The self accommodating martensitic microstructure of Ni-Ti shape memory alloys

Abstract

The martensitic microstructures of Ni-Ti shape memory alloys were examined by transmission electron microscopy and found to be self accommodating conglomerates of large primary (or first formed) plates and smaller secondary martensite plate clusters. While the morphology of the primary plate groups is that of a “hollow” triangular arrangement of three plates, that of the secondary martensite plate groups is a “solid” triangular arrangement of three plates. The present investigation, which focuses on the crystallography, self accommodation and autocatalytic 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 〈111〉c axes. The observed plate group is the most self accommodating of all possible autocatalytically nucleated plate groups. Additionally, in conformity with the crystallographic restrictions for autocatalysis determined in this work, all intervariant interfaces in the secondary martensite plate group are twin planes derived from mirror planes of the cubic parent phase. The proposed autocatalytic mechanism can be applied in general to determine the preferred self accommodating martensite plate clusters in any alloy system.