Preparation Of TiAl Sheet By Elemental Foil Metallurgy And Its Molecular Dynamics Study

Due to its advantages of high strength,low density,good oxidation resistance,and other characteristics,TiAl alloys had attracted much attention in the erospace engine field.However,the preparation of TiAl alloy sheet was a difficult problem,due to its high brittleness at room temperature,narrow hot working window and difficult processing.In this research,the TiAl alloy sheets was prepareed via elemental foil metallurgy,explore the preparation process and heat treatment organization control process of TiAl alloy sheets.Combined with molecular dynamics(MD),the diffusion mechanism in the preparation process of sheets and the microscopic deformation mechanism of the phase interfaces in sheets were studied.The findings provided a theoretical basis for the preparation of TiAl sheets.The following innovative results were achieved:(1)The optimized composite parameters of multilayer Ti/Al foil strip were:0.5 mm single pass deformation,0.16 m/s speed,and 62.5%total deformation.The Ti and Al foils in the multi-layer Ti/Al composite sheets were synergistically deformed,and the interface was well combined.In addition,mechanical occlusion was the bonding mode of the interface,and there was no diffusion layer.Combined with the molecular dynamic simulation of the diffusion reaction process of Ti-Al and Ti-Al-Nb,1000℃ was determined as the appropriate temperature for the diffusion reaction.(2)The optimized diffusion reaction process of TiAl alloy sheet was:1000℃for 48 h.TiAl alloy sheet was mainly composed of TiAl phase and Ti3Al phase.The calculation of the two-dimensional mismatch degree of the TiAl and Ti3Al phases,it was found that the mismatch degree of the TiAl and Ti3Al phases should be as small as possible,so as to obtain an alloy containing more y(TiAl)phase.(3)After heat treatment at different temperatures and holding times,the TiAl alloy sheet structure mainly consisted of γ phase and α2/γ lamellar structure.The distribution of Al in the alloy had an obvious influence on the formation of the α2/γlamellar structure.Areas with low Al content transformed into a lamellar structure during heat treatment.However,the Al concent increased with the increase of holding time,leading to more formations of TiAl phases in the area,forming thick lamellar structure in the area.After heat treatment at 1350℃ for 1 h,it was found that the mismatch between the TiAl and Ti3Al phases was low,which made it easier to get more γ(TiAl)phase in the alloy.The TiAl and Ti3Al phases were found to have an obvious preferred orientation,that is.Ti2Al|0(?)0|//TiAl|(?)(?)2].(021)Ti3AI//(1(?)1)TiAl.(4)Combined with the orientation relationship between phases observed by TEM,the results of MD simulation of the tensile,compressive and nanoindentation deformation processes of the TiAl alloy sheet showed that the γ(TiAl)/α2(Ti3Al)coherent interface and semi-coherent interface in the TiAl allov sheet had different deformation and fracture mechanisms.The Young’s modulus and hardness of the TiAl alloy sheet were 187.66 and 6.1 GPa respectively.Based on the simulation data,it was found that the interface bonding strength of the γ(TiAI)/α2(Ti3Al)semicoherent interface in TiAl alloy sheet were significantly higher than those of the coherent interface,and the atomic structure of the semi-coherent interface helped increase the strength of the TiAl alloy sheet.(5)The diffusion reactions of the TiAl-Nb sheet were:1000℃ for 48 h+1400℃.for 24 h.The alloy structure was mainly composed of a relatively coarse γ/α2 lamellar structure and B2/β phase.After heat treatment at 1410℃ for 3 min+1150℃for 2 h,the lamellar structure in the alloy was refined.The coherent interface was formed between TiAl,Ti3Al and B2/β.The mismatch between the three was low as well,which made it easier to get more γ(TiAl)phase in the alloy.The TiAl and Ti3Al phases after heat treatment at 1410℃ for 3 min+1150℃ for 2 h had an obvious preferred orientation,that is,Ti3Al[0(?)0]//TiAl[10(?)],(002)Ti3Al//(1(?)1)TiAl.