Damping characteristics of directionally solidified MnNi alloy exhibiting multiply phase-transformation behavior

Abstract

In the process of directional solidification, the degree of element segregation can be changed by altering the drawing rate to affect the phase transformation behavior of the alloy. In this paper, four kinds of MnNi alloy specimens were prepared by directional solidification at different drawing rates. The phase transformation behavior of the alloy was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermal expansion analysis and dynamic mechanical analysis (DMA). The microstructure of the alloy was observed by metallographic microscope, scanning electron microscope and transmission electron microscope. The results show that the microstructure of directionally solidified MnNi alloy has the characteristics of polymorphy. There are multi-scale twins from micron to nanometer in the alloy, and their coordinated movement makes the internal friction of twins 3-6 times than that of rolled alloy. It is found that during heating, the alloy undergoes fct1→fcc phase transformation in the interdendritic spacings of Mn-poor region while fct2→fco→fcc multi-stage phase transformation occurs in Mn-rich dendrite. This multiply phase transformation behavior makes the damping capacity of directionally solidified MnNi alloy 2-6 times than that of rolled MnNi alloy in a wide temperature range.

Keywords

• mnni damping alloy
• directional solidification
• phase transformation
• twin microstructure