Sufrace Alloying Mechanism During Metal Ion Implantation Into Metal At Elevated Temperatures

The metal ion implantation into metal at elevated temperatures is an important surface alloying process. The metal ion is implanted into metal that is heated by ion bombardment with the large ion current density or auxiliary heater. Compared with conventional ion implantation, the implantation depth at elevated temperatures is deeper and intermetallics that are benefit to the improvement of surface performance can form during metal ion implantation into metal at elevated temperatures. At present, the researches about the metal ion implantation into metal at elevated temperatures concentrate on the improvement of implantation process and the properties of the implanted layer. However, the modification mechanisms of the implanted layer are not fully understood. The mass transfer and the rule of the intermatellics formation and transformation during metal ion implantation into metal target at elevated temperatures determine the composition and construction of the implanted layer, which affect the performance of the implanted layer significantly. To improve and control the process of metal ion implantation into metal and make a further use of the process of metal ion implantation into metal at elevated temperatures in the surface alloying, the mechanisms of mass transfer and the rules of intermetallics formation and transformation have been studied by the theory analysis and the experimental measurement.A mass transfer model has been built up for metal ion implantation into metal at elevated temperatures, based on the transport of ions in matter and the radiation enhanced diffusion theory. With the model, the ion implantation process at elevated temperatures is simulated by the Dynamics Monte Carlo method. Using a maximum allowed atomic fraction simulates the local saturation behavior in the crystal target. The radiation enhanced diffusion coefficient is obtained by taking into account the linear annealing defects. The concentration-depth profiles of the implanted species are determined from the diffusion equations for the implanted species and nonequilibrium vacancies. The nonequilibrium vacancy source function and the surface sputtering effects are introduced in the diffusion equations. The concentration-depth profiles of Cr, Ni, and Fe ions implantation into Al and Al ions implantation into Fe were calculated at the temperatures of room temperature to 700 ℃, at the implantation energy of 2-120 keV and with the implantation doses of 2×10~16—1 ×10~18 ions cm"". The calculated results are consistent with the experimental ones. The effects of temperature, implantation energy and implantation dose on the radiation enhanced diffusion are also discussed and the errors generated from calculations andexperiments are analyzed.Due to the characteristics of interface reaction during metal ion implantation into metal at elevated temperatures, the effective heat of formation model is built to predict intermetallics formation during metal ion implantation into metal at elevated temperatures by introducing the conception of effective heat of formation. The effective heat of formation is calculated by the standard heat of formation of the intermetallics predicted to form multiplying the ratio of the concentration of the limiting element at the reaction interface and the concentration of the limiting element in this intermetallics. The intermetallics Al^Fe^ HfAb, MoAln, NbAls, NiAl3, TaAh and ZrAls are reasonably predicted to form during metal Fe, Hf, Mo, Nb, Ni, Ta and Zr ions implantation into metal Al at the implanted temperature of 300-600 °C, at the implanted energy of 50-140 keV, at the current density of 10-60 uA cm"2, with the implanted dose of 10I7-1018ions cm"2 and the prediction results are consistent with the experimental results. Considering the metastable phase formed during metal ion implantation into metal at elevated temperatures, the effective heat of formation model predicts formation of the intermetallics CrnAlge at the temperature of 40fr °C, and C^Alu at above 400 °C. By taking the nucleation barrier at solid state reaction interface into account, intermetallics VAI3 is predicted to form by the effective heat of formation model. The biphase M0AI12, M0AI5 and TiA^, TiAl formed during the metal Mo and Ti ion implantation into metal Al at above 600 °C, respectively, where the effective heat of formation model is not valid.Al ions with the implanted energy of 120 keV are implanted into Fe at the temperatures of room temperature to 500 °C to the implanted doses of 5><10i6 ions cm"2 and lxlO17 ions cm"2 by using the 400 keV accelerator and Al ions with the implanted energy of 45 keV are implanted into Mg alloy AZ31 at the temperatures of RT and 300 °C to the implanted doses of 2><1016-lxl017 ions cm'2 ions by using MEVVA source. The concentration-depth profiles of implanted species are obtained from Rutherford Backscattering Spectrometry (RBS) and the phase structure of samples is analyzed by X-ray diffraction analysis (XRD). And the concentration-depth profiles and the phase formation are calculated by the mass transfer model and effective heat of formation model given in this dissertation, respectively. The results of measurement and simulation show that with the implantation dose of 5x1016 ions cm"2, at 250 °C, the maximum concentration of the implanted Al in Fe samples is 6 at.% with a implantation depth of 160 nm; with the implantation dose of 1 xlO17 ions cm"2, with the temperature increasing from 250 °C to 500 °C, the maximum concentration of the implanted Al in Fe keeps constant of 10 at.% and the implantation depth increases from 180 nm to 200 nm; the intermetallics could be detected at 250 °C, with the implantation dose of lxlO17 ions cm"2 and at 500 °C, with the implantation doses of 6x1016 ions cm"2 and l><1017 ions cm"2; at room temperature and 300°C, with the implantation dose of l*10!7 ions cm"2, the maximum concentration of implanted Al in Mg alloy is 10 at.% and 8 at.%, the implantation depth is 840 nm and 1200 nm, respectively; at room temperature, with the implantation dose of l><1017 ions cm"2, intermetallics Al^Mgn starts to precipitate, while at 300 °C, with the implantation dose of2*1016 ions cm"2, the intermetallics Al^Mgn begins to precipitate; the calculated concentration-depth profiles and the prediction of phase formation by using the mass transfer model and effective heat of formation model are consistent with the experimental results.