Martensitic Transformation in Aged CuZnSn Shape Memory Alloys
Institute of Metallurgy and Materials Science, Polish Academy of Science, 25 Reymonta St., 30-059 Krakow, Poland
The effect of addition of up to 12 at.% Sn on the β1 structure and the reversible martensitic transformation in ternary brasses and their response to ageing at 200°C was investigated. The quenched alloys showed modulations in the <100> β directions and corresponding rectangular ordered L21 domains. The alloys containing less copper transformed to 2H martensite that retained its reversible character during cycling. However, the alloy with higher copper content which underwent the martensitic transformation above the R.T. was of 18R type and did not retransformed to the β1 phase on heating. Ageing caused a lowering of characteristic martensitic transformations temperatures accompanied by a widening of transformation histeresis. The above changes of characteristic temperatures during ageing are faster for alloys with higher tin content. The microstructure observation proved, that the above changes were closely related to almost simultaneous precipitation the of 3R bainite and y phase. The bainite, as compared with the matrix, was strongly depleted in tin and enriched in copper, while zinc changes were small. The y phase as compared with the matrix was strongly eriched with tin and depleted with copper while zinc content was slightly lowered.
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