Kohji Kawahara

X-ray diffraction characterization of the decomposition behavior of γMn phase in a Mn-30at.%Cu alloy

In MnCu alloys with more than about 70 at. % Mn, antiferromagnetic phase transition and the closely associated fcc-fct phase transformation take place in the {gamma}{sub Mn} phase which is stable at higher temperatures. The decomposition of {gamma}{sub Mn} phase into Mn-rich and Cu-rich fractions in aged MnCu alloys has been extensively investigated and the boundaries of the metastable miscibility gap for {gamma}{sub Mn} phase were also outlined by means of the Vickers hardness data of aged MnCu alloys. However, the Mn contents of the two decomposed fractions confined by the gap boundaries describe the decomposition behavior during aging only qualitatively, because both Mn content and volume fraction of the decomposed products are influenced not only by aging temperature, but also by alloy composition, aging time and the possible precipitation of {alpha}{sub Mn} phase. The purpose of the present work is to characterize the co-existence of the decomposed two phases by an accurate X-ray scanning, and hence trying to clarify the decomposition process of {gamma}{sub Mn} phase quantitatively.

The damping behavior of a Ni-50 at.%Ti shape memory alloy

In order to utilize the shape memory alloys (SMAs) for prospective damping applications, a near equiatomic NiTi alloy was investigated on a dynamic mechanical analyzer in the infrasonic frequency range. A high damping capacity stable to the variation of temperature can be obtained during a two-stage phase transformation and in the martensite state approaching to the martensite-parent phase transformation temperature in the NiTi SMA. The transient damping peaks associated with the phase transformation in the NiTi SMA shows a strong frequency dependence in both solutionized and annealed specimens. The frequency dependence parameter, n, was found to be in the range 0.3 – 0.6. For the applied strain amplitudes from 8.5 × 10 –6 to 5.0 × 10 –5 a clear strain amplitude dependence was not observed in the damping peaks, except for the solutionized specimens. While the damping capacity of the martensite state, characterized at 300 K, showed obvious frequency dependences in both the solutionized and annealed specimens, only the annealed specimens manifested the strain amplitude dependence. The different damping behavior was attributed to the internal structure of martensite, including dislocations, the twinning types and variant interfaces, changeable with the prior thermomechanical treatments.