Towards understanding the effects of magnesium addition on microstructural and thermal properties of NiAlCr alloys in as-cast and heat treated states.

The effects of magnesium addition on the microstructure, hardness, chemical composition and thermal expansion coefficient of NiAlCr alloys in as-cast and heat-treated states were studied. NiAlCr alloys containing 1, 2.5 and 3.8 at.% Mg were prepared via atmosphere-controlled induction melting following vacuum arc melting. According to the XRD results, as-cast alloys were mainly composed of martensite, β-NiAl and γ′-Ni 3 Al. High amounts of Mg in alloys resulted in a slight increase in CTE. Increasing Mg also changed the morphology of the interdendritic Ni 3 Al phase from continuous network to discontinuous structure, resulting in Ni 2 Mg precipitates with hardness of 1280 HV 0.002 . In the furnace cooling procedure following the annealing procedure at 1050 °C for 96 h, martensite completely transformed into β-NiAl with α-Cr precipitates. The single phase Ni 3 Al transformed into Al 0.9 Ni 4.22 -Ni 3 Al lamellar structure and Ni 2 Mg phase disappeared. The presence of Mg in solid solution did not influence CTE of the alloys. [ABSTRACT FROM AUTHOR]