Materials Science Forum Vols. 654-656

Abstract: Bulk metallic glasses (BMGs) and high entropy alloys (HEAs) have unique structures at the length scales of micro- and nano-metre, and exhibit unique properties, which make them potential materials for structural applications. The tensile ductility of dendrite/BMG composites can be greatly improved by Bridgman solidification. The BCC structured HEA with a composition of Ti0.5AlCoCrFeNi exhibits ultrahigh fracture strength which is competitive to most of the BMGs, moreover, the strength can be sustained at high temperatures. The phase changes of HEAs are closely related to the atomic packing efficiency (APE).

Abstract: The RE4Fe72B22M2 (RE=Y, Dy or Nd; M=Nb or Ta) alloys in ribbon and rod forms were prepared by melt spinning and water cooling copper mold casting methods, respectively. Amorphous structure was observed in all ribbon samples, but the results from rod samples indicated that the composition has an important effect on the glass formability. All rod samples with RE=Y or Nd were partly amorphous, while bulk metallic glass (BMG) rod with a diameter of 2 mm was obtained for the Dy4Fe72B22Nb2 alloy. The rare earth and doping elements on the glass formability have been discussed. The effects of post-annealing on the microstructure and magnetic properties of these alloys have been investigated. The results presented here provide a viable method to get fully dense bulk hard magnets.

Abstract: We investigated characteristics of Ti-Ni-Zr amorphous thin films before and after annealing, in order to search for novel Ti-Ni-Zr thin film metallic glasses (TFMGs) that exhibit a shape memory effect after crystallization. We found that the Ti42Ni47Zr11 TFMG exhibited a stable shape memory effect after crystallization. This alloy showed a glass transition with the glass transition and crystallization temperatures of 703 and 760 K, respectively. Moreover, this alloy showed a one-step martensitic phase transformation of B2 to B19', when annealed at 973 K for 3.6 ks. The martensitic phase start temperature and reverse martensitic phase start temperature were 296 and 352 K, respectively. In addition, this alloy showed a recovery strain of about 2.4%.

Abstract: Zr-based bulk glassy (BG) alloys show high tensile strength and a high Charpy impact value. In this study, the free volume changes for the hypoeutectic Zr60Cu30Al10 BG alloy during isothermal annealing below glass transition temperature (Tg) have been investigated by positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements. The positron lifetime of hypoeutectic and eutectic BG alloys is almost the same, although the atomic volume ratio, estimated by the density for the eutectic BG alloy is different from that for the hypoeutectic BG alloy. The CDB spectra show a marked difference between eutectic and hypoeutectic BG alloys; that is, the spectrum of the hypoeutectic BG alloy is closes to that of Zr metal than that of the eutectic BG alloy. This result exhibits that the elemental fraction of Zr atoms around free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. The CDB ratio profiles for the hypoeutectic BG alloy during annealing shows no appreciable change. The same trend was observed in the eutectic BG alloy.

Abstract: The microstructure of Fe61Co10Zr2,5Hf2,5Nb2W2B20 amorphous alloy in the form of ribbons obtained by classical melt spinning and plates obtained by an induction suction method were investigated using X-ray diffraction. The type of structural defects were studied by analysis of the magnetization characteristics near ferromagnetic saturation of the sample. It was shown that the presence of structural defects is strongly correlated with sample thickness and production process. It was shown that ribbons with cooling rate between 105-106 K/s have point type defects, wires obtained with lower cooling rate between 101-102 K/s, have linear type defect (quasi-dislocation dipoles). crystallization.

Abstract: Sheets of a Mg60Cu29Gd11 alloy were produced by twin roll casting with all operational variables, except roll speed, being kept constant. As a function of the roll speed, the structure of the as-cast sheet changed from being crystalline to fully amorphous and then back to crystalline. Through careful selection of the casting speed that is suitable for the selected alloy system and with which the exit temperature of the sheet remains within the supercooled liquid region, a malleable sheet with no surface defects is produced. This work shows that twin roll sheet casting is a viable process for the production of magnesium-based bulk amorphous sheet in a continuous manner and on an industrial scale.

Abstract: The application of moderate cooling rates to metal alloys of certain composition can generate metals that exhibit an amorphous microstructure on a bulk scale. This phenomenon is related to the avoidance of the nucleation of the competing crystalline phases associated with the alloy during solidification. This work describes the devitrification behaviour of the bulk glass forming Mg60Cu29Gd11 system through the use of a number of analytical techniques including DSC, laser confocal microscopy, SEM and XRD. Attention is drawn to the correlation between the more common analytical techniques and the observation of phase transformations on the surface of the metal, evident using a laser scanning confocal microscope fitted with a heating stage.

Abstract: We investigated consolidation behavior of gas-atomized Cu50Zr45Al5 metallic glassy alloy powders by a spark plasma sintering (SPS) process. Density of the sintered samples increased with an increase in sintering temperature. The nearly full density samples without crystallization could be attained by the SPS process at sintering temperature of 693 K under pressure of 600 MPa. The produced samples exhibited high-strength and met large-size requirement. The SPS process makes it possible to fabricate the large-size bulk metallic glasses without limitation of dimensions and alloy system.

Abstract: Some amorphous Fe-Cr-P-C coating films having high hardness and high corrosion resistance have been produced by a newly developed thermal spraying technique. In order to control the temperatures of the powder particles in the flame spray and the substrate, a newly developed cylindrical nozzle, with external cooling nitrogen gas, was mounted to the front end of the thermal spray gun. Fe70Cr10P13C7 films with various external cooling gas velocities between 20 m/s and 40 m/s exhibited entire amorphous structure without oxides and/or unmelted particles. Corrosion-resistance of the films was observed in immersion tests using various corrosive liquids.

Abstract: Bulk silicon is the material for microelectronics fabrication such as memory device. However, its optical properties are poor due to its indirect band gap. Since the photoluminescence from porous silicon at room temperature was first reported by Canham, silicon nanostructures have attracted considerable interest due to their potential applications in optoelectronic devices such as Si-based LEDs, solar cell. In the present study, the nanocrystalline silicons were synthesized by non-thermal plasma from gas phase. And Nitrogen plasma was applied to reduce the nonraidative recombination center which related to the emission efficiency. To confirm the effect of nitrogen plasma, the compositional, electrical and optical analysis of silicon nanocryatals layer were also investigated.