Effects of Cold Working and Heat Treatment on the Thermal Expansion Property of Fe-29%Ni-17%Co Low Thermal Expansion Alloy

摘要

The changes to thermal expansion and mechanical properties by cold working were investigated in the Fe-29%Ni-17%Co (wt%) low thermal expansion alloy. The Fe-29%Ni-17%Co alloy was cold rolled gradually and prepared for plating with reduction ratios of 0%, 20%, 40%, 60%, and 80%. The effect of annealing on the property was also studied. Thermal expansion was measured from 25 degrees C to 600 degrees C using a vacuum differential dilatometer. We found that the thermal expansion coefficient (alpha(30 similar to 400)) slightly decreased (at a reduction ratio of 20%) and then remarkably increased (above reduction ratios of 40%) with increasing reduction ratios of cold rolling. The thermal expansion coefficient (alpha(30 similar to 400)) sharply decreased after heat treatment. Yield and tensile strengths continuously increased and elongation decreased by cold rolling. Microstructural observation and XRD analysis results showed that the alpha phase significantly increased as the reduction ratio increased. The slight decrease of the thermal expansion coefficient below a reduction ratio of 20% could be explained by the destruction of short-range ordering and decrease in grain size. The significant increase in the thermal expansion coefficient with cold rolling attributed to the appearance of the alpha phase. The correlation between the microstructural cause and invar effect of the low thermal expansion behavior is also discussed.