Effect of aging time on microstructure and properties of as-cast Fe56.5Ni30Al10.5Nb3 shape memory alloy

Abstract

A Co-free Fe56.5Ni30Al10.5Nb3 (atom fraction/%) alloy was developed. In the preparation, the as-cast alloy was subjected to aging treatment at 650 ℃ and followed solution treatment directly. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to analyze the microstructure and phases of the alloy. At the same time, the hardness test, compressive strength test, superelasticity test and electron backscatter diffraction (EBSD) test were performed. The results show that, when the aging time range from 0 to 95 h, the alloy can exhibit superelasticity at room temperature with the maximum superelastic strain of 10.62%. There are nano-sized γ′ precipitates and fine β precipitates in the alloy. The γ′ phase is formed in the grain interiors and is most densely distributed when the aging time is 70 h. After the aging time exceeds 70 h, the γ′ phase enters the growth stage. The β phase can be formed both in the grain interiors and in the grain boundaries, and the amount of β precipitates increases with the aging time. The precipitation of the β phase can improve the hardness, but its formation in the grain boundaries is detrimental to the strength. With the increase of aging time, the hardness of the alloy increases continuously, and the compressive strength and superelastic strain first increase and then decrease，and reach the maximum value at the time of 70 h. The nano-sized γ′ precipitates and strong {100} texture in the alloy have a great influence on its superelasticity.

Keywords

• shape memory alloys
• aging
• superelasticity
• fe-ni-al-nb
• texture