Longitudinal zero thermal expansion in Re-Fe (R = Tb, Er) eutectic alloys with high fracture resistance

Longitudinal zero thermal expansion (LZTE) alloys hold unique application potentials due to their size stability along the longitudinal direction and thermal expansion compatibility inside the radial plane. However, they are rare and usually exist in an ordered intermetallic phase that suffers from poor mechanical properties. Here, two novel alloys, Tb0.05Fe0.95 and Er0.04Fe0.96, were designed and fabricated by hypo-eutectic reaction. The Tb0.05Fe0.95 alloy possesses axial zero thermal expansion with an ultralow coefficient of thermal expansion (α1 = 0.029 × 10-6 K-1, 110 to 425 K) and a large ultimate compressive stress (δUS) of 0.80 GPa with a strain limit of 3.9%, and the Er0.04Fe0.96 alloy exhibits axial zero thermal expansion (α1 = -0.33 × 10-6 K-1, 110 to 330 K) and an ultimate compressive stress (δUS) of 0.73 GPa with a large strain limit of 13.8%. Further studies show that the [001] axis of the hexagonal R2Fe17 phase (R = Tb, Dy) tends to nucleate and grow along the maximum temperature gradient direction, where the negative thermal expansion of the R2Fe17 phase is neutralized by the positive thermal expansion of α-Fe and leads to the LZTE. The fractured surface shows that the plastic α-Fe phase hinders the enlargement of micro-cracks from the brittle R2Fe17 phase, and it leads to plasticity while the matrix R2Fe17 phase gives rise to high strength. Furthermore, the present LZTE alloys are stable under thermal circulation, which has application prospects for high-precision engineering.