Effects of Alloying Elements on the Formation of Core-Shell-Structured Reinforcing Particles during Heating of Al–Ti Powder Compacts

Abstract

To prepare core-shell-structured Ti@compound particle (Ti@compoundp) reinforced Al matrix composite via powder thixoforming, the effects of alloying elements, such as Si, Cu, Mg, and Zn, on the reaction between Ti powders and Al melt, and the microstructure of the resulting reinforcements were investigated during heating of powder compacts at 993 K (720 °C). Simultaneously, the situations of the reinforcing particles in the corresponding semisolid compacts were also studied. Both thermodynamic analysis and experiment results all indicate that Si participated in the reaction and promoted the formation of Al–Ti–Si ternary compounds, while Cu, Mg, and Zn did not take part in the reaction and facilitated Al3Ti phase to form to different degrees. The first-formed Al–Ti–Si ternary compound was τ1 phase, and then it gradually transformed into (Al,Si)3Ti phase. The proportion and existing time of τ1 phase all increased as the Si content increased. In contrast, Mg had the largest, Cu had the least, and Si and Zn had an equivalent middle effect on accelerating the reaction. The thicker the reaction shell was, the larger the stress generated in the shell was, and thus the looser the shell microstructure was. The stress generated in (Al,Si)3Ti phase was larger than that in τ1 phase, but smaller than that in Al3Ti phase. So, the shells in the Al–Ti–Si system were more compact than those in the other systems, and Si element was beneficial to obtain thick and compact compound shells. Most of the above results were consistent to those in the semisolid state ones except the product phase constituents in the Al–Ti–Mg system and the reaction rate in the Al–Ti–Zn system. More importantly, the desirable core-shell structured Ti@compoundp was only achieved in the semisolid Al–Ti–Si system.

Core-Shell-Structured Particle Reinforced A356 Matrix Composite Prepared by Powder-Thixoforming: Effect of Reheating Temperature

“…As the temperature rises, the primary particles are gradually separated by the increased liquid phase, i.e., the primary particles are separated by the 3Ss ( Figure 1 b). During partial remelting, the size decrease of primary particles from their partial melting and the size increase from mergence and/or Ostwald ripening are in a dynamic competition status [ 13 , 14 ]. The present results indicate that the former factor is smaller than the latter one, and thus, the primary particle size continuously increases.…”

“…Our previous investigation on the 2024Al-Ti system proposed that stress reached 15.89 GPa when a 2 μm Al 3 Ti shell formed around a 9.28 μm Ti powder, which was higher than the theoretical strength of Al 3 Ti phase (14.4 GPa) [ 13 ]. In addition, the difference in volume expansion ratio was very large for forming different compounds, i.e., 2.55 for Al 3 Ti, 1.8 for τ1 phases (according to the composition of Al 5 Si 12 Ti 7 ) and a value varied between 2.55 and 1.8 for (Al,Si) 3 Ti with its composition [ 14 ]. That is, the generated stress in (Al,Si) 3 Ti phase is larger than that in τ1 phase.…”

“…The authors investigated the microstructural evolutions of Ti-A356 and Ti-2024 compacts during partial remelting, and the effects of alloying elements such as Si, Cu, Zn, and Mg on the formation of the Ti@Al 3 Ti p [ 12 , 13 , 14 ]. The results indicated that Si element participated in the reaction between Ti powders and Al melt, and an ideal core-shell-structured Ti@(Al-Si-Ti) p with a thick and compact Al-Si-Ti compound shell was obtained in A356 alloys after being properly heated at semisolid temperatures.…”

Core–Shell Particle Reinforcements—A New Trend in the Design and Development of Metal Matrix Composites

“…The in situ formation of core–shell reinforcements has been realized by a number of methods, including solid state sintering of powder mixtures under pressure [ 11 , 12 , 13 , 14 ], solid state sintering under pressure followed by hot deformation processing [ 15 ] (a combination with pre-heating [ 16 ] or post-deformation annealing [ 9 ] is possible), sintering of previously obtained metal@metal core–shell particles with an additional amount of the powder of the matrix [ 21 ], a solid state reaction between metallic rods and the surrounding matrix [ 19 , 20 ], thixoforming [ 22 , 23 , 24 ], friction stir processing followed by annealing [ 25 ], modification of a pre-existing skeleton followed by infiltration [ 26 ], a reaction between solid particles with a metallic melt [ 27 ] and a synthesis in a metallic melt from the added reactants [ 18 ].…”

“…An advantage of Ti@Al 3 Ti core–shell particles over Al 3 Ti particles is that both the soft Al matrix and the Ti core hinder the propagation of cracks nucleated in the Al 3 Ti shell, which delays the fracture of the composite. Chen et al [ 23 ] noted that, if the reinforcing particles have a core–shell structure with a soft Ti core and a hard Al 3 Ti intermetallic shell, they should have a strengthening effect similar to that of monolithic Al 3 Ti particles. However, cracks generated in the Al 3 Ti phase upon loading are constrained within the shell, which makes the cracks shorter than those crossing the monolithic Al 3 Ti particles with the same diameter as the core–shell particles.…”