Compressive Mechanical Properties of NiAl/Cr(Nb) Alloy Prepared by Rapid Solidification Processing

Abstract

It is well known that inadequate strength at elevated temperatures and low fracture toughness at ambient temperatures are the major deficiencies preventing the B2 aluminide NiAl as an advanced structural material used at high temperatures. [1–3] In order to promote its practical use, a promising NiAl based alloy system, i.e., the IP alloy reinforced by Laves phase has been developed. [4–10] The common characteristics of these alloys are that all of them exhibit higher melting points (about 1460°C), lower densities (6.20 ∼ 6.35 g/cm 3 ), higher thermal conductivity and excellent oxidation resistance compared to the Ni-based superalloys. Furthermore, these alloys can be processed by a series of different production routes, which may be used for property variation within a wide range. [8–10] However, the previous relevant studies proved that the alloys prepared by conventional ingot casting showed coarse-grained microstructure and then exhibited serious brittle fracture at ambient temperatures. On the other hand, the alloys processed by powder metallurgy or mechanical heat treatment with finer microstructure usually improved room temperature brittleness but seriously lowered yield stresses at high temperatures. [6,7] Therefore, for these alloys, to find an efficient way to optimize mechanical properties is necessary to promote their practical use. It is suggested that the mechanical properties of these alloys depend sensitively on the grain size of NiAl matrix and distribution of strengthening Laves phase which is controlled by processing parameters, such as temperature, time and cooling rate. [7] Rapid solidification to produce bulk metallic alloys is known for a long time to result in refined microstructure, extended solubility, reduced microsegregation, and formation of metastable phases. Injection casting, as a popular rapidly solidified technique to fabricate bulk amorphous alloys, can provide relatively high cooling rates of about 10 2 ∼ 10 3 Ks –1 that is significantly higher than 10 –1 ∼ 10 –2 Ks –1 for conventional casting. [11] Sun et al. [12] and Dai et al. [13] have investigated the effects of refined eutectic interlamellar spacing and grain size on the mechanical properties of the Ti-based alloys, and found that the optimized microstructure induced by Cu mold casting was responsible for the improved room-temperature mechanical properties. Recently, Huai et al reported that the mechanical properties of eutectic NiAl/Cr(Mo, Hf) alloy could be optimized by rapid solidification process, i.e. suction casting. [14] However, the suction casting process could not provide enough high cooling rate as injection casting. Accordingly, in the current study, an IP alloy with the composition of NiAl-Cr/Nb alloy was chosen and prepared by injection casting, and its microstructural morphology and mechanical properties were investigated. Due to the limited amount of sample material available, compression tests were performed only. The compression testing results demonstrate that the resultant alloy exhibits improved compressive ductility and yield strength at room temperature as compared with that of the same alloy prepared by conventional ingot casting. Meanwhile, the yielding strength of resultant alloy is slightly lowered at high temperatures. The test results are quite reproducible.