Fabrication,Characterization And Mechanical Behaviors Of Intermetallic-based Laminated Composites In Ti-Ni System

Laminated composites are a kind of novel composites proposed on the basis of studying the microstructure and strengthening and toughening mechanism of the shell nacre.Among them,laminated composites composed of alternatively arranged intermetallics and tough metals or alloys possess not only high hardness and stiffness of intermetallics but also high toughness and ductility of ductile phases,thus considered to have great application prospect in the armor protection field.In order to meet the demand of the new generation armor material,in this study,using pure Ti and Ni foils as the original reaction materials via hot pressed sintering,Ti₂Ni/TiNi and Ni₃Ti/TiNi laminated composites were fabricated for the first time with the resultant TiNi as the strengthening and toughening phase.Meanwhile,the formation processes,microstructure evolutions,and mechanical responses under different loading conditions of these two kinds laminated composites were also systematically studied.The main research results are as follows:In the early stage of hot pressed sintering process,Ni₃Ti and Ti₂Ni intermetallics preferentially nucleate and grow at the Ti/Ni interface through diffusion reaction.Subsequently,TiNi nucleates at the interface of Ni₃Ti/Ti₂Ni and grows by consuming both Ni₃Ti and Ti₂Ni.When the Ti and Ni foils are not completely consumed,the three intermetallic compounds show a parabolic growth.When the atomic ratio of the original Ti and Ni layers is within the range of 1²,Ni,Ni₃Ti and Ti layers gradually disappear during the diffusion reaction and finally the Ti₂Ni/TiNi composite is formed.When the atomic ratio of the original Ti and Ni layers falls within the range of 0.33¹,Ti,Ti₂Ni and Ni gradually disappear until the Ni₃Ti/TiNi laminated composite is fabricated.Before the complete consumption of Ni₃Ti in the fabrication of Ti₂Ni/TiNi laminated composite and that of Ti₂Ni in the fabrication of Ni₃Ti/TiNi laminated composite,the TiNi layer in the formed transition laminated composites has two crystal structures:B2 and B19'.In the finally fabricated Ti₂Ni/TiNi and Ni₃Ti/TiNi laminated composites,TiNi layers are only B19'-structured and B2-structured,respectively.Meanwhile,the crystal structures of Ti₂Ni and Ni₃Ti stay stable during the whole fabrication process.In the TiNi layers of the Ti₂Ni/TiNi laminated composite,there is only Ti₂Ni precipitates;while in those of the Ni₃Ti/TiNi laminated composite,the precipitates are mainly Ni₄Ti₃ with different morphologies and a small amount of Ni₃Ti and Ti₂Ni.There exists a strong relation between the phase transformation behavior of laminated composites,the crystal structures of TiNi,and the types of precipitates in TiNi layers.During fabrication of Ti₂Ni/TiNi and Ni₃Ti/TiNi laminated composites,the mechanical properties and fracture mechanisms of the formed transition laminated compositesl are closely related to the types,thickness,strength,and ductility of the different layers in the composites.During tensile processes,the ductile phases in the composites can effectively prevent the rapid expansion of the crack in the brittle phases,and the brittle phases can deform in coordination with the ductile phases by generating transverse cracks,inclined cracks and delamination,thus improving the tensile properties of the laminated composites.By adjusting the original thickness of Ti and Ni foils,Ti₂Ni/TiNi and Ni₃Ti/TiNi laminated composites with different TiNi contents can be fabricated.The compressive strength and ductility of Ti₂Ni/TiNi laminated composites in both perpendicular and parallel directions increase with the increase of TiNi content,and all of these composites exhibit excellent compressive properties.In the perpendicular direction,the shear failure is the main fracture mechanism;while in the parallel direction,the failure mechanism is the local shear of TiNi layers and interlayer splitting of Ti₂Ni layers.The compressive properties of Ni₃Ti/TiNi laminated composites in the perpendicular direction are obviously better than those in the parallel direction.When compressed in the parallel direction,the lower bonding strength of Ni₃Ti interlayer makes it prone to serperating,resulting in poor compression properties of Ni₃Ti/TiNi laminated composites in this direction.In the AO direction?Arrester Orientation?,the toughening mechanisms of Ti₂Ni/TiNi and Ni₃Ti/TiNi laminated composites are different.However,both composites exhibit high fracture toughnesses.The deflection and“bridging”effect of TiNi on cracks enable Ti₂Ni/TiNi laminated composites to show high fracture toughness.Whereas,the delamination of Ni₃Ti layers leads to the deflection of the main crack and the high strength and ductility of TiNi layers,resulting in the high fracture toughness of Ni₃Ti/TiNi laminated composites in the AO direction.