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In this work, the thermal stability of nanocrystalline (NC) AZ31/TiB2 magnesium matrix composites was investigated, while the microstructure evolution and mechanical properties were analyzed. The results indicated the AZ31/TiB2 still maintained NC structure after annealing at 350 °C for 180 min. Even at the high annealing temperature of 400 °C and 450 °C for 180 min, their average grain size just reached about 124 nm and 155 nm, indicating excellent thermal stability. The TiB2 particles with sub-micron size uniformly distributed in Mg matrix had no change in size and no reaction with matrix during the annealing treatment. Due to the strong Zener pinning effect of TiB2 particles, the grain growth of Mg matrix at high temperature was effectively inhibited. Meanwhile, the solution and precipitation behavior of Al atoms were completed in a short time, due to the existence of many grain boundaries and structural defects. By calculation, the grain growth kinetics was described by the kinetics equation D 8 - D 0 8 = k t and the activation energy Eg for grain growth was 131.6 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Due to their excellent thermal stability, the decrease in both compressive yield strength and ultimate compressive strength was no more than 12.2% after annealing treatment. Even annealing at 450 °C for 180 min, the CYS and UCS of the samples were still above 283 MPa and 295 MPa, respectively. [ABSTRACT FROM AUTHOR]

Mechanical milling was used to prepare nanocrystalline AZ31 magnesium (Mg) alloy with titanium (Ti) addition. During milling, the crystallite size of the AZ31 Mg matrix decreased steadily with the increase of milling time. After milling for 110 h, the crystallite size of the Mg phase of the alloy with 18 wt.% and 27 wt.% Ti additions was refined to 86 nm and 66 nm respectively, leading to the formation of a nanocrystalline Mg matrix microstructure. Meanwhile, the size of Ti particles added in the alloy also decreased gradually with the increase of milling time, and the Ti dispersions presented a positive effect on the refinement of the Mg matrix grains. By milling for 110 h, the average size of Ti particles was refined to less than 1 μm in the AZ31 Mg alloy with 18 wt.% Ti addition. After mechanical milling, the as-milled powders were cold pressed into green compacts, and the mechanical properties were characterized by Vickers' hardness and uni-axial compressive tests. It was found that the micro-hardness of mechanically milled AZ31 Mg alloy with 27 wt.% Ti addition reached about 147 Hv, which was almost three times larger than that of the as-received AZ31 Mg alloy. Meanwhile, the yield strength of mechanically milled Ti-added AZ31 Mg alloy was seen to be as high as 293 MPa at room temperature and 60 MPa at 300 °C, respectively. [ABSTRACT FROM AUTHOR]

The method of mechanical milling was applied to synthesize nanocrystalline Mg x Ti 100 − x ( x = 95, 90, 85) composite powders. The results reveal that mechanical milling is an effective method for preparing Mg–Ti composite powders, which consist of nanocrystalline Mg matrix and fine dispersed Ti particles. Moreover, the microstructure evolution and morphology of the as-milled powders were observed, and the corresponding mechanisms were also discussed. After milling for 60 h, the crystallite size of the matrix Mg in Mg x Ti 100 − x ( x = 95, 90, 85) composite powders was refined to 105 nm, 84 nm, and 76 nm, respectively. Meanwhile, the average size of the dispersed Ti particles in Mg x Ti 100 − x ( x = 90) composite powders was refined to about 1 μm. Based on the XRD data, the solid solubility of Ti in Mg was calculated by using the Vegard's law. For all these Mg–Ti composite powders, the solid solubility of Ti in Mg seemed to be closely related with the milling time and the content of Ti. In the 60 h-milled Mg x Ti 100 − x ( x = 95, 90, 85) composite powders, the solid solubility of Ti in Mg was estimated to be 0.56 at.%, 1.32 at.% and 2.35 at.%, respectively. [ABSTRACT FROM AUTHOR]

In this work, vanadium particles (VP) were utilized as a novel reinforcement of AZ31 magnesium (Mg) alloy. The nanocrystalline (NC) AZ31–VP composites were prepared via mechanical milling (MM) and vacuum hot-press sintering. During the milling process, the presence of VP contributed to the cold welding and fracture mechanism, resulting in the acceleration of the milling process. Additionally, increasing the VP content accelerated the grain refinement of the matrix during the milling process. After milling for 90 h, the average grain size of AZ31-X wt % Vp (X = 5, 7.5, 10) was refined to only about 23 nm, 19 nm and 16 nm, respectively. In the meantime, VP was refined to sub-micron scale and distributed uniformly in the matrix, exhibiting excellent interfacial bonding with the matrix. After the sintering process, the average grain size of AZ31-X wt % VP (X = 5, 7.5, 10) composites still remained at the NC scale, which was mainly caused by the pinning effect of VP. Besides that, the porosity of the sintered composites was no more than 7.8%, indicating a good densification effect. As a result, there was little difference between the theoretical and real density. Compared to as-cast AZ31 Mg alloy, the microhardness of sintered AZ31-X wt % VP (X = 5, 7.5, 10) composites increased by 65%, 87% and 96%, respectively, owing to the strengthening mechanisms of grain refinement strengthening, Orowan strengthening and load-bearing effects. [ABSTRACT FROM AUTHOR]

In this work, nanocrystalline AZ31 magnesium alloy powders, reinforced by submicron TiB2 particles, were prepared via mechanical milling. It was found that TiB2 particles stimulated the fracture and welding of AZ31/TiB2 powders, leading to the acceleration of the milling process. Meanwhile, the TiB2 particles were refined to submicron-scale size during the milling process, and their distribution was uniform in the Mg matrix. In addition, the matrix was significantly refined during the milling process, which was also accelerated by the TiB2 particles. The formation of grain boundary segregation layers also led to the weakened TiB2 peaks in the XRD patterns during the mechanical milling. The grain sizes of AZ31–2.5 wt % TiB2, AZ31–5 wt % TiB2 and AZ31–10 wt % TiB2 powders were refined to 53 nm, 37 nm and 23 nm, respectively, after milling for 110 h. Under the combined effect of the nanocrystalline matrix and the well-dispersed submicron TiB2 particles, the AZ31/TiB2 composites exhibited excellent micro-hardness. For the AZ31–10 wt % TiB2 composite, the micro-hardness was enhanced to 153 HV0.5. [ABSTRACT FROM AUTHOR]

AZ61 Mg and AZ61 Mg with 10 at. %Ti addition (AZ61-10 at. %Ti) were mechanically milled and a comparative study on microstructure evolution was performed. During mechanical milling, Mg grain refining/nanocrystallization and the dissolving of Mg 17 Al 12 precipitates, with the formation of Al supersaturated Mg solid solution, were observed for both materials due to mechanical milling. As analyzed by density dislocation, Ti dispersions hinder the movement of dislocations and grain boundaries and accelerate the dislocation pile-up. Therefore, Ti addition accelerated Mg grain refining, meanwhile, the ultimate Mg grain (43.47 nm) with Ti dispersions was smaller than that (69.19 nm) without Ti dispersions based on X-ray diffraction and transmission electron microscopy results. As the scanning electron microscope and X-ray diffraction exhibiting, the dissolving of Mg 17 Al 12 precipitates was expedited by Ti dispersions and the dissolving of Al in Ti phase was verified by high angle annular dark field. Moreover, during mechanical milling, Ti particulates entranced Mg particles gradually by the welding and cracking effect. After mechanical milling, Ti phase, dispersed in Mg matrix, were smashed to approximately 274.0 nm and Ti supersaturated Mg solid solution was synthesized with the solid solubility being 1.07 at. %. [ABSTRACT FROM AUTHOR]

Nanocrystalline AZ61 powders, dispersed with submicron/nanometer Ti particulates, were mechanically milled and a comparative study on thermal stability and mechanical properties was performed. After mechanical milling, Ti dispersions were cracked to submicron/nanometer particulates and Mg grains were refined to nanocrystalline. As illustrated by high-resolution transmission electron microscopy, coherent lattice and semi-coherent lattice interfaces between magnesium and titanium were achieved. Based on high angle annular dark field and corresponding energy dispersive spectrometer maps, Al dissolved into Ti particulates after mechanical milling. Moreover, the emergence of Ti 3 Al phase was verified by X-ray diffraction during mechanical milling and heat treatment. According to heat treatment results, this nanocrystalline magnesium composite exhibits a preeminent thermal stability among nano-structured magnesium alloys. After annealed at 673 K for 600 min, the composite still remained nanocrystalline with average grain size being 68.6 nm. Due to such microstructure, the composite revealed prominent mechanical property with the hardness being 146.6 HV. Simultaneously, the hardness of annealed composite remained high, with the value being 134.7 HV, which could ascribe to the excellent thermal stability. [ABSTRACT FROM AUTHOR]

The nanocrystalline AZ61 containing 10 at.%Ti composite powders were synthesized by mechanical milling. The results indicate that mechanical milling is available to prepare AZ61-10 at.%Ti composite powders with nanocrystalline Mg matrix, fine dispersed Ti particles and supersaturated solid solutions of Al and Ti in Mg matrix. The microstructure and morphology evolution of magnesium matrix and Ti particles were observed by SEM and TEM. Meanwhile the solid solubility and grain size evolution of Mg matrix were calculated by XRD, with the corresponding mechanism being analysed. Specifically, the Mg grain and the Ti particles were refined with the increasing milling time. The Mg grain was refined to approximately 58.2 nm and the Ti particle size was reduced to around 558 nm after 110 h mechanical milling. From XRD results of the as-milled powders, the supersaturated solid solution of Ti and Al in Mg matrix were detected. Generally, the Al might be completely dissolved into the Mg matrix after 110 h mechanical milling. Correspondingly, the solid solubility of Ti in Mg matrix was estimated to be 3.18 at.%. The micro-hardness of as-milled composite powders increased with the increasing milling time. The micro-hardness could reach 120.42 HV being 2.1-fold the micro-hardness of raw AZ61 alloy. [ABSTRACT FROM AUTHOR]