A Meso-scale Mechanics Based Study Of Material Size Effect And Phase Transformation–Deformation Coupling

Size effect and phase transformation – deformation coupling are two typical problems and challenges for the development and application of advanced engineering metallic materials.To explore solutions to these two problems,mechanical behaviors of 316 L stainless steel thin wires and a transformation induced plasticity high entropy alloy(TRIPHEA)were experimentally studied and modeled.The microscopic mechanisms of the two problems were revealed through development and application of an advanced testing apparatus and application of advanced experimental observation techniques,respectively.Based on the microscopic mechanisms and the rule of mixtures,mesoscale mechanical behaviors of “phases” that represent distinct features driving the size effect or TRIP assisted deformation were characterized.Mesoscale mechanics composite models were proposed and well described the macroscopic mechanical properties of the studied materials under the influence of size effect or phase transformation – deformation coupling,respectively.The outcome of this study provides data support and research methodology for pursuing solutions to the above two challenging problems and has high potential of wide application in engineering practice.A tension-torsion fatigue testing apparatus for testing of multiaxial cyclic mechanical behavior of microscale specimen was first developed and then used for systematic exploration of tension,torsion,tension-torsion cyclic and fatigue behaviors of 316 L stainless steel thin wires with different diameters and grain sizes.Two types of opposite size effect,i.e.“smaller is weaker” and “smaller is stronger”,were revealed to occur subsequently along with the decrease of number of grains across diameter of thin wires.A mesoscale analog composite model was proposed based on the equivalent mesoscale mechanical behaviors of the two “phases” – interior and surface grains and combined with the classic Hall-Petch grain size effect equation.The model revealed surface grain softening due to lack of grain boundary strengthening as the main microscopic mechanism of the“smaller is weaker” size effect,and well described the size effect on the deformation behaviors of thin wires under tension and torsion.A multiaxial fatigue life prediction model with size effect taken into account was further proposed and well predicted the reduced multiaxial fatigue life of thin wires with decreasing diameter.Effect of microstructure,cooling temperature and magnetic ordering and their interaction on the metastability and thermally induced phase composition of a TRIP-HEA was revealed through in situ real-time neutron diffraction in combination with electron backscatter diffraction(EBSD),superconducting quantum interference device(SQUID)and thermodynamic calculation.With such understanding,TRIP-HEA specimens with different thermal stability of FCC phase and thus different initial phase composition were produced.Deformation mechanisms during tension of those specimens were further explored through in situ real-time neutron diffraction,and effect of FCC thermal stability on the phase transformation,deformation and their interaction was revealed.Deformation mechanisms of TRIP-HEA include TRIP and dislocation slip of FCC phase and twinning and dislocation slip of HCP phase.Persisting TRIP of FCC phase and the strain-hardening potential of the increasing HCP phase are the main contributions to the remarkably persisting bulk strainhardening.A semi-empirical dual-phase mesoscale composite model was proposed based on the experimental observations of phase transformation – deformation coupling.The model is based on the unified hardening behavior of FCC phase,FCC/HCP two-phase stress constraints,and FCC phase stress-dependent transformation at small strains,and unified macroscopic strain hardening at large strains.The model well described the stress-strain behaviors of TRIP-HEA specimens with different thermal stability.