Study On The Process Of Zr And Cr Implantation And The Mechanical Properties Of Implanted Layer On The Surface Of TC18 Titanium Alloy

TC18 titanium alloy is increasingly used to manufacture the aircraft undercarriage due to its excellent comprehensive performance.However,poor fatigue resistance limits its application.The surface state of TC18 titanium alloy is an important factor affecting its fatigue performance.Surface scratches,high roughness and other surface defects can reduce its fatigue life,while residual compressive stress can improve its fatigue life.This paper proposes to strengthen the surface of TC18titanium alloy by ion implantation technology,and studies the influence of the surface state after ion implantation on the fatigue performance of TC18 titanium alloy.In this research,TC18 titanium alloy was implanted with Cr and Zr ion at different fluences(1.2¹9.0×10¹⁶ ions/cm²).SEM,AFM,XRD and residual stress analyzer was used to characterize the surface morphology,composition,phase structure and residual stress state.The nanomechanical property of TC18 titanium alloy before and after implanted was analyzed by nano-indentor under different loads?5 mN,10 mN and 15 mN?.The effect of Cr and Zr ions on the fatigue performance of TC18 titanium alloy was analyzed by using the fatigue-testing machine and analyzed combining with the change of surface state of TC18 titanium alloy before and after ion implantation.Based on Monte Carlo method,the movements of Cr and Zr ions in TC18 titanium alloy were studied.The results showed that the sputtering phenomenon occurred on the surface of TC18 titanium alloy during ion implantation,which affected the surface morphology of the substrate.When Cr and Zr ion implantation fluence increased to 5.5×10¹⁶ ions/cm² and 3.6×10¹⁶ ions/cm²,respectively,obvious cracks and erosion pits begin to appear on the surface of TC18 titanium alloy.As the implantation fluences continues to increased,the number and size of cracks and etch pits increased.Further studies showed that the surface roughness of substrate first decreased,then increased and then decreased with the increase of Cr and Zr ion implantation fluence.Upon Cr ion implantation fluence of 5.5×10¹⁶ ions/cm² and Zr ion implantation fluence of 3.6×10¹⁶ ions/cm²,substrate surface roughness?RMS?was the smallest,which was 36.74 nm and 34.96 nm respectively.Phase structure analysis showed that no new phase was found in the surface of TC18 titanium alloy after Cr and Zr ion implantation.Lattice distortion caused by Cr and Zr ion implantation made the residual compressive stress value and lattice parameters on the surface of TC18 titanium alloy increased with increasing of ion implantation fluence and the XRD diffraction peak was shifted to different angle.On the other hand,the dislocation density gradually decreased with increasing of implantation fluence.The results of nanomechanical properties analysis showed that after Cr and Zr ion implantation,the nano-hardness and elastic modulus of TC18 titanium alloy showed a trend of increasing first and then decreasing with increasing of implantation fluence.Cr and Zr solid solution strengthening improved the hardness of substrate.However,the dislocation density gradually decreased with increasing of implantation fluence and effect of radiation damage reduced the nanohardness and elastic modulus.When Cr and Zr ion implantation fluence was 3.56×10¹⁶ ions/cm²,the surface hardness and elastic modulus of TC18 titanium alloy showed the highest value under different loads.Fatigue test results showed that when the ion implantation fluence was 3.6×10¹⁶ ions/cm²,the anti-fatigue properties of TC18 titanium alloy were improved under the combined influence of low roughnessand high residual compressive stress.It can be seen from the S-N curve and calculating fatigue life that the strength of TC18 titanium alloy before ion implantation was 407 MPa?fatigue limit is 5×10⁶ cycles?,and that the strength of TC18 titanium alloy after Cr and Zr ion implantation were 413 MPa and 431 MPa,respectively.The fitting results were basically consistent with the fatigue test results.This indicated that Zr ion implantation had the greatest improvement on the fatigue life of TC18 titanium alloy at this implantation fluence.