On the stability of Ni 3 Ge
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M. C. Chaturvedi Department of Mechanical Engineering, University of Manitoba, Winnipeg, Canada R3T 2N2 (Received 3 May 1989; accepted 28 June 1989) Ni3Ge-containing nickel-germanium alloys with 12.10 to 16.8 at. % Ge were investigated. Nickel-saturated Ni3Ge had a lattice parameter of 3.57015 x 10~10 m and contained 22.5 at. % Ge over the temperature range 650 to 1100 °C. However, the precipitates in the initial stages contained 24.44 at. % Ge and were almost cubic in shape. The strengthening effects of Ni3Ge were appreciable. In alloys containing higher percentages of germanium, precipitates were observed in as-quenched specimens. The hardness after quenching was high and did not change appreciably with time. Hardness was reduced in the presence of aluminum.
I. INTRODUCTION Intermetallic compounds such as Ni3Al have recently been receiving considerable attention as they show excellent promise as high temperature materials. Ni3Ge is similar in many respects to Ni3Al. Both Ni3Ge and Ni3Al have the ordered Ll 2 structure. At room temperature Ni3Ge is brittle, but at higher temperatures it can be plastically deformed.1'2 According to Shashkov,2 the transition from the brittle to the plastic state is due to the disappearance of directional atomic bonds. Thus, like Ni 3 Al, Ni 3 Ge also passes from the brittle to the plastic state at about 550 °C. In this investigation, the nickel saturated compound Ni3Ge was produced in six nickel-germanium alloys and the effects on structure, composition, morphology, and hardness were examined.
polished and etched specimens were examined in SEM. The lattice constants were determined from x-ray diffraction patterns of strain-free powder samples, using silicon as the standard at 21 °C. The lattice parameter, a, was calculated by Nelson-Riley extrapolation. Hardness (VPN) was determined using a 5 kg load. The average of at least ten measurements was selected for each sample. III. RESULTS AND DISCUSSION A. Composition of nickel-rich Ni3Ge The alloy containing 14.35 at. % Ge was aged at several temperatures as a function of time and the lattice parameters of the two phases present in this alloy in the aged condition were measured. The lattice parameters are shown as functions of time in Fig. 1. Some of the earlier
II. EXPERIMENTAL The alloys were prepared either by induction melting or by arc melting 99.99% pure nickel and germanium. The ingots were vacuum sealed in quartz tubes, homogenized at 1100 °C for up to 230 h, and quenched in water. The homogenized alloys were analyzed in the SEM. An average of five to ten measurements on each alloy was taken. The compositions were 12.10, 12.77, 14.35, and 15.68 at. % Ge. One of the alloys contained 15.62 at. % Ge and 8.21 at. % Al. Another alloy had the nominal composition, 16.8 at. % Ge. Some of the alloys were aged at 650, 656, 706, 749, and 851 °C in vacuum sealed quartz tubes for various lengths of time. In order to prepare extraction replicas, the alloy specimens were mechanically polished in 0.25 /urn diamond paste, etched by immersing
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