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Abstract: The effect of bismuth (Bi) addition on primary Mg2Si phases in Mg–Si alloys was investigated. It is found that proper amount of Bi in the alloys is efficient in refining the microstructure of Mg–Si alloys. According to the present experiment, the optimal modification effect is obtained when the Bi content in the alloy is 0.5%. The size of primary Mg2Si is significantly decreased to 15μm or less, and its morphology is changed from coarse dendritic shape to polyhedral shape. However, when the Bi content is increased to or exceeds 0.8%, it is found that many primary Mg2Si become coarse again. Moreover, a small amount of Mg–Bi intermetallic compounds in the form of Mg3Bi2 are formed in the eutectic microstructure of the alloys with 0.5, 0.8 and 1%Bi. [Copyright &y& Elsevier]

Abstract: The TiB2 particulate reinforced magnesium matrix composites (MMCs) were fabricated by adding a TiB2–Al master alloy processed via self-propagating high-temperature synthesis (SHS) reaction of Al–Ti–B systems with the different molar ratio of Ti to B (n Ti:n B =1:2 and n Ti:n B =1:2.5, respectively) to molten magnesium and using the semisolid slurry stirring technique. When the molar ratio of Ti to B is 1:2 in TiB2–Al master alloy, the TiB2–Al master alloy and TiB2p/AZ91 composites contained a certain amount of transient TiAl3 phase; however, when the molar ratio of Ti to B is 1:2.5, the TiAl3 phase was almost completely eliminated and the distribution of TiB2 particulates generally appeared to be more homogeneous throughout the magnesium matrix. [Copyright &y& Elsevier]

Abstract: The reactive infiltration synthesis of steel matrix composite reinforced with in situ TiB2–TiC particulates by the self-propagating high-temperature synthesis (SHS) reaction of the Ni–Ti–B4C system during casting was investigated. The as-cast microstructures of the in situ processed composites reveal a uniform distribution of TiB2–TiC particulates with a fine size of ∼5.0μm. Although, microstructural characterization shows the absence of macro-pores and blowholes, the Ni content in the system has a significant effect on the presence of micro-porosity in the composites. Moreover, the results show that the densities of the composites are lower than those of medium carbon steel and high-speed (HS) steel. Although, the hardness values of the composites are significantly higher than that of medium steel and lower than that of HS steel, the wear resistance values of the composites are higher than those of medium steel and HS steel. [Copyright &y& Elsevier]

Abstract: The modification effect of KBF4 on the primary and eutectic Mg2Si in Mg–Si alloy is better than that of K2TiF6. This may be the reason of the presence of B in melts. However, the modification effect of KBF4 is greatly weakened by the simultaneous addition of K2TiF6 due to the decreasing of B content in the melts by the formation of TiB and TiB2. [Copyright &y& Elsevier]

Magnesium metal matrix composites (MMCs) reinforced with various fractions of 10, 15 and 20 vol.% B4C particulates fabricated by powder metallurgy (P/M) technique were investigated. Microstructure characterization of the composites revealed necklace distribution of B4C particulates in the matrix material and the presence of minimal micro-porosity. The X-ray diffraction (XRD) showed the formation of MgO and MgB2 in B4C/Mg composites. Moreover, the results revealed that the hardness and wear resistance of the composites were higher than those of as-cast Mg ingot and increased with increasing amount of B4C particulates from 10 to 20 vol.%. [Copyright &y& Elsevier]

Abstract: In situ TiB2 particulate locally reinforced steel matrix composites were successfully fabricated by self-propagating high-temperature synthesis (SHS) reactions in a Fe–Ti–B system containing 10, 30, 50 and 70wt.% Fe, respectively, in the molten high chromium alloy steel during casting. The locally reinforced regions of the Fe–TiB2 composites with different Fe contents in the Fe–Ti–B system consist of three different regions, i.e. the Fe-rich region, the fine TiB2 particulate region and the larger TiB2 particulate region. The portion of the Fe-rich region decreased and the larger TiB2 particulate region became unclear with the increase in Fe content in the Fe–Ti–B system. In addition, only a small amount of micropores is found in the fine TiB2 particulate region of the Fe–TiB2 composite synthesized in a 70Fe–Ti–B system, and the Fe content has obvious effect on the wear resistance of the locally reinforced region of the Fe–TiB2 composites. [Copyright &y& Elsevier]

Abstract: The modification of in situ formed Mg2Si in Mg–Si alloys with yttrium (Y) was investigated. As-cast microstructure analysis of Mg–5wt.% Si alloys indicates that all the primary Mg2Si crystals are surrounded by a layer of α-Mg dendritic halos, which is again surrounded by the eutectic Mg2Si+Mg structures. When the Y content is 0.1 or 0.4wt.%, the sizes of primary and eutectic Mg2Si do not significantly reduce in Mg–5wt.% Si alloys. When the Y content is 0.8wt.%, the mean size of primary Mg2Si decreases from more than 100μm to about 30μm or less; furthermore, the Chinese script type Mg2Si particle also exhibits modified morphology as a fine fiber. When the Y content is further increased to 1.2wt.%, the primary Mg2Si becomes coarser again and even larger than that in the unmodified alloy; however, the eutectic Mg2Si still exhibits modified morphology as a fine size. The size and amount of Mg–Y point compounds are increased with the increasing of Y content in the alloys. In addition to distributing in α-Mg phase, the Mg–Y point compounds are also present in primary Mg2Si dendritic crystal. In the unmodified alloy, the coarse primary Mg2Si is formed by the preferred growth that occurs at the tips of branches; however, when the alloy is modified with 0.8wt.% Y, the preferred growth manner of the primary Mg2Si was depressed, while another type of growth, isotropic growth manner, is enhanced in the modified alloy. [Copyright &y& Elsevier]

Abstract: Magnesium metal matrix composites (MMCs) reinforced with 10, 20 and 30 vol.% TiB2 particulates, respectively, were fabricated by powder metallurgy. The microstructure, porosity, hardness and abrasive wear behavior of the composites were evaluated. Microstructural characterization of Mg MMCs showed generally uniform reinforcement distribution. As compared with pure Mg, the hardness (HB) values of Mg MMCs reinforced with 10, 20 and 30 vol.% TiB2 particulates were increased by 41%, 106% and 181%, respectively. The abrasive wear tests showed that the wear resistance of Mg MMCs is increased with the increasing of the reinforcement volume fraction. This was due to the strong particulate-matrix bonding and high hardness of the TiB2 particulate. [Copyright &y& Elsevier]

Partial remelting behaviors of Mg–11Al–0.5Zn alloy and TiB2/Mg–11Al–0.5Zn composite are investigated by the isothermal holding at the temperatures of 510 and 545 °C. The result shows that, although, the TiB2 addition has no influence on the remelting characteristics of eutectic and liquidus temperatures of the composite, it has a great effect on the remaining liquid located at the grain boundary. With a processing temperature of 545 °C, the remaining liquid is dramatically decreased in the composite, compared with that in the unreinforced matrix alloy. Furthermore, the solid α-Mg grain size in the composite does not significantly reduce compared with that in the matrix alloy; however, the microhardness of the α-Mg grain is increased in the partial remelting microstructure in the composite. Although most of the TiB2 particulates are present in the remaining liquid phases distributed at the grain boundaries, some liquid islands are entrapped inside α-Mg grains. As a consequence, the TiB2 particulates can eventually reside within the grains after complete solidification. However, it is difficult that TiB2 particulate is directly incorporated (engulfed) into the Mg–11Al–0.5Zn matrix alloy during the regular solidification process. Furthermore, the DTA, SEM and XRD results reveal that no reaction occurs between the TiB2 particulate and matrix alloy during the partial remelting process, which indicates that TiB2 particulate is thermodynamically stable at the Mg–11Al–0.5Zn matrix alloy. [Copyright &y& Elsevier]

A TiB2 particulate reinforced magnesium matrix composite (MMCs) was fabricated by adding a TiB2–Al master alloy processed via self-propagating high-temperature synthesis (SHS) reaction in Al–Ti–B system into molten magnesium and using the stir casting technique. The results showed that the TiB2–Al master alloy and TiB2/Mg composite contained a certain amount of transient TiAl3 phase. When the Al–Ti–B system was incorporated with an appropriate C, TiB2 and TiC reinforcing phases were simultaneously synthesized and the TiAl3 phase was almost completely eliminated. [Copyright &y& Elsevier]