Mechanism of dislocation channel-induced irradiation assisted stress corrosion crack initiation in austenitic stainless steel

The mechanism by which dislocation channeling induces irradiation assisted stress corrosion cracking was determined using Fe–13Cr15Ni austenitic stainless steel irradiated with protons to a dose of 5 dpa and strained at high temperature in both argon and simulated boiling water reactor normal water chemistry environments. Straining induced dislocation channels that were characterized by digital image correlation and confocal microscopy. Dislocation channels were found to be either continuous across the boundary, discontinuous, or discontinuous with slip in the boundary. Discontinuous channels were found to contain the least amount of strain but have the highest propensity for initiating cracks. Discontinuous dislocation channel–grain boundary intersections were shown to have the highest local stress. TEM in-situ straining of irradiated steels and atomistic simulation of dislocation–grain boundary interaction provided supporting evidence that channels that were unable to transfer strain underwent cracking. The inability of channels to relieve stress, by either slip in the adjacent grain or in the grain boundary, resulted in high local stresses and increased susceptibility to stress corrosion cracking initiation.     

Surface roughness evolution during early stages of mechanical cyclic loading

The effect of crystal size and initial dislocation density on surface roughness evolution in FCC single crystals during the early number of cycles of mechanical cyclic loading is investigated using three dimensional discrete dislocation dynamics simulations. Crystals having size less than 2 μm show early development of surface slip localization, while larger ones show a more uniform distribution of surface steps. The surface roughness is found to increase with increasing number of loading cycles with larger crystals showing a high roughening rate compared to smaller crystals. Double cross-slip is observed to be the main mechanism that derives the development, growth and thickening of surface slip bands. The maximum surface height, which is an indicator of the surface stress concentration is observed to increase linearly with the number of loading cycles and quadratically with the crystal size for the simulated number of cycles. Finally, the results are shown to be in agreement with experimental results and provide further physics based understanding on the mechanisms controlling the evolution of the surface roughness.     

A research on the creep properties of titanium matrix composites rolled with different deformation degrees

(TiB + La₂O₃)/Ti composites were in situ synthesized and deformed with different deformation degrees. The influence of TiB whisker orientation and grain refinement on the creep properties of titanium matrix composites (TMCs) are discussed. The creep test reveals that the steady state creep rate of TMCs first decreases and then increases with the increase of deformation degree, which can be attributed to competing effects: TiB whisker rotating to the rolling direction, α plate grain boundary hindering and pinning dislocations can all decrease the creep rate, however, dislocation movement on the α plate grain boundary and dislocation emitting from the α plate grain boundary can both increase the creep rate.     

振动时效机理研究

分析了残余应力的产生及常规消除残余应力方法的机理,研究了在循环应力下材料塑性变形特征,以及频加载率和应力集中对材料塑性变形的影响.结果表明,振动时效是在材料的弹性范围内发生的微塑性变形,微塑性变形只在材料的部分晶粒内发生,在亚晶粒内部形成位错带,亚晶界发生旋转形成"晶粒",亚晶界的位错开通,形成一个个微观屈服小区,晶格扭曲减少,从而使工件中的残余应力峰值降低并得到均布.     

集成电路塑封模具错位、偏心问题探讨

就集成电路封装过程中塑封模具的错位和偏心问题做探讨,分析导致错位和偏心的各种原因,并就如何使塑封模具的错位、偏心问题对产品的质量影响降到最低提出了具体的改善措施.     

Anisotropic and tensile flow behaviors of Mg alloy AZ31B thin sheet in H24 condition at elevated temperatures

In this work, a series of experiments was performed to explore the effects of anisotropy, strain rate, and temperature on microstructure change and associated mechanical response of a rolled AZ31B-H24 Mg alloy sheet under tension. Tensile tests were carried out on specimens in the 0, 45, and 90° to the rolling direction, using initial strain rates in the range of 4 × 10⁻³ to 1 × 10⁻¹ s⁻¹ at temperatures of 250 and 370 °C. Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. Resultant microstructures, such as degree of dynamic recrystallization, grain growth, and shape of the grain, were associated with temperature, strain rate, and tensile loading direction. The initial texture influenced the variations in changes in microstructure and mechanical properties upon testing in different directions. The specimens upon testing in the 45° to the rolling direction yielded higher m-value, lower strength, and greater elongation to failure under all test conditions.     

Effect of strain rate on microstructure evolution of a nickel-based superalloy during hot deformation

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s⁻¹ at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s⁻¹, the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ3 boundaries at the intermediate strain rate of 1 s⁻¹, while the fractions of Σ3 boundaries were much higher at both the lower strain rates ($\dot{\epsilon }⩽0.1{\text{s}}^{-1}$) and higher strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$).