Decomposition of high temperature γMn phase during continuous cooling and resultant damping behavior in Mn74.8Cu19.2Ni4.0Fe2.0 and Mn72.4Cu20.0Ni5.6Fe2.0 alloys

Abstract

The temperature dependent changes of logarithmic decrement δ and Young's modulus E were measured for continuously cooled Mn74.8Cu19.2Ni4.0Fe2.0 and Mn72.4Cu20.0Ni5.6Fe2.0 (at%) alloys, after solid solution holding at 1173 K. Microstructure of those alloys, cooled to room temperature in different rates, was characterized by X-ray diffraction and electron microscopy. Ni apparently inhibited the decomposition of high temperature γMn phase during cooling, so lower fcc-fct transformation temperatures (Tt) were observed in Mn72.4Cu20.0Ni5.6Fe2.0 alloy. Besides, several fct phases with different lattice parameters or Tt temperatures, i.e. an appearance of compositional heterogeneity in the Mn-enriched fraction, were also manifested in the alloy with Ni of 5.6 at%. Large amounts of (011) twinning plates were observed as a typical feature in the microstructure of Mn74.8Cu19.2Ni4.0Fe2.0 alloy. However, tetragonal distortion caused mis-fitting at the twinning boundaries resulted in a rapid reduction of logarithmic decrement in the vicinity of 273 K. Meanwhile, a larger amount of precipitated Cu-enriched phase confined the formation and growth of twinning plates in the fct phases, and hence reduced logarithmic decrements. In contrast, a tweed microstructure in Mn72.4Cu20.0Ni5.6Fe2.0 alloy sustained the high damping capacity in a wide temperature range near 273 K, irrespective of the cooling rates.