Twin roll casting has been used to produce near-net-shape sheet of Mg - 3 to 9wt%Al based alloys (i.e. AZ31, AM60 and AZ91 alloys), and this was followed by thermomechanical processing (i.e. hot rolling and heat treatment) of the as-cast sheet. The microstructures of both the as-cast and hot-rolled Mg alloys have been characterized so as to investigate the effects of near-rapid solidification from twin-roll casting and the subsequent thermomechanical processing on the morphology, size and distribution of the microstructural components. Compared to similar alloys processed by conventional ingot casting and rolling technology, these alloys produced via twin-roll casting have been found to exhibit homogeneity of microstructure, refined grain size, reduced segregation and a dense distribution of fine particles within the Mg solid solution. Mechanical properties have also been evaluated and have been shown to be significantly improved as a result of the associated improvements in microstructure.

Previous authors have established that prior nitriding or nitrocarburising will enhance the thickness of the vanadised coating formed on tool steels by the thermal diffusion (TD) process. However, whereas the single TD treatment produces a uniform surface layer of vanadium carbide, the combined treatments result in a complex vanadium carbonitride coating. Such a coating can be expected to exhibit a hardness that is lower overall and decreases away from the surface with increasing nitrogen to carbon ratio. An experimental study was undertaken to assess the relative merits of nitriding, nitrocarburising and carburising prior to TD vanadising on the steel AISI H13. The study demonstrates the specific advantages of carburising, previously untried as a pre-treatment. It produces the thickest coating (about twice as thick as that due to vanadising alone) and, perhaps more importantly, the coating is essentially vanadium carbide and exhibits a uniform high hardness across its entire span.

Boriding has been employed with some success to increase the service life of commercial forging dies fabricated from the hot work steel AISI H13. This steel is poorly suited to the treatment, however, since an undesirable intermediate layer is formed between the hard boride coating and the core. In practice, a boride coating thickness well below that required for the application has been imposed by the need to minimise the thickness of this layer. An experimental study was undertaken with the aim of improving the effectiveness of a boriding treatment by engineering the subsurface of AISI H 13 steel. It was established that a carburising pretreatment markedly improved the microstructural features of the steel subsurface. The formation of the undesirable layer was completely inhibited within the range of boride thicknesses employed in commercial practice and severely limited at the higher thicknesses better suited to forging applications.

Under conditions of applied current flow, oxygen can be transported through a ZrO2 sensor so as to create a localized, relatively oxygen-rich atmosphere at the measuring electrode and thereby provide protection for the sensor in hostile reducing atmospheres. The rate of dissipation of this protective atmosphere can be controlled by the use of a porous ceramic muffle around the sensor's measuring electrode. Analysis of the output signal from the sensor not only allows the oxygen content in the bulk atmosphere to be determined, but can also yield much useful information relevant to the operation of the sensor, such as the onset of electrode limiting and electronic conduction.

Reaction Bonding is a direct solid-state process for forming strong, vacuum-tight seals between ceramics and metals. Several potential new application areas for the process are discussed, which involve bonds formed between a wide range of materials - metals such as steels, copper, titanium, platinum, and gold, and the ceramics Al2O3, PSZ and silicon nitride.

When operating the reaction-bonded zirconia sensor as an electrolytic cell, in the temperature range 250–500°C, a relationship is shown to exist between oxygen partial pressure at the external platinum electrode and the cell overpotential occurring as a result of an applied cathodic potential of between 0.1 and 4 V. To investigate the nature of this relationship, the variation in sensor resistance polarization Rp and cathodic current flow I with oxygen partial pressure is measured over the range of pO2 of 1 to 10−3 atm. The magnitude of current flowing through the sensor is related to a number of variables - I α pO2, T, (VB-η), 1/Rp, where pO2 is oxygen partial pressure at the cathode, T is temperature, VB is applied cathodic potential, and η is sensor overpotential. All response is recorded as a change from a “reference condition”, applying when anode and cathode are both at air atmosphere; this allows compensation to be made for polarization effects normally occurring when operating the sensor as an electrolytic cell. It is shown that there is a linear relationship between the cell overpotential η and oxygen partial pressure, and that the applied potential VB will determine both the magnitude of η and the electrode reaction rates of the sensor. In this mode of operation, the sensitivity of the electrode reaction to oxygen partial pressure is shown to be increased dramatically, such that usable response can be achieved well below the normal minimum operating temperature for zirconia sensors.

The formation and resulting mechanical strength of solid state metal-ceramic reaction bonds of alumina to platinum are investigated in terms of the effects of three main parameters — bonding temperature, time at temperature and contact pressure. Also the effect of the subsequent operating temperature on the bond strength is examined. An optimum bonding regime can be devised to create platinum-alumina bonds of optimum strength and durability, suitable for use in practical bonding applications.

A technique is described whereby zirconia oxygen sensors with platinum electrodes can be protected from deterioration in reducing atmospheres, typical of metallurgical heat treatment conditions. By the application of current to the sensor, the technique provides a form of anodic protection, while at the same time the oxygen concentration in an atmosphere varying in temperature and composition can be continuously detected and measured.

The solid state reactions that take place between many ceramics and metals can be utilised to produce strong vacuum-tight joints that maintain their strength and durability even at elevated temperatures. In this context platinum can play an especially important role because of its high melting point and resistance to chemically aggressive environments. Some industrial applications of the solid state bonding process using platinum are described in this article, which also includes some data on the properties of joints made in this way.