Effect of Composition on the Internal Friction and Young's Modulus in γ Phase Mn-Cu Alloys

A detailed investigation has been made regarding the variation in internal friction and Young's modulus with temperature in manganese–copper alloys containing 11.74–26.08 wt.-% Cu. It was confirmed that the transformation temperature for the fct → fcc reverse martensitic transformation in quenched specimens is reduced from 129 to – 76° C (402 to 197 K) with increase in Cu content from 11.74 to 26.08 wt.- %. This is in good agreement with the results of previous workers. The internal friction was found to exhibit a ‘main peak’ and a ‘sub-peak’. The former appeared at ∼ 3°C (276 K) in all specimens containing < 21 wt.-% Cu, while the latter was observed at the temperature of the reverse martensitic transformation (T_t ) in each specimen. It is concluded from detailed analysis that the main peak is due to a relaxation process associated with the movement of the {10l}-twin boundaries in the fct phase, the activation energy and the frequency factor being, respectively, ∼ 5.15 × 10⁴ J“g atom and 2.4 × 10¹³ S⁻¹ for a sample containing 11.74 wt.-% Cu. The nature of the sub-peak, however, is not well understood. It is tentatively suggested that it may be closely connected with the movement of interfaces separating the fcc phase from the fct phase. The anomaly in Young's modulus observed at T_t can be satisfactorily explained in terms of a strain-dependent exchange model and the fairly close connection between the martensitic transformat ion and the magnetic transition in this alloy is pointed out.