Introduction of nanotwins into nanoprecipitations strengthened CoCrNiMo_(0.2) alloy to achieve strength and ductility trade-off:A comparative research

Normal strengthening methods through precipitations and deformation obviously enhance the strength of metallic materials while resulting in the sacrifice of ductility,and synergistic improvement of strength and ductility is currently an urgent requirement.Herein we developed a cryogenic deformation combined with an annealing method to fabricate CoCrNiMo_(0.2) medium entropy alloy,which achieved an ultrahigh strength of 1.8 GPa with synergistic improvement in strength and ductility.Microstructure,mechanical performance,and strengthening mechanisms of the developed alloys were investigated compared with that prepared by the regular room temperature deformation method.It was found that high-density nanotwins were produced in CoCrNiMo_(0.2) MEA via cryogenic deformation.Fine grains,hard precipitations,and high volume fraction of nanotwins greatly strengthened the alloy,obtaining a yield and ultimate tensile strength of 1400 MPa and 1800 MPa.Ductility improvement of the developed alloy was mainly attributed to the production of deformation nanotwins due to the lower stacking fault energy,which greatly increases the dislocation storage ability,and thus,the ductility of the alloy was enhanced.

Dynamic oxidation mechanism of carbon fiber reinforced SiC matrix composite in high-enthalpy and high-speed plasmas

This work employed an inductively coupled plasma wind tunnel to study the dynamic oxidation mechanisms of carbon fiber reinforced SiC matrix composite(C_(f)/SiC)in high-enthalpy and high-speed plasmas.The results highlighted a transition of passive/active oxidations of SiC at 800–1600℃and 1–5 kPa.Specially,the active oxidation led to the corrosion of the SiC coating and interruption of the SiO_(2) growth.The transition borders of active/passive oxidations were thus defined with respect to oxidation temperature and partial pressure of atomic O in the high-enthalpy and high-speed plasmas.In the transition and passive domains,the SiC dissipation was negligible.By multiple dynamic oxidations of C_(f)/SiC in the domains,the SiO_(2) thickness was not monotonously increased due to the competing mechanisms of passive oxidation of SiC and dissipation of SiO_(2).In addition,the mechanical properties of the SiC coating/matrix and the C_(f)/SiC were maintained after long-term dynamic oxidations,which suggested an excellent thermal stability of C_(f)/SiC serving in thermal protection systems(TPSs)of reusable hypersonic vehicles.