To improve the surface quality for aluminum alloy 6061(Al6061) in ultra-precision machining, we investigated the factors affecting the surface finish in single point diamond turning(SPDT)by studying influence of the precipitates generation of Al6061 on surface integrity and surface roughness.Based on the Johnson-Mehl-Avrami solid phase transformation kinetics equation, theoretical and experimental studies were conducted to build the relationship between the aging condition and the type, size and number of the precipitates for Al6061. Diamond cutting experiments were conducted to machine Al6061 samples under different aging conditions. The experimental results show that, the protruding on the chip surface is mainly Mg_(2)Si and the scratches on the machined surface mostly come from the iron-containing phase(α-, β-AlFeSi).Moreover, the generated Mg_(2)Si and α-, β-AlFeSi affect the surface integrity and the diamond turned surface roughness. Especially, the achieved surface roughness in SPDT is consistent with the variation of the number of AlFeSi and Mg_(2)Si with the medium size(more than 1 μm and less than 2 μm) in Al6061.
Heterostructures of alloyed composites,comprising heterogeneous domains with dramatically different constitutive properties,hold remarkable potential to expand the realm of material design systems and resolve the tradeoffbetween strength and ductility.This study introduces an innovative materials design method for synthesizing gradient pseudo-precipitates heterostructure(GPHS)in non-heat-treatable Al-2.5%Mg alloys.Utilizing cost-effective mild steel as both the diffusion source and protective layer,this heterostructure is achieved through pin-less friction stir-assisted cyclic localized deformation process.Exogenous Fe atoms diffuse across the interface by friction stir-induced heat conduction,forming Fe-Al second-phase particles in the Al alloy matrix.A rapid inter-diffusion mechanism is activated in conjunction with dense dislocation walls,grain boundaries,and sub-structures,resulting in the formation of pseudo-precipitates.These pseudo-precipitates are ultimately dispersed in a gradient distribution throughout the entire thickness of the Al alloy matrix induced by localized incremental deformation.The GPHSed Al-2.5%Mg alloy exhibits an enhanced synergy of strength and ductility,with a uniform elongation increase from 11%to 21.2%,while maintaining the strength.Multiple strengthening and hardening mechanisms,such as solid solution strengthening,dislocation hardening,and second phase strengthening,work synergistically to promote mechanical performance.Notably,the hetero-deformation between hard pseudo-precipitates and soft Al alloy matrix induces additional strain hardening,leading to high ductility.This work provides a fresh perspective on the design and fabrication of high-performance alloys with advanced heterostructures,especially for non-heat-treatable alloys.
The chemical boundaries inside the ultrafine spinodal decomposition structure in metastable β-Ti alloys can act as a new feature to architect heterogeneous microstructures.In this work,we combined two semi-empirical methods,i.e.,the d-electron theory and the e/a electron concentration,to achieve the spinodal decomposition structure in a metastable β Ti-4.5Al-4.5Mo-7V-1.5Cr-1.5Zr(wt.%)alloy.Utilizing the spinodal decomposition structure,the aged Ti-Al-Mo-V-Cr-Zr alloys showed multi-architectured α precipitates spanning from micron-scale(primary α_(p))to nano-scale(secondary α_(s))that were uniformly distributed in the β-domains.Being compared with the forged sample,the multi-scale heterogeneous microstructure enables the aged β-Ti alloy to have ultra-high strength(yield strength ~1366 MPa and ultimate tensile strength ~1424 MPa)and an appreciable ductility(~9.3%).Strengthening models were proposed for the present alloys to estimate the contribution of various microstructural features to the measured yield strength.While the solid solution strengthening,β-spinodal strengthening,and back stress strengthening made comparable contributions to the strength of the forged alloy,the back stress strengthening was the predominant strengthening effect in the aged alloy.This alloy design approach based on chemical boundary engineering to construct multi-architectured α precipitates provided an effective strategy for achieving an outstanding combination of ultra-high strength and ductility in metastable β-Ti alloys.
The coarsening behavior and strengthening effect of L1_(2)-Ni_(3)(Ti,Al)precipitates in a face-centered-cubic(FCC)(FeCoNi)_(92)Al_(2.5)Ti_(5.5) high entropy alloy have been systematically investigated.The coherent L1_(2) precipitates,uniformly distributed throughout the FCC matrix,consistently retain a spherical shape.The coarsening rate coefficient of precipitate is determined by employing the Philippe-Voorhees(PV)model,suggesting excellent thermal stability.Furthermore,the elemental partitioning and compositional evolution of the L1_(2) precipitates is analyzed by atom probe tomography,which identify aluminum(Al)as the slowest diffusion species during the coarsening process.In addition,the precipitation strengthening effect is quantified to ascertain the optimal size of the precipitates.Our study enhances the understanding of precipitate coarsening in high entropy alloys,presenting valuable insights into their thermal stability and mechanical properties.
Yuqi LiuFeng WangSongyang ChenHui WangZhiping XiongKhurram YaqoobZhangwei WangMin Song
