MARTENSITIC-TRANSFORMATION IN AGED CUZNSN SHAPE-MEMORY ALLOYS

The effect of addition of up to 12 at.% Sn on the beta(1) structure an d the reversible martensitic transformation in ternary brasses and the ir response to ageing at 200 degrees C was investigated. The quenched alloys showed modulations in the <100> beta directions and correspondi ng rectangular ordered L2(1), domains. The alloys containing less copp er transformed to 2H martensite that retained its reversible character during cycling. However, the alloy with higher copper content which u nderwent the martensitic transformation above the R.T. was of 18R type and did not retransformed to the beta(1) phase on heating. Ageing cau sed a lowering of characteristic martensitic transformations temperatu res accompanied by a widening of transformation histeresis. The above changes of characteristic temperatures during ageing are faster for al loys with higher tin content. The microstructure observation proved, t hat the above changes were closely related to almost simultaneous prec ipitation the of 3R bainite and gamma phase. The bainite, as compared with the matrix, was strongly depleted in tin and enriched in copper, while zinc changes were small. The gamma phase as compared with the ma trix was strongly eriched with tin and depleted with copper while zinc content was slightly lowered.