Martensite-to-martensite transformations in Cu&.sbnd;Zn alloys

Abstract

Single crystal specimens of β′ CuZn alloys deformed above M s eventually transform into a single variant of stress-induced martensite (SIM). Specimens cooled to M f initially exhibit domains of four variants, each of which becomes a single variant region upon loading. In both cases (above M s and below M f ), additional loading causes the onset of “advanced deformation”—the formation of “pink” bands in the single crystal regions of the SIM and deformed thermal martensite. These bands are identified as a martensitic phase, α s , which is stress induced and reversible on unloading. The formation of the α s martensite is associated with a second stress plateau in the stress-strain curves of single crystals loaded above M s . This double pseudoelastic phenomenon gives rise to strains up to 17%, which are completely recoverable. In some instances the α s does not completely reverse on unloading, and internal markings appear in the α s bands which are elastic twins that reverse on load cycling. Mechanical properties associated with the double pseudoelastic phenomenon in single crystals, and a crystallographic analysis of the α s martensite are reported and discussed. A mechanism for the transformation from the 9R thermal or stress-induced martensite into the α s martensite is proposed. Also, a stress-temperature phase diagram has been determined, and a triple point for the β′, SIM, and α s phase regions is suggested.