Institute of Metallurgy

Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.

Abstract A study of the possibility of the existence of stacking faults in b.c.c. crystals on {110} and {112} planes has been performed, representing the lattice by a central force interaction between atoms. The same study was also carried out for {111} planes in f.c.c. crystals. The results for the f.c.c. lattice are in full agreement with the predictions based on a hard-sphere model. However, the results for the b.c.c. lattice are very different and suggest that no stable instrinsic stacking faults of the same type as in f.c.c. crystals can exist either on {110} or on {112} planes in b.c.c. crystals.

Abstract For a number of metallic polycrystalline aggregates, it is shown experimentally that [sgrave]f, the flow stress at constant strain, is related to the grain diameter l by where [sgrave]0 and k are constants. This has the same form as the relationship between the lower yield point, when this occurs, and grain size. An explanation is given by an extension of Taylor's theory to allow for the resistance at the grain boundary to the formation of a slip band. Dislocation-locking and a small number of slip systems are shown to favour a strong grain-size dependence of [sgrave]f and this explains the variation of this dependence amongst the common metals and alloys.

Abstract A calculation is made of the length of plastic zone needed to accommodate a given plastic displacement at the root of a notch in a uniformly stressed solid. In an optimum range of stress this zone is about 1000 times larger than the plastic displacement and about five times longer than the notch. The distribution of plastic-elastic strain in the yielded region can be represented by an inverted pile-up of dislocations. The results are related to the problem of notch-brittleness in steel and it is concluded that a condition of ‘ far-reaching ’ yield should replace the condition of general yield for starting a fracture. Various factors important to notch-brittleness are briefly discussed.

Abstract The normal spectral absorptance of a number of metal, ceramic and polymer powders susceptible to be utilised for selective laser sintering (SLS) technique was experimentally determined. The measurements were performed with two laser wavelengths of 1.06μm and 10.6μm obtained by using two lasers – Nd‐YAG and CO2 respectively. The change in the powder absorptance with time during laser processing was also investigated. The effect of the absorptance characteristics on the sintering process is discussed.

Abstract The dynamic behavior of a multiple-capacity passive system is compared to that of a single-capacity system of appropriate “time constant with a finite time delay.” Such a comparison indicates that the latter system reproduces the dynamic behavior of the multiple-capacity system with sufficient accuracy for most engineering purposes. This fact makes it possible to characterize such systems by a single parameter, the ratio of the time constant (L) of the single-capacity system to the delay (D). Values of the control constants for various degrees of stability are plotted against this ratio (L/D) for proportional, proportional-plus-integral, proportional-plus-integral-plus-derivative type regulators. An electronic analog computer was used in obtaining the data.

Abstract A relaxation-type calculation of the structure of the dislocation core has been made for the ½ 〈111〉 screw dislocation in b.c.c. crystals, using a variety of central-force potentials. Two stable configurations were found, corresponding to the centre of the dislocation being along either the left-hand or the right-hand type of three-fold screw axis in the crystal. These two configurations differed only in the very centre. For both configurations and for all potentials, the core structure possessed three-fold symmetry, the largest displacements being in the directions in which displacements on (211) type planes were in the twinning sense. The structure can be described by a combination of large displacements on {110} type planes, plus ‘stacking faults’, 1–2b wide on {211} type planes in the twinning sense only. An investigation of the effect of boundary conditions showed that any errors caused by incomplete relaxation were negligible, and that changing the initial dislocation position or the position of the boundaries did not affect the final core structure. Four potentials were used, similar in form but having a five-fold variation in the depth of the minimum. The core structure was similar for all of these : the only changes were in the relative magnitude of the displacements and in the ‘stacking fault’ width which varied between, approximately, 1.2b and 2b. This core structure therefore appears to be general for b.c.c. crystals, and the implications of these results on the movement of dislocations are briefly discussed.

A new kind of Al-ion battery with carbon paper as the cathode, high-purity Al foil as the anode and ionic liquid as the electrolyte is proposed in this work. The significance of the presented battery is going to be an extremely high average voltage plateau of ca. 1.8 V vs. Al(3+)/Al.

<title/>Stainless steel, Co-Cr and Ni-Cr alloys have played an important role in many industrial sectors to combat environmental degradation. However, low hardness and poor wear properties have impeded their tribological and tribochemical applications. Conventional thermochemical treatments can be used to significantly harden these passive alloys but at the expense of their corrosion resistance due to precipitation induced depletion of Cr in the matrix. Research in 1980s led to the discovery of a new expanded austenite phase, i.e. so called S-phase with combined improvement in wear and corrosion resistance. Recent research has revealed that S-phase can be formed not only in stainless steels but also in Co-Cr alloys and Ni-Cr alloys. It is the purpose of this paper to critically review the S-phase surface engineering of stainless steels, Co-Cr alloys and Ni-Cr alloys. Particular attention will be paid to the structure, formation conditions, supersaturation, hardening mechanisms and metastability of S-phase. Based on the discussion of the chemical, mechanical, tribological and tribochemical properties of S-phase, the importance of the S-phase surface engineering technology is demonstrated by examples. Finally, future directions towards more stable and thicker S-phase layers will be discussed.

Abstract Stacking faults on the (111) planes of several face-centred cubic metals and alloys have been introduced by cold work, and estimates of the stacking fault probability α, have been obtained from changes produced in the Debye-Scherrer spectrum. The faulting probability increases on alloying, from one plane in 300 in copper, to one plane in 25 for some high solute content alloys containing zinc, aluminium, tin or germanium. Both neutron irradiation (5×1019 n.v.t.) and ‘quenched-in’ vacancies have little significant effect on the faulting parameter. Lowering the temperature of deformation increases the faulting probability but in copper the faults ‘anneal-out’ at room temperature after several hours. Line broadening analysis shows that the dislocation density is increased on alloying and also by lowering the deformation temperature. It is suggested that the faulting may be accounted for by the regions between separated half dislocation. Ribbons of stacking faults as given by extended dislocations and ‘infinite’ stacking faults produced by splitting of dislocations at high stresses are both considered and estimates of fault energy made. Some work on segregation of solute atoms to faults is reported with particular reference to the alloys silver-gold and copper-aluminium.

Zinc oxide (ZnO) has been considered as one of the potential materials in solar cell applications, owing to its relatively high conductivity, electron mobility, stability against photo-corrosion and availability at low-cost. Different structures of ZnO materials have been engineered at the nanoscale, and then applied on the conducting substrate as a photoanode. On the other hand, the ZnO nanomaterials directly grown on the substrate have been attractive due to their unique electron pathways, which suppress the influence of surface states typically found in the former case. Herein, we review the recent progress of ZnO nanostructured materials in emerging solar cell applications, such as sensitized and heterojunction architectures, including those embedded with promising perovskite materials. The remarkable advancement in each solar cell architecture is highlighted towards achieving high power conversion efficiency and operational stability. We also discuss the foremost bottleneck for further improvements and the future outlook for large-scale practical applications.

(1952). Mathematical theory of stationary dislocations. Advances in Physics: Vol. 1, No. 3, pp. 269-394.

Introduction Stress corrosion cracking is a phenomenon that is of interest to a wide range of metal users. When it occurs under service conditions, often without any prior indication of impeding failure, its effect may be catastrophic.

Abstract Cup-and-cone fracture in single-phase ductile metals appears to originate at holes formed by drawing away of material from non-metallic inclusions, as suggested by Tipper. In copper, the holes expand under the triaxial stresses in the neck and coalesce in a macroscopic fissure; in α iron fine cracks are formed by the stress concentrated at the holes. In coarsegrained material shear cracks are formed on the surface of the neck. Pure polycrystalline aluminium separates at the neck of a tensile specimen by slipping-off along a plane of shear. This is thought to be the usual mode of failure in materials in which work-hardening has been exhausted.

Abstract The crystallographic characteristics of deformation twinning are derived by considering the atomic movements which occur at the moving interface as a twin propagates. This is facilitated by making use of the notation of the tensor calculus, and general expressions, valid for all crystal structures, are obtained giving the magnitude of the twinning shear and relating the twinning elements for both type I and type II twinning. The atomic shuffles, which in general must accompany the twinning shear in both single and multiple lattice structures, are examined in detail and expressions are derived for their magnitudes and directions for the cases of the four classical orientation relationships associated with deformation twinning. The use of these expressions in predicting operative twinning modes is described and the relations between this theory and other recent theories of the crystallography of deformation twinning are discussed.

A system of ferromagnetic β phase Ni–Co–Al alloys with an ordered B2 structure that exhibits the shape memory effect has been developed. The alloys of this system within the composition range Ni (30–45 at. %) Co–(27–32 at. %) Al, undergo a paramagnetic/ferromagnetic transition as well as a thermoelastic martensitic transformation from the β to the β′(L10) phase. The Curie and the martensitic start temperatures in the β phase can be controlled independently to fall within the range of 120–420 K. The specimens from some of the alloys undergoing martensitic transformation from ferromagnetic β phase to ferromagnetic β′ phase are accompanied by the shape memory effect. These ferromagnetic shape memory alloys hold great promise as new smart materials.

Abstract From measurements of the radius of curvature of extended dislocation nodes observed by transmission electron microscopy, stacking-fault energies have been determined for a number of copper and silver base aluminium and zinc solid solution alloys and also for a number of nickel-cobalt alloys. Rough estimates of about 40, 25 and 150 erg cm−2 respectively can be made of the stacking-fault energies of copper, silver and nickel. The accuracy of the measurement and the possibility of segregation of solute atoms to stacking faults are discussed.

Carbon-containing NiCo2S4 hollow-nanoflake structures were fabricated by a one-step solvothermal method using CS2 as a single source for sulfidation and carbonization. The reaction mechanism for the hollow structure with carbon residues was explored based on the formation of a bis(dithiocarbamate)-metal complex and the Kirkendall effect during solvothermal synthesis. The NiCo2S4 nanoflake electrode exhibited a high specific capacitance of 1722 F g-1 (specific capacity 688.8 C g-1) at a current density of 1 A g-1 and an excellent cycling stability (capacity retention of 98.8% after 10 000 cycles). The as-fabricated asymmetric supercapacitor based on NiCo2S4 nanoflakes and activated carbon electrodes revealed a high energy density of 38.3 W h kg-1 and a high power density of 8.0 kW kg-1 with a capacitance retention of 91.5% and a coulombic efficiency of 95.6% after 5000 cycles, highlighting its great potential for practical supercapacitor applications.

Abstract The dynamical theory of diffraction contrast is applied to elastic inclusions. The visibility of such inclusions is discussed, and the best conditions for obtaining information about the state of strain around them derived. It is shown how strains can be measured around prismatic dislocation loops, and G.P. 2-type precipitates. An Appendix shows how it may be possible to derive information about the type of strain field surrounding a precipitate by a study of its image.

Abstract A criterion for the transition in fracture mode from ductile to cleavage is calculated for fracture at a notch. It is concluded that the temperature-dependence of this transition probably arises more from that of the Peierls-Nabarro stress required to move a free dislocation in α-iron than from the temperature-dependence of the locking of e dislocation source. The transition temperature is found to be a function of grain size, the friction on a free dislocation, the strength of the dislocation locking, the degree of triaxiality of the applied stress and the elastic constants.