Effect of double aging on mechanical properties and microstructure of EV31A alloy

To improve the ductility of a commercial Mg−rare earth alloy EV31A(Mg−3Nd−1.5Gd−0.3Zn−0.5Zr),a heat treatment method called double aging is explored,and its effect on mechanical properties and microstructure of the alloy is studied.Ultimate strength and elongation of the alloy can be increased to 288 MPa and 6.6%by the optimum double aging process,compared to 273 MPa and 4.9%after single aging.Time consumption of the aging process is also significantly decreased from 16 h(single aging)to 2 h.HAADF-STEM characterization shows that the primary precipitate isβ'phase,which is similar toβ'phase in Mg−Nd binary alloy.By double aging,theβ'phase is finer and more densely distributed compared with single aging,with approximately double density and half size,which explains the improvement in strength and ductility.

Two-stage double peaks ageing and its effect on stress corrosion cracking susceptibility of Al-Zn-Mg alloy

Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking （SCC） susceptibility of AI-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η ′phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6% （33.2 MPa） and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. AI-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates （MPts）, coarsened and discontinuous grain boundary precipitates （GBPs）, as well as the narrow precipitate free zone （PFZ） in the second peak ageing condition. 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Strength-Toughness Improvement of 15-5PH Stainless Steel by Double Aging Treatment

To obtain better strength-toughness balance of 15-5PH stainless steel,a double aging treatment is proposed to investigate the mechanical properties and microstructure evolution.In this study,Cu precipitates and reversed austenite played a determining role to improve strength-toughness combination.The microstructure was observed using electron backscattered diffraction,transmission electron microscopy and scanning transmission electron microscopy.The volume fractions of Cu precipitates and reversed austenite were calculated with Thermo-Calc software and measured by X-ray diffraction.The results showed that the reversed austenite is formed at the martensitic lath boundaries and its volume fraction also increases with the increase of the aging temperature.At the same time,the size of the Cu precipitates gradually increases.Compared with the traditional single aging and double aging treatment,double aging treatment of 15-5PH stainless steel can increase the toughness while retaining the necessary strength.During double aging of 550℃×4 h+580℃×1 h,15-5PH stainless steel has the best strength and low-temperature(-40℃)toughness match.Its yield strength,ultimate tensile strength and the Charpy impact energy are 1.037 GPa,1.086 GPa and 179 J,respectively.

Effect of heat treatment on microstructural evolution,mechanical properties and tribological properties of H13 steel prepared using selective laser melting

H13 tool steel was successfully prepared by selective laser melting(SLM)technology.The effects of heat treatment on the microstructure,mechanical properties,and tribological properties of SLMed H13 steel were investigated.The heat treatment process involved a solution treatment and a double aging treatment of the deposited H13 tool steel prepared by SLM.The aim is to optimize the microstructure and mechanical properties of SLMed H13 steel.Due to the rapid heating and cooling effects of SLM,carbide precipitation in the deposited H13 steel was not uniform and residual stresses were present.The purpose of the solution treatment is to dissolve the solution at a high temperature to eliminate the residual stresses and defects introduced by the SLM-forming structure.The solution treatment and first aging treatment produced the precipitation of small carbides at the grain boundaries and inside the crystals,which increased the hardness of SLMed H13 steel.The hardness increased from 538±4.0 HV of the as-deposited sample to 548±5.8 HV of samples after the first aging treatment.Accordingly,the ultimate tensile strength and the elongation at break decreased from 1882 MPa and 11.5%in the as-deposited sample to 1697 MPa and 7.9%in those after the first aging treatment,respectively.Furthermore,the friction coefficient and wear rate in the as-deposited sample decreased from 0.5160 and 2.36×10^(–6)mm^(−3)N^(−1)m^(−1)to 0.4244 and 1.04×10^(–6)mm^(−3)N^(−1)m^(−1),respectively.However,the distribution of carbides inside the crystals was not uniform.The second aging treatment adjusted the morphology of carbide precipitation and made it more uniform,but the precipitation of carbides grew and settled at the bottom of the grain boundaries.The hardness decreased to 533±6.7 HV compared with that with the first aging treatment,but the ultimate tensile strength and plasticity reached a balance(1807 MPa,14.05%).Accordingly,the friction coefficient and wear rate also showed a stable and decreasing trend(0.4407,0.98×10^(–6)mm^(−3)N^(−1)m^(−1)).

Controllable magnetic properties of cobalt ferrite particles derived from layered double hydroxide precursors

Cobalt ferrite CoxNi1-xFe2O4 （x = 0, 0.5, 1 ） particles with controllable magnetic properties have been prepared by calcination of co-substituted NiFe^2＋Fe^3＋ -layered double hydroxide （NiFe^2＋Fe^3＋-LDH） precursors prepared via a scalable method involving separate nucleation and aging steps （SNAS）. Their structural and magnetic characteristics were investigated by powder X-ray diffraction （XRD）, scanning electron microscopy （SEM） and vibrating sample magnetometry （VSM）. Measurements of magnetic properties show that the saturation magnetization （Ms） and coercivity （He） of the calcined products increased with increasing cobalt content. The LDH precursor-based product obtained by calcination of a mixture of CoFe^2＋Fe3^＋-LDH and NiFe^2＋Fe^3＋ -LDH powders with a Co/Ni molar ratio of 1：1, exhibits a moderate value of Ms and an increased value of He compared to the corresponding values for an Ni0.5Co0.5Fe2O4 material prepared by calcination of a Co0.5Ni0.5Fe^2＋Fe^3＋-LDH precursor, and a physical mixture of CoFe2O4 and NiFe2O4 with a Co/Ni molar ratio of 1 ： 1. These results may provide a way to regulate magnetic anisotropy of ferrite spinels by varying the composition of the LDH precursors.