Papers by Keyword: Sinterability

Abstract: One strategy to make PM titanium components competitive in terms of mechanical properties is the addition of suitable alloying elements. PM offers the possibility to adapt the alloy composition in order to achieve the required properties. In this study, different alloying elements were introduced into α/β titanium alloys and fatigue behaviour was evaluated. Four-point bending fatigue tests with a stress ratio of 0.2 were performed on specimens manufactured by metal injection moulding (MIM) and shot peening. Results showed an enhanced sintering activity of Ti-6Al-7Nb by adding small amounts of iron. The impact on fatigue properties was evaluated. The increase of oxygen in Ti-6Al-7Nb from 0.15 wt.% to 0.45 wt.% caused a decrease in elongation from 16% to 6%. However the fatigue strength at 10⁷ cycles is just slightly reduced from 450 to 350 MPa. The addition of 0.5 wt.% yttrium powder to gas atomized Ti‑6Al‑4V powder led to a noticeable refinement of the microstructure of the sintered parts, due to the formation of Y₂O₃ particles, which hinder grain growth. In spite of a slightly higher residual porosity, the microstructural refinement increased the fatigue strength at 10⁷ cycles from 450 to 470 MPa. At 10⁶ cycles, the fatigue strength increased even from 705 to 765 MPa. The addition of yttrium did not result in a higher oxygen pick up, which indicates a scavenging of oxygen from the titanium matrix by formation of Y₂O₃ during sintering. Contrary to the fatigue strength results, the scavenging effect led to a decrease in tensile strength of about 70 MPa. The microstructure of fatigue-tested specimens was characterized by using optical and scanning electron microscopy.

Abstract: A powder metallurgical process was used to fabricate Metal Matrix Composites (MMCs). A 2124 aluminium alloy was reinforced with 5 and 10 vol.% of Al₂O₃ (40-70nm) to form Metal Matrix Nano Composites (MMNCs) as well as 10 and 15 vol.% of SiC (1-10µm) to fabricate low micron MMCs. It was observed that the nano-sized Al₂O₃ particles were evenly dispersed in the aluminium matrix while a lot of loose SiC particles settled on the grain boundaries in the low micron MMCs. The relative density of all the composites increased due to sintering, however full densification was not achieved. This result was attributed to the hindered motion of dislocations, grains and grain boundaries by reinforcing particles. The 2124-Al/10%-SiC composite was cold extruded and the extruded part fractured. A metallographic evaluation was carried out and it was deduced that the mode of failure was intergranular cracking. Hardness tests performed after sintering indicated that hardness increased with an increase in volume fraction of reinforcement in the matrix. Annealing of the extruded part resulted in a decrease in hardness.

Abstract: Lead-zinc mine tailings were used to produce ceramsite by sintering method to reduce their environmental impact. The properties of tailings from Beishan Lead-zinc mine located at the north-west of Guangxi Province in southern China have been studied. The sinterability and expansion property of the main raw materials was studied during the controlled trial burn tests. The results show that lead-zinc mine tailings are not suitable for making ceramsite alone, and should be added with the clay material. The sintering temperature has a significant influence on expansion property. The optimum firing temperature to make ceramsite is from 1140 °C to 1150 °C according to the apparent density index, and the heavy metals are properly stabilized in ceramsite. It is a viable approach for making ceramsite with lead-zinc mine tailings and clay.

Abstract: Dense iron-containing hydroxyapatite (HA)/titanium composites were synthesized via pressureless sintering at a relatively low temperature using nanosized HA powders and Ti-Fe mixed powders. XRD analysis showed that desirable Ti phase still remained in the HA matrix. The addition of iron improved the densification by enhancing the sinterability of titanium, and reduced the decomposition rate of HA and the interaction between HA and titanium. The mechanical tests showed that both the flexural strength and fracture toughness of the composites were significantly improved. The Ti-Fe reinforcing particles exhibited plastic stretching and bridged an advancing crack, making a significant contribution to the improvement of mechanical properties of the composites.

Abstract: The properties of anorthite-based glass/ceramic with different amounts of CeO₂ addition had been investigated in this paper. The glass/ceramics with small amounts of CeO₂ addition could be well sintered at 850~900°C. The water absorption was examined to evaluate the sintering behavior of the samples. The crystalline phase,microstructure, dielectric properties, thermal expansion and bending strength were characterized by X-Ray diffraction (XRD), scanning transmission electron microscope (SEM), impedance materials analyzer, thermal expansion analyzer and flexural strength analyzer. The results showed the 1wt % CeO₂-doped glass/ceramics sintered at 900 °C have optimum properties of ε = 6.282, tanθ = 2.18×10^-3 at 1 MHz and a thermal expansion coefficient of 3.32×10^-6 °C^-1 (25~500 ^{oC) matched with silicon chip (3.4×10-6} °C ^-1). Obviously, CeO₂ could be used as a suitable sintering aid that improves densification of the glass/ceramic composites.

Abstract: MgAl2O4-SiC Composites were prepared by using magnesium aluminate spinel and silicon carbide powder as starting materials and the sintering mechanism was investigated at 1600°C in reduction atmosphere. Sintered samples were analyzed using XRD, SEM and EDS. The results show that Alumina-rich spinel and SiCN are formed due to the reactions between spinel and SiC. Weight loss of samples was detected due to SiO formation and volatilization. The bulk density of samples decreases and apparent porosity increases as increasing the content of SiC.

Abstract: MgAl₂O₄-SiC composites were prepared by using MgAl₂O₄ and SiC powder as main starting materials, being pressed to cylindrical specimens of Φ30mm×30mm at 200 MPa and heated respectively at 1400°C and 1500°C for 3 hours in air atmosphere. The room temperature physical properties of sintered samples were tested according to related national standards. The phase compositions were analyzed by XRD. The microstructure of the fracture of the samples was observed by SEM. The results showed that MgAl₂O₄-SiC materials could sintered well in air atmosphere. The mechanism is that SiC was oxidized to form active SiO₂, which reacted with MgAl₂O₄ to produce liquid cordierite promoting the sintering of the materials. The sinterability of MgAl₂O₄-SiC materials was overall improved with increasing the sintering temperature. When the content of SiC of MgAl₂O₄-SiC materials was 10%, the cold crushing strength and bulk density of the samples reached maximums and apparent porosity reached a minimum; the content of SiC was more than 10%, the sinterability of samples deteriorated.

Abstract: The effect of holding time variation and sintering temperature on the mechanical properties and sinterability of yttria stabilized tetragonal zirconia doped with 1 wt. % MnO₂ was investigated. Samples were sintered at 1150-1500 °C with holding times varying from 12 and 120 minutes. Comparing to the MnO₂-doped Y-TZP, longer holding time resulted in enhanced densification of the undoped Y-TZP samples. In the case of MnO₂-doped Y-TZP however, Bulk density, Young’s modulus, Vickers hardness and fracture toughness results show the beneficial effect of MnO₂ in enhancing the densification of Y-TZP ceramics.

Abstract: The sinterability of magnesium oxide (MgO) doped hydroxyapatite (HA) ranging from 1 to 10 wt% when sintered at 1150°C was investigated in terms of phase stability, bulk density, Young’s modulus, Vickers hardness and fracture toughness. The addition of up to 1 wt% MgO as sintering additive was found to be beneficial in promoting the densification of HA. Further addition of MgO in the HA matrix would deteriorate its densification properties. Similar results were observed for its stiffness and Vickers hardness. Nevertheless, the fracture toughness of HA was greatly enhanced by the incorporation of 5 wt% MgO. An increased toughness of up to 35% was obtained for the MgO-doped HA when compared to the undoped HA. This improvement is associated to the smaller grain size of the doped sample as compared to the undoped HA.

Abstract: The aim of this work is to study the phase stability and sinterability of bismuth oxide (Bi2O3) doped HA ranging from 0.05 wt% to 1 wt%. The green samples were sintered in air at temperature ranging from 1000oC to 1400oC. In this experiment, the results from XRD analysis revealed that the stability of HA phase was disrupted when addition of 0.3, 0.5 and 1.0 wt% Bi2O3 were used and when samples sintered above 1100oC, 1000oC and 950oC, respectively. In general, HA containing 0.5 wt% of Bi2O3 and when sintered at 1000oC was found to be beneficial in enhancing densification, Young’s modulus, Vickers hardness and fracture toughness. Throughout the sintering regime, the highest value of relative bulk density of 98.7% was obtained for 0.5 wt% Bi2O3-doped HA when sintered at 1000oC. A maximum Young’s modulus of 119.2 GPa was observed for 0.1 wt% Bi2O3-doped HA when sintered at 1150oC. Additionally, 0.5 wt% Bi2O3-doped HA was able to achieve highest hardness of 6.04 GPa and fracture toughness of 1.21 MPam1/2 at sintering temperature of 1000oC. Furthermore, the Young’s modulus of HA was found to vary linearly with bulk density.