High temperature damping in some body centred cubic metals.

The literature on the internal friction of martensite, the theory of microplasticity, and the microplasticity of iron-based alloys is reviewed. Low-frequency internal friction, strain relaxation and microplasticity tests were conducted on a 0.4% carbon martensite. Modifications to the Ke pendulum made strain relaxation and microplasticity tests possible. Amplitude-dependent damping at temperatures of 20 to 130°C was explained by coherent precipitation of ? -carbide. Koster type experiments at temperatures of 125-188°C support the pinning model of Beaulieu. The peak in the temperature range 210-250°C at a frequency of 1Hz observed in bainite was shown to be analogous to the 200-250°C peak present in martensite. The background damping is discussed, a method for its removal presented and the width of the isolated peaks analysed to determine their width parameter B. The separate effect of initial stress and testing temperature on the strain relaxation at constant stress for 0.4% carbon martensite at temperatures up to 255°C are interpretted by thermally activated unpinning of dislocations. Curves at 110 and 137°C were used to determine the parameter B, and these values in conjunction with those obtained from the peak width suggest that the distribution in the relaxation time is in ?. The internal friction of martensite was analysed in terms of the current theories for the cold-worked peak and it was found to be consistent with the theories proposed by Schoeck, and Ino and Sugeno. Microplasticity tests on martensite between 20 and 387°C show that, depending upon testing conditions, reversible and irreversible bowing of dislocations can occur. Attempts were made to explain this bowing, and hence the internal friction of martensite, in terms of the double kink mechanism of microplasticity. An unidentified peak at 300°C for 1 Hz was discovered in an Fe - 28% Cr. alloy quenched from 1100°C