Bimodal Titanium Alloys Based On Eutectic Transformation: Semi-solid Sintering Mechanism And Microstructure-mechanical Property Regulation

Pursuing higher strength and ductility in titanium alloys is the main aim for the researcher in order to meet the more stringent requirements.Therefore,the exploration of bimodal/multi-modal microstructural titanium alloys with high strength and non-sacrifice of plasticity has become a significant topic in the field of materials science and engineering.Recent studies have shown that in order to achieve excellent comprehensive mechanical properties,research trend of materials microstructure is to develop composite structure and morphology in addition to grain refinement and bimodal/multi-modal structure.On this account,semi-solid sintering?SSS?technique based on eutectic transformation was proposed in this paper.In other words,the amorphous/nanocrystalline alloy powders prepared by mechanical alloying?MA?were subjected to SSS via spark plasma sintering system,aiming at obtaining bimodal eutectic structural titanium alloys.Consequently,nano-/ultrafine lamellar bimodal eutectic structure in titanium alloy with high strength and ductility was obtained by the regulation of the alloy systems,chemical composition,amorphous content,SSS temperature and other factors.This mainly stems from that these factors can greatly affect the highly dense random-packed structure and the atomic long-range ordered structure of semi-solid eutectic liquid phase,and further affect the proportion,morphology and scale of lamellar eutectic structure during cooling.Firstly,according to Ti-Co binary phase diagram,the target ternary alloy(Ti_76.75Co_23.25)₈₃Fe₁₇ was designed based on?-Ti and Ti₂Co eutectic reaction,and mechanically alloyed amorphous/nanocrystalline powder was subjected to SSS.In order to regulate the microstructure and optimize the mechanical property,the alloy composition was re-adjusted to multi-component?Ti63.7Fe17Co19.3?87.8Nb12.2 and?Ti63.7Fe17Co19.3?82Nb12.2Al5.8with constant contents of?-stabilizer Nb and the minor Al addition for promoting the formation of amorphous phase with highly dense random-packed structure.By the systematic study on SSS temperature and milling time of the alloy powders,a bimodal structural(Ti_63.7Fe₁₇Co_19.3)₈₂Nb_12.2Al_5.8 alloy with local eutectic structure was successfully prepared at1080?by SSS of 50h-milled alloy powder.Its microstructure was composed of equiaxed Ti₂?Co,Fe?phase surrounded by the local ultrafine lamellar eutectic structure consisting of?-Ti and Ti?Fe,Co?together with spheroidal residual?-Ti.Corresponding mechanical properties were ultimate compressive strength of 2315 MPa,yield strength of 1580 MPa,and fracture plasticity of 26.2%,respectively.Surprisingly,due to the solid solution effect of Fe and Co atoms,expected eutectic reaction between?-Ti and Ti₂?Co,Fe?was suppressed and replaced by the hypoeutectic reaction of?-Ti and a small amount of Ti?Fe,Co?in the matrix.Secondly,in order to maximize the eutectic reaction between?-Ti and Ti?Fe,Co?,the alloy compositions were adjusted to the(Ti_70.56Fe_29.44)₉₀Co₁₀,(Ti_63.5Fe_26.5Co₁₀)_87.8Nb_12.2,and(Ti_63.5Fe_26.5Co₁₀)₈₂Nb_12.2Al_5.8,according to the Ti-Fe phase diagram.The results showed that multicomponent alloy with higher component number was more likely to form eutectic liquid phase with more highly dense random-packed structure during high-temperature SSS,and then easier to form ultrafine/nano lamellar eutectic matrix during cooling.The comparative study on the microstructure and properties of quinary titanium alloys prepared at different SSS temperatures showed that the sintered bulk alloys had the same phase constitution and content.However,the microstructure of the alloy sintered at 1100?showed finer grains with more uniform distribution than those sintered at 1080?and 1150?,which caused better comprehensive mechanical properties.The optimized SSS temperature 1100?was applied to the quinary titanium alloy powders with different milling times.The results showed that,with the increase of the amorphous content in alloy powders under longer milling time,the eutectic structure in the bulk alloys was evolved from irregular eutectic?a small amount of coarse-grained lamellar eutectic?fine-grained cellular eutectic?nano-/ultrafine bimodal typical lamellar eutectic structure.Especially,a resultant bimodal eutectic structural alloy was successfully prepared at1100?by SSS of 45h-milled alloy powder.The bimodal microstructure are composed of micron-sized Ti₂?Co,Fe?embedded into nano-/ultrafine lamellar eutectic matrix containing ultrafine?-Ti and Ti?Fe,Co?lamellae,which consists of a coherent interface with an orientationalrelationships:?110?_-Ti??110?_Ti?Fe,Co?,?11?0?_-Ti??11?0?_Ti?Fe,Co?,?200?_-Ti??100?_Ti?Fe,Co?.Thirdly,the study on SSSed no-eutectic structural titanium alloy showed that the microstructure was mainly composed of coarse-grained equiaxed?-Ti and short rod-like Ti₂?Co,Fe?phase distributed on?-Ti grain boundary.The alloy exhibited far lower mechanical properties than those of bimodal eutectic titanium alloy,due to the stress concentration and microcracks in the hard and brittle Ti₂?Co,Fe?phases during loading.At the same time,the same compositional titanium alloys fabricated by solid-state sintering,copper mold casting and hot isostatic pressing also were studied;the results showed that the solid sintered titanium alloys exhibited poor comprehensive mechanical properties due to its complete equiaxed grains.The study on copper mould cast alloys showed that both microstructures were composed of coarse-grained Ti?Fe,Co?/?-Ti surrounded by the ultrafine lamellar eutectic matrix of?-Ti and Ti?Fe,Co?,and the mechanical properties showed a lower performance indicators than the SSSed counterparts,due to the lack of dispersion strengthening of hard Ti₂?Co,Fe?particles and the coherent strengthening effect of lamellar eutectic.For the hot isostatic pressed titanium alloys,they exhibited the similar microstructure characteristic as the SSSed counterparts but lower mechanical properties compared with bimodal eutectic titanium alloys,because of grains coarsening induced by the instable equilibrium of eutectic liquid phase and inhomogeneous distribution of the local crystal phases and other factors.In a word,these comparative studies indicated that the SSS technique and resultant nano-/ultrafine lamellar eutectic structure are the fundamental reason of high strength and ductility for bimodal titanium alloy.Finally,according to crystallization,densification and SSS mechanism of the as-milled alloy powders,the formation principle of bimodal eutectic structure during SSS was revealed.The fabrication of bimodal titanium alloy via SSS can be divided into five stages,i.e.,rearrangement of powder particles,densification and crystallization of amorphous powder,grain growth of crystallized phases,formation of eutectic liquid phase?semi-solid state?,and rapid solidification of eutectic liquid phase into nano-/ultrafine lamellar eutectic matrix and the sustained growth of remaining solid-phase Ti₂?Co,Fe?during cooling.By doing so,the bimodal structure of concomitant nano-/ultrafine lamellar eutectic and micron-sized Ti₂?Co,Fe?was successfully obtained.The alloy exhibited excellent compressive mechanical properties with ultimate compressive strength of 2897MPa,ultra-high yield strength of 2050MPa,and fracture strain of 23%,together with an optimal tensile fracture strength of 920MPa and a elongation of 1.6%.This indicates that the bimodal eutectic titanium alloy has balanced strength and ductility,and its comprehensive mechanical properties are superior to those of bimodal titanium alloys reported so far.The results showed that the strengthening mechanisms of the bimodal eutectic structural titanium alloy by SSS are mainly classified as three aspects.Firstly,decomposition of the superdislocation,formation and annihilation of antiphase boundary,and occurrence of cross-slip can accelerate the atoms movement in the form of paired dislocations and repeat transformation of order?disorder,resulting into so-called ordered strengthening effect of B2 superstructured Ti?Fe,Co?.Secondly,based on the theory of coherent strengthening,it was found that both the content of eutectic structure and the length of lamellae can affect the mechanical properties of the alloys.It was calculated that the SSSed nano-/ultrafine lamellar eutectic had high critical shear stress,which was characterized by the accumulation of large number of dislocations in the grain boundaries and the tearing and elongation of fracture morphology.Finally,dislocation by passed mechanism of equiaxed hard and brittle Ti₂?Co,Fe?particles embedded into eutectic matrix can produce a large number of dislocation rings and increase the flow stress around the Ti₂?Co,Fe?particles,achieving the strengthening effect by the fragmentation and even peeling of the particles.In addition,the precipitation strengthening of the stress-induced Laves phase also cannot be neglected.The proposed SSS technique based on eutectic transformation can provide a new approach for the preparation of bimodal titanium alloys with higher strength and ductility.The design criterion of chemical composition,regulation concept of microstructure and properties can also provide valuable insight into the development of metal composite materials with high comprehensive performance.Therefore,the SSS technology is expected to be applied to high-melting point alloy systems.