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The present work deals with the high temperature deformation behavior of Fe-11.15Mn-5.6Al-0.07C (wt.%) triplex ferrite-based lightweight steel in the temperature range of 800–1100 °C under the strain rate of 0.001 to 0.1s−1. The compressive high temperature flow curves under the various thermomechanical conditions were accompanied by a considerable fractional softening. According to the detailed microstructural analysis, the observed flow softening was discussed relying on the occurrence of dynamic strain induced transformation and dynamic recrystallization. In this respect, a sine hyperbolic Arrhenius-type constitutive model was developed considering the three dimensional variation of the materials’ constant with strain, strain rate and temperature. This provided a proper condition for accurate assessment of the strain compensation mechanisms. The capability of the modified and un-modified constitutive models in prediction of the high temperature flow behavior of experimented low density steel were compared. According to the verified model, activation energy (Q) maps were developed and discussed in correlation with the characterized microstructure evolutions. The Q-plots were divided into three domains and a transition range was recognized at ~1100–1250 K, the extent of which decreased with increasing imposed strain. The low energy domains were attributed to the (i) activation of load transition as an effective strain compensation mechanism and the occurrence of dynamic austenite to ferrite transformation, and (ii) the high dislocation annihilation rate at high temperatures.

The high temperature deformation behavior of a duplex ferrite based low density steel was investigated and the governing restoration mechanisms were determined. Toward this end, the isothermal hot compression tests in the temperature range of 800–1000 °C under the strain rate of 0.001–0.1 s−1 were executed. Interestingly, an appreciable ferrite grain refinement was achieved through thermomechanical processing and the ferrite phase was dynamically recrystallized through continuous mechanism at various temperature and strain rates. The ferrite grain size decreases from ∼900 μm (initial microstructure) down to ∼35 and 50 μm for the microstructure deformed at 800 °C/0.001s−1 and 1000 °C/0.001s−1, respectively. The occurrence of grain refinement within the austenite was also observed which increased the stability of austenite against thermally induced martensitic transformation during quenching. The deformation induced transformation of the austenite to ferrite was also characterized as one of the involved refinement mechanisms which was intensified under the higher strain rates at lower temperature of 800 °C. Considering the portion of the fine ferrite grain resulted from strain induced transformation, the grain size under the lower strain rate of 0.001s−1 decreases down to the ∼14 μm. Finally, the room temperature mechanical properties of the thermomechanically processed specimens was assessed through miniaturized shear punch testing method. It was found that the workability of deformed specimens was significantly improved owning to the simultaneous occurrence of dynamic recrystallization and deformation induced transformation.

The present work deals with emphasizing the role of substructure development in fragmentation of the cast structure in Mg-Sn-0.3Li-3Zn alloy during thermomechanical processing. In this respect, a new micro-mechanism and a conceptual model were proposed for dendrite fragmentation, which was supported by 3D-tomography, electron back scattered diffraction, and angle selective back-scattered imaging analyses. Balancing of the dihedral angle of the non-equilibrium triple junctions led to the formation of grooves at the dendrite arms, followed by penetration of α-Mg matrix through the grooves and finally resulted in fragmentation of the specified dendrite arm. In fact, despite the conventional expectation regarding the high contribution of brittle fracture and shearing in fragmentation of the dendrites at low temperature regime, it was observed that the substructure development in a medium-to-high stacking fault energy material greatly participated in thermally-activated disintegration of the dendritic cast-structure.

The present work deals with proposing a new refinement mechanism in triplex microstructure of a ferrite-based low density steel in the course of thermomechanical processing. Toward this end, the material was deformed in compressive mode at 500 °C down to various strain levels and the microstructures was studied step by step with the aim of advanced characterization methods. Owing to the partially transformation of austenite to martensite, the softer lamella-type ferrites were encapsulated between austenite and/or martensite laths. It was found that the imposed strain was primarily accommodated in ferrite phase, due to the strain incompatibility between ferrite and martensite and the constraints coming from martensite, and the higher load-bearing capability of ferrite. This was led to substructure development within the ferrite, and groove formation through boundary splitting mechanism. Finally, the stepped ferrites were fragmented at higher imposed strain, and such constraint induced ferrite refinement was led to the formation of the grains down to the size of 0.5–2 μm.

The Gibbs energy model as an effective thermodynamic function along with detailed microstructural analyses were employed to assess the capability of strain induced transformations in a ferrite based lightweight steel. In this respect, the material was deformed in a warm temperature regime in compression mode and the resultant microstructures were studied in details. The progressive and extensive development of the banded structure was led to the partitioning of the ferrite phase and also the shearing of the parent austenite. The shear strain localization was effectively released through simultaneous transformation of ferrite to austenite and austenite to ferrite transformations as an effective strain compensation mechanism. This was led to appreciable flow softening in corresponding flow curves. The capability for bidirectional transformation resulted from the specified local elemental partitioning of Al and Mn and the resultant exactly same levels of Gibbs energies of each constituent phases.