Thermal shock behaviour of mullite–cordierite refractory materials

Abstract

The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of K _Ic measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.     

Half-Tone Reproduction on Diazo Materials

A simplified screening method providing an easy means of producing good quality screened masters. A general consideration of half-tone diazo printing.     

Oxidation of MCrAlY coatings on Ni based superalloys

Abstract

Industrial gas turbine engines used for power generation generally employ Ni based superalloys for the turbine blades. The operating conditions for these blades are very arduous with high temperatures (>900°C) leading to oxidation and corrosion. Therefore in order to increase the service life of components, coatings are employed which allow the use of Ni based superalloys at higher temperatures and therefore more efficient engines. These systems are very complicated and in order to understand coating performance and service life, many modelling approaches have been utilised. However, there is still a lack of understanding of the thermally grown oxide (TGO) with regards to its chemistry, microstructure, adhesion strength and mechanical properties. Therefore a more detailed understanding of the TGO would be useful for both empirical and computational modelling. The effect of compositional changes in the MCrAlY bond coat and their effect on the TGO have been studied. Two different MCrAlY coatings have been examined after aging at representative operation temperatures. A number of analytical techniques have been used including, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray and electron backscattered diffraction. A dual beam scanning electron microscope has also been utilised for both TEM sample preparation and microstructural evaluation. These techniques will allow a better understanding of the microstructure of the TGO, ultimately leading to improved predictions of coating behaviour and service life.     

Atom probe characterisation of high temperature materials

Abstract

An atom probe is capable of quantitatively analysing materials at the atomic level. Modern atom probes are derived from the field ion microscope, and are coupled with time-of-flight mass spectrometers, permitting identification of individual atoms. The introduction of position-sensitive detectors enables the reconstruction of a small volume of the sample owing to simultaneous determination of the x, y, and zcoordinates and the mass to charge ratios of individual atoms. This paper focuses on the application of atom probe techniques to the microstructural analysis of high temperature materials. Illustrations include carbide precipitation in creep resistant power plant steels and analyses of model and commercial multicomponent nickel based superalloys. It is demonstrated that atom probe field ion microscopy and atom probe tomography are valuable techniques in the development and understanding of technologically important alloys for high temperature service.     

Effect of Ni additives on pressureless sintering of SHS ZrB2

Abstract

Pressureless sintering of ultrafine zirconium diboride ZrB₂ produced by self-propagating high temperature synthesis (SHS) was carried out over a temperature range of 1573 to 1873 K using a nickel additive. The additive improved densification behaviour and a maximum densification of 88% was achieved at 1873 K. The XRD pattern showed formation of Ni₃Zr phase during pressureless sintering. The microhardness of sintered ZrB₂ was found to increase with Ni content and a maximum hardness of 1150 kg mm^?2 was found at 40 wt-%Ni addition. The coefficient of thermal expansion of different sintered samples was also investigated.     

Engineering nanostructured thermal spray coatings: process–property–performance relationships of ceramic based materials

Abstract

Nanostructured powders were deposited using thermal spraying to produce coatings having internal features of nanosized dimensions. Several ceramic based materials were studied, including WC–12 wt-%Co, TiO₂, hydroxyapatite, Al₂O₃–13 wt-%TiO₂ and yttria stabilised zirconia. The effect of the thermal spray conditions on the microstructure, phase composition, properties and performance was investigated. Key nanostructural features of the coatings were identified and their potential benefit in contributing to enhanced behaviour explored. Issues relating to design strategies and process control for engineering these types of coatings with performance characteristics tailored for targeted applications are discussed.     

Creep kinetics in compression of sand cast commercial Zn–Al alloys

Abstract

The compressive creep behaviour of sand cast ZA8, ZA12, and ZA27 alloys was investigated. Primary creep contraction increased with aluminium content and stress and, with the exception of ZA27, decreased with temper ature. Secondary creep rates for ZA8 and ZA27 were similar and 33% less than for ZA12. Creep kinetics obeyed an empirical equation ln t = C′ — n ln (σ) + Q/RT where t is the time to a selected strain, σ is the nominal stress, T is the absolute temperature, R is the gas constant, and C′, n, and Q are material constants for each alloy. For total deformations up to 1%, the overall creep resistance increased in the order ZA27 > ZA12 > ZA8. In terms of secondary creep rate, the compressive creep resistance of ZA12 was similar to its tensile creep resistance as reported in the literature, but for ZA27 the creep rate in compression was generally slightly higher.     

Microstructure and creep resistance of Mg–10Zn–4Al–0.15Ca permanent moulding alloy

Abstract

In this paper, the microstructure of Mg–10Zn–4Al–0.15Ca (wt-%) (ZAC104015) magnesium alloy was characterised in the as cast condition. Thermal analysis was performed to determine the liquidus, solidus, and intermediate reactions that occur upon cooling. The results show that this alloy is composed of α-Mg and two intermetallic phases containing calcium τ'₁ and τ'₂. These two intermetallic phases have the same type of crystal structure as that of phase τ (Mg₃₂(Al,Zn)₄₉). The thermal stability of the phases τ, τ'₁, and τ'₂, were also evaluated. This experimental study indicates that the phase τ'₂ is more stable than τ and τ'₁ at elevated temperature and directly related to creep resistance improvement.     

Temperature tests on package lid seal materials over one year duration

Abstract

The design of lid seal system of any package transporting irradiated nuclear fuel is a major aspect of the containment safety case under normal and accident conditions. Consequently, when BNFL, now known as British Nuclear Group Sellafield, BN-GS, decided on a change in lid seal material, year long proving trials on the new material were conducted, these simulating actual service and accident temperature conditions. Several years ago, BN-GS with the Rubber and Plastic Research Association (RAPRA) developed an elastomer called EPDM 30H which could perform at temperatures below ?40°C and had excellent resistance to radiation, as later proven by long term testing. EPDM 30H material has been used in many site applications, with its first transport package application being on the Excellox 6 type, commissioned in 1991. Towards the end of the 1990s, BN-GS became aware that the fluorocarbon grade of lid seal material, used on several other packages, may be discontinued by the manufacturers and so they took the decision to replace the seals on the NTL 11 type packages with EPDM 30H. Consequently, BN-GS embarked on an extensive programme of seal testing which went beyond any it had previously carried out. Comprehensive data were available on radiation resistance and performance at low temperatures, but additional data were needed on its behaviour at elevated temperature over periods of about one year. A number of test sets were assembled comprising seals in representative lid seal grooves. Most of the test sets were fitted with EPDM 30H seals but another grade of EPDM seal was also included. Two test sets were continuously maintained at temperatures of 120 and 150°C, for one year. Other test sets were maintained at 60, 90, 120, 140, 160 and 180°C, but periodic inspection and compression set measurements were taken. At the end of one year the continuously heated seals were subjected to a thermal cycle corresponding to the thermal accident safety cases studies. These tests demonstrated that the EPDM 30H material had lower compression set characteristics than the other grade of EPDM and that EPDM 30H was a suitable seal material for all irradiated LWR fuel transport packages currently operated BN-GS.     

Thermal analysis applied to estimation of solidification kinetics of Al–Si aluminium alloys

Abstract

Evaluation of solidification kinetics by thermal analysis is a useful tool for quality control of Al–Si melts before pouring provided it is rapid and highly reproducible. Series of thermal analysis records made with standard cups are presented that show good reproducibility. They are evaluated using a Newton's like approach to get the instantaneous heat evolution and from it solidification kinetics. An alternative way of calculating the zero line is proposed which is validated by the fact that the latent heat of solidification thus evaluated is within 5% of the value calculated from thermodynamic data. Solidification kinetics was found highly reproducible provided appropriate experimental conditions were achieved: high enough casting temperature for the cup to heat up to the metal temperature well before solidification starts; and equal and homogeneous temperatures of the metal and of the cup at any time in the temperature range used for integration.