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Graphical abstract Highlights • Ultrafine-grained Ti/AZ31 magnesium matrix composites were prepared by powder metallurgy. • AZ31 alloys distributed uniformly by submicron/nanometer sized titanium were obtained. • The strengthening mechanisms for Ti/AZ31 composites at room temperature were analyzed quantitatively. • Outstanding strength at elevated temperature of ultrafine-grained Ti/AZ31 composites was confirmed experimentally. Abstract The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ∼310 nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66 nm to 800 nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341 MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573 K is 82 MPa and ultrafine Ti dispersions are responsible for the enhanced strength. [ABSTRACT FROM AUTHOR]

The nanocrystalline AZ31 magnesium (Mg) alloy with 18 wt% titanium (Ti) addition was isothermally annealed in the temperature range of 300–400 °C for different durations. A study on the thermal stability, including solid solubility evolution, phase transformation, grain growth kinetics and hardness evolution, was carried out. The microstructure evolution at elevated temperatures was investigated by the means of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and high angle annular dark field (HAADF). Based on lattice parameters evolution and TEM observation, it was confirmed that the Ti element precipitated from Mg matrix as Ti phase. After the material being annealed at 400 °C for 300min, as to the original Ti particulates, there is no segregation and growth with the average particulate size being approximately 367 nm, meanwhile, magnesium matrix shows limited grain coarsening with the average grain size increasing from 95 nm to 210 nm. In addition, the grain growth kinetics could be well described by the kinetics equation, D 7 − D 0 7 = k t . The activation energy for grain growth was calculated to be 120 kJ/mol, which is about 28 kJ/mol greater than that of pure Mg. Simultaneously, the stability of mechanical property was investigated by microhardness evolution at different annealing temperatures. It was found that after this material being annealed at 400 °C for 300min, the microhardness remained 105HV, which is two times larger than that of the as-cast AZ31 Mg alloy. [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]

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]

Nanocrystalline Ti/AZ61 magnesium matrix composite, prepared by mechanical milling, was annealed in the temperature range of 573–723 K for various durations. During annealing treatment, diffusion of Al and Ti elements, formation of precipitates and grain growth behavior were observed and the evolution of hardness was obtained. Based on microstructural evolution, the mechanism for the precipitating of Ti and Al from magnesium matrix was analyzed. And the corresponding schematic illustration of microstructural evolution model was proposed. By the observation from high-resolution transmission electron microscopy, it was found that dispersing nanometer sized particulates, precipitated from Ti and Al supersaturated Mg solid solution, pined up magnesium grain boundaries. During annealing treatment, the nano-scaled precipitates together with original submicron Ti particulates hindered the movement of grain boundaries. Thus, the thermal stability for magnesium grains was improved. After annealing treatment at 723 K for 600 min, this magnesium matrix composite with the average grain size of ∼93 nm was still nanocrystalline. In addition, influence of temperature and time on hardness was investigated. After annealing treatment at 723 K for 600 min, the hardness of annealed Ti/AZ61 composite is approximately 116 HV, presenting outstanding thermal stability. [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]