Chia-Jung Hu

Synthesis of Mg-Cu-Ti based amorphous alloys by mechanical alloying technique

This study examined the behavior of amorphous Mg-Cu-Ti based alloys synthesized by mechanical alloying technique. The microstructural evolution during mechanical alloying of the mixed powders was investigated by both X-ray diffraction and scanning electron microscopy. The phase stabilities of the as-milled powders were analyzed by the differential scanning calorimetry. Differential thermal analysis on the 12-h milled ternary Mg-Cu-Ti powders reveals endothermic peaks in the Mg40Cu40Ti20 and Mg40Cu35Ti25 at around 550 K and no such a peak is observed in the another ternary compositions. For the ternary and quaternary Mg-Cu-Ti based alloys, several amorphous powders were found to exhibit a wide supercooled liquid region before crystallization. The temperature interval of the supercooled liquid region defined by the difference between Tg and Tx, i.e. Tx-Tg, is 54K for Mg40Cu40Ti20, 53K for Mg40Cu35Ti25, 51K for Mg40Cu35Ti23B2, 62 K for Mg40Cu35Ti21Sn4, and 74 K for Mg40Cu35Ti21Ni5. As the results demonstrated, the small addition of Sn and Ni significantly improved the glass forming ability of the Mg-Cu-Ti based amorphous alloys.

Effect of composition and heat treatment on the phase formation of mechanically alloyed Cr-B and Mo-B powders

Blended elemental Cr-B and Mo-B powders in atomic ratio of 67:33, 50:50, and 20:80 were subjected to mechanical alloying up to 60 h and subsequent heat treatment to investigate effect of composition and heat treatment on the phase formation of Cr-B and Mo-B powders. It was studied by X-ray diffraction and differential thermal analysis. Mechanical alloying these powder mixtures for 60 h leads essentially to a amorphous structure except for the Mo20B80 powder, which creates a partially amorphous MoB4 structure. Annealing at lower temperatures relieves the strains cumulative in the milled powders and creates no new phase. The structures obtained after annealing the milled powders at higher temperature vary and depend on the overall composition of the powder mixtures. Annealing the milled Mo-B powders having greater Mo content ends up with a dissociation reaction at higher temperature.

Preparation of SiC/MoSi2 composites by mechanical alloying and its fracture properties

Ball milling was utilized to make composite powders from either elemental Mo, Si, C powders or compound MoSi2 and SiC powders. The milled powders were hot-pressed in a vacuum furnace to produce 10 to 30 vol.% SiC-reinforced MoSi2 composites. The influence of microstructure on the indentation fracture toughness of the fabricated SiC/MoSi2 composites was investigated. The SiC particles present in the consolidated compound composite are larger than those in the elemental composite while the pores observed in the former composite are fewer than in the latter. The overall values of fracture toughness measured on the compound composites are higher than those of elemental composites. The major reason for the greater toughness of compound composites is due to the larger SiC particles and fewer pores in these materials.

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, − 50, − 100 and − 150 °C. It was found that plane-stress,KC, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestKC value at all temperatures among the three alloys. TheKC values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,KIC values of the F2 alloy at − 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at −150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aKC value of 106 MPa m1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.