| In 2015,Dierk Raabe,a well-known steel research scientist from Max Planck Institute in Germany,proposed the concept of high-entropy steel in combination with the design concept of high-entropy alloys.In 2017,the research group introduced the entropy effect on the basis of traditional stainless steel to prepare high-entropy and medium-entropy stainless steel series.In this paper,combining the characteristics of traditional precipitation hardening stainless steels that generally contain a large amount of alloying elements,the composition is further adjusted,and the precipitation hardening medium entropy stainless steel is prepared using a vacuum arc melting furnace.The content of Cr in the composition is 18 at.%,and the content of Fe is 60at.%.In order to increase the entropy of the alloy composition system,Mn,Cu,Co,Ni,Mo,Al,and Ti are added in nearly equimolar ratios,which contain up to 9 Kinds of different elements.And compared with the structure,hardness and corrosion resistance of traditional precipitation hardening stainless steel 17-4PH prepared under the same melting process.First,the study found that there was no obvious second phase in the solidified microstructure and the microstructure with solid solution treatment above 1120℃of precipitation hardening medium-entropy stainless steel Fe60Cr18Cu3（MnCoNiMoAlTi）19,which was a simple BCC solid solution with significant aging hardening effect.After solution and aging heat treatment（1120℃+520℃）,the hardness increases from 495HV before aging to 621 HV,which is much higher than the 17-4PH maximum aging hardness of 460 HV.TEM analysis shows that the age hardening mechanism in medium-entropy stainless steel comes from a large number of nanoscale ordered B2phases dispersed and precipitated in the matrix.However,the study also found that the high-temperature solid solution temperature of the developed precipitation hardening medium-entropy stainless steel should not be lower than 1060℃,and a large amount of brittle intermetallic compound Cr9Fe42Mo2Ti5 phase will be precipitated in the structure below this temperature.Secondly,the corrosion resistance of the heat-treated sample at 1120℃+520℃in 10 wt.%H2SO4 solution and 3.5 wt.%Na Cl solution was studied.In 10 wt.%H2SO4solution,the corrosion current density of the polarization curve of precipitation hardening medium-entropy stainless steel is 32.95μA/cm2,and the charge transfer resistance Rct value in electrochemical impedance spectroscopy is 1102Ωcm2,which is significantly lower than the corrosion current density of the comparative component17-4PH with 121.29μA/cm2,especially the Rctvalue increased by nearly 20 times compared with the 52Ωcm2 of 17-4PH,indicating that the passive film on the surface of the precipitation hardened medium-entropy stainless steel is more resistant to charge transfer,the passive film is more stable,and the corrosion resistance is better.After the sample size of 8×5×3mm was soaked in 10 wt.%H2SO4solution at 60℃for 192 h,the soaking experiment found that the 17-4PH smelting sample was basically dissolved,while the precipitation hardening medium-entropy stainless steel sample had no obvious size change,which further proved its extremely excellent corrosion resistance of sulfuric acid.The precipitation hardening medium-entropy stainless steel in 3.5 wt.%Na Cl solution also has significantly lower corrosion current density,higher pitting potential,wider passivation zone and higher Rct value.After scanning the polarization curve to the overpassivation potential 1.3 V,the pitting morphology of the surface of the passivation layer was observed.It was found that there were fewer pitting pits on the surface of the precipitation hardening medium-entropy stainless steel.The maximum pitting size was about 80×75μm,and the maximum depth was about 29.5μm,much smaller than 17-4PH pit size is about 720×150μm,and the maximum depth is about123.4μm.The XPS analysis results of the passive film show that the excellent corrosion resistance of precipitation hardening medium-entropy stainless steel comes from the combined effect of the high content of Crhy3+,Crox3+and Cu+ox,Tiox4+,Alox3+and Moox6+alloying element ions in the passive film.Finally,the influence of the content of C,Al and Ti alloying elements on the precipitation hardening mechanism of precipitation hardening medium-entropy stainless steel is studied.It is found that unlike the hardening mechanism of traditional steel materials,carbide precipitation is not the main factor in the age hardening of precipitation hardening medium-entropy stainless steel.It is clear that Al and Ti are the main elements of precipitation hardening medium-entropy stainless steel,and they must be combined to obtain better strengthening effect.In summary,this paper proposes a precipitation hardening medium-entropy stainless steel composition with a simple BCC solid solution phase and its solution treatment method,which has obtained significantly better hardness and corrosion resistance than 17-4PH stainless steel.It is clear that its excellent corrosion resistance comes from the combined action mechanism of various alloying elements added in the passivation film,and the effect of the combined addition of Al and Ti elements on the precipitation strengthening mechanism is revealed.It provides a new idea for the development of new precipitation hardening stainless steel,which is expected to have broad application prospects in the future. |
