Microstructure and thermomechanical behavior of Heusler phase Ni2AlHf-strengthened NiAl-Cr(Co) alloy produced by HIP of plasma-spheroidized powder

Abstract

This study demonstrates that homogeneous high-purity micropowders with grain size of 10–45 µm from refractory NiAl-Cr(Co)-Hf alloy can be produced by combustion elemental synthesis and further treated in a thermal plasma flow. The optimal parameters of plasma spheroidization of the synthesized powders are shown at which the resulting spherical alloy particles are characterized by high sphericity (~ 98%) and the absence of satellites or internal porosity. The spherical powders were consolidated by HIP. The alloy structure was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The fine structure and features of segregation of hardening nanoparticles of Ni2AlHf (the Heusler phase) and hafnium solid solution (ssHf) in the NiAl matrix were thoroughly studied. Furthermore, in situ examination of structural phase transformations occurring in the as-HIP NiAl-based alloy was carried out upon heating in the temperature range of 25–850 °C. The α-Cr phase nanoparticles are formed inside the NiAl grains via two different mechanisms: (1) spinodal decomposition of solid solution at 250–450 °C resulting in segregation of the Guinier-Preston zone rich in Cr and its subsequent transformation into α-Cr precipitates 25–170 nm in size; (2) heterogeneous nucleation and growth of α-Cr nanocrystallites 7–40 nm long on prismatic dislocation loops at 750–850 °C.The thermomechanical properties of the alloy were studied in the temperature range of 596–1100 °C on a Gleeble System 3800 testing system. The correlation between the thermal load and the high-temperature creep rate, as well as the temperature dependences of the Young's modulus (E), the offset yield strength (σ0.2), and the rate sensitivity coefficient (m), were ascertained. The Young's modulus (E), the ultimate tensile strength, the offset yield strength σ0.2, and the proportional limit of the alloy being examined during uniaxial compression test of a cylindrical sample at 750 °C and at true strain rate of ~ 0.01 s−1 were as high as 137.8 GPa, 682, 455 and 358 MPa, respectively.