Precipitate-induced R-phase in martensitic transformation of as-spun and annealed Ti51Ni49 ribbons

Differential scanning calorimetry (DSC) results indicate that a two-step B2 → R → B19′ martensitic transformation and a one-step B19′ → B2 transformation exhibit in as-spun and in 200–600 °C annealed Ti51Ni49 ribbons. Guinier-Preston (GP) zones and Ti2Ni precipitates are formed in ribbons annealed at ≦300 °C and ≧400 °C, respectively, and a conspicuous increase of DSC transformation peak temperature occurs in between 300 °C and 400 °C. The sizes of GP zones and Ti2Ni precipitates increase with increased annealing temperature. Transmission electron microscope (TEM) observations show that GP zones can induce the R-phase and both of them are formed along 〈1 0 0〉B2 directions. DSC and TEM tests show that Ti2Ni precipitates can induce the R-phase more than GP zones and the induced R-phase plates are also found along 〈1 0 0〉B2 directions. Experimental results show that the growing direction of R-phase plates is strongly confined by that of GP zones and Ti2Ni precipitates. The length of R-phase plates can reach about 2 μm in 300 °C annealed ribbon.