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The metastable α' phase and dislocation characteristics (e.g., density and constituents) are of vital importance for the mechanical responses of (α+β) titanium alloys. In this work, to reveal the in-depth decomposition mechanisms of dislocations and metastable α' and their influences on mechanical properties in a Ti-6Al-4V (α+β) alloy, the thermal stability of different microstructures tailored by various cooling approaches were investigated utilizing scanning electron microscope, electron backscattered diffraction, transmission electron microscope and X-ray diffraction line profile analysis. The results showed that the initial characteristics of α' and α laths and dislocation density were influenced by the cooling methods remarkably. The thermal stability of Ti-6Al-4V alloy increased with decreasing cooling rate. The improvement in thermal stability can be ascribed by the decrement in dislocations, partitioning of the alloying elements and grain orientations variation of the α' lath and α phase. It is also found that the plastic strain accommodation of β→α' transformation was dominated by dislocations. During stabilization annealing treatments, the , and dislocations simultaneously decomposed. The decomposition of dislocations and metastable α' phase during various stabilization annealing and the particular twins and stacking faults microstructures formed during quenching have a great influence on the properties of the studied Ti-6Al-4V alloy.

In the present work, a double-pass continuous expansion extrusion forming (CEEF) process was proposed for an Al-Mg-Si alloy, in which the diameter of rods was gradually expanded. The microstructural evolution, mechanical properties and deformation characteristics were investigated by utilizing microstructural observations, mechanical testing and a finite element method coupled with a cellular automata model. The results showed that the strength and ductility of the double-pass CEEF processed Al-Mg-Si alloys were improved synchronously, especially in artificially aged alloys. The grain size of the processed Al-Mg-Si alloy rods was refined remarkably by continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX), and the homogeneity of microstructure was gradually improved with increasing number of processing passes. The artificially aged alloy processed with double-pass CEEF and water quenching contained fine (sub)grains and high-density dislocations, which resulted in more needle-shaped β” precipitates and a larger precipitate aspect ratio than the as-received and air-cooled CEEF alloys owing to the different precipitation kinetics. The severe cumulate strain and microshear bands were found to accelerate CDRX and GDRX for grain refinement between adjacent positions of the parabolic metal flow due to the special temperature characteristics and large shear straining during the CEEF process.

The precipitation evolutions of Mg-Al-Zn alloys play essential roles in their mechanical properties, corrosion performance, formability, plastic deformation mechanisms and texture development. In the present work, the precipitation evolutions of AZ80 magnesium alloy during both non-isothermal and isothermal processes were unraveled by utilizing in situ electrical resistivity monitoring, hardness testing, differential scanning calorimetry and microstructural characterization. The results showed that discontinuous precipitation (DP) and continuous precipitation (CP) occurred competitively during non-isothermal and isothermal processes. The precipitation of dominant β-Mg17Al12 phase during non-isothermal processes was highly dependent on the thermal history. During isothermal processes, the precipitation behavior of AZ80 magnesium alloy could be considered as the functions of holding temperature and time. At lower temperatures, massive DP and CP were gradually formed to equally strengthen the alloy. At higher temperatures, the Ostwald coarsening was characterized in the later stages and indicated to slightly soften the alloy. Isothermal time-temperature-precipitation curves and quantitative precipitate evolution were estimated to unravel precipitation characteristics and their strengthening functions.

The flow softening is an important phenomenon during hot deformation of metallic materials. In the present work, a more evident flow softening of Ti-6Al-4V alloy when deformed in two-phase region was observed in coarser α grain sample, which was attributed to an accelerated dynamic transformation from harder α phase into β phases. Notably, full β microstructure was observed in coarse grain samples at strain of 1.2, while retained α phase was observed in fine α grain specimens. In the views of thermodynamics and crystallographic analysis, the in-depth mechanisms of dynamic transformation were further investigated.