The effect of grain size on the damping capacity of Fe-17 wt%Mn

Abstract

The grain size dependence of the damping capacity of Fe-17 wt%Mn steel was investigated. A high damping capacity was measured in the ultra-fine grained steel, despite its lower volume fraction of ε martensite and lower density of ε variant boundaries and ε/γ phase boundaries. Dilatometry of the ultra-fine grained Fe-17 wt%Mn steel revealed that the ε↔γ phase transformation was largely suppressed. The features of the damping spectra were related to the anti-ferromagnetic transition in the γ phase, the thermo-elastic ε↔γ phase transformation and the motion of grain boundaries in the ultra-fine grained microstructure. The damping spectrum of the ultra-fine grained Fe-17 wt%Mn steel was dominated by grain boundary damping effects.

Introduction

In engineering design for machines and vehicles, vibration and noise reduction technology is given much attention as it prevents undesirable fatigue failures and it promotes user comfort. Materials with a high damping capacity are therefore being employed in the manufacture of metallic parts of machines and vehicles. High damping Fe-Mn based alloys are considered ideal for the purpose as they combine a high tensile strength with a high damping capacity [1], [2].

The intrinsic mechanical damping or internal friction of a material is defined as the capacity of a material to convert the mechanical energy of vibrations into heat that is dissipated in the material [1], [3]. The conversion of a cyclic externally-induced elastic strain to heat is believed to result from the movement of point defects (i.e. alloying elements and vacancies), point defect complexes, line defects (i.e. dislocations), or planar defects (i.e. stacking faults, grain boundaries and phase boundaries) in the microstructure of high damping capacity materials. The principal source of damping in Fe-Mn alloys is still under discussion [2], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The standard viewpoint is that the damping is related to the ε phase volume fraction in the Fe-Mn alloy. The movement of the ε/γ phase boundaries, the boundaries between ε phase orientation variants and the stacking fault boundaries in the ε phase and in the γ phase are also believed to be involved in the damping in Fe-Mn alloys. The quantitative relationship between the damping capacity and the volume fraction of ε [2], [4], [5], [6], [7] suggests that the defect properties of the ε phase rather than the γ phase play a central role in the damping. This viewpoint is not universally accepted. For example, Wen et al. [19] proposed a damping mechanism which is not related to the ε phase volume fraction. They report that the damping capacity is related to variations of the vacancies concentration and the segregation of carbon atom in Fe-Mn alloys. In the present contribution, the effect of grain size on the damping capacity was studied. A reduction of the grain size was found to suppress the ε↔γ phase transformation and result in a high damping capacity which was not related to the ε phase volume fraction, but determined mainly by grain boundary damping.