Stress induced hexagonal martensite in CuZnAl

It has been shown that it is possible to induce a hexagonal long range ordered martensite phase in a CuZnAl alloy with M_S= +50°C and an electron concentration e/a=1.48. From measurements of the critical stress and of the retransformation temperature to β after reheating, it is concluded that the hexagonal and the original 3R martensite have the same formation free energy. This implies that the two structures cannot be described by their difference in stacking fault density (compared to a face centered lattice), and an energy difference which is proportional to it. It is probable that either the interaction between neighbouring stacking faults or an electronic contribution from the Fermi surface interaction with the Jones zone plays an important role.     

New evidence for GP zones in binary AlLi alloys

The DSC results described above are thought to present a relatively complete and persuasive picture of the characteristics of an AlLi phase which is metastable with respect to δ′. The phase is not present immediately after quenching; it appears at room temperature with kinetics that are similar to those of GP zones in other Al base alloys, it dissolves in the 75–150°C temperature range with an enthalpy typical of GP zones, and the kinetics of its dissolution are also similar to those of other GP zones. The DSC measurements are not an artifact caused by the dissolution of δ′, which is significantly more stable and which is also probably present. Finally, the existence of GP zones was predicted independently on sound theoretical grounds. According to the above results, optimum conditions for observation of the zones would be after long-term aging of a 2 to 2.5 wt% alloy in the 50 to 75°C temperature region; TEM examinations of such samples are currently underway.     

Thermal properties of Mg–Li and Mg–Li–Al alloys

Abstract

The present work is a study of the thermal properties of Mg–xLi–y Al with x= 4, 8 and 12 wt-% and y= 0, 3 and 5 wt-% as a function of temperature in the range 20–375°C. The thermal diffusivity and coefficient of thermal expansion (CTE) have been measured and the thermal conductivity calculated. The thermal diffusivity of all alloys decreases with an increasing content of lithium. The CTE of the single phase alloys Mg–4Li and Mg–12Li has a linear character, and the CTE of Mg–12Li is higher than that of Mg–4Li. The influence of thermal stresses in the two phase alloy Mg–8Li is perceptible in terms of temperature dependence of the CTE. In Mg–4Li–3Al and Mg–4Li–5Al, an influence of the solution of AlLi phase on all the studied thermal properties has been found.