Effect of warm rolling on microstructures and properties of the high strength invar alloy

Phase transformation is noneffective for strengthening the invar alloys with single-austenite phase. How to greatly increase the mechanical strength of the alloys without worsening their low thermal-expansion property is crucial for their industrial applications. In this study, warm rolling was tried to increase crystal defects in the invar alloy, in which more low-energy twin boundaries were expected to form by means of interaction between these crystal defects during solution treatment. The twin boundaries interfere the network connected by random grain boundaries to prevent unhomogeneous precipitation of the secondary phase along them during aging and the resultant poor strengthening effect. Our results revealed that warm rolling in the temperature range of 650 °C–800 °C indeed increased mechanical strength of the invar alloy, together with a decrease in its coefficient of thermal expansion (CTE). The mechanical strength and the CTE parabolically varied with the increased warm-rolling temperature. The sample, which was warm-rolled at 750 °C, followed by solutionizing at 1250 °C for 3 h and then aging at 650 °C for 7 h, presented the maximum ultimate strength of 921 MPa, the best elongation of 11.2%, and almost the minimum CTE of 3.2 × 10-6/°C in the range of −50 to 100 °C. These values increased by about 21% and 24%, and decreased by about 20%, respectively, compared to those without warm rolling.