In order to obtain air-hardening steel with good cold formability, the microstructure evolution and mechanical properties of cold rolled LH800 steel during the batch annealing were studied by means of scanning electron microscopy, electron back scatter diffraction, transmission electron microscopy, and other technical means. The results showed that the ferrite + carbide structure was obtained by annealing between 600 ℃ and 700 ℃, the tensile curve had an obvious yield plateau, and the length of the yield plateau decreased with annealing temperature increasing. As the annealing temperature increased, the nanoscale carbides inside the ferrite grain gradually decreased, the coarse carbides at the grain boundary gradually increased, the volume fraction of low-angle grain boundaries gradually decreased, and the kernel average misorientation (KAM) value gradually decreased. When the annealing temperature exceeded 700 ℃, the microstructure was ferrite + martensite + carbide, and no yield plateau appeaed in the tensile curve. As the annealing temperature continued to increase, the volume fraction of martensite gradually increased, as did the KAM value. The mechanical property analysis showed that, after annealing at 700 ℃ for 4 h, air-hardening steel had the lowest yield strength and tensile strength, the highest elongation, and the best cold-forming performance. Based on the evolution of the microstructure and the nanoscale carbides of cold-rolled LH800 steel during the annealing process, this work revealed the essence of the yield-plateau phenomenon of LH800 steel and obtained its key process parameters of best cold-forming-performance.

Four TiAl alloys with different Mo contents were designed, and the microstructure and mechanical properties of these MoTiAl alloys were studied by scanning electron microscope, nanoindentation, and hot compression simulation methods. Results show that with increasing the Mo content, the content of. phase is gradually decreased, while that of beta phase is gradually increased. The Mo element mainly exists in the form of beta phase in the TiAl alloy. During the hot isostatic pressing process, the Mo element is diffused from the. and a 2 phases to the beta phase. The nanoindentation hardness of Mo-TiAl alloy reaches the maximum when the Mo content is 1.59at%, and it is negatively correlated with the interlamellar space. As the content of Mo element increases, the flow stress of Mo-TiAl alloys decreases, and the TiAl alloys with 2.11at% and 3.94at% Mo addtion have poor plasticity due to the Al element segregation.

Medium Mn steel (MMS) is a new category of the third-generation advanced high strength steel (3rd AHSS) which is developed in the recent 1-2 decades due to a unique trade-off of strength and ductility. Thus, this steel grade has a wide application potential in different fields of industry. The current work provides a fundamental study of the effect of hot-rolling on the inclusion deformation inMMSincluding a varied 7 to 9 mass pctMn. Specifically, the deformation behavior of different types of inclusions (i.e., Mn(S,Se), liquid oxide (MnSiO₃), MnAl₂O₄, and complex oxy-sulfide) was investigated. The results show that both MnSiO₃ and Mn(S,Se) are soft inclusions which are able to be deformed during the hot-rolling process but MnAl₂O₄ does not. The aspect ratio of soft inclusions increases significantly from as-cast to hot-rolling conditions. When the maximum size of different inclusions is similar, Mn(S,Se) deforms more than MnSiO₃ does. This is due to a joint influence of physical parameters including Young's modulus, coefficient of thermal expansion (α), etc. However, when the maximum size of one type of inclusion (e.g., MnSiO₃) is much larger than another one (e.g., Mn(S,Se)), this maximum size of soft inclusions plays a dominant role than other factors. In addition, the deformation behavior of dual-phase inclusion depends on the major phase, i.e., either oxide or sulfide. Last but not least, empirical correlations between the reduction ratio of the thickness of plate, grain size, and aspect ratio of oxide and sulfide inclusions after hot-rolling are provided quantitatively. This work aims to contribute to the 'inclusion engineering' concept in the manufacturing of new generation AHSS.

TiAl alloys undergo cyclic temperature changes during use, the process of which can be simulated by the thermal shock test. A systematic investigation of the thermal shock behavior of four Mo-containing TiAl alloys is conducted. The increase in Mo content from 1.0% to 4.0% causes the gradual decrease in the volume fraction of γ/α2 lamellar colony, while the volume fraction of equiaxed γ and βo phases gradually increases. At the same time, the thermal shock resistance of the TiAl alloys decreases as the Mo content increases. After thermal shock, cracks often occur within the lamellae and extend in a zigzag manner for TiAl−1.0Mo and TiAl−1.5Mo alloys. Their thermal shock resistance is enhanced by crack deflection, bridging, and microcrack shielding. For TiAl−2.0Mo and TiAl−4.0Mo alloys, cracks occur at the grain boundaries or within the γ phase and extend straight, with the result that these two alloys have worse thermal shock resistance than the other two alloys due to the limited effect of microcrack shielding. In addition, the microstructure stability of TiAl alloys after thermal shock is discussed, and there is a critical value of Mo content between 3.13% and 5.67%, which inhibits the βo → ω phase transition.

The mechanical response, on a microscopic and macroscopic level, and the deformation-induced martensitic transformation (DIMT) were investigated in multi-phase medium Mn steels (MMnS) with 6, 8 and 9 wt% Mn using in situ high-energy synchrotron x-ray diffraction during tensile loading. Prior to the in-situ analysis, a similar heat treatment finishing with an intercritical annealing was imposed on all MMnS. The initial microstructure prior to tensile loading was investigated by electron backscatter diffraction analysis. The volume fraction of austenite (gamma) after the heat treatment decreases from 60.2% to 50.7%, and 23.6% with increasing Mn content from 6 to 8 and 9 wt% Mn, respectively. This is mainly due to the difference in the formation of athermal alpha '-martensite. Athermal epsilon-martensite also formed in the MMnS with 8 and 9 wt% Mn, whereas no athermal epsilon-martensite formed in the MMnS with 6 wt% Mn. The alloys have quite different deformation behavior due to the different microstructures, and the majority of the load is carried by the phase that forms a continuous network throughout the steel, which in turn influences the DIMT. These results reveal the importance of assessing both phase-specific strain/stress and the inherent mechanical stability of the austenite in order to predict the macroscopic mechanical properties of the steel. As an example, this is witnessed by the comparison of MMnS9 and MMnS8. Austenite in MMnS9 bears about half the load as compared to austenite in MMnS8 during early deformation due to a continuous network of athermal alpha '-martensite resulting in significant load partitioning from austenite to alpha '-martensite. Thus, the mechanical driving force for DIMT in MMnS9 is reduced and therefore causes lower DIMT kinetics in MMnS9 than in MMnS8, even though MMnS9 has lower inherent austenite stability.

The flow stress–strain curves appear to be sensitive to deformation conditions. The ratio of critical strain to peak strain εc/εp follows a linear relationship except when the temperature is 1240 °C and the strain rate is 0.001 s−1. During the deformation, the fragmentation and decomposition of γ/α2 lamellae are related to recrystallization of α2 and γ laths in the lamellae and the γ → α2 phase transformation, the former depends on dislocation slip and twinning, and the latter is related to temperature, local stress concentration and diffusion time. As for the recrystallization mechanisms, the γ phase is discontinuous dynamic recrystallization (DDRX) mode, while the α2 phase relies on continuous dynamic recrystallization (CDRX) mode. The β phase has more low-angle grain boundaries (LAGB) during deformation, indicating the continuous coordinated deformation, and this explains the enlarged hot working window of the TiAl alloy (1165–1240 °C/0.001 ~ 1 s−1 and 1120–1165 °C/0.001–0.4 s−1).

Changes in the mechanical stability of austenite with strain state is a significant aspect that needs to be addressed in the use of medium Mn steel in automobile body construction. New insights are provided in this paper to understand strain-state-dependent austenite stability in a 5 wt% Mn-containing steel based on the analysis of the Schmid factor by electron backscatter diffraction and the calculation of the work done by the applied stress. Four strain states, namely uniaxial tension, simple shear, plane strain, and equibiaxial stretch, were considered and digital image correlation was used to capture the strain field during the entire deformation process. Results show that the mechanical stability of austenite decreases when strain state changes from uniaxial tension to plane strain, and further decreases in the case of equibiaxial stretch due to the increasing work done by the applied stress and the increasing Schmid factor. The austenite acquires the highest mechanical stability when it is deformed at the simple shear state, which corresponds to the lowest work done by the applied stress. The obtained knowledge promotes the basic understanding for the comprehensive application of medium Mn steels in the automobile industry.

The isothermal oxidation behavior of two TiAl alloys (as-HIP and as-RHT) were compared to explain the effect of microstructure on the oxidation resistance of TiAl alloy. After hot-pack rolling and cyclic heat treatment, the size of lamellar colonies was refined from 35.4 mu m to 21.5 mu m, and the beta/B2 phase was effectively removed. It is concluded that the as-RHT TiAl alloy has better oxidation resistance than the as-HIP TiAl alloy. The main reason is due to refinement of lamellar colony size, elimination of beta/B2 phase, uniform distribution of Nb and Mo, and the crushing of Y compounds.

Medium Mn steel (MMnS) is the good choice for car manufacturers to meet the requirements of reducing the weight of automobiles. Quenching & Partitioning (Q&P) process is an effective method to stabilize austenite in advanced steel, thus prompting the comprehensive mechanical properties of advanced steel. In this article, the Q&P process is applied to the MMnS to explore potential mechanical properties. The effect of austenitizing temperature, one of the significant parameters of Q&P process, on the microstructure and mechanical properties of MMnS was investigated. According to microstructural analyse results, all of the MMnS specimens processed by Q&P treatment with different austenitizing temperatures could obtain multi-phase microstructure, including alpha '-martensite, e-martensite and austenite. Furthermore, the highest volume fraction of austenite was observed in the MMnS processed by Q&P treatment at the austenitizing temperature of 920 degrees C. Due to the facilitated transformation-induced plasticity effect resulted from the high volume fraction of austenite with the austenitizing temperature of 920 degrees C, the MMnS obtained the high strength, high plasticity and sustaining work-hardening rate.