Preparation And Mechanical Behavior Of Ti-Ni-Al Intermetallic Laminates

Metal-Intermetallic-Laminate composite（MIL）is composed of intermetallic layers and ductile metal layers alternately arranged,which not only has the high hardness and high stiffness of intermetallic,but also has the good toughness and toughness of metals.At the same time,MIL also has the advantages of low density and high specific strength.These advantages make it have broad application prospects in aerospace structural materials,armor protection and other fields.On the basis of the research group’s research on Ti-Al₃Ti composites,in order to further improve the mechanical properties of MIL composites and solve the shortcomings of poor designability of Ti-Al₃Ti composites,this paper prepared Ti-Ni-Al by hot pressing sintering process.Ternary MIL composite material,studied the reaction process and microstructure evolution of this ternary system,and analyzed the reaction mechanism from the perspective of thermodynamics and kinetics,and finally tested its mechanical properties such as microhardness and compressive strength.The main research results are as follows:In the reaction of the Ti-Ni-Al ternary system,the types of intermetallic generated by the Ni-Ti reaction are relatively stable:Ni Ti,Ti₂Ni and Ni₃Ti.As the reaction progresses,the Ni-Ti intermetallic layer gradually becomes thicker.The thickness is about 15μm～30μm.The intermetallic generated by Ni-Al are:Ni₂Al₃,Ni Al₃,Ni Al,Ni₃Al,and the products are mainly affected by the reaction time.As the reaction progresses,the intermetallic layer evolves according to the steps of Ni Al₃+Ni₂Al₃to Ni2Al₃and then to Ni Al+Ni₃Al.The thickness of the intermetallic layer formed by the Ni-Al reaction is about 60μm to 140μm,which is higher than the thickness of the Ni-Ti intermetallic layer,indicating that the Ni-Al diffusion reaction in the Ti-Ni-Al ternary stack reaction system The rate is higher than that of Ni-Ti.After the reaction,a large number of Kirkendall holes appeared at the center of the original Al layer,proving that Al is the main diffusion element in this system,but a small amount of Kirkendall holes were also observed in the center of the original Ni layer,indicating the total outflow of atoms in the Ni layer greater than the total inflow.It was found that when the temperature increased from 710℃to 900℃,the Ni-Al reaction rate did not change significantly,while the Ni-Ti reaction rate increased significantly.The reason may be that in the Ni-Ti diffusion reaction,Ni,as aβ-stable element,diffuses into the Ti6Al4V layer,reducing the phase transition temperature of Ti6Al4V to between 710 and900°C.Therefore,a large amount ofα-Ti toβ-Ti phase transformation occurs in the Ti6Al4V layer at 900℃,and the diffusion rate of Ni toβ-Ti is much higher than that of Ni toα-Ti,which makes the Ni-Ti at 900℃The rate of Ti diffusion reaction is higher than 710℃,and finally a thicker Ni-Ti intermetallic layer is formed.This can be used as an effective method to control the proportion of each product in the ternary MIL system in this study,for example,the reaction temperature can be increased when more Ni-Ti intermetallic are required;more Ni-Al intermetallic can be obtained in Extend reaction time at lower temperature.Ti₂Ni structure appears inside Ti6Al4V.When the temperature is 710℃,the Ti₂Ni structure is network and island;when the temperature is 900℃,the Ti₂Ni structure is coarse needle.However,in the control experiment of pure titanium,layered Ti₂Ni was only formed at the boundary,and the Ti₂Ni structure did not appear inside the pure titanium.This is because Ti6Al4V itself is anα+βtitanium alloy,and Ni atoms preferentially diffuse toβ-Ti,and continue to diffuse into the interior of the Ti6Al4V layer throughβ-Ti and grain boundaries as fast channels,while the reaction diffusion of Ni andα-Ti Slower,the inhomogeneous diffusion caused by this difference in diffusion speed eventually forms a Ti₂Ni structure inside Ti6Al4V.Pure titanium is a singleαphase,and the growth of its intermetallic is controlled by lattice diffusion,so Ni atoms diffuse along the layer to the interior of titanium,forming a layered Ti₂Ni.Mechanical experiments show that the mechanical properties of the materials in this study are affected by their processes,mainly because different intermetallic are produced under different processes,and the Ni-Al intermetallic that account for the most are the same as the compressive strength of TNA-MIL.The properties are most closely related.For example,samples containing high hardness Ni₂Al₃exhibit high modulus and low fracture toughness as a whole,and samples containing Ni Al and Ni₃Al,two low-hardness intermetallic,exhibit lower modulus and higher strain.This proves that it is effective to control the overall mechanical properties by controlling the product.Observing the cracks,it can be found that the tough layer hinders the crack growth,delays the fracture of the material,and effectively improves the mechanical properties of the material.The compressive strength of the Ti-Ni-Al composite prepared in this study reaches up to 1900 MPa,and the fracture toughness reaches0.35,which is higher than the compressive strength of 1400 MPa and the fracture toughness of 0.23 of the Ti-Al₃Ti laminate in previous studies.