Effects of Temperature and Grain Size on the Transformation and the Deformation Behaviors of the TRIP (Transformation Induced Plasticity) Steels

Two types of TRIP steels, a commercial 304L stainless steel and an Fe-10%Cr-5%Ni-8%Mn steel, have been used to investigate the effects of temperature and grain size on the transformation and mechanical behaviors. 304L SS was used to investigate the effects of temperature ranging from 300K to 77K, while the Fe-10%Cr-5%Ni-8%Mn steel was used for the investigation of the effects of the grain size at 300K.

At 203K, the fcc grains of the 304L SS with {200} plane normal parallel to the loading direction are preferred for the fcc to bcc transformation and the {200} plane normals of the newly-formed bcc grains are also concentrated along the loading direction. For the fcc to hcp transformation, the fcc grains transform within a wider range of orientation distribution and the hcp basal plane normals are primarily orientated with an anlge between about 10° to 50° to the loading direction.

At 77K, two components, <110> and <111>, in contrast to only <110> component at room temperature, develop in the fcc austenite during the deformation. For bcc martensite, the grains are primarily oriented with (100) plane normal parallel to the loading direction. At high strain levels, the bcc phase becomes less textured when the fcc grains with less favorable orientations start to transform. It is illustrated that the phase transformation dominates the texture evolution during the low temperature deformation. The fcc to hcp transformation occurs in a wide angle range and a high shear stress is favored for the hcp phase formation.

To investigate the effect of grain size on transformation/deformation behavior, the Fe-10%Cr-5%Ni-8%Mn TRIP steels with different grain sizes (50µm, 350nm, and 250nm) were prepared and in-situ neutron diffraction was performed. The evolution of phase fractions and lattice strains for both austenite and martensites was investigated at 300K. The ultrafine-grained (350nm and 250nm) samples show a combination of high strength and good elongation. The martensite formation and the concurrent load partitioning between the austenite and the newly-forming martensite phases were identified as the source of the strain hardening that facilitates the high ductility maintained in the UFG steel.