The Use of Transient Liquid Phases in Powder Metallurgy

Liquid phase sintering is most widely known in its variant „persistent liquid phase sintering“, in which case the liquid phase is present in constant quantity during the entire isothermal period. There is however also the variant „transient liquid phase“, the melt being present only for a short period in the first stage of sintering and then solidifying through diffusional processes. In this presentation, the preconditions for both variants are presented, in particular with regard to the starting materials. The benefits of transient liquid phases are described, both for sintering – to accelerate material transport, contact formation and microstructural homogenization compared to standard solid state sintering – and for transient liquid phase bonding, a brazing variant which is an attractive method for joining porous powder compacts. Both techniques are highly useful in particular for ferrous powder metallurgy precision components, etc.

Advances in Transient-Liquid-Phase Bonding of Ultra-high Temperature ZrC Ceramics

Full exploitation of the many attractive engineering properties of ultra-high temperature ceramics (UHTCs) requires that they can be joined. This paper explores progress in identifying joining strategies based on the use of transient liquid phases (TLPs). Wetting studies are used to explore the suitability of specific liquids for joining, while bonding studies provide the ultimate test. Sintering aids in the UHTC provide a major potential obstacle to successful joining, and dissolved impurities in the TLP can also complicate the joining process. Nonetheless, we show that well-bonding interfaces can be achieved when ZrC ceramics are bonded at 1673 K using a Ni/Nb/Ni multilayer interlayer.

Processing of Refractory Metal Borides, Carbides and Nitrides

Transition metal borides, carbides and nitrides are candidates for very high temperature applications. A review of various processing techniques to fabricate dense monolithic and composite materials is presented. In particular, we focus on reactive hot pressing (RHP) which allows synthesis and densification to be achieved simultaneously. We report the RHP of composites in the Ti-B-N, Zr-B-C and Zr-B-Si-C systems using the reactions of Ti/BN, Zr/B4C (Si, SiC particulate) powder mixtures at moderate pressures and temperatures. Substantial reductions in processing temperature may be realized from those in excess of 1800°C down to as low as 1200°C by exploiting a combination of transient liquid phases, plasticity in a non-stoichiometric ZrC and enhanced transport in a sub-micron microstructure.

An Investigation on the Sintering Behavior of High Strength Al-Zn-Mg-Cu Alloy Prepared from Elemental Powders

In the present work, the sintering behavior of high strength Al-5.6Zn-2.5Mg-1.6Cu (in wt.%) alloy compacts was investigated. Green parts made by pressing of mixed elemental powders were sintered at different temperatures between 400 and 600 °C. The effect of the sintering temperature on density, dimensional change and microstructure of the Al alloy was studied. Microstructural evaluation was accompanied by XRD and DSC methods in order to determine the temperature and chemical composition of the liquid phases formed during sintering. It was found that three transient liquid phases are formed at 420, 439 and 450 °C. Microstructural study revealed the progressive formation of sintered contacts due to the presence of the liquid phases, although the green compact expands as a result of the melt penetration along the grain boundaries. While Zn melts at ~420 °C, the intermetallic phases between Al and Mg were found to be responsible for the formation of liquid phase and the dimensional change at higher temperatures.