Work Production and Shape Memory Effect During Thermal Cycling of TiNi-Based Alloys Under Symmetric Scheme

Influence of thermal cycling under symmetric scheme for work production on functional properties and work output in Ti – 51.5 at. %Ni and equiatomic TiNi shape memory alloys was studied. To produce a work output samples were subjected to thermal cycling through the temperature range of martensitic transformation under symmetric scheme in the mode of torsion. The stress acting on cooling was 50 MPa, and the stress acting on heating was 200 MPa. The data obtained had shown that alloy composition influenced values of shape memory effect, work output and residual strain variation. It was found that in Ti₅₀Ni₅₀ alloy values of shape memory effects were three times larger and work output was four times higher than in Ti_48.5Ni_51.5 alloy. For instance the value of work output was 10 MJ/m³ in Ti₅₀Ni₅₀ alloy and it was 2.5 MJ/m³ in Ti_48.5Ni_51.5 alloy. It was observed that in Ti_48.5Ni_51.5 alloy during thermal cycling the plastic strain varied non-monotonically and reached 0.6 % after 30 cycles. Equiatomic TiNi alloy demonstrated monotonic dependence of residual strain on cycle number and 3.3 % of residual strain was accumulated by 30 cycles. The data obtained had shown that thermal cycling Ti₅₀Ni₅₀ alloy under symmetric scheme did not influence a variation in functional properties and work output in studied alloys. It should be noted that thermal cycling under asymmetric scheme with the same stress regime led to degradation of functional properties of the alloy, to decrease in the work output and resulted in accumulation of large residual strain (more that 50 %). Therefore, thermal cycling the TiNi alloy in symmetric scheme allows one to produce a high work output and significantly depress an accumulation of residual strain.