Abnormal Twinning Behavior and Constitutive Modeling of a Fine-Grained Extruded Mg-8.0Al-0.1Mn-2.0Ca Alloy under High-Speed Impact along Various Directions

To understand the mechanical and twinning behaviors of a fine-grained extruded Mg-8.0Al-0.1Mn-2.0Ca alloy under high-speed impact,impact tests were carried out using a split Hopkinson pressure bar,and microstructures at strains of 0.05,0.10 and 0.20 were obtained using a series of stop rings manufactured by high-strength steel.The stress response and twinning behavior are closely related to loading direction and applied strain rate.The true stress-true strain curves are s-shaped in extrusion direction(ED)and c-shaped in transverse direction(TD),showing apparent anisotropy,while the yield strength is insensitive to loading direction.Almost identical strain-rate sensitivity is demonstrated by the stress in ED and TD.Interestingly,de-twinning is apparent as the applied strain increases to 0.20,and it is enhanced with increasing the applied strain rate.In contrast,the twin density in ED samples is clearly higher than that in TD samples.By modifying the terms of strain hardening and strain rate hardening in the classical JC model,an optimized model is built,which can accurately predict the stress response behavior of the studied alloy under high-speed impact along ED and TD.The correlation coefficient(R)and average absolute relative error(AARE)are 98.63%and 0.0199 for ED,and 96.88%and 0.0202 for TD,respectively.

Effect of long-period ordered stacking on twinning behavior and mechanical properties of Mg-Al-Y alloy during uniaxial hot compression

Two alloys with(T4-4h)and without(T4-8h)long-period stacking ordered(LPSO)phase were obtained by solution treatment of as-cast Mg-1Al-12Y alloy at 540℃ for 4 h and 8 h,respectively.The compressive tests(300-450℃,ε=0.01s^(-1))showed that the strengthening effect of LPSO in T4-4h alloy gradually disappeared with temperature increase.In addition,the corresponding compression deformation behavior of the two alloys was also investigated by electron backscatter diffraction(EBSD).It was found that the twinning of the two alloys was dominated by{1012}extension twins,and the large amount of lamellar LPSO in T4-4 h alloy inhibited the activation and growth of twins.The[0001]//compression direction(CD)texture formed after deformation is mostly attributed to the activation and growth of extension twins.By analyzing the activated twin variants,it was found that the activation of twin variants mainly depended on Schmid factor(SF).For twin variants with smaller SF,they may be activated due to local stress concentration.Based on the analysis results of in-grain misorientation axis(IGMA)and SF,the deformation mechanism of the alloys at elevated temperature is dominated by basal slip and non-basal slip.The stronger mechanical properties of T4-4h alloy than those of T4-8h are attributed to the obstruction of non-basal slip movement by LPSO.

Serration and Noise Behavior in Advanced Materials

The Chinese Materials Research Society（C-MRS）Conference（2015）was held in the Guizhou Park Hotel International Conference Center,Guiyang,China,from July 10-14,2015.This conference consists of 30symposia,including 4international symposia.As one of 4international symposia,＂Serration and noise behavior in advanced materials＂

Effect of Twin Boundary–Dislocation–Solute Interaction on Detwinning in a Mg–3Al–1Zn Alloy

In the present study,the influence of solute atoms together with dislocations at {101^-2} twin boundary（TB） on mechanical behavior of a detwinning predominant deformation in a Mg alloy AZ31 plate was systematically studied.The results show that a large number of {101^-2} twins disappear during recompression along the normal direction.Both the TB-dislocation interaction and TB-solute-dislocation interaction can greatly enhance the yield stress of the recompression along the normal direction（ND）.However,the solute segregation at {1012} TBs with an intensive interaction with 〈a〉 dislocations cannot further enhance the yield stress of ND recompression.The samples with TB-dislocation interaction show a similar working hardening performance with that subjected to a TB-solute-dislocation interaction.Both the TB-dislocation interaction and TB-solute-dislocation interaction greatly reduce the value of work hardening peaks during a detwinning predominant deformation.

Achieving excellent strength-ductility synergy in twinned NiCoCr medium-entropy alloy via Al/Ta co-doping

Alloying is an effective strategy to tailor microstructure and mechanical properties of metallic materials to overcome the strength-ductility trade-off dilemma.In this work,we combined a novel alloy design principle,i.e.harvesting pronounced solid solution hardening(SSH)based on the misfit volumes engineering,and simultaneously,architecting the ductile matrix based on the valence electron concentrations(VEC)criterion,to fulfill an excellent strength-ductility synergy for the newly emerging high/medium-entropy alloys(HEAs/MEAs).Based on this strategy,Al/Ta co-doping within NiCoCr MEA leads to an efficient synthetic approach,that is minor Al/Ta co-doping not only renders significantly enhanced strength with notable SSH effect and ultrahigh strain-hardening capability,but also sharply refines grains and induces abnormal twinning behaviors of(NiCoCr)_(92)Al_(6)Ta_(2) MEA.Compared with the partially twinned NiCoCr MEA,the yield strength(σy)and ultimate tensile strength(σUTS)of fully twinned Al/Ta-containing MEA were increased by~102%to~600 MPa and~35%to~1000 MPa,respectively,along with good ductility beyond 50%.Different from the NiCoCr MEA with deformation twins(DTs)/stacking faults(SFs)dominated plasticity,the extraordinary strain-hardening capability of the solute-hardened(NiCoCr)_(92)Al_(6)Ta_(2) MEA,deactivated deformation twinning,originates from the high density of dislocation walls,microbands and abundance of SFs.The abnormal twinning behaviors,i.e.,prevalence of annealing twins(ATs)but absence of DTs in(NiCoCr)_(92)Al_(6)Ta_(2) MEA,are explained in terms of the relaxation of grain boundaries(for ATs)and the twinning mechanism transition(for DTs),respectively.