Microstructural Evolution And Strengthening-softening Mechanisms Of In-situ Ti₂AlN/TiAl Based Composites

The ternary nanolaminated compounds MAX phase possess a remarkable and unusual combination of metallic and ceramic properties due to their unique microstructure,thus receiving extensive attention.Introducing MAX phase intoγ-TiAl matrix alloy by in-situ method has been regarded as one of the effective methods to improve and enhance the properties ofγ-TiAl based alloys.To conduct researches on synthesis mechanism,microstructural evolution and mechanical properties for MAX phase reinforced TiAl based composites is believed to have great significance for promoting development,design and application of high-performance TiAl based composites.In view of this,the MAX phase,Ti₂AlN,was used as reinforcement and the Ti₂AlN/TiAl based composites were prepared by reactive arc-melting method in this paper.The mechanism of in-situ synthesis for the composites and the influence of Ti₂AlN on the solidification process of TiAl matrix were investigated.The distribution pattern and growth mechanism of Ti₂AlN were studied.While grain refinement mechanism of the Ti₂AlN/TiAl based composites was also discussed.Through mechanical properties testing at room and high temperatures,the strengthening-softening and fracture mechanisms of the TiAl based composites were investigated.Finally the microstructural evolution during solid solution treatment and thermal exposure was researched.Characteristics of different types of secondary precipitates and influence of the precipitates on microstructure and properties of TiAl matrix were investigated.The major conclusions are listed as follows:The results of thermodynamic analysis for Ti-Al-TiN system show that Ti₂AlN can be in-situ synthesized in this system and is the most stable one.The primary phase of Ti-48Al-2Cr-2Nb matrix alloy is changed due to the introduction of Ti₂AlN phase.The solidification process is turned from L→L+?+Ti₂AlN→L+?+?+Ti₂AlN→?+Ti₂AlN→?+?+Ti₂AlN→?₂+?+Ti₂AlN for 3vol.%Ti₂AlN/TiAl based composite into L→L+?+Ti₂AlN→?+Ti₂AlN→?+?+Ti₂AlN→?₂+?+Ti₂AlN for 4vol.%Ti₂AlN/TiAl based composite and L→L+?+Ti₂AlN→L+?+?+Ti₂AlN→?+?+Ti₂AlN→?₂+?+Ti₂AlN for5vol.%Ti₂AlN/TiAl based composite.Ti₂AlN would first precipitate from the Ti-Al melt and act as heterogeneous nucleation sites,thus promoting the nucleation of primary phase.The microstructure of in-situ synthesized Ti₂AlN/TiAl based composites consists of fine?₂+?lamellar colony and a small number of?phases distributed at colony boundary and rod-like Ti₂AlN particles dispersed in the colony.Compared with the TiAl matrix alloy,the microstructure of TiAl based composites is significantly refined,and microsegregation of Al in the composites is less significant.Ti₂AlN particles grow in a layer-by-layer manner and exhibit a rod-like morphology owing to the restriction of adjacent?phase.With increasing reinforcement content,the aspect ratio of Ti₂AlN particles increases.The coherency between Ti₂AlN and?phases and the terraced structure of Ti2AlN phase provide condition for Ti2AlN as the heterogeneous nucleation sites in the TiAl matrix melt,thus improving the nucleation rate of?phase and refining the colony size of TiAl matrix.Introduction of Ti₂AlN makes the room and high temperatures properties of TiAl based composites higher than those of Ti-48Al-2Cr-2Nb matrix alloy.With the increasing of reinforcement content,the compressive yield strength of the composites continues to increase,while compressive strength,tensile fracture strength,compressive fracture strain and tensile fracture strain all present a trend of firstly increase and then decrease.4vol.%Ti₂AlN/TiAl based composite shows the highest compressive strength（2555.75 MPa）,compressive fracture strain（25.19%）,tensile fracture strength（670.68 MPa）and tensile fracture strain（0.39%）,and displays better match between strength and plasticity.The high-temperature tensile yield strength of the composites increases continuously,and the high-temperature tensile strength shows an increase firstly then decrease,while the elongation displays a trend contrary to tensile yield strength.The composite with 5vol.%Ti₂AlN has the highest yield strength and4vol.%Ti₂AlN shows the highest tensile strength,which are 260.3 MPa and 267.6 MPa higher than those of the TiAl matrix alloy,respectively.3vol.%Ti₂AlN/TiAl based composite presents an elongation even higher than the TiAl matrix alloy.The deformation and fracture mechanisms of the Ti₂AlN/TiAl based composites at room temperature are associated with loading condition and Ti₂AlN content.With the increasing of Ti₂AlN content,fracture mode of the TiAl based composites under compression changes from mixed mode of translamellar,interlamellar and trans-granular fracture（3vol.%and 4vol.%Ti2AlN）into brittle fracture（5vol.%Ti2AlN）,and the characteristic of brittle fracture under tension composed of translamellar fracture and fracture of Ti₂AlN particles becomes more pronounced.Tensile fracture behavior for the TiAl based composites at 800°C is also related to Ti₂AlN content.With the Ti₂AlN content increasing,fracture mode of the composites changes from a mixed mode consisted of translamellar,interlamellar and intergranular fracture and ductile dimple fracture（3vol.%）into cleavage fracture（5vol.%）.Grain refinement strengthening of TiAl matrix,load transfer strengthening of Ti₂AlN particles,solid solution strengthening of interstitial nitrogen atom,dislocation strengthening and cooperative strengthening effect are the strengthening mechanism of the Ti₂AlN/TiAl based composites at room temperature.Moreover,the grain refinement strengthening plays a major role.The improvement of plasticity at room temperature can be attributed to the microstructure refinement of TiAl matrix and inhibition of crack propagation by dispersed Ti₂AlN particles.The strengthening of TiAl based composites at 800°C is associated with enhancement of load transfer effect and resistance of dislocation and mechanical twin motion caused by lots of?/?and?/?₂ lamellar interfaces in the fine-grained TiAl matrix.After thermal exposure at 900°C for long period of time,the Ti₂AlN/TiAl based composite has higher?₂ content and more stable lamellar structure than the Ti-48Al-2Cr-2Nb matrix alloy.The?₂+?lamellar structure of the TiAl matrix alloy degrades significantly by phase transformation of?₂→?during thermal exposure.?₂ lamella is decomposed by parallel decomposition,and?/?₂ interface is bulged and dissolved.With time prolonging,the?lath merges into a wider one and a plenty of dislocation nets form in?matrix.In Ti₂AlN/TiAl based composite,slight parallel decomposition of?₂ lath and Ti₂AlN precipitation at?/?₂ interface occur via?₂→?and?₂→γ+Ti₂AlN phase transformations,respectively.After long-term exposure,no obvious degradation of lamellar structure is observed.During thermal exposure,second precipitates,Ti₂AlN and Ti₃AlN are formed in the TiAl based composite.Ti₂AlN precipitates distribute at?/?₂ interface with rod-like and nano-size morphologies and at colony boundary with granular morphology.The nano-sized Ti₂AlN at?/?₂ interface has crystallographic orientation relationship of with?phase,（111）_γ//（0001）_{Ti2 AlN}and[101]_γ//[1120]_{Ti2 AlN}.The Ti3AlN phase with needle-like morphology distributes in?matrix and shares a crystallographic orientation relationship of with?phase,（001）_{Ti 3AlN}//（001）_γand<100>_{Ti3 AlN}//<100>_γ.The morphology and distribution of second nitride precipitates are determined by distribution of nitrogen atom,crystal structure and misfit of nitride precipitates and TiAl matrix.The Ti₂AlN phases precipitated at lamellar interface retard the coarsening of lamellae,thus improving the stability of lamellar structure.The nitride precipitates at colony boundary play a role in pinning boundary and hindering its migration,thus inhibiting the discontinuous coarsening of lamellar colony.The degradation of lamellar structure leads to continued decrease of hardness for Ti-48Al-2Cr-2Nb alloy,while due to the strengthening effect caused by lots of nitride precipitates,the hardness of Ti₂AlN/TiAl based composite after thermal exposure 100 h increases and is higher than that before thermal exposure.