Investigation On Normal Pressure And High Pressure Martensitic Transformation Of Fe-Ni Alloys

First,martensitic transformation is the basis of the hardening of steels via heat treatment.However,compared to Fe-C martensite that shows enhanced hardening with carbon content,Fe-Ni martensite shows very limit hardening and is almost independent of Ni content.Second,it is demonstrated by large numbers of investigations that carbon-independent superstrengthening can be realized via substructure.Concerning that Fe-Ni martensite commonly contains high density of twinned lamellae（in the midrib）,the traditional viewpoint on the none-hardening ability needs to be clarified.Third,as an important variable that is able to influence the thermodynamics for martensitic transformation independent of alloy element,high pressure can widely tune the size of the substructure and in turn the strengthening.It is thus necessary to address the principles that the pressure effect on the Fe-Ni martensite.Finally,in this paper,Fe-Ni alloys with different nickel contents（Fe-5Ni,Fe-10Ni,Fe-15Ni,Fe-33Ni）were chosen to subject to thermal cycling under normal atmospheric pressure and high pressure of 1-4 GPa（after holding at at 1200℃-30 min,quenched into water under normal pressure and slowly cooling at 10℃/s for high pressure）.The microstructures of the final products were characterized by optical microscope,X-ray diffractometer,scanning electron microscope and transmission electron microscope.The following research results are obtained:（1）Lath martensite was obtained in Fe-10Ni and Fe-15Ni by atmospheric water quenching,and lenticular martensite was obtained in Fe-33Ni.The relationship between martensite（α’）and parent austenite（γ）accords with Kurdjumov-Sachs:（0 1 1）_α’//（1 1 1）_γ,[1 1 1]_α’//[1 0 1]_γ.Lath martensite is a typical dislocation martensite and lenticular martensite is twinned martensite.Twinned laths exist in both kinds of martensite,which come from the KS variants with twinned relationship,rather than{1 1 2}<1 1 1>deformation twins.（2）Lenticular martensite under atmospheric pressure consists of midrib region,twinned region and dislocation region.The midrib region is a high-density twinned variants with 3-4 times of remarkable strengthening（308-432 HV）,and the hardness of twinned region and dislocation region is 262-287 HV and 238-270 HV respectively.（3）Twinned lath martensite is obtained by high pressure martensite transformation of Fe-Ni alloy.Martensite and parent austenite also follow the classical Kurdjumov-Sachs relationship:（0 1 1）_α’//（1 1 1）_γ,[1 1 1]_α’//[1 0 1]_γ.The lath martensite obtained under high pressure is mainly twinned laths,and the laths with twinned relationship is KS twinned variants.Unlike twinned lath martensite.（4）Pressure promotes the selection of martensite twinned variants pair of Fe-Ni alloy lath.With the increase of pressure,the content of twinned variants increases and the size decreases.High-pressure martensitic transformation improves the strengthening effect,which is 1-3 times higher than that of normal-pressure water-quenched samples,and the improvement range is affected by nickel content.