Effects of Thermal Ageing in β-Phase in Cu-Al-Ni Single Crystals

Abstract
The effect of isothermal ageing of β-Cu-Al-Ni alloys at low temperatures (473K and 573K) on the martensitic transformation has been studied using differential scanning calorimetry and transmission electron microscopy. Three single crystals with different Al content (13.5 , 13.6 and 13.8 wt%) and the same Ni concentration (4%) were used. Before ageing, the samples were annealed at 1070K and quenched to room temperature. The transformation temperatures remained nearly constant during the initial stages of ageing and started to increase noticeably after a given ageing time, which is shorter in the samples with higher Al content. For the alloy with highest Al content, the Ms decreased slightly at the beginning of ageing at 473K. The transformation to two coexisting martensitic structures, β'(18R) and γ' (2H) has been observed. The proportion of each one changes with the composition and ageing, the γ' phase being promoted for increasing ageing times and temperatures. A good correlation exists between the initial ageing stage, in which there is no evolution of transformation temperatures, with the coexistence of the two martensites. The formation of the two kinds of martensites is related to the short range reordering processes taking place during ageing, specially of as quenched next nearest neighbours disordered Cu-Al pairs. The results would indicate that the disordered Cu-Al pairs stabilize the β' martensite and when these pairs recover the correct positions, the γ' becomes the stable martensite. The high increase of transformation temperatures with ageing at 573K is related to two different precipitation processes observed.