Study On The Internal Friction Behaviors And Mechanisms Of Thermoelastic Martensitic Transformation

Cu-based shape memory alloys have been a promising class of functional materials widely studied and applied in recent years due to their outstanding shape memory effect, super-elasticity, high damping capacity etc. Thermoelastic martensitic transformation is one of the most important features of these alloys. The study on the mechanical relaxation behaviors of Cu-based shape memory alloys is of great significance to reveal the process of thermoelastic martensitic transformation and to expand these fields of applications.Cu-Al-Ni-Mn-Ti and Cu-Al-Mn-Zn-Zr etc., adopted as experimental materials, were studied by means of heat treatment, organizational analysis, X-ray diffraction, and dynamic mechanical measurement.During the variable-temperature measurement of thermoelastic martensitic transformation internal friction, if measuring frequency is more than 0.045Hz, there is only a single peak that corresponds, in temperature spectra, to the minimum of relative dynamic modulus and decreases with increasing frequency, but the position of the peak is frequency independent; if measuring frequency is less than 0.045Hz, there exist two distinct internal friction peaks, one corresponds to the minimum of relative dynamic modulus and the maximal values of the peak increase rapidly with decreasing frequency at frequency spectra, which is only related to the phase transition rate; the other corresponds to the inflection point of relative dynamic modulus and the maximal values of the peak distributes symmetrically in logarithmic frequency spectra, only associated with the anelastic movement of phase interface,. When heating rate is 0.75℃/ min, the loss modulus of martensitic transformation in the alloy is M 2 = 0.316δM, the semi-peak width isΔlog10ωτ=1.735. Compared with Debye relaxation peak, the relaxation peak gets apparently broaden. As increasing the heating rate, the relaxation time decays quickly at first, and then gradually to an equilibrium value. If the heating rate is between 0.25 and 12℃/ min, the relaxation time varies from 4.449 to 0.112s.When internal friction of parent phase is isothermally measured, the intrinsic internal friction decays with the decrease of frequency, but an internal friction peak really arises within lower frequencies and falls as the isothermal temperature decreases. The frequency shift phenomenon emerges in the internal friction measurement, indicating that the relaxation in parent phase belongs to classical anelastic relaxation. Compared to martensitic transformation relaxation, the anelastic relaxation is caused actually by diffusion, which adjusts the atomic order degree in parent phase to meet the requirements of the parent structure to martensitic transformation.We have established a generalized phase transition kinetic equation and calculated the volume fraction variation in the case of stress-induced thermoelastic martensitic transformation, based on the relationship of martensitic transformation to stress, temperature, and time variables, in order to analysis the internal friction of martensitic transformation.