S. D. Prokoshkin, K. Inaekyan, Moscow Institute of Steel and Alloys, Moscow, Russia; A. Korotitskiy, V. Brailovski, Ecole de technologie superieure, Montreal, QC, Canada
The binary Ti-50.26 and 50.60at%Ni shape memory alloys were studied using X-ray diffractometery and differential scanning calorimetery (DSC) after cold rolling (logarithmic thickness reduction from e=0.25 to 2.1) and post-deformation annealing in the 200 to 700oC temperature range.
The study shows that moderate cold-rolling strains (up to e=0.72) result in a shift of the reverse martensitic transformation range towards higher temperatures combined with a decrease in the latent heat of martensitic transformation. This treatment leads to a partial martensite stabilization accompanied with a growth of the austenite fraction at the expense of the martensite one, as a direct result of deformation. The B19’-martensite X-ray lines do not change their positions, but widen, while modifying their intensity ratios. When the strain exceeds e=0.5, an amorphous halo appears amplifying with the strain growth, that correlates well with an increase in the crystallization heat effect measured by DSC.
Varying the cold rolling strains and post-deformation annealing temperatures leads to different structural states of the B2-austenite prior to the martensitic transformation: (1) strain-hardened, recovered or polygonized dislocation substructure, and (2) recrystallized coarse-grained, submicrocrystalline (grain size from 0.1 to 1 microns) or nanocrystalline (grain size lower than 100 nm) grain structure. The lattice parameters of the martensite formed from the austenite containing a well-developed dislocation substructure or ultra-fine grained structure differ from those of the martensite formed from the non-dislocated coarse-grained recrystallized austenite. These differences result in the different calculated values of the martensitic transformation lattice strains and therefore theoretically possible recovery strains. However, no matter what thermomecanical treatment is applied, the martensite lattice parameters are the same sensitivity to the temperature variations in the -180 to +20oC temperature range.
Summary: Martensite lattice parameters and transformation lattice strains are calculated for binary Ti-Ni alloys with different dislocation substructures (as-deformed, recovery and polygonized) and structures (nano, submicro, micro and recrystallized) resulted from thermomechanical processing consisting in cold rolling (0.25 to 2.1 of logarithmic thickness reduction) and post-deformation annealing in the 200 to 700 deg.C temperature range.
