The role of dendritic morphology and segregation in FcC-FCT transformation and damping capacity of MnCu based alloys

An alloy with a nominal composition of 70Mn24.95Cu3Al2Zn0.05Ce (at%) was prepared using vacuum induction melting (VIM) technology, or followed by directional solidification (DS) processing at withdrawal rates of 20 and 100 µm/s. Further, VIM, DS20 and DS100 alloys were aged at 703 K for 2 h. The microstructure, fcc-fct transformation and damping capacity of VIM and DS alloys have been investigated comparatively. The results show that the microstructure of VIM alloy mainly comprises equiaxial £-MnCu dendrites while that of DS20 and DS100 ones is primarily composed of columnar £-MnCu dendrites, and the directional effect of such columnar dendrite is obviously strengthened with increase in withdrawal rate. Two and three compositional segregations are present in VIM and DS alloys respectively, and fine ¡-Mn phase is formed in DS100 one. The starting fcc-fct transformation temperature of the alloy bears a relationship of T_tVIM > T_tDS20 > T_tDS100. The stepped fcc-fct transformations occur and couple to promote the formation of phase transformation damping step due to compositional segregation, which is more obvious in DS alloys than in VIM one. The twin relaxation peak damping capacity of VIM and DS20 alloys is similar but evidently higher than that of DS100 one. The damping capacity of long columnar dendrite especially at 100 µm/s is also degraded due to strong grain boundary blocking effect. There exists a relationship of Q¹¹ _VIM > Q¹¹ _DS20 > Q¹¹ _DS100 for damping capacity of VIM, DS20 and DS100 alloys at room temperature over the whole strain amplitude range.