On the strengthening and slip activity of Mg-3Al-1Zn alloy with pre-induced{1012¯}twins

{1012¯}twins were introduced into the magnesium(Mg)plate AZ31 via pre-rolling along its transverse direction.The plates,both with and without the pre-induced{1012¯}twins,were subjected to uniaxial tension along different directions.Using crystal plasticity modeling,we found that the strengthening effect of the pre-induced{1012¯}twins on the macroscopic flow stress primarily arised from the increased slip resistance caused by the boundaries,rather than the orientation hardening due to the twinning reorientation(although the latter did make its contribution in some specific loading directions).Besides,the pre-existing{1012¯}twins were found,by both experiments and simulation,to promote the activity of prismatic and pyramidal<c+a>in the parent matrix of the material.Further analysis showed that the enhanced non-basal slip activity is related to the{1012¯}twin boundaries’low micro Hall-Petch slope ratios of non-basal slips to basal slip.With the critical resolved shear stress(CRSS)obtained from crystal plasticity modeling and the orientation data from EBSD,a probability-based slip transfer model was proposed.The model predicts higher slip transfer probabilities and thus lower strain concentration tendencies at{1012¯}twin boundaries than that at grain boundaries,which agrees with the experimental observation that the strain localization was primarily associated with the latter.The present findings are helpful scientifically,in deepening our understanding of how the pre-induced{1012¯}twins affect the strength and slip activity of Mg alloys,and technologically,in guiding the design of the pre-strain protocol of Mg alloys.

Microalloying Al alloys with Sc:a review

As a kind of important light alloys,the Al alloys exhibit mechanical properties that are closely related to the microstructures.Changing the main alloying elements and adjusting heat treatments are usually approaches to tune the microstructure and hence artificially control the mechanical properties.However,the windows for the two approaches have become quite narrow,after extensive studies in the last half of century.Microalloying has become the most promising strategy to further modify the microstructure and improve the mechanical properties of Al alloys,among which the element of scandium(Sc)is especially powerful.In this paper,the recent progresses in Al alloys microalloyed with Sc are briefly reviewed,focusing on the microstructural characterization,strengthening response,and underlying mechanisms.The possible key research points are also proposed for the further development of Al alloys microalloyed with Sc and other rare earth elements.

Impact of thermal exposure on the microstructure and mechanical properties of a twin-roll cast Al-Mn-Fe-Si strip

Al-Mn-Fe-Si strips were fabricated via both the twin-roll casting(TRC)and the more conventional route,direct-chill casting(DC).The two types of strips prepared were subjected to thermal exposure at a series of temperatures.Uniaxial tensile tests after the thermal exposure showed that while the DC strip presented a~74%decrease in the yield strength and~35%decrease in the ultimate tensile strength(UTS)after being exposed to 350℃for 12 h,the TRC strip,in contrast,maintained its strength at temperatures up to~460℃ for the same duration.Systematic microstructure characterization revealed that the different thermal stability in the strength of the two types of strips arised from their distinct evolution in grain morphology and second phase particles during the thermal exposure.The calculation based on Cahn-Lücke-Stüwe(CLS)model suggested that due to the highly supersaturated solute atoms,at the beginning of the thermal exposure,the TRC strip experienced a strong solute drag which reduced the grain boundary migrating velocity to a value that is orders of magnitude smaller than that in the DC strip.With the progress of the thermal exposure,the solute atoms precipitated out,forming densely distributed second phase particles.For one thing,these particles stabilized the grain structure by inducing Zener pinning pressure which could be ten times higher than that in the DC strip,depending on the temperature.For another,they acted as dislocation obstacles and compensated for the strength loss owing to decreasing solution hardening.Both effects contributed to the TRC strip’s fairly stable strength regarding thermal exposure below 460°C.The present work could guide the direct application of the TRC strips in the industry.The results should also be helpful for the development of a fundamental framework for designing advanced TRC Al strips with improved mechanical properties at elevated temperatures.

拟一维等熵Euler流的小Mach数极限

本文在BV∩L1框架下研究拟一维等熵Euler管道流初值问题解的小Mach数极限.在初始值全变差有界且远离真空以及Mach数和管道截面全变差充分小的假设下,通过解的L1-稳定性和标准Riemann半群方法的应用,本文严格证明拟一维等熵Euler管道流初值问题的熵解可以按照小Mach数展开且其系数满足一带源项的线性声波方程组.