Use of neutron diffraction for stress measurements in thin and thick thermal sprayed coatings

Abstract

Thermal spraying is a widely used and cost effective technique for the surface protection of engineering components. The spectrum of applications is vast: corrosion protection, wear resistance and abrasion resistance, thermal barriers, electrical (dielectric) coatings, etc. Process induced residual stress has long been recognised as an important factor influencing the integrity and overall performance of coatings. Residual stress generation during thermal spraying is a complex phenomenon. Significant efforts have been made to improve understanding of the evolution of residual stresses during deposition and to develop practical models for numerical prediction of stress distributions in coatings. Owing to the high penetrating power of neutrons and spatial resolution in the millimetre and submillimetre range, neutron diffraction is, perhaps, the most versatile method for stress determination, and has been used extensively for experimental validation of theoretical predictions. Examples of neutron diffraction residual stress results are presented to illustrate the capabilities of the technique: a thin (～0·3 mm) Mo/Mo₂C composite HVOF coating, several examples of millimetre thick ceramic and metallic coatings, and thick coatings (～10 mm) of iron made by spray forming.     

Thermal stability and age hardening of supersaturated AlCrN hard coatings

Abstract

During advanced machining processes (high speed and dry cutting), the temperature at the cutting edge can exceed 1000°C. For modern protective hard coatings, thermal stability is of major interest. Equally important are superior mechanical properties, such as hardness, remaining at a high level over a wide temperature range. AlCrN coatings perform well in cutting tests and show excellent oxidation resistance as well as good tribological behaviour. In this work, supersaturated cubic Al_0.7Cr_0.3N coatings deposited by cathodic arc evaporation are studied. The phase and microstructure evolution of the material is investigated up to 1450°C using a combination of differential scanning calorimetry, thermal gravimetric analysis, mass spectrometry, X-ray diffraction and analytical transmission electron microscopy. During annealing up to 925°C, hexagonal AlN precipitates are formed at grain boundaries. At higher temperatures, a transformation of the remaining cubic AlCrN matrix into Cr via Cr₂N takes place, accompanied by a release of nitrogen. After annealing up to 1450°C, the AlN grains coarsen and coalesce around the Cr and Cr₂N grains. The results explain the superior cutting performance by the formation of precipitates, but also demonstrate the limitations in usage at high temperature regimes due to decomposition. Nevertheless, the substitution of Cr in the CrN lattice by Al has proven to increase the decomposition resistance significantly. Finally, nanoindentation experiments reveal that AlCrN coatings retain hardness beyond the stage of residual stress recovery up to 900°C, demonstrating an age hardening process.     

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

Abstract

The microstructural transformations and the dimensional evolution of green specimens obtained by pressing mixtures of austenitic and ferritic stainless steel powders have been investigated by sintering at 1120 and 1240°C. Dilatometry experiments show that the linear shrinkage is influenced by the amount of ferritic powder. Moreover, during sintering Ni diffuses into the ferritic grains causing austenite destabilisation and the formation of a mixed constituent, whose constitution has been investigated by means of EDXS and interpreted on the basis of the Schaeffler diagram. Sigma phase also forms during sintering of the duplex mixtures.     

Effects of strain aging on the structure and mechanical properties of PM 92·5W–5Ni–2·5Fe heavy alloys

Abstract

This paper describes the effects of strain aging on the mechanical properties and the microstructure of forged 92·5W–5Ni–2·5Fe and its heavy alloys microalloyed with cobalt. The investigation was performed on cold rotary forged rods deformed 15, 20 and 30% and strain aged at temperatures from 673 to 1273 K for 1·8–32·4 ks. The results show that for these alloys, there is a temperature range from 773 to 873 K in which maximum ultimate strength and hardness can be attained. Furthermore, the strain aged alloys have shown strength and hardness increase at a temperature of 973 K in a time period of 10·8 ks. The fracture analysis has shown the presence of predominant transgranular fracture of the tungsten phase and γ-phase in the strain-aged alloys in comparison with the forged alloys. The results indicate that interface and tungsten phase strengthening are predominant mechanisms of strain aging.     

Conference Report - 4th European Continuous Casting Conference

Abstract

In this research carbothermic reduction of mechanically activated hematite–graphite–copper mixture was investigated. The effects of Copper and milling time on reduction behaviour of mixtures were studied by differential thermal analysis and thermogravimetry experiments. SEM and XRD techniques were also used to evaluate microstructure and phase constituent of the samples. By mechanical activation of the hematite–graphite–copper mixture the reduction temperature could be decreased >200°C. The presence of copper in mixture has an additional effect on the initiation of the Boudouard reaction at lower temperatures. Thus the gaseous reduction of Hematite with CO was started at a temperature as low as 790°C when Cu was used in the activated mixture.     

Effect of Mg on the evolution of non-metallic inclusions in Mn–Si–Ti deoxidised steel during solidification: experiments and thermodynamic calculations

Abstract

The effects of Mg on the evolution of non-metallic inclusions in Mn–Si–Ti deoxidised steels during solidification were investigated in a study based on experiments and thermodynamic calculations. The inclusions were composed of the MgO–MnO–Ti₂O₃–TiO₂ oxide, MnS, and TiN. With the increase of Mg concentration in steels, the phases of oxide inclusions were changed, in the order of pseudobrookite (Ti₃O₅–MnTi₂O₅), ilmenite (MgTiO₃–MnTiO₃–Ti₂O₃), spinel (Mg₂TiO₄–MgTi₂O₄–Mn₂TiO₄–MnTi₂O₄) and MgO. Thermodynamic calculations for inclusion evolutions were in good agreement with the experimental results.     

Oxide ionic conductivity and crystallographic properties of tetragonal type Bi2O3-based solid electrolyte doped with Ho2O3

Abstract

In the present work, the authors have investigated the binary system of (Bi₂O₃)_1–x(Ho₂O₃)_x. For the stabilisation of the tetragonal type solid solution, small amounts of Ho₂O₃ were doped into the monoclinic Bi₂O₃ via solid state reactions in the stoichiometric range 0·01≤x≤0·1. The crystal formula of the formed solid solution was determined as Bi(III)_4–4xHo(II)_4xO_6–2xVo_(2+2x) (where Vo is the oxide ion vacancy) according to the XRD and SEM microprobe results. In the crystal formula, stoichiometric values of x were 0·04≤x≤0·08, 0·03≤x≤0·09, 0·02≤x≤0·09 and 0·04≤x≤0·09 for annealing temperatures at 750, 800, 805 (quench) and 760°C (quench) respectively. The four probe electrical conductivity measurements showed that the studied system had an oxide ionic type electrical conductivity behaviour, which is increased with increasing dopant concentration and temperature. The obtained solid electrolyte system has an oxygen non-stoichiometry characteristic, and it contains O^2– vacancies, which have disordered arrangements in its tetragonal crystal structure. The increase in the amount of Ho₂O₃ doping and temperature causes an increasing degree of the disordering of oxygen vacancies and a decrease in the activation energy E_a.     

Effect of alloying elements on properties and microstructures of Ti–V–Cr burn resistant alloys

Abstract

Burn resistant Ti alloys have been developed over the last 10 years. The aim of the present paper is to study the effects of alloying elements on properties and microstructures of Ti–V–Cr burn resistant alloys. The alloys Ti–35V–15Cr, Ti–25V–15Cr, and Ti–25V–15Cr–3Al usually show simple β phase structures, and recrystallisation finishes at 800°C. The Ti–25V–15Cr alloy has good workability, tensile properties, and thermal stability compared with the Ti–35V–15Cr and Ti–25V–15Cr–3Al alloys. There are α precipitates in the Ti–25V–15Cr alloy after exposure at 500°C for 100 h, which leads to a decrease in the thermal stability.     

Behaviour of surface oxide scale before roll bite in hot rolling of steel

Abstract

Hot stalling rolling tests using mild steel slabs with thin and thick scale layers, and cold stalling rolling simulation tests using lead slabs coated with brittle lacquer layers, have been conducted to investigate the behaviour of the surface oxide scales before hot rolling. The effects of rolling reduction, temperature, scale thickness and structure, and slab geometry were examined. The experimental results indicate that the oxide scale can either adhere and deform with the parent steel, or delaminate from the parent steel, or suffer from cracking before hot rolling, depending on the hot strength of the scale and the stress status in the scale, which depend on rolling reduction and temperature, and scale thickness and structure. Therefore, the cracks observed in the oxide scales on the steel slab after hot rolling are generally the combined results of cracking before rolling and further cracking in the roll gap. On the basis of comparative analysis, the prerolling behaviour of the oxide scales in a laboratory scale rolling mill and in an industrial hot strip mill is discussed.     

Impression creep of Sn–40Pb–2·5Sb peritectic solder alloy

Abstract

The power law creep behaviour of the Sn–40Pb–2·5Sb peritectic solder alloy was investigated using an impression test apparatus. The tests were carried out under constant stress in the range 17 to 39 MPa and at temperatures in the range 296 to 363 K. Assuming a power law relationship between the impression velocity and stress, power law stress exponents in the range 1–3 were determined. Analysis of the data showed that for all loads and temperatures, the activation energy was stress independent with values in the range 51–56 kJ mol^-1. Based on the stress exponents obtained and activation energy data, it is proposed that grain boundary diffusion is the major mechanism for creep of the Sn–Pb–Sb peritectic alloy under these test conditions.