Influence of Heat Treatment on the Structure and Properties of a High-Temperature Intermetallic Nickel-Based Alloy

The study tested single crystals of intermetallic nickel alloy VIN4 with crystallographic orientation [001] after the liquid metal cooling and heat treatment. By means of electron microscopy, X-ray diffraction and physico-chemical phase analysis, we find that after the heat treatment, i. e. hardening at (T_s-(50 - 60))°C, where T_s = 1310°C, for 5 hours, then cooling in the air, there occurs redistribution of the chemical elements in the alloy structure and allocation of γ'-phase, as well as the additional P-phase and γ-phase. Structural type of P-phase is presented as Cr₉Mo₂₁Ni₂₀. Allocation of such phases is typical for high-rhenium superalloys which contain, as a rule, up to 50% of Re and up to 20% of W. We assume that P-phase is a source of premature origin and distribution of cracks to superalloys. Comparison of the results of mechanical tests, as well as the microstructural examination of samples, shows that the additional phase of P-type has a beneficial effect on tensile strength, probably owing to its morphology and location, considerably increasing tensile and yield strength at room temperature. Consequently, plates like P-phase slow down the advance of dislocations in case of sample deformation. Moreover, we examine phase containing Re, Mo, Ni - bal. This phase is identified by a X-ray diffraction method as γ_к with the period of a crystalline lattice a_γK=0,361nm. Besides the observed increase in long rupture life at 1200°C, there is a consequence of additional dispersing hardening of alloy due to allocation of secondary γ'-phase. Also, the microstructure formed after the heat treatment provides an increase in tensile strength, yield strength at room temperatures and 1200°C in ~35...50% and long rupture life at 1200°C and stress σ=40MPa in 20...25%.

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