Research On Microstructure And Mechanical Properties Of Copper/Carbon Steel Namocomposite

Although high strength can be readily achieved via high density grain boundaries or phase boundaries,most of the nanostructured steels suffer from poor ductility.How to break through the strength-ductility trade-off have been a long-term hot issue in the research field of nanostructured steels.It has been reported that the nanoscale metastable austenite could transformed to martensite induced by stress or strain in nanobainitic steels and nanograin stainless steels,leading to enhanced ductility.Hence obtain uniformly distributed nanoscale metastable austenite is a effective path to achieve nanostructured steels with high strength and ductility.However it is tricky to acquire such metastable austenite in nanostructured steels by traditional cold deformation and heat treatment process.If processing coarse grained metastable austenitic steels by severe plastic deformation(SPD),the austenite would transform to martensite during deformation and nanograins without transformation ability are usually formed.Gaining a large amount of metastable austenite in nanograined carbon steels is particularly challenging because the processing must include austenization during which the nanograins grow rapidly.As a consequence,the nanograins no longer exist after austenization.In this study,nanolamellar Cu/T10 steel composite with the average layer thickness of about 69 nm were fabricated by hot pressing,forging,rolling and wire drawing.Due to the Cu barely intermixes or reacts with Fe or C and keep geometric instability at high temperatures,Cu suppressed the grain growth of austenite during austenization and lead the austenite maintain nanoscale.Subsequent austenization and quenching enable the formation of a high volume fraction of uniformly distributed nanoscale metastable retained austenite.The nanolamellar Cu/carbon steel composite with well ductility was successfully fabricated and the transformation behavior and the deformation mechanisms of the composite was explored by in situ high-energy X-ray diffraction.In addition,the Cu/martensite high strength composite was obtained by quenching in liquid nitrogen to make the retained austenite transform to martensite.On this basis,the nanolamellar Cu/65Mn steel composite was designed and fabricated.The main results are as follows:A high volume fraction(more than 64%)of retained austenite with the morphology of thin film or island has been retained after quenching in steel layer of the Cu/T10composite.The DSC results have showed the M_s of the retained austenite was?5°C and revealed a wide temperature range(4°C to-80°C)for thermally activated transformation.The stress-strain curve of the specimen tensiled at room temperature can be divided into three stages:initial elastic deformation,yielding at a constant stress and work-hardening.In situ tensile tests performed in the synchrotron X-ray beam have shown that the stress-induced martensitic transformation started inhomogeneously(Lüders-like band)at the beginning of the plastic deformation,and was followed by a homogeneous transformation until fracture.Tensile tests at various temperatures reveal that the mechanical behavior of the Cu/T10 steel composite is strongly temperature-dependent.It is suggest that the deformation mechanism of the composite is a result of the competition between transformation and slip of the retained austenite.The temperature crossover(M_s~?)from stress-to strain-induced transformation has been identified as 60°C.When the test temperature is below 60°C,the critical stress required to deform austenite is linearly increased with the increasing temperature.The plastic deformation is mainly carried by so-called stress-induced martensitic transformation.If the test temperature exceeds 60°C,the plastic deformation of austenite enters the so-called strain-induced martensitic transformation regime.The deformation mechanism is the synergistic coupling of martensitic transformation and dislocation slip,which is the origin of high work hardening rate and hence the superior ductilityThe Cu/T10 steel nanocomposite with the microstructure of Cu/martensite is obtained after liquid nitrogen quenching.The maximum lattice strain of the martensite is 1.93%during deformation which is about 30%higher than that in other steels previously reported,owing to the larger aspect ratio of martensite in the Cu/T10 steel nanocomposite.After liquid nitrogen quenching,the Cu/65Mn composite exhibits a combination of high strength of 1220 MPa,ductility of 6.4%and electrical conductivity of 60%IACS(International Annealed Copper Standard),which is comparable to the properties of high strength Cu wires reported in literature.