Effects of mechanical deformation on dislocation density and phase partitioning in 4130 steel

Interrupted tensile tests were performed on an AISI 4130 pressure vessel steel and investigated by neutron diffraction and scanning microscopy techniques. Analysis of the neutron diffraction patterns reveal a partitioning of ferrite and martensite phases resulting from deformation. A modified Williamson-Hall approach was used to model the broadening of Bragg peaks associated with the two phases as a function of applied strain, revealing an order of magnitude increase in their dislocation densities when the material was strained beyond the ultimate tensile strength (UTS). Lattice strains measured in the ferrite phase were consistently larger than those measured in the martensite phase for all the applied strain levels investigated. Moreover, a strain-induced phase transformation from a predominately martensitic steel to a ferritic steel was observed, with the average martensite phase fraction of an as-received specimen going from 78% to 22% when pulled to failure. Electron Backscatter Diffraction (EBSD) and Scanning Kelvin Probe Force Microscopy (SKPFM) were used to characterize the microstructure and phase fractions of ferrite and martensite associated with the various strain levels. These results agree well with those obtained from neutron diffraction and demonstrate the utility of SKPFM to distinguish between metallic phases with similar crystal structures that may be difficult to detect using conventional methods such as EBSD.