Strengthening of the Fe-Ni Invar Alloy Through Chromium.

Invar alloys with both high strength and low thermal expansion are urgently needed in fields such as overhead power transmission, aero-molds, and so on. In this paper, Cr was introduced as a cost-efficient alloying element into the Fe-36Ni binary invar alloy to increase its mechanical strength. Our results confirmed that fine Cr7C3 precipitants, together with some Fe3C, in the invar alloy aged at 425 degrees C could be obtained with a short aging time. Those precipitants then grew and aggregated at grain or sub-grain boundaries with an increase in aging time. Simultaneously, mechanical strength and coefficient of thermal expansion (CTE) parabolically varied with the increase in aging time. The sample aged at 425 degrees C for 7 h presented a maximum strength of 644.4 MPa, together with a minimum coefficient of thermal expansion of 3.30 x 10(-6) K-1 in the temperature range of 20-100 degrees C. This optimized result should be primarily attributed to the precipitation of the nanoscaled Cr7C3.