Effect of Cu addition on microstructural evolution and hardening of mechanically alloyed Al−Ti−O in-situ composite

The microstructure formation and strengthening of an Al−5wt.%TiO₂ composites with additions of 5 wt.% Cu and 2 wt.% stearic acid (as a process control agent, PCA) during mechanical alloying and subsequent thermal exposure were studied. The powder composites were prepared by high-energy ball milling for up to 10 h. Single line tracks of the powders were laser melted. Optical and scanning electron microscopy, XRD analysis and differential scanning calorimetry were used to study microstructural evolution. The results showed that the Cu addition promotes an effective mechanical alloying of aluminum with TiO₂ from the start of milling, resulting in higher microhardness (up to HV 290), while the PCA, on the contrary, postpones this process. In both cases, the composite granules with uniform distribution of TiO₂ particles were formed. Subsequent heating of mechanically alloyed materials causes the activation of an exothermic reaction of TiO₂ reduction with aluminum, the start temperature of which, in the case of Cu addition, shifts to lower values, that is, the transformation begins in the solid state. Besides, the Cu-added material after laser melting demonstrates a more dispersed and uniform structure which positively affects its microhardness.     

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al−Zn−Mg−Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al₃(Sc,Zr) and Al₃Zr dispersoids. The volume of Al₃Zr particles is less than that of Al₃(Sc,Zr) particles, but the distribution of Al₃(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.     

Simulation on microstructure evolution and mechanical properties of Mg−Y alloys: Effect of trace Y

The influence of trace Y on the microstructure evolution and mechanical properties of Mg_100?xY_x (x=0.25, 0.75, 1.5, 3, 4, 5, at.%) alloys during solidification process was investigated via molecular dynamics (MD) simulations. The results show that the Mg_100?xY_x alloys are mainly characterized by a face-centered cubic (FCC) crystal structure; this is different from pure metal Mg, which exhibits a hexagonal close packed (HCP) structure at room temperature. Among these alloys, Mg_99.25Y_0.75 has a larger proportion of FCC cluster structures, with the highest fraction reaching 56.65%. As the content of the Y increases up to 5 at.% (Mg₉₅Y₅ alloy), the amount of amorphous structures increases. The mechanical properties of the Mg_100?xY_x alloys are closely related to their microstructures. The Mg_99.25Y_0.75 and Mg₉₇Y₃ alloys exhibit the highest yield strengths of 1.86 and 1.90 GPa, respectively. The deformation mechanism of the Mg?Y alloys is described at the atomic level, and it is found that a difference in the FCC proportion caused by different Y contents leads to distinct deformation mechanisms.     

Microstructural evolution and microsegregation in directional solidification of hypoeutectic Al−Cu alloy: A comparison between experimental data and numerical results obtained via phase-field model

The objective of the work is focused on predictions of microsegregation, solidification speed, dendritic arm spacings and dendrite morphology by phase-field model. The numerical results were compared with experimental data. The experimental values for cooling rates and effective partition coefficient were adopted during calculations. The results of microsegregation through phase-field model show excellent agreement with the experimental data. Such excellent agreement is because cooling rates, effective partition coefficient and back-diffusion of solute are considered in the model. For solidification speed, the calculation results show good agreement with the experimental data. Tertiary dendritic arm spacing calculated with phase-field model is compared with experimental data. The results show good agreement between them. The dendrite arm spacing varies with position because high cooling rates are responsible for the refinement effect on microstructure. Finally, two-dimensional simulation produced a dendrite that is similar to that found in the experiment.