Exceptional thermal stability and enhanced hardness in a nanostructured Mg-Gd-Y-Zn-Zr alloy processed by high pressure torsion

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr(wt.%) alloy is processed by solution treatment and high pressure torsion(HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K,grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg_(5)RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.

Ordered Sn distribution adjacent to the precipitate-matrix interface in a Mg-9.8wt.%Sn alloy

The major interface betweenβ-Mg_(3)Sn precipitate plate and theα-Mg matrix in a Mg-9.8wt.%alloy has been investigated using aberrationcorrected scanning transmission electron microscopy and first-principles calculations.It is found that Sn atoms orderly distribute in the single layer of theα-Mg matrix immediately adjacent to the broad surface ofβat the early stage of ageing.These Sn atoms substitute Mg atoms located at the centers of equilateral triangles constituted by three Mg columns in the outmost layer ofβ.First-principles calculations suggest that the ordered Sn distribution is energetically favored and it not only decreases the interfacial energy of theβ-matrix interface but also hinders the occurrence of 1/3<01■0>αshear that thickens theβplate.

Machine learning assisted design of γ′-strengthened Co-base superalloys with multi-performance optimization

Designing a material with multiple desired properties is a great challenge,especially in a complex material system.Here,we propose a material design strategy to simultaneously optimize multiple targeted properties of multi-component Co-base superalloys via machine learning.The microstructural stability,γ′solvus temperature,γ′volume fraction,density,processing window,freezing range,and oxidation resistance were simultaneously optimized.

Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy

Mg-9Al-6Sn-3Zn （wt%） alloy was extruded and heat treated in T5 and T6 conditions, and its mechanical properties and microstructures were investigated. The extruded product can be slightly strengthened by the T5 treatment as a result of sparse and heterogeneous precipitation. Significant increase in strength is achieved by the T6 treatment, and this is mostly attributed to the formation of lamellar discontinuous Mg17Al12 precipitates. The segregation of Al and Zn at grain boundaries is responsible for the discontinuous Mg17Al12 nucleation. The T6-treated alloy exhibits a tensile yield strength of 341 MPa and an ultimate tensile strength of 409 MPa, together with an elongation to fracture of 4%.

Heterogeneous lamella design to tune the mechanical behaviour of a new cost-effective compositionally complicated alloy

A heterogeneous lamella(HL)design strategy was applied to manipulate mechanical properties of a new cost-effective Fe_(35)Ni_(35)Cr_(25)Mo_(5)compositionally complicated alloy(CCA).The HL structure was produced by single-step heat treatment(800℃for 1 h)after cold rolling.This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix(5-20μm),and regions containing ultra-fine grains or subgrains(200-500 nm)together with nanoprecipitates(20-500 nm)and annealing twins.As compared with other cost-effective CCAs,the 800℃annealed sample with HL structure demonstrated a comparable tensile property,with yield strength over 1.0 GPa and total elongation of~13%.Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation ofσphase at the shear bands with a high density of lattice defects(e.g.high-density dislocation walls and deformation twins).The latter restricted the growth of recrystallized grains,leading to the formation of ultrafine subgrains within the HL structure.The high yield strength resulted from the multistage hetero-deformation induced(HDI)strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates.The ductility was originated from the coexistence of multiple deformation mechanisms,which started with dislocation slip and formation of stacking faults at the initial stage,followed by nano-twinning at the higher strain level.This HL design strategy,comprising composition and thermomechanical process designs,and the resultant microstructure tuning,open a broader window for the development of cost-effective CCAs with enhanced performance.

Electron beam irradiation induced metastable phase in a Mg-9.8 wt%Sn alloy

Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase,designated asβ’’phase,induced to form by electron beam irradiation in a Mg-9.8 wt.%Sn alloy.This phase is spherical in three dimensions,having a D019 structure with the lattice parameters of a=0.642 nm,c=0.521 nm and space group of P63/mmc.Its chemical formula is Mg_(3)Sn,like theβ’metastable precipitate phase.The orientation relationship between theβ’’phase and theα-Mg matrix is such that■_(β)’’//■_(α)and(0001)_(β)’’//(0001)_(α).Its formation involves solely the ordering of Sn atoms in the solid solution magnesium matrix.First-principles calculations indicate that the formation of theβ’’phase is energetically favored.