A Study On Ion Implantation Of Titanium Alloys For Reactor Materials And Mechanisms On Corrosion And Wear Resistance

Titanium and its alloys offer a unique combination of desirable mechanical properties, low density, and high corrosion resistance, which make them more attractive candidate materials for structural application in the field of nuclear energy. Wear and corrosion of titanium-based alloys used as structural materials of nuclear reactor occur inevitably due to the severe serving conditions, which destroys the surface of the materials and brings nuclear accidents. The concepts of high burn-up, long duration time and zero broken were putted forward recently, which made improving the corrosion and wear resistance behavior of titanium-based alloys are necessary.The aim of the present investigation is to mainly understand the surface characteristics, corrosion and tribological behavior of two new titanium-based alloys: TA16 and TA17, developed in China, under equilibrated conditions in a simulated nuclear reactor environment. The research contents consist of the following aspects: Tribological and corrosion properties of the two titanium-based alloys implanted by N, Ni, Ta, Al and Nb single implantation and Ta+Ni co-implantation; Influence of post-annealing treatment on tribological and corrosion properties of the as-implanted alloys; Influence of implantation temperature on tribological and corrosion properties of the alloys. The main results are as follows:1. The phases formed during nitrogen ion implantation at low fluences mainly consist of Ti₂N andα-Ti, and a homogenous distribution of small precipitates of TiN form as the implantation fluences increasing. The critical value of fluence transition between the two kinds of nitrides varies with different implantation conditions and alloying element. It has been found that nitrogen implantation induces the formation of a TiN layer which acts as diffusion barrier that blocks the path-ways for a Ti migration from the metal to the solution which normally occurs during anodic polarization.In addition, wear resistance has been improved obviously when implantation fluence is above 8×10¹⁶ ions/cm². The great improvement in the wear resistance is correlated with the decrease in the friction coefficient and the increase of microhardness. The increase of resistance to wear and microhardness is caused by the implantation of nitrogen leading to the formation of nitrides and radiation defects.2. Tantalum implantation effectively suppresses the overall anodic reaction in the active and passive regions. At higher tantalum fluences, the effects are gradually diminished because of elimination of tantalum in the implanted layer by sputtering. While, nickel implantation, which causes precipitation of the intermetallic Ti₂Ni in the implanted layers, preferentially promotes the passivation of titanium-based alloys. It has been demonstrated that co-implantation effectively improved the corrosion resistance of titanium-based alloys in an aggressive reducing environment.Nickel implantation can't effectively improve the wear resistance of the alloys. However, the wear resistance has been improved significantly by tantalum implantation. In the co-implantation, the best wear resistance has been attained after tantalum implantation at 1×10¹⁷ ions/cm², but the influence of nickel implantation fluences on wear resistance during the co-implantation is ignorable.3. For aluminium implantation, the corrosion potential of the alloy increased significantly. The change of the corrosion potential is inappreciable as the fluences increasing, but for the passive current density, it's obvious. The corrosion potential increases slightly but the passive current density decreases greatly as the fluences increasing after niobium implantation. The passive current density increased after niobium fluence reaching to a certain extent, which is similar to that of aluminium implantation. The corrosion resistance become better after 5×10¹⁷ Al⁺/cm² or 1×10¹⁷ Nb⁺/cm² implantation.The improvement of wear resistance of the alloy after aluminium implantation is minute. Although the fluctuation of the friction coefficient of niobium-implanted sample is obvious at low fluences, it dies down at a higher fluence and the friction coefficient keeps below 0.2 during the experiment, so the wear resistance has been improved significantly.4. The effect of annealing treatment on corrosion resistance of the nitrogen implanted sample is ignorable when the annealing temperature is below 600℃, however, it becomes obvious at 600℃and above. The corrosion resistance of the samples at lower fluences which have poor corrosion resistance initially become better, but the corrosion resistance of the higher fluence samples which have better corrosion resistance initially become reduced, which maybe attribute to the size of the formed nitrides. The change of the passive current density is ignorable as the implantation temperature increasing. However, the corrosion potential reduces with increasing of the implantation temperature under the condition of the test.The improvement of the microhardness of the as-implanted samples is significant when annealing temperature attains to 600℃. The influence of annealing treatment on the friction coefficient of the sample implanted with nitrogen under the same implantation condition is inappreciable at 500℃or below, but after annealing at 500℃and above, the value of the friction coefficient is so small that the sample could not been broken under the condition of the experiment. The effect of the implantation temperature on the tribological behaviors is similar to that of the annealing treatment.