Ordering-induced Elinvar effect over a wide temperature range in a spinodal decomposition titanium alloy

Abstract

Temperature-independent elastic modulus is termed as Elinvar effect, which is available by tuning the continuous spin transition of ferromagnetic alloys via composition optimization and the first-order martensitic transformation of shape memory alloys via plastic deformation. However, these reversible mechanisms are restricted generally in a narrow temperature range of less than 300 K. Here reports, by tuning a spinodal decomposition in a Ti-Nb-based titanium alloy via aging treatment, both the Elinvar effect in a wide temperature range of about 500 K and a high strength-to-modulus ratio of about 1.5% can be obtained by a continuous and reversible crystal ordering mechanism. The results demonstrate that the alloy aged at 723 K for 4 h has a nanoscale plate-like modulated β+α" two-phase microstructure and its elastic modulus keeps almost constant from 100 to 600 K. Synchrotron and in-situ X-ray diffraction measurements reveal that the crystal ordering parameter of the α" phase increases linearly with temperature from 0.88 at 133 K to 0.97 at 523 K but its volume fraction keeps a constant of about 33.8%. This suggests that the continuous ordering of the α" phase toward the high modulus α phase induces a positive modulus-temperature relation to balance the negative relation of the elastically stable β phase. The aged alloy exhibits a high yield strength of 1200 MPa, good ductility of 16% and a high elastic admissible strain of 1.5%. Our results provide a novel strategy to extend the Elinvar temperature range and enhance the strength by tuning the crystal ordering of decomposition alloys.