acid was dissolved in a boiling hydrous solution of 5 vol.% HF up to the saturation point. After cooling, the samples were etched in the fresh corrosive for 20-60 s. For TEM the atomized powders were electrolytically embedded into a Ni foil by a method similar to those already described in [4-6]. One liter of electrolyte consisted of 250 g NiSO 4, 30 g NiC12 and 30 g H 3BO 3, with a pH value of about 3. Ni bulk material was used as the anode. The powders were fixed by drying a few drops of electrolyte on a sheet steel which was connected as cathode. To avoid the formation of pores in the coating, the electrolyte was heated to ~ 50 `C. After 8 h of galvanization with a current density of 200 A/m 2 a foil of approx. 300 pm thickness was obtained (fig. lb). The preparation was continued with the usual procedures
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