THE SELF ACCOMMODATING MARTENSITIC MICROSTRUCTURE OF NI-TI SHAPE-MEMORY ALLOYS

The martensitic microstructures of Ni-Ti shape memory alloys were exam ined by transmission electron microscopy and found to be self accommod ating conglomerates of large primary (or first formed) plates and smal ler secondary martensite plate clusters. While the morphology of the p rimary plate groups is that of a ''hollow'' triangular arrangement of three plates, that of the secondary martensite plate groups is a ''sol id'' triangular arrangement of three plates. The present investigation , which focuses on the crystallography, self accommodation and autocat alytic mechanism related to the formation of the secondary martensite plate groups, has revealed these to be plate groups which are coupled by compressive strain along the [1 1 1], axes. The observed plate grou p is the most self accommodating of all possible autocatalytically nuc leated plate groups. Additionally, in conformity with the crystallogra phic restrictions for autocatalysis determined in this work, all inter variant interfaces in the secondary martensite plate group are twin pl anes derived from mirror planes of the cubic parent phase. The propose d autocatalytic mechanism can be applied in general to determine the p referred self accommodating martensite plate clusters in any alloy sys tem.