Study On Microstructure And Properties Of Mg-Y(-Gd)-Zn And CoCrFeNi-M Alloys Controlled By Ordered Phase

The mutual exclusion between strength and ductility is the technical bottleneck in the development of high-performance metal structural materials.With the rapid improvement of development quality and level in China's manufacturing industry,the exploration of strengthening and toughening methods has attracted great attention.Against this background,the strengthening effect of ordered phase was used as the breakthrough point and two representative systems,Mg-Y?-Gd?-Zn and Co Cr Fe Ni-M,were selected as the research objects.The various methods were used to regulate the ordered phase,whose formation and evolution were systematically analyzed.Based on the correlation of composition,process,microstructure,and properties,the main strengthening mechanisms were revealed and various effective strengthening and toughening methods were explored.Firstly,Mg-Y-Zn-Ti alloys with different microstructure characteristics were prepared based on the transformation mechanism of the LPSO phase and the heat treatment process.The effect of the LPSO phase morphology on the mechanical properties and corrosion behavior was systematically discussed.The block-shaped LPSO phase distributed along the grain boundaries in the as-cast alloy transformed into the disordered rod-shaped LPSO phase during heat treatment at high temperature,while solution treatment at low temperature caused the precipitation of lamellar LPSO phase with the same orientation in the grain interior.In contrast,the rod-shaped LPSO phase was more conducive to the compatible deformation between adjacent grains and the occurrence of kink deformation.Corrosion resistance is a decisive factor restricting the application range and service life of magnesium alloys.The ideal morphology of the LPSO phase,which is beneficial to both the mechanical properties and corrosion resistance,was determined by analyzing corrosion behavior.The alloy with the rod-shaped LPSO phase exhibited more uniform corrosion and better corrosion resistance due to the compact surface film and the uniform distribution of the LPSO phase.The lamellar LPSO phase arranged in the matrix increased the proportion of cathodes and destroyed the compactness of the surface film,resulting in an increase of the corrosion rate and severe localized corrosion.It was found that the LPSO phase with continuous distribution not only acted as a cathode in micro-galvanic corrosion to accelerate the corrosion but also served as a corrosion barrier.The cast Mg-Gd-Y-Zn-Ti alloys were designed by combining the cooperative strengthening of LPSO and?'phases and introducing grain refinement.The Mg₅?Gd,Y,Zn?eutectic phase in the as-cast microstructure transformed into the short lath-shaped LPSO phase after solution treatment.The second phase strengthening of the LPSO phase and the solid solution strengthening of Gd and Y elements in the matrix significantly improved the mechanical properties,especially the ductility.The refining effect of Ti was discussed by setting the concentration gradient and combining comparative experiments.It was found that Ti had a better refinement effect than Zr.Especially in the high-temperature treatment,Ti could effectively inhibit the growth of grain.With the increase of Ti content,the grain refinement effect was gradually obvious.The alloys with high Ti content maintained a similar grain size before and after solution treatment.The effect of fine grain strengthening was reflected in the mechanical properties,and the addition of an appropriate amount of Ti was beneficial to improve the strength and ductility of the alloy.After peak-aging treatment,the dispersed?'phase precipitated from the matrix interacted with the coexisting 14h-LPSO phase,during which the alloy obtained the maximum strength.The tensile strength of the alloy containing0.4wt%Ti exceeded 300 MPa while maintaining an elongation of 8%.The L1₂ type Co-Cr-Fe-Ni-Ti alloys with the precipitation strengthening of lamellar?phase were designed by applying the above cooperative strengthening concept to the Co Cr Fe Ni-M system.By controlling the alloying element and aging process,the?'and?phases were synchronously adjusted.The cold-rolled alloys with 70%and 85%reduction were fully recrystallized after high-temperature annealing,but the grain size,the number of twins,and the dislocation density all vary with cold-rolling reduction.In this condition,the fine grain strengthening and solid solution strengthening induced by Ti in FCC solid solution were the main strengthening mechanisms.The nano-scale?'phase particles and lamellar?phase with similar chemical constituents were respectively precipitated in the grain interior and near the grain boundaries.Both the?'and?phases had a demand for alloying element Ti.The volume fraction of the two phases increased as the content of Ti increased,and the higher aging temperature accelerated the growth of the?'phase.The aged S-Ti0.15 alloy exhibited an ideal combination of strength and ductility,and its tensile strength reached 1 GPa while maintaining an elongation of about 45%.The quantitative analysis results indicated that more than 80%of the strength increment came from the precipitation strengthening caused by the?'and?phases,but too many?phases would reduce the ductility.Based on the properties of alloying elements Al and Si and combined with the precipitation strengthening of nanoparticles,the B2-type Co-Cr-Fe-Ni-Al-Si alloys were designed via combined addition to alleviating the dependence of strengthening and toughening design on thermomechanical processing.The evolution of the microstructure was analyzed by regulating the Si content.The alloy transformed from single-phase FCC solid solution to FCC+BCC/B2dual-phase structure,and the volume fraction of the BCC/B2 phase was positively correlated with the Si content.Before brittle fracture occurred,the traditional composite model was consistent with the increase of the actual strength.A proper amount of BCC/B2 phase was conducive to obtaining good comprehensive mechanical properties.The Vickers hardness enhanced from 143 HV to 826 HV with increasing Si content,and the specific wear rate was reduced by two orders of magnitude accordingly.The BCC and B2 structures coexisted in the form of a spinodal structure.The addition of Si caused the coherent precipitation of nano-scale BCC particles from the B2 ordered matrix,in which the particle and matrix were Cr-rich and Al-Ni-rich phases,respectively.The coherent interface and size characteristics of the spherical particles determined that the precipitation strengthening would depend on the shear mechanism.The second phase strengthening induced by the transition from the FCC to BCC/B2 phases and the precipitation strengthening together constituted the main strengthening mechanisms.