Solidification Behaviour and γ Precipitation in Nickel-Rich γ′ (Ni 3 (Al,Ti))

Abstract

In the binary nickel-aluminium system, solidification based on primary γ′ (Ni₃Al) is difficult to achieve because of the close proximity of a peritectic and a eutectic reaction at the melting point of Ni₃Al. However, the present work shows that addition of titanium as a substitute for aluminium gave rise to a range of alloys which froze with primary separation of γ′. Under most freezing conditions, these adopted a cellular solidification mode. At low titanium levels (∼ 4 at.-%), nickel-rich γ′ displayed solid-state precipitation of γ as {100} platelets. Growth of the precipitate obeyed the coarsening law: <disp-formula> <mml:math> <mml:mrow> <mml:mover> <mml:mi>l</mml:mi> <mml:mi>¯</mml:mi> </mml:mover> <mml:mo>∝</mml:mo> <mml:msup> <mml:mi>t</mml:mi> <mml:mrow> <mml:mn>0·40</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0·05</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> </disp-formula> where l is mean precipitate spacing and t is ageing time at 870° C. At higher titanium levels (∼ 12 at.-%) the γ′ formed as cellular dendrites which remained singlephase on subsequent ageing. The interdendritic regions were composed of γ′ in a γ matrix.