On the micromechanism of superior strength and ductility synergy in a heterostructured Mg-2.77Y alloy

Heterostructured metals and alloys are a new class of materials in which mechanical behaviors between the heterogeneous regions are significantly different,and the mechanical properties of bulk materials are superior to the superposition of individual regions.In this paper,three distinct types of heterostructures were constructed in Mg-2.77Y(wt.%)alloy by applying simple thermomechanical processing.Namely,Type I:the non-recrystallized grains of several tens of microns were embedded in the micron-scaled recrystallized grains that were distributed along shear bands and dispersed near grain boundaries;Type II:the aggregations of micron-scaled recrystallized grains were surrounded by the non-recrystallized grains;Type II:the micron-scaled recrystallized grains dominated the microstructure,and the non-recrystallized regions with diameters of tens of micrometers were surrounded by those fine recrystallized grains.Mechanical tests showed that the material with type III heterostructure had the optimal combination of yield strength and uniform elongation.This is attributed to its remarkable hetero-deformation induced(HDI)strengthening and dislocation strengthening.At the initial stage of plastic deformation(engineering strain below 4%),the rapid accumulation of geometrically necessary dislocations(GNDs)at the interfaces between recrystallized and non-recrystallized regions and between neighboring recrystallized grains lead to the significant HDI strengthening.As deformation proceeded,the HDI strengthening effect gradually decreased,and the traditional dislocation strengthening that was caused by GNDs accumulation at grain boundaries became significant.In-situ electron back-scattered diffraction(EBSD)testing revealed that the non-basal slip in the non-recrystallized regions became more remarkable in the late stage of deformation,which improved ductility and strain hardening of the alloy.These findings provide new insight into the design of high-performance hexagonal close-packed structural materials by using the concept of HDI strengthening.

Achieving exceptional strength and ductility combination in a heterostructured Mg-Y alloy with densely refined twins

Metals and alloys with heterogeneous microstructures are an emerging class of materials that exhibit exceptional mechanical properties,owing to the novel scientific principle of hetero-deformation induced(HDI)strengthening and hardening.For magnesium alloys,due to their low recrystallization temperature,poor ductility at room temperature,limited cold workability,and the tendency to generate strong basal texture during deformation,it is difficult to obtain heterostructures without relying on precipitation of the second phases.Here,three heterostructured Mg-2.9Y(wt.%)materials with varying accumulative equivalent true strains,i.e.,5%-5 cycles,7.5%-5 cycles,and 10%-5 cycles materials were fabricated via applying five complete triaxial compression cycles to the bulk alloy.The 5%-5 cycles material with an accumulative equivalent true strain of 0.37 is featured with long twin lamellae embedded in coarse grains.When the accumulative true strain increases to 0.72,a heterogeneous structure composed of long and short twin lamellae is formed inside the 7.5%-5 cycles material.As the equivalent true strain further increases to 1.01,the 10%-5 cycles material exhibits a mixed structure with densely refined twin lamellae embedded in the coarse-grained matrix.The room-temperature uniaxial tensile tests show that the yield strength of the materials processed by triaxial cyclic compression(TCC)has been significantly improved compared to that at the initial state,whereas ductility was not significantly sacrificed without the subsequent heat treatment.The dense and refined twin lamellae that serve as hard domains in this material provide a high density of interfaces and impede dislocation motion effectively.This results in significant HDI strengthening and hardening.These findings provide new insight into the design of heterostructured hexagonal close-packed materials with both high strength and good ductility.

Synergy effect of multi-strengthening mechanisms in FeMnCoCrN HEA at cryogenic temperature

High-entropy alloys(HEAs)have attracted great research interest owing to their good combination of high strength and ductility at both room and cryogenic temperatures.However,expensive raw materials are always added to overcome the strength-ductility trade-off at low temperatures,leading to an increased production cost for the cryogenically used alloys.In this work,a series of nitrogen-doped Fe Mn Co Cr HEAs have been processed by homogenization annealing,cold rolling and recrystallization annealing followed by water quenching.The microstructural evolution and mechanical properties of the alloys are studied systematically.The Fe_(49)Mn_(30)Co_(10)Cr_(10)N1alloy shows excellent mechanical properties at both 293 K and 77 K.Particularly,the yield and ultimate tensile strength of 1078 and 1630 MPa are achieved at the cryogenic temperature,respectively,while a satisfactory uniform elongation of 33.5%is maintained.The ultrahigh yield strength results from the microstructure refinement caused by the activation of athermal martensitic transformation and mechanical twinning that occur in the elastic regime together with the increased lattice friction due to the cryogenic environment.In the plastic regime,the dynamic Hall-Petch effect caused by twinning,martensitic transformation,and reverse transformation together with the high barrier to dislocation motion jointly contribute to the ultrahigh tensile strength.Simultaneously,the transformation induced plasticity(TRIP)and the twinning induced plasticity(TWIP)effects jointly contribute to the ductility.The design strategy for attaining superior mechanical properties at low temperatures,i.e.by adjusting stacking fault energy in the interstitial metastable HEAs,guides the development of high-performance and low-cost alloys for cryogenic applications.