F. Castro

Sinter-HIP of α-alumina powders with sub-micron grain sizes

Abstract The densification behaviour of different nanoscale pure alumina powders by sinter-HIP to obtain specimens with sub-micron grain size has been investigated. The sinterability of these powders to determine the minimum temperature to obtain close porosity has been determined by dilatometry and by pressureless sintering in air. A good correspondence between minimum temperature to obtain closed porosity and minimum temperature to obtain full density by sinter-HIP has been determined. Different green performing routes has been analysed, uniaxial pressing and colloidal pressure filtration, being the green density obtained of 56% TD and 62% TD respectively. From the powders analysed Taimicron TM-DAR is the powder with its maximum sinterability and can be fully densified by sinter-HIP at temperature as low as 1250xa0°C and 150 MPa. The specimens obtained at the lowest temperature presented a grain size of 0.45 μm, a hardness of 23–24 GPa and fracture toughness K IC =3.5 MPa m 1/2 .

Oxidation resistance of SiC ceramics sintered in the solid state or in the presence of a liquid phase

The present paper reports on the oxidation resistance of liquid phase sintered SiC. The results are compared to the behavior shown by solid state sintered samples after oxidation under exactly the same conditions. The microstructural characteristics of the oxide scales formed on both kinds of SiC- ceramics are also reported. The specimens used for oxidation treatments were obtained in fully dense form through pressureless sintering or hipping at a temperature of 1,850 C in the case of liquid phase sintered SiC (LPSSC) and at 1,950 C for solid state sintered SiC (SSC). As expected SSC is more resistant to oxidation than LPSSC, mainly due to the crystallinity of the oxide product formed on the surface of the specimens and the absence of an intergranular phase in the as-sintered materials. LPSSC, containing YAG as intergranular phase, is more resistant to oxidation at low temperatures than the samples with additions of Y{sub 2}O{sub 3} and SiO{sub 2}. At higher temperatures however this tendency is reversed due to the massive formation of Y{sub 2}Si{sub 2}O{sub 7} that covers all the surface of the sample providing a better oxidation resistance than the glassy phases formed on the Y{sub 2}O{sub 3} and Al{sub 2}O{sub 3}morexa0» containing samples.«xa0less

Nitridation of SiC for the production of SiC-Si3N4 Nanocomposites

Abstract High pressure sintering of SiC green compacts, both with and without additions of Al 2 O 3 and SiO 2 , has been carried out in a HIP press under a nitrogen atmosphere. The amount of conversion of SiC to Si 3 N 4 has been quantified after nitridation by XRD, and the results were interpreted in terms of phase stability diagrams taking into account the chemical composition of the specimens, the carbon activity and the nitrogen and the oxygen partial pressure. As a result of the process the final microstructure of the sintered materials was observed to consist of a combination of SiC and Si 3 N 4 grains in the nanosize range and whose relative volume fractions depend on sintering conditions and the initial chemical composition. A minor proportion of free carbon, in the form of both amorphous carbon and nanosized graphite grains, was also part of the microstructure. This method is therefore regarded as a means for the production of SiC-Si 3 N 4 nanocomposites with varying amounts of SiC, α-Si 3 N 4 and β-Si 3 N 4

Processing Routes for Obtaining Novel High Performance Mn-Containing PM Steels

A Mn-containing master alloy (MA) has been specially designed, through thermodynamic and metallurgical criteria, for obtaining high performance low alloy PM steels by SPSS or DPDS. This MA exhibits improved characteristics with respect to ferromanganese and other Mn carriers for alloying PM steels preventing oxidation, keeping a high compressibility of the powder mixture and providing opportunities for low temperature processing. The improved sinterability through the formation of a transient liquid phase leads to dimensional stability and high reproducibility of mechanical properties after sintering at 1120°C. The microstructural development of the PM steels was studied during the sintering cycles. The final microstructure of these PM steels, after defined sintering cycles, was characterised by LOM while the mechanical properties of the consolidated materials were determined by tensile testing.

Effect of the Bi content on the mechanical properties of a Sn-Zn-Al-Bi solder alloy

Because of environmental and health concerns, some alternative solder alloys, named lead-free ones, are being developed. Among them, the Sn-Zn-Al system has been studied and reveals promising properties. Selection of the solder alloys, for the electronic industry applications, is conditioned by their mechanical properties due to the stress produced in service. The studied alloys were produced by melting the pure elements in a resistance furnace, under inert atmosphere, and pouring in a steel mould. The samples were heat treated for homogenization of the microstructure. The produced alloys were analyzed by XRF spectrometry and Scanning Electronic Microscopy (SEM/EDS) for chemical and microstructural characterization. In this work the presence of bismuth, in the range of 0-7 weight %, was evaluated in what concerns to the as-cast and homogenized microstructures and mechanical properties. The mechanical properties of the produced alloys, in the as-cast and homogenized conditions, have been determined by tensile strength and hardness tests. The results obtained showed that bismuth has an important effect on the mechanical behavior of the alloys, namely, in the transition from ductile to brittle behavior. This behavior was correlated with the samples microstructures.

Development of PM Stainless Steels with Improved Properties through Liquid Phase Sintering

The use of boron for successfully obtaining high density PM stainless steels with improved mechanical properties and corrosion resistance is presented. Boron is added as part of master alloys which have been specifically designed to provide the formation of wetting liquid phases with excellent characteristics for producing controlled densification and alloying of 316L and 304L austenitic stainless steels. The as-sintered density and properties of these alloys is determined by the amount of master alloy, the chemical composition of the stainless steel powder, the sintering temperature and time. The microstructural development and alloy homogenisation are determined by the chemical composition of the Fe-based powder and the chemical reactions taking place between the basic powder and the master alloy particles during high temperature sintering. The use of this master alloy is shown to lead to stainless steels with outstanding combinations of strength and ductility. The influence of alloying and the sintering conditions on the final microstructure, density, corrosion resistance and tensile properties is also discussed.

Leaching of brasses in long-term direct contact with water

Copper-zinc alloys (alfa-beta brasses) are currently used to produce house water piping and further parts, that might be in direct contact with drinking water. Lead is added to these alloys in order to improve their machinability. It is well known that lead is unhealthy for humans and, according to EU Council Directive 98/83/EC on the quality of water for human consumption, the admissible Pb content in drinking water has to decrease during the next years because of its high toxicity. The interest in improving public health evocated the need of developing new ecological materials for application on water piping and systems because extensive research indicates that the occurrence of increased Pb content in drinking water can be due to the release (leaching) of Pb from leaded copper alloys. As a consequence of such findings, the dissolution /erosion behaviour of a commercial sanitary brass tap, submitted to a dynamic test simulating the service conditions, has been studied. For that purpose an equipment that enables a long ‐ term direct contact with water has been designed and developed. The chemical composition of the tap and of the final residue produced during the test have been determined by XRF spectrometry and the solutions analysed by atomic absorption spectrometry. Results concerning metal values dissolution have been compared with the limit values established by the legislation. For the experimental conditions, results show that Pb level in leaching solution is lower than the admissible value indicated for the quality of water for human consumption.

Experimental Phase Diagram of the Ternary Bi-Sn-Zn

Bi-Sn-Zn is one of the systems being used as substitute of the traditional lead solders. Therefore a deeper knowledge of its phase diagram is a priority. Due to the lack of data about the thermodynamic properties of terminal solid solutions as well as on their phase boundaries, only binary contributions were utilized and the ternary terms were ignored on the published assessments [1, 2]. Samples corresponding to several vertical sections were prepared and DTA, SEM/EDS and XRD at room and high temperatures were performed. It was concluded that ternary terms should be held in consideration in a future assessment of the system.