Materials Science Forum Vols. 534-536

Abstract: Dilatometric curves of iron-copper compact made from elemental or pre-alloyed powder were compared in order to study its expansion mechanism. The compacts from a mixture of elemental iron and copper powders showed the maximum expansion at the copper powder content of 7.44mass% and at the sintering temperatures above the copper melting-point. In the case of the compacts from a mixture of iron pre-alloyed powder of (Fe- 3.44mass%Cu) and copper powder, the maximum expansion which is lower than the former case was obtained at the additive copper powder content of 4.00mass%. But the compact from a mixture of iron pre-alloyed powder of (Fe-7.18mass%Cu) and copper powder showed only shrinkages at any additive copper powder content. The penetration of liquid copper into the interstices between solid grains was observed in all sintered compacts. The iron-copper compacts containing the same content of copper powder showed a very different expansion behavior depending on the amount of copper content in the starting iron raw powder.

Abstract: Nd-Fe-B type powder was sintered using spark plasma sintering method. Fabricated compact sintered at the temperature of 700 °C, is found to be a composite magnet with Nd-Fe-Co-B and α-Fe. The compact sintered at 700 °C shows slightly low coercivity and large remanent magnetization comparing to the compact sintered at 600 °C due to the formation of α-Fe phase, resulting in the large maximum energy product. Maximum energy product tends to decrease with decreasing thickness of sintered compacts below 0.5 mm in thickness.

Abstract: By the high frequency induction heated combustion synthesis (HFIHCS) method, dense nanostructured TaSi2-SiC composite was synthesized within 2 minutes and in a single step from powders of TaC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of an induced current and mechanical pressure. Highly dense TaSi2-SiC with relative density of up to 97% were produced under a 60MPa pressure and induced current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.

Abstract: SnO2 ceramics were co-doped with the aliovalent ions of CoO and Nb2O5 and the grain growth behavior of the SnO2 was investigated. When only CoO was doped, the grain growth exponent of SnO2 was 3. As the amount of Nb2O5 increased, the exponent changed from 3 to 2 when 0.505 mol% of Nb2O5 was added. The further addition of Nb2O5 changed the exponent from 2 to 3. When Nb2O5 content was 0.505 mol%, of which the grain growth exponent was 2, it is believed that an iso-electric point is formed without grain boundary segregation, since the respective space charges, generated by Nb5+ and Co3+ might compensate each other.

Abstract: In this study, aliovalent ions of Nb2O5 and CoO were co-doped in SnO2 with different Nb2O5/CoO ratios and the grain growth rate of SnO2 was examined. The maximum grain growth rate was observed to be around Nb2O5/CoO = 0.25. Here, the results will be discussed in connection with the defect concentration and the change in diffusion rate-determining species with different Nb2O5/CoO ratios.

Abstract: Homogeneous microstructures of the PM compacts are difficult to attain when mixed elemental powders are used. This study examined the microstructures of pressed-and-sintered and MIM products that contain Ni and Mo. Ni-rich areas, which were lean in carbon and were soft, were found easily in regular specimens. Gaps or cracks near the Ni-rich or Mo-rich areas were also frequently observed. This problem got worse when Ni and Mo particles were large and were irregular in shape. By using ball milling treatment and ferroalloy powders, the microstructure homogeneity and mechanical properties were improved. The addition of 0.5wt%Cr further improved the distribution of Ni because Cr reduced the repulsion effect between nickel and carbon. With the elimination of Ni-rich areas, more bainites and martensites were formed and mechanical properties were significantly improved.

Abstract: Pulsed Current Sintering (PCS) process possesses some problems that need to be resolved. We, therefore aims at understanding phenomena of PCS process by presenting some basic data on in situ sintering behavior of PCS. In order to observe in situ sintering behavior of PCS, a special graphite mold equipped with thermo couple and electrodes were designed to measure the temperature, electric current and voltage inside the powder during PCS process. We apply three types of raw materials, especially for ZnO (thermoelectric material) as semiconductor, Al2O3 as non-conductor and WC (Tungsten Carbide) as good conductor. The observation succeeded and some valuable data were obtained. The results showed that the temperature in the Al2O3 powder is 100 K higher than the graphite mold at the temperature of 1473K and ZnO powder is 150 K higher than the graphite mold at the temperature of 1373K. The electric current and voltage were measured for each powder during PCS process. In addition, their electric resistance properties were calculated. The electric resistance showed different behavior.

Abstract: Powder grades pre-alloyed with 1.5-3 wt% chromium are suitable for PM steel components in high performance applications. These materials can be successfully sintered at the conventional temperature 1120 °C, although well-monitored sintering atmospheres with low oxygen partial pressures (<10-17-10-18 atm) are required to avoid oxidation. Mechanical properties of the Cralloyed PM grades are enhanced by a higher sintering temperature in the range 1120-1250 °C, due to positive effects from pore rounding, increased density and more effective oxide reduction. A material consisting of Astaloy CrM, which is pre-alloyed with 3 wt% Cr and 0.5 wt% Mo, and 0.6 wt% graphite obtains an ultimate tensile strength of 1470 MPa combined with an impact strength of 31 J at density 7.1 g/cm3, after sintering at 1250 °C followed by cooling at 2.5 °C/s and tempering.

Abstract: In this study, the properties of sintered parts produced by Powder Injection Molding (PIM) from the feedstocks of steatite powders with water based binders were investigated. The steatite powder solid loading was 58 vol. %. The properties investigated were density, % size change, tensile and bending strengths. Sintering in a high temperature furnace at different temperatures, heating rates and sintering times have been carried out. Densities of sintered parts have been measured by using Archimedes’ principle. Maximum attained relative density was 96 % at 1275 °C sintering temperature, 5 °C/min heating rate and 3 hours sintering time. At this sintering condition, linear shrinkage was 17.6 %, tensile strength was 16.7 MPa and bending strength was 130.6 MPa.

Abstract: It is well known that PM stainless steels have lower corrosion resistance than the corresponding wrought steels, since they are affected by the presence of the open porosity. A way to obtain a surface densification is the addition of a small quantity of boron (from 0,3 to 0,5%wt.) to the stainless steel. The presence of boron produces a liquid phase phenomenon that results in a final microstructure consisting of a boron-rich phase network surrounding the stainless steels grains. Close to the surface, a boron-free layer was observed in which pores are very few, closed and round. This leads to an improvement in the steel corrosion resistance.