Solidification and the Structure-Phase State of Ni3Al–Ni–NiAl Alloys with Chromium, Molybdenum, Tungsten, Rhenium, and Cobalt

Abstract

The ternary Ni–Al–AE phase diagrams, where AEs are the main alloying elements of high-temperature cast (γ' + γ) γ'-Ni3Al-based alloys, are analyzed. Principal differences between the solidification schemes in the vicinity of the γ'-Ni3Al and β-NiAl phases and the differences in the characters of the influence of the main AEs at their contents to 10 at % on the phase composition and the structure of multicomponent γ'-Ni3Al-based VKNA-1V, VKNA-25, VKNA-4U, and IM730 alloys are detcted. The structural base of all the alloys are (γ' + γ) dendrites. In the first-type cobalt-free VKNA1V alloys, as well as in IC221M, IC218, IC438, and IC6SX alloys, coarse single-phase γ'-Ni3Al precipitates, which are the degenerate eutectic L$$ \rightleftarrows $$ (γ' + γ) + γ', form in the interdendritic space. In the second-type cobalt-containing alloys at the same (~17 at %) aluminum content (VKNA-4U, VKNA-25) or in the chromium-containing cobalt-free alloys with an increased (~21 at %) aluminum content (IM730), the following two-phase precipitates form: the nonequilibrium β-NiAl phase, which represents the degenerate L$$ \rightleftarrows $$ (γ' + β) + β eutectic, forms inside γ'-Ni3Al. The following solid-phase transformations are found to develop intensely in the second-type alloys on heating in the temperature range 1200–1300°C: the dissolution of the β-phase precipitates in the primary γ'-Ni3Al precipitates, the dissolution or a weakening of the interdendritic γ'-phase precipitates, and a change in the morphology and orientations of the γ-phase precipitates in dendrites and the interdendritic space.