Composition And Microstructure Design Of Ti-Al-Nb-V-Zr-Sn-Cr-Mo Alloys Based On The High-Throughput Experimental Methods And Its Strengthening-Toughening Mechanism

High strength and toughness titanium alloys,which usually used to make the main structural parts of aviation,is one of the important trends of advanced titanium alloys.The key aircraft models in our country have the definite demand for it.But the key issue is how to improve the strength and toughness synergistically.To solve this problem,two important perspectives can be considered,“composition design”and“microstructure regulation”for strengthening-toughening.In this study,under the guidance of material genetic engineering concept,a variety of high-throughput experimental methods were integrated.A new alloy was designed based on the TB17 alloy,the fifth generation high-strength titanium alloy.The mechanism of strengthening-toughening and failure mechanism of different microstructures in the metastableβ-Ti alloy were systematically analyzed by using various characterization methods.The main research works and results are as follows.（1）The effects of two elements,Cr and Mo,on the content and morphology ofαphase in the Ti-Al-Nb-V-Zr-Sn-x Cr-y Mo alloy system and the mechanism of action were investigated.Combining the high-throughput experimental method of diffusion multiples and high-throughput characterization tools such as Micro-XRF,the basic data of“composition-microstructure-properties”in the Ti-Al-Nb-V-Zr-Sn-x Cr-y Mo alloy system were quickly obtained.The correlation model of"composition-microstructure characteristics"under solution and aging treatment were established by using the machine learning method of BP-neural method.The results show that the higher the[Mo]_eq value of the alloy,the moreβphase was retained after solution treatment,and when[Mo]eq>8,the alloy can obtain a singleβ-phase after solution atβphase region.After aging treatment,the secondaryαphase precipitates from the metastableβmatrix,which plays a strengthening effect.If the alloys have the same[Mo]_eq,the higher the Mo element content,the more“long and thin”shape of the precipitatedαphase;the higher the Cr element content,the more"short and thick"theαphase,and the higher the hardness of the alloy.Combining the results of machine learning and the effect of Mo and Cr elements on microstructure characteristics and mechanical properties,a new titanium alloy with a[Mo]_eq of 9 and a nominal composition of Ti-4Mo-4Cr-3Al-2Nb-1.2V-1Zr-1Sn（Ti-4321）was designed.（2）The correlations between processing-microstructure-mechanical properties of Ti-4321 alloy were studied.A metastableβgrain was obtained using the solution treatment（ST）processing,achieving high plasticity of the alloy;the elongation of the alloy treated with solution at 830℃ reached more than 25%.A needle-like structure with diffuse distribution of needle-likeα_s was obtained by theβphase region solution and then aging treatment（BSTA）,achieving ultra-high strength of the alloy,the strength of the alloy after aging treatment at 500℃ for 8 h reached more than 1500 MPa.A bimodal structure consisting of spherical primaryα_p phase and needle-like secondaryα_s phase was obtained byα+βphase region solution and then aging treatment（STA）.After solution treatment at 780℃ for 1 h,and then aging treatment at 530℃ for 8 h,the tensile strength of the alloy reached1425 MPa while maintaining elongation of about 13%and fracture toughness of 61.3 MPa·m^1/2.A lamellar structure consisting of lamellar primaryαphase and needle secondaryαphase was obtained byβannealing-slow cooling-aging（BASCA）treatment,and an ultra-high fracture toughness of 116.3 MPa·m^1/2 was achieved after annealing at840℃ for 2 h,followed by cooling to 530℃ for 8 h at a cooling rate of2℃/min.,and the tensile strength of 1123 MPa and 16.9%elongation.Comparative analysis shows that Ti-4321 alloy has a better strength and toughness match than other typical high tenacity titanium alloys.（3）The room temperature plastic deformation modes of Ti-4321 alloy in various microstructure and its strengthening-toughening mechanism were investigated.The results show that,for the metastable matrix,martensite-induced plasticity（TRIP effect）is the main mechanism.Increasing matrix stability by introducingα-phase increases the critical induced stress of martensite and weakens the TRIP effect.For the needle-like microstructure,theβ/α_s phase interface hindering the motion of matrix dislocations is the main strengthening mechanism,and the strengthening effect is mainly related to the size ofα_s phase.The smaller the size ofα_sphase,the higher the density of the phase interface,the more it can disperse the dislocation plugging at the phase interface,and the better the strengthening effect.For the bimodal microstructure,a large number of dislocations in theβmatrix would generate during deformation,and these dislocations are impeded by interfaces such asβgrain boundaries,sub-grain boundaries,β/α_s phase interfaces,andβ/α_p phase interfaces during the slipping.These various interfaces hindered the movement of dislocation in the matrix,providing high strength to the alloy,are the main strengthening mechanism.The sphericalα_p phase coordinates the overall deformation by rotating,being elongated,slipping,and twinning,which provides high ductility to the alloy.For the lamellar microstructure,a large amount of dislocation slip is initiated within bothβandαphases during deformation,and dislocations are impeded by theβ/α_p andβ/α_s interfaces during slipping,and forming a pinning structure.Under the action of continuous external stress,dislocations plugged at the phase interface,grain boundaries,α_l colony intersections,etc.induce the formation of micro-voids.The thickerα_lcolony can hinder the crack expansion and also coordinate the deformation by bending the whole colony.The cracks will be repeatedly deflected and extended by the hindrance of the lamellarαphase and induced by the interfacial microporosity at the crack tip front,realizing the high toughness of the alloy.