Investigation On High Pressure Martrnsitic Transformation Of Fe-Cr Alloys

Martensite has played an important role in the development of steel for thousands of years.However,Fe-Cr binary alloys with excellent oxidation resistance are well known unable to be effectively hardened by martensitic transformation even under extremely fast cooling>10~4℃/s.Besides the absence of carbon makes solid solution strengthening and structural strengthening less effective,phase transformation is absent as Cr content exceeds 12 wt.%where high temperature austenite phase zone does not exist.Therefore,Fe-Cr alloys must rely on alloying valuable elements such as W,Ni etc.to realize a good combination high temperature oxidation resistance and strength,but causing unexpected rising in cost and difficulty of production and recycling.As a thermodynamic variable independent of chemical composition,the pressure can enhance the driving force for martensitic transformation and decrease the M_S point by providing extra work to increase the change in Gibbs free energy,and consequently,tune the substructure and strength of martensite.It is thus the present investigation to research effects of different pressure(1,2,3,4 GPa)thermal cycles induced martensitic substructure and properties in binary Fe-15wt.%Cr alloy.It was characterized by X-ray diffraction,scanning electron microscope and trasmission electron microscope and the hardness,high temperature oxidation resistance will be measured.The following conclusions were reached:1)Different pressure thermal cycles induced twinned lath martensite with different thicknesses in binary Fe-15wt.%Cr alloy,twinned laths are twin-related martensitic variants,following K-S orirntation relationship and having(110)boundaries.With the pressure increases,the thicknesses of blocks are further reduced,but the deneity of dislocation increases.In addition to lath martensite,ferrite structure with high dislocation density was made in the samples under the pressure of 1 GPa.The average block thicknesses under the pressure of 2,3,4 GPa were 138 nm,44 nm and 31 nm.2)The hardneing effect of the samples under different pressure thermal cycles are distinct,hardness value is gradient,and it gradually increases with the raise of pressure.The hardness values are:210 HV-290 HV,280 HV-330 HV,320 HV-500 HV,420 HV-600 HV,the hardness of samples under the pressure of 4 GPa is increased by a factor about 4 times compared to matrix(150 HV).3)Compared with binary Fe-15wt.%Cr alloy,the samples under the pressure of 4 GPa are oxidized in the air at 500℃ for 1h,it shows excellent oxidation resistance.A technology with both high hardness and high temperature oxidation resistance was prepared in binary Fe-Cr alloys.