Elinvar effect in Ti_50-xNi₄₁Co₉Nb_x shape memory alloys

Based on the observation of Elinvar effect in non-magnetic Co doped TiNi alloys, we added a small amount of Nb to Ti₅₀Ni₄₁Co₉ alloy with low Co content to regulate the phase transformation behaviors and induce Elinvar effect. The effects of Nb content on the phase transformation behaviors and the temperature dependences of elastic moduli of Ti_50-xNi₄₁Co₉Nb_x shape memory alloys were investigated. The Ti₅₀Ni₄₁Co₉ alloy was found to exhibit only the B2→R phase transformation during cooling and R_S decreased gradually with the increase of Nb content. As the Nb content increased to 1.5 at.%, the B2→R phase transformation was suppressed and converted to a continuous transformation over a broad temperature range, which accompanied with the electric resistivity abnormally increased and the appearance of some bright “particle”-like dots during cooling. The temperature dependence of elastic modulus also decreased with the increase of Nb content. As a result, Ti_48.5Ni₄₁Co₉Nb_1.5 alloy showed the Elinvar effect over a wide temperature range from 304 °C to 35 °C, which could be attributed the success to the gradual modulus softening caused by the continuous transformation compensated the normal modulus hardening of matrix.