Toughening Mechanism Investigations Of Intermetallic Al₃Ti Alloy

With the increasing requirement of the lightweight materials in the aerospace,armor protection and high-speed transportation fields,Ti-Al intermetallics with low density,high specific strength and high specific modulus have got more and more attentions,which shows the bright application prospects.Among the Ti-Al system,intermetallic Al₃Ti alloy exhibits the lowest density?3.36 g/cm³?,highest elastic modulus?216 GPa?,superior melting point?1360??and excellent oxidation resistance,which can be regarded as the most potential lightweight material.However,due to its stable tetragonal D0₂₂ crystalline structure,the necessary dislocation slip system for plastic deformation is inhibited at room temperature,and shows intensive brittleness and poor plastic deformation capacity,which severely limits its engineering applications.Therefore,in order to improve the plastic deformation capacity of Al3Ti alloy at room temperature and investigate the toughening mechanism,based on the concept of the metal matrix composite,the ductile Al phase reinforced Al₃Ti matrix composite?Al/Al₃Ti?was fabricated by reactive foil sintering in vacuum in this work.Furthermore,in order to simultaneously improve the compressive and tensile plastic deformation capacities of Al₃Ti alloy at room temperature and investigate the toughening effect and toughening mechanism of the ductile NiTi fiber on the Al₃Ti alloy,the continuous shape memory alloy NiTi fiber reinforced Al3Ti matrix composite?CSMAR/Al3Ti?was fabricated via reactive foil sintering in vacuum in this work.On this basis,different heat treatments were performed on the as-fabricated Al/Al₃Ti and CSMAR/Al₃Ti composites for further improving the plastic deformation capacity of the Al3Ti alloy,and finally raised a series of toughening methods.Additionally,the toughening effect and toughening mechanism raised in this work were investigated by the systematical microstructure characterizations and mechanical property tests.Furthermore,the results of this work also resulted in the scientific advances for promoting the application of the Al₃Ti alloy in the lightweight materials field as well as providing the corresponding approach to improve the ductility of other brittle intermtallics.For the Al/Al₃Ti composite,due to the introduction of the ductile Al phase with appropriate volume fraction?6.4 vol%?,the inherent brittle deformation mode of Al₃Ti alloy was converted into the mixture mode of brittle deformation and plastic deformation of the6.4%Al/Al3Ti composite,and the failed composite shows an obvious 45°shear failure.Mechanical property testing results indicates that the 6.4%Al/Al3Ti composite can effectively improve the compressive plastic deformation capacity of Al3Ti alloy,and the compressive stress-strain curves of the samples shows the apparent plastic deformation characteristic,however,the tensile plastic deformation capacity of Al3Ti alloy is not significantly improved by the ductile Al phase resulting from the relatively high brittle characteristic of Al3Ti alloy.Furthermore,owing to the appropriate Al3Ti grain size and the precipitation strengthening effect of the?-Al₂O₃ phase,the compressive stress-strain curve of the 750?/AC sample shows more significantly yield behavior and relatively long plastic deformation stage,and manifests the noticeable plastic deformation characteristic,which indicates that the compressive plastic deformation capacity of the Al3Ti alloy can be further improved by the heat treatments.Owing to the unique coupling of thermal effect and athermal effect caused by the high-energy electropulsing treatment?EPT?,the atomic diffusion and dislocation movement of Al3Ti matrix in the Al/Al3Ti composite can be improved,which is beneficial to optimize the microstructure and enhance the<100>and<112>crystal orientation and further improve the twinning deformation capacity of the Al3Ti alloy.Compared to the as-fabricated Al/Al3Ti composite,the compressive stress-strain curve of the EPT sample treated by 450 Hz shows more obvious plastic deformation characteristic,and the compressive strain is significantly improved by the EPT with 450 Hz.Furthermore,mechanical property testing results indicate that the plastic deformation capacity of the EPT samples are much better than that of the conventional heat treatment?CHT?samples under the same treatment condition.Therefore,based on the toughening method by the ductile Al phase,the unique electroplastic effect of the EPT can further improve the plastic deformation capacity of Al3Ti alloy at room temperature.The CSMAR/Al3Ti composite consists of NiTi fiber,Al3Ti layer,eutectic area,interfacial reaction layer and a small amount of residual Al phase.Eutectic area is made up of Al3Ti and Al3Ni intermetallics,the Al3Ni phase shows a non-textured structure,while the Al3Ti phase shows a strong<001>crystal orientation structure and apparent fibrous texture.Owing to the gradual decreasing distribution characteristic of the grain size from the eutectic area to the interface of the CSMAR/Al3Ti composite,the crack propagation can be effectively blunted and deflected during the tensile deformation process,which is beneficial to improve the tensile property of the Al₃Ti alloy at room temperature.TEM experimental results indicate that the interfacial reaction layer of the CSMAR/Al3Ti composite exhibits a multi-phase mixture structure of various Ti-Al and Ni-Al intermetallics,and the types of the phases in the interfacial reaction layer decrease with increasing distance away from the NiTi fiber side to the eutectic area side.TEM and HRTEM experimental results demonstrate that due to the atomic substitution effect between Ti atoms and Ni atoms,the continuous Ti2Ni layer with some lattice distortions and edge dislocations formed between NiTi fiber and interfacial reaction layer.Furthermore,the good interfacial metallurgical bonding and the interfacial multi-phase intermetallic mixture structure characteristic of the CSMAR/Al3Ti composite is beneficial to transfers loading,decreases local stress concentration and improves local deformation capacity during the deformation process.Mechanical property testing results indicate that the interfacial characteristics of CSMAR/Al3Ti composite can simultaneously improve the compressive and tensile plastic deformation capacities of the Al3Ti alloy at room temperature.The annealing treatment investigations indicate that the multi-phase intermetallic mixture toughening structure of Al3Ti alloy can be achieved by the solid-state diffusion phase transition on the as-fabricated CSMAR/Al3Ti composite.Owing to its unique gradually distributed microstructure and multi-layer structure characteristics,the strength and toughness simultaneously increase from the eutectic area to the interface of the multi-phase intermetallic mixture toughening structure,and the crack propagation can be effectively blunted and deflected from the eutectic area to the newly formed intermetallic layer during the tensile deformation process.Furthermore,the newly formed intermetallic layer is beneficial to transfers loading,decreases stress concentration and improves local deformation capacity during the deformation process.Therefore,based on the toughening method by the NiTi fiber,the multi-phase intermetallic mixture structure achieved by the annealing treatment on the as-fabricated CSMAR/Al3Ti composite can further improve the plastic deformation capacity of the Al₃Ti alloy at room temperature.