Journal of Powder Materials

On the Properties and Synthesis of Nanostructured W-Cu alloys by Mechanical Alloying(II) Sintering Behavior of MA NS W-Cu Composite Powders: J Korean Powder Metall Inst. 1998;5(2):89-97.

79 View
0 Download

Abstract PDF: Sintering behavior of nanostructured(NS) W-Cu powders prepared by mechanical alloying (MA) was investigated as a function of sintering temperature. MA NS W-2owt%Cu and W-3owt%Cu composite powders with the crystal size of 20-30 nm were annealed at 900°C, and thermal characteristics of those powders were investigated by DSC. Sintering behavior of MA NS W-Cu composite powders was investigated during the solid-state sintering and the Cu-liquid phase sintering. The new nanosintering phenonenon of MA W-Cu powders at solid-state sintering temperature was suggested to explain the W-grain growth in the inside of MA powders. The sintering densification of MA NS W-Cu powders was enhanced at Cu melting temperature by arrangement of MA powders, i.e., the first rearrangement of MA powders was occurred, and then the rearrangement of W-grains in the sintered parts was also took place during liquid-phase sintering, i.e., the second rearrangement was happened. Due to the double rearrangement process of MA NS W-Cu powders, the high sintered density with more than 96%o was obtained and the fine and high homogeneous state of W and Cu phases was achieved by sintering at 1200 °C.

Synthesis and Microstructural Characterization of Mechanically Milled (Ti_52Al_48)_100-x-xB (x=0,0.5,2,5) Alloys: J Korean Powder Metall Inst. 1998;5(2):98-110.

80 View
0 Download

Abstract PDF: Ti_52Al_48 and (Ti_52Al_48)_100-xB_x(x=0.5, 2, 5) alloys have been Produced by mechanical milling in an attritor mill using prealloyed powders. Microstructure of binary Ti_52Al_48 powders consists of grains of hexagonal phase whose structure is very close to Ti_2Al. (Ti_52Al_48)_95B_5 powders contains TiB2 in addition to matrix grains of hexagonal phase. The grain sizes in the as-milled powders of both alloys are nanocrystalline. The mechanically alloyed powders were consolidated by vacuum hot pressing (VHP) at 1000°C for 2 hours, resulting in a material which is fully dense. Microstructure of consolidated binary alloy consists of gamma-TiAl phase with dispersions of Ti_2AlN and A1_2O_3 phases located along the grain boundaries. Binary alloy shows a significant coarsening in grain and dispersoid sizes. On the other hand, microstructure of B containing alloy consists of gamma-TiAl grains with fine dispersions of TiB_2 within the grains and shows the minimal coarsening during annealing. The vacuum hot pressed billets were subjected to various heat treatments, and the mechanical properties were measured by compression testing at room temperature. Mechanically alloyed materials show much better combinations of strength and fracture strain compared with the ingot-cast TiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.

Effect of Parameters for Dense Bleposit by Plasma: J Korean Powder Metall Inst. 1998;5(2):111-121.

62 View
0 Download

Abstract PDF: Thick and dense deposit of higher than 97% of theoretical density was formed by induction plasma spraying. To investigate the effects of powder morphology on the density of deposit, two different kinds of Yttria-Stabilized-Zirconia powder, METCO202NS (atomized & agglomerated) and AMDRY146 (fused & crushed), were used and compared. After plasma treatment, porous METCO202NS powder was all the more densely deposited and its density was increased. In addition to the effect of powder morphology, the process parameters such as, sheath gas composition, probe position, particle size and spraying distance, and so on, were evaluated. The result of experiment with AMDRY146 powder, particle size and spraying distance affected highly on the density of the deposit. The optimum process condition for the deposition of -75 µm of 20%-Yttria-Stabilized-Zirconia powder was 120/201/min of Ar/H_2 gas rate, 80 kW of plasma plate power, 8 cm of probe position and 150 Torr of spraying chamber pressure, at which its density showed 97.91% of theoretical density and its deposition rate was 20 mm/min. All the results were assessed by statistical approach what is called ANOVA.

On The Creep Threshold Stress in Secondary Recrystallized ODS MA NiAl: J Korean Powder Metall Inst. 1998;5(2):122-128.

91 View
0 Download

Abstract PDF: NiAl based ODS (Oxide Dispersion Strengthened) intermetallic alloys have been produced by mechanical alloying (MA) process and consolidated by hot extrusion. Subsequent thermomechanical treatments have been applied to induce secondary recrystallization in an attempt to improve creep resistance in this material. The creep behavior of secondary recrystallized MA NiAl has been investigated and compared with those of as-extruded condition. Minimum creep rate were shown to be approximately two orders of magnitude lower than that in as-extruded condition. The improvement in creep resistance is believed due to the grain coarsening, restricting of dispersoid coarsening as well as increase in grain aspect ratio. Creep threshold stress behavior, below which no measurable creep rate can be detected, has been discussed on the basis of particle-dislocation interaction theory. The threshold stress becomes negligible after secondary recrystallization in MA NiAl, presumably due to dispersoid coarsening and a decrease in grain boundary area during secondary recrystallization.

Elevated Temperature Compressive Properties of Al-Ti Alloys Prepared by Mechanical Alloying: J Korean Powder Metall Inst. 1998;5(2):129-132.

57 View
0 Download

Abstract PDF: The elevated temperature compressive tests were carried out in order to investigate the deformation behavior and microstructural characteristics of Al-8%Ti, Al-12%Ti and Al-16%Ti (wt%) alloys, which were mechanically alloyed and consolidated by vacuum hot pressing, A13Ti intermetallic phases were formed with sizes of few hundred nanometers in the mechanically alloyed Al-Ti alloys. The compressive strength of mechanically alloyed AA-Ti alloys increased with decroasing the temperature and with increasing the strain rate. The strain rate sensitivities of Al-8%Ti, Al-12%Ti and Al-16%Ti alloys were measured 0.02,0.03, and 0.14, respectively, at 350°C.

Effect of Seeding on Microstructural Development of Silicon Nitride Ceramics: J Korean Powder Metall Inst. 1998;5(2):133-138.

57 View
0 Download

Abstract PDF: The effect of beta-Si_3N_4 seeding on microstructural development of silicon nitride based materials has been investigated. In particular, to observe more distinctly the abnormal grain growth in pressureless sintering, fine alpha-Si_3N_4(mean particle size: 0.26 µm) powder classified by sedimentation method was used. It was possible to prepare silicon nitride with abnormally grown grains under low nitrogen pressure of 1 atm thanks to the heterogeneous nucleation on Si_3N_4 seed particles. The size and morphology of silicon nitride grains were strongly influenced by the presence of beta-Si_3N_4 seed and overall chemical composition. For specimens with initially low beta-content, the large grains grew without a significant impingement by other large grains. On the contrary, for specimens with initially high beta-content, steric hindrance was effective. The resulting microstructure was less inhomogeneous and characterized by unimodal grain size distribution.

Evaluation of Mechanical Properties and Resistance to Thermal Shock of YBCO-Ag Superconductors: J Korean Powder Metall Inst. 1998;5(2):139-144.

87 View
0 Download

Abstract PDF: We have evaluated the role of Ag additions on the strength, fracture toughness, elastic modulus and resistance to thermal shock of YBa_2Cu_3O_7-x(YBCO) superconductor. Addition of 10 vol.% Ag improved strength and fracture toughness, whereas, decreased elastic modulus of YBCO. In addition, YBCO-Ag composites improved resistance to thermal shock probably due to enhanced strength, fracture toughness and thermal conductivity as a result of Ag addition. It is to be noted that YBCO-Ag made by mixing with AgNO_3 solution showed slightly higher strength, fracture toughness and resistance to thermal shock, compared to that made by mixing with metallic Ag powder. These improvements are believed to be due to the microstructure of more finely and uniformly distributed Ag particles.

Metal Injection Molding of Nanostructured W-Cu Composite Powders Prepared by Mechanical Alloying: J Korean Powder Metall Inst. 1998;5(2):145-153.

49 View
0 Download

Abstract PDF: W-Cu alloy is attractive to thermal managing materials in microelectronic devices because of its good thermal properties. The metal injection molding (MIM) of W-Cu systems can satisfy the need for mass production of the complex shaped W-Cu parts in semiconductor devices. In this study, the application of MIM process of the mechanically alloyed (MA) W-Cu composite powders, which had higher sinterability were investigated. The MA W-Cu powders and reduction treated (RT) powders were injected by using of the multicomponent binder system. The multi-stage debinding cycles were adopted in N_2 and H_2 atmosphere. The isostatic repressing treatment was carried out in order to improve the relative density of brown parts. The brown part of RT W-Cu composite powder sintered at 1100°C had shown the higher sinterability compared to that of MA powder. The relative sintered density of all specimens increased to 96% by sintering at 1200°C for 1 hour. The relationship between green density and the sintering behavior of MA W-Cu composite powder was analyzed and discussed on the basis of the nanostructured characteristics of the MA W-Cu composite powder.

급냉법에 의한 고온용 Mg 합금의 개발 전망: J Korean Powder Metall Inst. 1998;5(2):154-156.

70 View
0 Download

PDF

Previous issues