Microstructure And Mechanical Properties Of TiNbMo(Zr,Si) Complex Concentrated Alloys Fabricated By Powder Metallurgy

As a novel high-temperature alloy design strategy,the refractory high entropy alloys(RHEAs)have attracted extensive research attention all over the globe.Being at the early stage of research,the RHEAs are revealed to have lots of outstanding problems which would significantly hinder the actual application of this promising alloy.In this work,we aimed at coping with the high density,high cost,difficulty in fabrication and processing,and designed and fabricated the light-weighted Ti-rich refractory complex concentrated alloys,using relatively low-cost and lighter Ti,Nb,Mo,and Zr elements.Combing the ab-initio calculation method and experimental investigations,the microstructures and properties of the Ti-Nb-Mo-Zr quaternary alloys and Ti-Nb-Mo ternary alloys were carefully examined.According to the calculated elastic properties,the Zr element has the most significant influence.The elastic moduli would drop,and the ductility would be improved with the increase of Zr content,however,adding too much Zr would destabilize the BCC solid solution structure.The Ti content has a similar effect with Zr,while the Nb and Mo have the opposite effect on the alloy,and the effect of Mo is calculated to be more significant.Based on the calculated composition-properties relationship,Ti-Nb-Mo-Zr quaternary alloys and Ti-Nb-Mo ternary alloys were designed.After optimizing the fabrication procedures,the fabrication route of "mixing at ball-milling and alloying by sintering" was established.The microstructure characterizations showed that the Ti-Nb-Mo-Zr alloys are single-phase BCC solid solution alloys while the Ti-Nb-Mo alloys exhibited dual-phase microstructures.The mechanical properties of these alloys agreed well with the ab-initio prediction.Considering the density,strengths,and ductility,the TI70 alloy and the T4NM alloy were the more promising alloys,thus,more in-depth investigations were carried out.To further strengthen the TI70 alloy,two-scale silicide reinforcements were successfully tailored by adding 1 at.%,2 at.%,and 4 at.%Si element(denoted as TI70-Six alloys).According to the microstructure characterizations,the T170-Six alloys exhibited a dual-phase microstructure consisting of matrix and silicides.The matrix was determined to be the random BCC solid solution phase.The S1-type(Ti,Zr)5Si3 silicides were precipitated and distributed dispersively at the grain interior,while the lath-like and particle-like S2-type(Ti,Zr)6Si3 silicides were distributed at the grain boundary.The crystallographical information of the silicides was revealed for the first time by atomic-resolution STEM and the elastic properties were predicted by ab-initio calculations(the Young's moduli of the S1 and S2 silicide were predicted to be 250.3 GPa and 232.3 GPa,respectively).Investigations on the interface also shed a light on the interface structure and the formation mechanism of the silicide-Ti system.The formation mechanism of the lath-like dispersion silicide and the lath-like boundary silicide could be attributed to the solution and precipitation of the Si element during the fabrication process,while particle-like boundary silicide was formed from the in-situ reaction between the excessive Si and the matrix.The TI70-Six alloys strengthened by the two-scale silicides exhibited remarkable strength enhancement not only at ambient temperature but also at high temperatures.The compressive yield and fracture strengths of the TI70-Si4 alloy were increased to 1550 MPa and 2003 MPa at ambient temperature.Moreover,the TI70-Si4 alloy exhibited ultrahigh strengths of 1048 MPa and 437 MPa at 600? and 800?,which were increased by 61.1%and 111.2%compared with TI70 alloy,respectively.The superior strengths can be mainly attributed to the coordinated strengthening mechanisms(mainly by Hall-Petch strengthing and the load-transfer strengthening)of two-scale silicide reinforcements and novel network microstructure.However,the drastic softening of the matrix would lead to the low strengths at 800?.To cope with the high-temperature softening of the high Ti content alloys,the T4NM alloy consists of a BCC-structured(Ti,Nb,Mo)phase and an FCC-structured Ti-C phase.According to the S/TEM investigation,the(Ti,Nb,Mo)phase was determined to be a random BCC solid solution phase formed by inter-diffusion of elements during the sintering process,while the FCC-Ti(C)phase is an FCC-structured Ti with C atoms distributed randomly inside the octahedral interstices.The phase transformation mechanism was rationalized by ab-initio calculations to be attributed to the energy variation caused by interstitial C atoms.T4NM alloy exhibited a compressive yield strength of 1874 MPa and a fracture strength of 2050 MPa at room temperature.The high-temperature yield strengths were tested at 500?,700?,800?,and 900? were 1405 MPa,1154 MPa,913 MPa,and 508 MPa,respectively.According to the nanoindentation tests,the(Ti,Nb,Mo)phase has a Young's modulus of 155.9±3.3 GPa and a nano-hardness of 6.6±0.2 GPa,which are considerably higher than the TI70 alloy.The FCC-Ti(C)phase,on the other hand,possesses a much higher Young's modulus(233.1±22.2 GPa)and nano-hardness(17.8±1.9 GPa).Micro-pillar compression tests also demonstrated the ultrahigh strengths of the FCC-Ti(C)phase:a critical resolved shear strength(CRSS)of 1.1±0.1 GPa.The fractography characterizations and the TEM characterizations on the deformed T4NM alloy indicated that the deformation mechanism of the(Ti,Nb,Mo)phase is dislocation slipping,while the deformation mechanism of the FCC-Ti(C)phase is dislocation slipping and staking fault deformation.The generalized stacking fault energy(GSFE)curves of the pure FCC-Ti and C-doped FCC-Ti were calculated to evaluate the mechanical properties.It can be concluded from the GSFE curves that,the {111}<112>-type stacking fault is more likely to be formed in the FCC-Ti(C)phase.Doping C in the FCC-Ti has a relatively insignificant influence on the energy barrier of the stacking fault formation,while could remarkably increase the ?osf,indicating that doping C atoms could hinder the nucleation of HCP-Ti.According to the {111}<110>-type GSFE curve,the ideal slide stress of the FCC-Ti and FCC-Ti(C)were calculated showing that doping C could effectively hinder the dislocation movement in the {111}<110>slip system and realize the strengthening of the mechanism.