Plastic flow and microstructure of cast nickel aluminides at 1273 K
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I.
INTRODUCTION
ALLOYSbased on the intermetallic compound Ni3AI are presently receiving considerable attention owing to their potential use as structural materials at elevated temperatures. 1-7 The grain boundary embrittlement inherent to the stoichiometric compound has been alleviated by lowering the aluminum content and by adding small amounts of boron. 1'3'7 The mechanical properties can be further improved by alloying with hafnium. 8 Cast Ni3Al-type alloys have been successfully processed by rolling at room temperature with intermediate anneals. 3 Initial reductions in area between anneals (typically 1 hour at 1323 K) were usually 15 pct. Much larger reductions per pass are possible once the cast structure has been removed. Another way of producing Ni3Al-type alloys is to hot extrude canned alloy powder. ~ Isothermal forging as well as superplastic behavior have been demonstrated for nickel aluminides processed in this manner. 9 The hot-forging of cast nickel aluminides has so far not been successful. Even at forging temperatures close to the melting point (1668 K) severe cracking and disintegration of the ingots occurred. 9 Empirically, the formability of alloys is usually improved by prior homogenization of the ingots, reductions in strain rate, and increases in temperature. In the present work we decided to concentrate on the effect of homogenization and strain rate on the flow and fracture behavior of nickel aluminides. Strain rates were varied between 10 -5 and 10 -~ s -~, which covers isothermal forging as well as creep conditions. It will be seen that as-cast structures retain their integrity at higher strain rates than homogenized ones, and possible reasons for this will be discussed. II.
pared from approximately 99.95 pct pure metals and a Ni-4 wt pct B master alloy by arc-melting and drop-casting (in argon) into copper chill molds (length 100 mm, diameter 9.5 ram). Cylindrical specimens (diameter 7 to 9 mm, height 9 to 12 mm) were machined from the ingots. Some specimens were homogenized in vacuum for 5 hours at 1473 K prior to testing. Many specimens contained a center pipe, and metallographic examination was always carried out well away from such defects. A summary of the investigated alloys which includes grain size and casting information is found in Table I.
B. Mechanical Testing Constant displacement-rate compression tests were performed in vacuum (10 -3 Pa) with a hydraulic testing machine. The test temperature, 1273 K, was usually reached in 1 hour. Assuming homogeneous deformation, true specimen areas and stresses were calculated from the loaddisplacement curves recorded by an x-y recorder. In some tests considerable specimen barreling or cracking was observed. In order to facilitate comparison with other tests, however, the assumption of homogeneous deformation was also made in these tests. Sample strains were calculated as (1, - ll)" 100 pct/1,, where 1, and 11 refer to the sample heights before and after testing, respectively.
C. Microstructural Examination Specimens were sectioned lo
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