The Shape-Memory Effect in a Titanium-35 wt.-% Niobium Alloy

The shape-memory effect has been studied in a titanium −35 wt.-% niobium alloy. The effect is observed under two different sets of conditions. First, when the fully martensitic alloy is deformed below M_s it recovers its original shape when heated above A_s Secondly, if the alloy is deformed between M_s and A_s (with M_s artificially depressed by a fine dispersion of ω-phase precipitates, which prevents the martensite forming on cooling through the expected M_s , a strain-induced martensite is formed and on heating above A_s the deformation is recovered. When the orthorhombic martensite forms from the cubic β phase, there is an expansion of the unit cell along two axes of the orthorhombic and a contraction along the third. The orientation relationship between the two phases predicts 12 possible variants and on stressing that variant will be formed which results in a relief of the stress, e.g. under a tensile stress those lattice planes with an increased spacing in the orthorhombic phase will tend to lie perpendicular to the stress axis. Deformation below M_s causes growth of certain variants at the expense of others and deformation between M_s and M_s results in different variants being produced under different stress conditions.