Thermoplastic effect in the thermal expansion of solidified gases

Hysteresis of the thermal expansion has been found, which disappears on thermal cycling and reappears again after holding the specimens at the measuring temperature for a long time (for 8 h). Experiments were carried out on polycrystalline argon with N2 and CO impurity (0.5−2 mole%) and also on pure polycrystalline krypton over the temperature range 2−12° K. The initial amplitude of the hysteresis (the relative increase in the thermal expansion coefficient) was about 14% in impure argon and 6% in pure krypton. The investigations were carried out for temperature cycles with different values of the temperature interval (0.2−2 °K) and with different heating (cooling) rates in the range 5·10−2−3·10−3 °K·min−1. The existence of hysteresis is associated with the possibility of the presence in the fee argon lattice of hcp phase regions near sources of internal stresses; dislocations, grain boundaries, etc. On changing the temperature, microplastic deformation takes place in the defect regions, caused by the tendency to reduce the proportion of hcp phase.

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