Journal of Powder Materials

Sintering: Densification and Grain GRowth: J Korean Powder Metall Inst. 1998;5(1):2-14.

67 View
0 Download

PDF

A Study on the Mechanical Properties and Contact Damage of Silicon Nitrides : 1. Effect of \alpha/\beta Phase Fraction: J Korean Powder Metall Inst. 1998;5(1):15-21.

92 View
0 Download

Abstract PDF: The effect of alpha/beta phase on the mechanical properties and contact damage of silicon nitrides Si_3N_4) was investigated. Silicon nitride materials were prepared from two starting powders, at selective increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse alpha-phase powder, essentially equiaxed alpha-Si_3N_4 grains, with limited, slow transformation to beta-Si_3N_4 grain; (ii) from relatively fine alpha-phase powder, a more rapid transformation to beta-Si_3N_4, with attendant grain elongation. The resulting micro-structure thereby provided a spectrum of alpha/beta phase ratios, grain sizes, and grain shapes. Fracture strength, hardness, and toughness were measured, and contact damage and strength degradation after indentation were investigated by Hertzian indentation using spherical indenter. A brittle to ductile transition in Si_3N_4 depended on alpha/beta phase ratio as well as grain size. Silicon nitride with elongated beta grains showed a superior, contact damage resistance.

Mechanical Properties and Contact Damage of Silicon Nitrides Nitrides : II. Effect of Microstructure: J Korean Powder Metall Inst. 1998;5(1):22-27.

72 View
0 Download

Abstract PDF: The effect of the alpha/beta phase fraction on the mechanical properties in silicon nitrides was investigated in part 1. In part II, we describe the role of microstructure on the mechanical properties and contact damage of silicon nitrides with coarse/equiaxed and coarse/elongated microstructures. Grain sizes and shapes were controlled by starting powder. Hertzian indentation using spherical indenter was also used to investigate contact damage behavior. Cone cracks from the spherical indentation were suppressed when the silicon nitride contains coarse and elongated grains. Coarse and elongated grains played an important role of cone crack suppression. The size of quasi-plastic zone does not depend on grain size or shape but depends on the fraction of alpha/beta phase. A quasi-plastic zone was consisting of microcracks by shear stress during indentation.

Fabrication of Agglomerated Cr_2O_3 Powder for Plasma Spray Coating by Spray Drying Process: J Korean Powder Metall Inst. 1998;5(1):28-34.

90 View
0 Download

Abstract PDF: Plasma sprayed ceramic coatings are widely used in various industrial fields to improve their properties or to reduce the production cost. The ceramic powders for plasma spray coating have been mainly manufactured by spray drying or fused+crushed process. In this study, chromium oxide which has better mechanical properties than those of the other ceramic was selected and agglomerated chromium oxide powders for plasma spray coating were produced by spray drying process with a various processing condition. The large hollow powders and the harsh surfaced powders are formed at high slurry feed rate more than 163 g/min. and low binder concentration less than 2wt%, respectively. These powders cause the considerable decrease of flowability and apparent density. The powders produced by spray drying process have the spherical shape with the mean size of 45 µm, but these are shown lower apparent density and flowability than the powders produced by fused+crushed powders. The plasma spray coated layers by spray dried powders are shown a different microstructure with that by fused+crushed powders in porosity shape, but their properties such as density, hardness and bond strength are similar.

The Effect of Mn on Microstructural Change in 93W-5.6Ni-1.4Fe Heavy Alloy: J Korean Powder Metall Inst. 1998;5(1):35-41.

50 View
0 Download

Abstract PDF: The effect of Mn on the densification and the microstructural change in W heavy alley was investigated with adopting the improved Mn-adding method. In order to avoid the pore formation problems associated with Mn powder mixing to the other constituent powders, Mn was added afterwards to the sintered heavy alloy; Mn powder was spread homogeneously on the surface of the sintered heavy alloy compact, and this Mn powder contained specimen was resintered at the same sintering temperature. As expected, the resintered specimen showed the pore free microstructure because Mn was reduced separately from the other constituent elements. It was also founded that W grains grew rapidly at the initial stage of resintering treatment due to the activated reprecipitation of the excess W atoms substituted by Mn atoms, but the growth rate of W grains was slowly lowered with the prolonged sintering time, especially, compared to the Mn free heavy alloy. Such a retardation of grain growth should be attributed to the decreased W solubility in the Mn contented matrix phase. Furthermore, Mn addition resulted in the decrease of contiguity by improving the wetting between matrix phase and W grain.

The Effects of Heat Treatment Temperature on Mechanical Property of 93W-6.3Ni-0.7Fe Heavy Alloy: J Korean Powder Metall Inst. 1998;5(1):42-49.

86 View
0 Download

Abstract PDF: A study on the improvement of the impact energy in 93W heavy alloy with a Ni/Fe ratio of 9/1 has been carried out as a function of heat treatment temperature. The obtained results were compared to that of the traditional alloy system in which the Ni/Fe ratio is 7/3 or 8/2. With increasing heat treatment temperature from 1150 to 1250°C, the impact energy of the alloy with the Ni/Fe ratio of 9/1 is remarkably increased from 42 to 72 J, which is higher than that of traditional alloy, up to 1180°C and then saturated. Fracture mode was also changed from brittle W/W boundary failure to W cleavage. The temperature showing the dramatic shrinkage by dilatometric anaysis of the heavy alloy with Ni/Fe ratio of 9/1 was found to be 1483 °C, which is higher than that (1460°C) of the heavy alloy with Ni/Fe ratio of 7/3. Auger Electron Spectroscopy showed that the segregation of impurities, such as S, P, and C in W/W grain boundary was considerably decreased with increasing heat treatment temperature from 1150 to l180°C. From the above results, it was found that the impurity segregation in W/W grain boundary played an important role on the decrease of impact properties, and the heat treatment temperature should be appropriately chosen, as considering the Ni/Fe ratio of the alloy, in order to get good impact properties.

Thermoelectric Properties of N-type 90% Bi_2Te_3+10% Bi_2Se_3 Thermoelectric Materials Produced by Melt spinning method and Sintering: J Korean Powder Metall Inst. 1998;5(1):50-56.

81 View
0 Download

Abstract PDF: A new process using rapid solidification (melt spinning method) followed by pressing and sintering was investigated to produce the n-type thermoelectric ribbons of 90% Bi_2Te_3+10% Bi_2Se_3 doped with CdCl_2. Quenched ribbons are very brittle and consisted of homogeneous Bi_2Te_3-Bi_2Se_3 pseudo-binary solid solutions. Property variations of the materials was investigated as a function of variables, such as dopant CdCl_2 quantity and sintering temperature. When the process parameters were optimized, the maximum figure of merit was 2.146times10-3K-1.

Finite Element Analysis for Forging Processes of Rapidly Solidified Al-Si Alloys: J Korean Powder Metall Inst. 1998;5(1):57-63.

47 View
1 Download

Abstract PDF: The densification behaviors of rapidly solidified Al-Si alloys under high temperature processing were investigated. In general, it was difficult to establish optimum process variables for forging condition through experimentation, because this was costly and time consuming. In this paper, to overcome these problems, we compared the experimental result to the finite element analysis for forging processes of rapidly solidified Al-Si alloys. The results of these simulations helped understand the distribution of relative density during various forging processes. This information is expected to assist in improving rapidly solidified Al-Si alloys forging operations.

Effect of Milling Medium Materials on Mechanical Alloying of Mo-25.0at%Si Powder Mixture: J Korean Powder Metall Inst. 1998;5(1):64-70.

93 View
0 Download

Abstract PDF: Milling media of steel and partially stabilized zirconia(PSZ) were used to produce Mo_3Si by mechanical alloying(MA) of Mo-25.0at%Si elemental powder mixture. The effect of milling medium materials on MA of the powder mixture have been investigated by XRD and DTA. The reaction rate and the end-product noticeably depended upon the milling medium material. The formation of Mo_3Si and Mo_5Si_3phases by PSZ ball-milling took place after 15 hr of MA and was characterized by a slow reaction rate as Mo, Si, Mo_5Si_3 and Mo_3Si coexisted for a long period of milling time. The formation of a new phase by steel ball-milling, however, did not take Place even after 96 hr of MA. DTA and annealing results showed that Mo_5Si_3 and Mo_3Si were formed after heating the ball-milled powder specimens to different temperatures. At low temperatures, Mo and Si were transformed into Mo_5Si_3. At high temperatures, the formation of Mo_3Si can be partially attributed to the reaction, 7Mo+Si+Mo_5Si_3-.4Mo_3Si . The formation of Mo_3Si and Mo5Si3 phases by mechanical alloying of the powder mixture and the relevant reaction rate appeared to depend upon the milling medium material as well as the thermodynamic properties of the end-products.

나노결정립 재료의 기술전망: J Korean Powder Metall Inst. 1998;5(1):71-74.

80 View
0 Download

PDF

Theoretical Analysis and Its Applications in Powder Forming: Hyeong-Seop Kim, Hong-Ro Lee, Byeong-Seon Cheon; J Korean Powder Metall Inst. 1998;5(1):83-88.

86 View
0 Download

PDF

Previous issues