Shape memory transformation in mechanically alloyed Ni-50at% Ti

Ni and Ti elemental powders have been mechanically alloyed within a horizontal attritor to produce a Ni-50at%Ti product that is amorphous upon XRD analysis. Neat treatment of this product at 500 degrees C within an Ar atmosphere has been shown to promote crystallisation into a Ni-Ti intermetallic of CsCl type parent phase crystallography. Upon cooling this parent phase undergoes a solid state thermoelastic shape memory type of phase transformation producing a monoclinic B19' martensite. This reversible phase transformation has been characterised by differential scanning calorimetry and displays a M-s <-> A(s) transformation temperature hysteresis of less than 10 degrees C. This is significantly lower than that associated with commercially produced ingot origin parent phase Ni-Ti, for which a 30 degrees C transformation hysteresis is expected. XRD analysis of the crystallised Ni-Ti indicates an increase ill the parent phase lattice parameter when compared to Ni-Ti synthesised by ingot route. It is proposed that the increased parent lattice parameter effectively reduces the overall lattice strain required for the martensitic phase transformation, enabling die transformation to occur at reduced undercoolings and superheats.