Hong-fei Sun

Mechanical properties and texture evolution during hot rolling of AZ31 magnesium alloy

Abstract Mechanical properties and texture evolutions of the as-rolled AZ31 Mg sheets were investigated. The results show that the grains of the sheets are significantly refined after hot rolling. The mechanical properties of the as-rolled samples are enhanced due to the grain size refinement. The intensity of basal texture decreases with the increase of deformation ratio, and double-peak type basal texture is discovered in the intermediate and large strain hot rolling processes. The formation of the texture is ascribed to the activities of prismatic and non-basal c + a > slips, which is the same as the 30% rolled and 50% rolled samples. The incline of basal planes exerts an effect on the mechanical anisotropy during tension along rolling direction (RD) and transverse direction (TD) at room temperature.

Grain refining and property improvement of AZ31 Mg alloy by hot rolling

Abstract Hot rolling of AZ31 Mg alloy was performed by using as-cast alloy ingot as the starting material. The microstructures and mechanical properties of the as-rolled alloy subjected to various rolling passes were investigated. The results show that the grain size of the alloy can be refined steadily with increasing rolling passes by dynamic recrystallization. With the steady refining of the grain size, both the mechanical strength and the plasticity of the alloy are improved correspondingly. In particular, when the grain size is reduced to about 5 μm after 5 rolling passes, the yield strength, ultimate tensile strength and tensile fracture strain of the alloy are 211 MPa, 280 MPa and 0.28 in the transverse direction, they are 200 MPa, 268 MPa and 0.32 in the rolling direction, respectively.

Microstructures and mechanical properties of pure magnesium bars by high ratio extrusion and its subsequent annealing treatment

Abstract Pure magnesium bars were prepared by two-pass cumulative high ratio extrusion with as-cast Mg as the original material and then the final as-extruded bars were annealed. The effect of extrusion deformation and annealing treatment on the microstructure, mechanical properties and fracture behaviour of the Mg were investigated by optical microscopy (OM), mechanical properties test and scanning electron microscopy (SEM), respectively. The results show that the grain size is obviously refined by the effect of dynamic recrystallization during the extrusion deformation. Thus the room-temperature mechanical properties and fracture behaviour of the material were significantly improved. After the first extrusion, the coarse as-cast grain size was reduced to 35 μm, and the yield strength (YS), ultimate tensile strength (UTS) and elongation of the bar achieved 84 MPa, 189 MPa, and 12%, respectively. After the further extrusion, the YS of as-extruded bar was over 120 MPa; however, the elongation decreased due to work hardening. Finally the grain size of the as-extruded bar was 9-10 μm after annealing treatment, and its YS, UTS and elongation of the bar achieved 124 MPa, 199 MPa, and 10.7%, respectively. The microstructures and mechanical performance of the material were enhanced obviously.

Development of flow stress model for hot deformation of Ti-47%Al alloy

Abstract The hot deformation behavior of a γ-TiAl based alloy (Ti-47%Al, mole fraction) was investigated by isothermal compression tests performed at elevated temperature of 900-1 200°C and strain rate of 0.001–0.02 s −1 . The effect of temperature, strain rate and strain on the flow stress of the alloy was evaluated. The higher the deformation temperature and the lower the strain rate, the smaller the deformation resistance. The stress exponent, n , and the apparent activation energy, Q , were determined as 2.6 and 321.2 kJ/mol by the sine hyperbolic law, respectively. Based on the experimental results by the orthogonal method, a flow stress model for hot deformation was established by stepwise regression analysis. Then the effectiveness of the flow stress model was confirmed by other experimental data different from those experimental data used to establish the model. And it was proved that the flow stress model can well predict the mechanical behavior and flow stress of the alloy during hot deformation.

Corrosion behavior of extrusion-drawn pure Mg wire immersed in simulated body fluid

The corrosion behaviors of extrusion-drawn pure magnesium wire soaked in simulated body fluid (SBF) and 0.9% NaCl solution were studied. The corrosion law of the pure magnesium wire immersed in SBF was investigated by measuring the average corrosion rate and the pH values of the solution after corrosion. It is found that the corrosion mechanism of the pure magnesium wire was pitting after the observation of corrupted surface morphology by scanning electron microscopy (SEM). Moreover, the corrosion law of extrusion-drawing pure magnesium wire in 0.9% NaCl solution implied that the smaller the grain size of the wire is, the better the corrosion resistance exhibits.

Working hardening behaviors of severely cold deformed and fine-grained AZ31 Mg alloys at room temperature

Abstract AZ31 Mg alloy extrusion wires were drawn to a maximum cumulative area reduction of 61% at room temperature, and the 61%–drawn sample was subjected to various annealing treatments for grain refinement. Tensile tests were performed on all the as-drawn and as-annealed samples at a constant strain rate at room temperature. The entire stress—strain curves of each investigated sample were analyzed for the dependence on drawn area reduction and mean grain size. The results show that the cold drawn samples exhibit a constant elastic modulus; however, the stresses are significantly dependent on the deformation level. The corresponding θ—σ curves (where θ is the strain hardening rate, dσ/dɛ) show extended stage II and suppressed stage IV of strain hardening. The recrystallized samples exhibit enhanced yield stress with the grain size refinement and typical hardening stages of polycrystalline metals: II, III, IV and V. Additionally, decreasing in stage IV with refining grain size is observed probably due to the contribution of grain boundaries slipping. The different hardening behaviors demonstrate the various hardening mechanisms between the cold drawn and recrystallized materials.

Characterization of Ti-50%Al composite powder synthesized by high energy ball milling

The Ti-50%Al (molar fraction) composite powder was synthesized by high energy ball milling (HEBM). The characteristics of the composite particles, including surface morphology, structure and components, were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The mechanochemical reaction between Ti and Al gradually took place through intermediate stages during milling, and resulted in the formation of disordered Ti/Al phases. Dissolution of Al in Ti (after 9 h milling), nanocrystalline Ti/Al solid solution and high density of dislocations were caused by HEBM. The element distribution of the mechanically alloyed Ti-50%Al composite powder was close to the initial composition, but the distribution was inhomogeneous.

Microstructure stability of cold drawn AZ31 magnesium alloy during annealing process

Abstract The microstructural evolution of heavily cold drawn AZ31 magnesium alloy wires was investigated during a wide range of annealing temperature from 200 to 450°C. The results show that the mean grain size of the as-annealed material is sensitive to the annealing temperature and cold drawn area reduction. Upon annealing symmetrical grain growth takes place in the AZ31 wires of cold drawn area reduction 12.2% under all the investigated annealing conditions, while three different stages of grain size refinement, normal grain growth and abnormal grain coarsening occur gradually in the materials with cold drawn area reduction 23.0%–60.5% with annealing temperature increasing. The increase of cold drawn reduction leads to the decrease of critical annealing temperatures for the three periods, and also enhances the start of abnormal growing grains preferentially in the severely drawn materials from the outer surface where larger amount of shear deformation constitutes the driving force for growth.

Structure and morphology of Ti-Al composite powders treated by mechanical alloying

Abstract The evolution in microstructure and composition of the milled Ti-Al composite powder with different milling time were investigated. It shows that with the milling time increasing, the initial powder underwent a successive change in its morphology from a flattened shape (2 h) to a fine, equiaxed and uniform one (above 6 h). The milled Ti-Al composite powder was nanocrystalline with the average crystallite size of about 17 nm after milling for 8 h. The evolution mechanism of Ti-Al composite powder was elucidated. The Ti(Al) solid solution is formed through a gradual and progressive solution of Al into Ti lattice. By differential thermal analysis on the ignition temperature of the reaction between Ti and Al as a function of milling time, it indicates that mechanical milling of the powders significantly lowered the ignition temperature of the reaction by refining its Ti-Al composite structure.

Characterization of TiAl-based alloy with high-content Nb by powder metallurgy

Abstract Powders with nominal composition of Ti-45Al-10Nb (mole fraction, %) were prepared by mechanical ball milling and were then consolidated by hot-pressed sintering in vacuum atmosphere. The microstructures of 12 h-milling composite powders and sintered bulk materials were characterized by OM, XRD, SEM and EDX. The results showed that the composite powders were completely homogeneous, and fine-grained Ti/Al/Nb composite powders were made. The microstructure of the consolidated alloy was composed of fine equiaxed grains with large pure Nb particles. And the results also indicated that the compressive strength and ductility of the sintered TiAl-based bulk material at RT could be improved effectively by adding 10% Nb element. The yield strength (σ 0.2 ) and fracture strength (σ b ) of the alloy were 842 MPa and 1314 MPa, respectively, and the compressive ductility (δp) was 12.4%.