Research On Copper Precipitation Behavior And Properties Of Cu-Bearing Antibacterial Austenitic Stainless Steels

Cu-bearing antibacterial stainless steels are a novel class of structure/function integrated materials developed in recent years.Copper(Cu)is a common alloying element in steels,and also one of essential trace metal elements in the human body.Through adding proper amount of Cu into the currently used medical stainless steels,the Cu-bearing antibacterial stainless steels can continuously release trace amount of Cu ions with bio-functions due to the inevitable corrosion in body environment.The related research has confirmed that the Cu-bearing stainless steels possess bio-medical functions such as anti-infection,inhibition of in-stent restenosis and promotion of osteogenesis.However,some basic material issues on this novel class steels still need to be deeply studied and understood.The objective of this research is to clarify the precipitation behavior and strengthening mechanism of Cu-rich precipitation in the Cu-bearing austenitic antibacterial stainless steels.Besides,the hot deformation behavior as well as the effect of Cu addition on overall properties of Cu-bearing stainless steels were also studied.These research results can provide a solid material basis for wider applications of Cu-bearing antibacterial stainless steels.The main studies and conclusions are as follows:(1)Study on effect of Cu addition on the overall properties of Cu-bearing austenitic antibacterial stainless steelsThe effects of Cu addition on mechanical properties,corrosion resistance and antibacterial performance of 316L stainless steel after solution and aging treatments were investigated.The results showed that the Cu addition(0-3.5 wt.%)and heat treatment had no obvious influence on the microstructure with complete austenite features.The strengths after solution treatment were almost similar,whereas the aging treatment obviously increased the strength due to formation of tiny Cu-rich precipitates.Meanwhile,the Cu-rich precipitates would destroy the uniform continuity of the passive film,and thus decrease the corrosion resistance.Besides,high Cu content(3.5 wt.%)and aging treatment(700 ?/6 h)were two compulsory processes to endow the 316L-Cu stainless steel good antibacterial performance.(2)Study on hot deformation behavior of Cu-bearing austenitic antibacterial stainless steelThe hot deformation behavior of Cu-bearing 316LN stainless steel(316LN-Cu)was studied in the temperature range of 950-1150 ? and strain rate range of 0.01-10 s-1.The flow stress of 316LN-Cu stainless steel was strongly dependent on the temperature and strain rate,and was found to increase with increase in strain rate,and decrease with increase in temperature.The activation energy of 316LN-Cu stainless steel during hot deformation was measured to be 533 kJ/mol.The additions of Cu and N could effectively enhance the activation energy.The large instability region and limited hot-working region of 316LN-Cu stainless steel were attributed to the addition of Cu.The optimum window for hot deformation is in the temperature range of 1100-1120 ? and strain rate range of 0.01-0.018 s-1 with a peak power dissipation efficiency of 38%.Fine and homogenized recrystallized grains could be obtained in this domain.Besides,the hot deformation constitutive equation of 316LN-Cu stainless steel could be described as:Z = ? exp(53428/RT)=1.1575 × 1018[sinh(0.0067 × ?)]3.914(3)Study on precipitation behavior of Cu-rich phase in Cu-bearing austenitic stainless steelThe coarsening behavior and strengthening mechanism of Cu-rich precipitations in Cu-bearing 316LN stainless steel after aging at different temperature for different time were systematically investigated by electrical resistivity,atom probe tomography(APT)and transmission electron microscopy(TEM).It was found that both hardness and mechanical strength of the steel increased to peak value within short aging time,and remained nearly unchanged with prolonged aging time.The TEM observations showed that the precipitation rate of Cu-rich phase was very quick.High number densities of spheroidal Cu-rich precipitates were homogeneously distributed in the austenitic matrix after short aging time and the interface between Cu-rich precipitates and austenitic matrix remained coherent during the aging process.APT analyses further revealed that the core composition of Cu-rich precipitates was almost pure Cu while the overall composition of Cu-rich precipitates was about 75%Cu.Compared with aging time,aging temperature had great effect on the coarsening of Cu-rich precipitation.As aging temperature was increased,the average radius of Cu-rich precipitates increased,and meanwhile the number density decreased significantly.The relatively slow growth and coarsening behavior of Cu-rich precipitates was largely attributed to the slower diffusion kinetics of Cu atoms and lower interfacial energy of Cu-rich precipitates in the austenite matrix.The lower volume fraction of Cu-rich precipitates in austenitic matrix contributed to the weak precipitation strengthening effect of Cu-rich phases.Besides,the strength contribution induced by coherency strengthening was larger than that of chemical strengthening.