Superior dynamic shear properties by structures with dual gradients in medium entropy alloys

Structures with single gradient and dual gradients have been designed and fabricated in an Al_(0.5)Cr_(0.9)FeNi_(2.5)V_(0.2) medium entropy alloy.Structures with dual gradients(with increasing grain size and a decreasing volume fraction of nanoprecipitates from the surface to the center)were observed to show much better dynamic shear properties compared to both structures with single grainsize gradient and coarse-grained structures with homogeneously distributed nanoprecipitates.Thus,the dual gradients have a synergetic strengthening/toughening effect as compared to the sole effect of a single gradient and the sole precipitation effect.Initiation of the adiabatic shear band(ASB)is delayed and propagation of ASB is slowed down in structures with dual gradients compared to structures with single gradients,resulting in better dynamic shear properties.A higher magnitude of strain gradient and higher density of geometrically necessary dislocations are induced in the structures with dual gradients,resulting in extra strain hardening.Higher density dislocations,stacking faults,and Lomer-Cottrell locks can be accumulated by the interactions between these defects and B2/L1_(2) precipitates,due to the higher volume fraction of nanoprecipitates in the surface layer of the structures with dual gradients,which could retard the early strain localization in the surface layer for better dynamic shear properties.

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.

Effect of nitrogen on corrosion behaviour of a novel high nitrogen medium-entropy alloy CrCoNiN manufactured by pressurized metallurgy

A novel high nitrogen medium-entropy alloy CrCoNiN, which had higher strength and slightly lower ductility than CrCoNi alloy, was successfully manufactured by pressurized metallurgy.The microstructure and corrosion behaviour were investigated by microscopic, electrochemical and spectroscopic methods. The results indicated that nitrogen existed in the form of Cr2N precipitates and uniformly distributed N atoms, and nitrogen alloying significantly refined the grain size. Besides, nitrogen enriched on the outmost surface of passive film and metal/film interface as ammonia （NH3 and NH4^＋） and CrN, respectively. The significant improvement of corrosion resistance of CrCoNiN was attributed to the lower metastahle pitting susceptibility together with thicker, less defective and more compact passive film.

拉伸变形时梯度结构的塑性协调

梯度结构是一种典型跨尺度微观结构,其内部不同尺度晶粒具有迥异的内禀塑性稳定性,塑性协调是其获得优异强韧性的关键微观机制.本研究采用同步辐射原位拉伸测试方法,对梯度结构在拉伸变形过程中,由表及里不同深度结构层的点阵应变演化进行了研究.结果表明,梯度结构拉伸时具有两类塑性协调响应:一是由逐层微观屈服导致的瞬态弹-塑性变形,使纳米结构层的弹-塑性应变范围延后和扩展;二是纳米结构表层的塑性局域化和软化.梯度结构塑性协调引起了层间和层内应力状态的转变,导致层间微观力学行为响应的显著差异,利用异质变形诱导应变硬化和林位错硬化,促使梯度结构中纳米结构表层获得与芯部粗晶层相当的拉伸均匀应变.

Deformation induced hcp nano-lamella and its size effect on the strengthening in a CoCrNi medium-entropy alloy

Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the Co Cr Ni medium-entropy alloy(MEA)under high strain rate and cryogenic temperature in the present study.Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase.Then,size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc Co Cr Ni MEA were investigated by molecular dynamics simulations.The overall strengthening was found to have two components:phase strengthening and extra interface strengthening,and the interface strengthening was observed to be always stronger than the phase strengthening.Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae.The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening.The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission.Nanotwins were observed to be formed in the embedded hcp nano-lamellae.Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems,resulting in higher strength for samples with smaller interspacing.