Cryogenic Deformation Behavior and Microstructural Characteristics of 2195 Alloy

Cryogenic deformation can improve the strength and plasticity of Al–Li alloy, although the underlying mechanism is still not yet well understood. The effects of cryogenic temperature on the tensile properties and microstructure of an Al–Cu–Li alloy were investigated by means of tensile property test, roughness measurement, scanning electron microscope (SEM), optical microscope (OM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM). The results indicated that the strength and elongation of the as-annealed (O-state) and solution-treated (W-state) alloys increased with the decrease in deformation temperature, where the increasing trend of elongation of the W-state alloy was more significant than that of the O-state alloy. In addition, a temperature range was observed at approximately 178 K that caused the strength of the W-state alloy to slightly decrease. The decrease in temperature inhibited the dynamic recovery of the Al–Cu–Li alloy, which increased the dislocation density and the degree of work hardening, thus improving the strength of the alloy. At cryogenic temperatures, the internal grain structure was more involved in the deformation and the overall deformation was more uniform, which caused the alloy to have higher plasticity. This study provides a theoretical basis for the cryogenic forming of Al–Li alloy.

Study of Texture Analysis on Asymmetric Cryorolled and Annealed CoCrNi Medium Entropy Alloy

CoCrNi equiatomic medium entropy alloy sheets were prepared by asymmetric rolling, cryorolling, and asymmetric cryorolling. The asymmetric cryorolled samples exhibited a noteworthy ultra-fine-grain heterogeneous lamella structure. The microstructure and corresponding hardness obtained by different rolling processes and subsequent annealing are compared. It can be seen from the results that the cryogenic deformation temperature had a stronger effect on the mechanical properties of the medium entropy alloys (MEA), compared with the shear strain caused by the asymmetric cryorolling. The effect of annealing temperature on texture components and volume fractions of the specially rolled samples was also analyzed. The result revealed that the recrystallized MEA exhibited similar texture components and the corresponding volume fraction, which indicated that the rolling process had limited influence on the formation of annealing texture. The recrystallized texture after annealing retained the deformation texture and twin related orientations appeared. Asymmetric rolled MEA showed strong random composition than symmetric rolled MEA regardless of rolling temperature. The recrystallized textures of the species obtained by the three rolling processes did not exhibit a significant dependence on the annealing temperature.

Fracture dynamics in an unstable, deglaciating headwall, Kitzsteinhorn, Austria

Processes destabilising recently deglaciated rockwalls, driving cirque headwall retreat, and putting high alpine infrastructure at risk are poorly understood due to a lack of in situ monitoring data. Deglaciation initiates internal stress redistribution and drastically increases atmospheric forcing rendering cirque headwalls particularly prone for rock slope failure. Here we present quantitative data from an unstable, recently deglaciated cirque headwall. We monitor the dynamics of a fracture at the north face of the Kitzsteinhorn (3203 m a.s.l.) over a period of 2.5 years. Two crackmeters measure horizontal and vertical crack deformation with a resolution of ±0.003 mm and are complemented by crack top temperature measurements. To decipher thermo-mechanical from cryogenic forcing, thermal expansion coefficients for both horizontal and vertical directions are calculated to derive purely thermo-mechanical deformation. Our data shows that fracture dynamics are dominated by thermo-mechanical expansion and contraction of the inter-cleft rock mass during snow-covered and snow-free periods. Significant deviations from thermo-mechanical behavior occur due to freeze-thaw action during spring and autumn zero curtain periods. Exceptional vertical deformation during these periods is triggered by rainfall events providing liquid water into the fracture system. Subsequent refreezing rather than hydrostatic pressure build-up is to the most likely cause of the mechanical response. Lower magnitude horizontal deformation occurs in autumn and early winter due to ice segregation. Irreversible fracture opening was not observed, however, enhanced cryogenic deformation in spring and autumn may lead to shallow, low magnitude rock detachments. Our results highlight the importance of liquid water intake in combination with subzero-temperatures on the destabilisation of glacier headwalls. We conclude that intense frost action and ice segregation are common processes in randkluft systems, serving as important preparatory factors of paraglacial rock slope instability.