Materials Science and Technology

Ni-based superalloys in turbine disc applications face increasing susceptibility to oxygen-assisted fatigue crack propagation due to increased turbine entry temperatures. The continued lack of understanding of the interplay between the factors operating during oxygen-assisted fatigue crack propagation limits: (1) development of lifing methodologies to accurately predict the fatigue performance of disc alloys/components and (2) associated disc alloy developments. An underpinning requirement to better understand the role of oxygen is to characterise the process of oxygen diffusion in the localised stress/strain state at the crack tip, which is related closely to microstructural features. The link between three-dimensional crack tomography, crack propagation rate and oxygen-related attack needs to be established. Quantitative models which include the interaction between fatigue–creep–oxygen attack need further development.

Analysis of real contact area and thermal resistance combined with experimentally derived interfacial heat transfer coefficient values led to the development of an advanced finite element based model to simulate the heat transfer at the oxidised tool/workpiece interface during hot steel rolling. An extensive progress review and building on the Sellars 1990's core assumptions are discussed. Today, oxide scale failure is predicted, taking into account the main physical phenomena such as stress directed diffusion, fracture and adhesion of the oxide scale. The separation loads within the scale metal/system are measured during testing. They are sensitive to the chemical composition of steel. The assumption of several parallel heat flow systems at the roll/stock interface remains the core model for today's research.

III-Nitrides are materials that have revolutionised the lighting industry allowing for the development of high brightness and efficiency white light emitting diodes (LEDs), enabling cost and energy savings at an unprecedented scale. However, there remain several obstacles to the further enhancement of the efficiency of LEDs, particularly for emission at longer wavelengths. The existence of polarisation fields as an inherent property of wurtzite III-nitride materials severely hampers LED performance. The origin of these fields due to the deviation from an ideal tetrahedral bonding structure and their relation to strain has been addressed in this review. The effect of the polarisation fields on the band structure of heterostructure quantum wells, known as the quantum confined stark effect, and its implications for the efficiency and spectral stability of LEDs have also been reviewed. Finally, the effectiveness and viability of several proposed methods of mitigating the harmful effects of the polarisation fields, such as the growth of III-nitrides on alternative planes, doping, strain engineering and growth of cubic GaN, have been addressed.

Lithium hydride remains theoretically the most efficient neutron shield material, showing immense application prospects. The properties of Lithium hydride neutron shielding material, such as use temperature, mechanical properties, thermal conductivity and irradiation swelling behaviour, were reviewed. Factors that affect the irradiation swelling of Lithium hydride were also discussed in this paper. The development trends of lithium hydride neutron shielding material were proposed.

Friction stir welding of AA5456 aluminium alloy with different thicknesses was investigated for single- and double-pass lap joint configurations. The influences of tool tilt angle in first and second passes, and rotational and welding speeds in second pass on metallurgical structure and joint strength were studied. The results indicated that tilt angle significantly influences material flow and imperfection formation, and accordingly controls the weld mechanical properties. The best results were achieved by tilt angle of 5° for single pass, and tilt angle, rotational speed and traverse speed of 5°, 250 rev min^− 1 an 50 mm min^− 1 respectively for double-pass. The characteristics of hooking and cold lap defects were used as criteria to recognise the influence of processing conditions on joint performance.

A protective Ni–Co alloy coating was successfully prepared on sintered neodymium–iron–boron (NdFeB) magnet via potentiodynamic electrodeposition in this paper. The microstructure and surface morphology analysis of Ni–Co alloy coating showed that the surface morphologies were quite compact and more uniform than that prepared by direct current (DC) electrodeposition; significantly, the chemical composition analysis indicated that the content of Co in the alloy coatings amounted to 33–34 wt-. Potentiodynamic polarisation and long term immersion test results revealed that the Ni–Co alloy coating could supply excellent corrosion protection for NdFeB substrate in 3.5 wt- NaCl solution. Overall, the Ni–Co alloy coating with higher Co content prepared by potentiodynamic electrodeposition exhibited better corrosion protection for sintered NdFeB magnet compared with that prepared by DC electrodeposition.

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10 nm, can be formed when the concentration of Zr is in the range of 0.015–0.030. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.

Deformation temperature and manganese content dependencies of twinning activity in two types of high Mn austenitic steels were investigated upon high velocity tensile loading. It was observed that nanotwin formation within previously formed twins dominates at subzero temperatures and significantly contributes to work hardening.

Fatigue is one of the most important life limiting factors for bearings. A number of different kinds of tests are used to assess the behaviour of bearing steels when subjected to repeated loading. The work presented here attempts to rationalise these test methods on the basis that they should all be related by the volume of material that is stressed. The analysis focuses on one particular steel, ‘AISI 52100’, which is the most common alloy used in the manufacture of bearings. It is demonstrated that a physically reasonable model can be produced using the neural network technique, in spite of some quite dramatic differences in sample geometry and loading conditions, thus establishing that simple tests can, in principle and practice, be used to estimate the performance during the fatigue experiments.

In this study, hot workability of Ti–35V–15Cr–0.3Si–0.1C alloy is investigated in the temperature range of 900–1150°C and strain rate range of 0.01–10 s^− 1 using processing maps. In the maps, the stability domain displays the feature of dynamic recrystallisation and dynamic recovery. The fraction of recrystallisation in this alloy is much larger than that in other β-Ti alloys, which can be attributed to the effect of titanium carbides by a ‘particle stimulated nucleation’ mechanism. Higher temperature and lower strain rate are helpful for breaking down the carbides and improving the workability. However, deforming >1100°C would result in grain coarsening. In instability domain, the occurrence of flow localisation, shear bands and cracking is discussed. The optimised processing window is in 1050–1100°C /0.01–0.1 s^− 1.

In the present study, the characterisation of phase stability and measurement of different thermophysical properties of alloy 740 has been carried out. The transformation temperatures including liquidus/solidus and corresponding enthalpy of transformation have been measured for different phase changes up to melting using dynamic calorimetry. Further, the enthalpy increment data have been measured in the temperature range of 473–1473 K to obtain the heat capacity using static calorimetry. The present calorimetric data have been analysed in corroboration with the results obtained using JMatPro and Thermo-Calc simulation. In addition, the temperature dependence of other thermophysical properties such as thermal expansivity, density, thermal diffusivity and thermal conductivity are also measured in the range of 300–1473 K using thermomechanical analyser and laser flash method.

The effects of solution treatment and large strain rolling (LSR) on the microstructure and mechanical properties of Mg–10.73Li–4.49Al–0.52Y alloy were investigated. Results showed that after solution treatment, α-phase and AlLi phase were dissolved into β matrix, which led to the increase of strength. With the increased temperature of LSR, new phase MgLi₂Al (θ-phase) and AlLi phase precipitated from the matrix in turns. The Mg–Li alloy rolled at 623 K with 75 reduction showed the ultimate tensile strength of 328 MPa, which was more superior to many other Mg–Li alloys. The excellent strength could be explained by the mechanisms of solution strengthening and fine grain strengthening.

The effect of Zr addition on the toughness and microstructure of coarse grained heat affected zone (CGHAZ) in Al deoxidised low carbon steel was investigated. In this present work, the specimens (Zr free and Zr added) were subjected to weld thermal cycle with heat input of 54, 80 and 100 kJ cm^− 1 at 1350°C peak temperature using thermal simulator. The typical inclusions were characterised by scanning electron microscopy, and the chemical composition of the inclusions was determined by energy dispersive spectroscopy. The nanoscale precipitates were analysed by transmission electron microscopy and energy dispersive spectroscopy. It was verified that the occurrence of Zr in steel mostly exists in the form of oxide inclusions, but a few in the form of solid solution state and Zr containing precipitates when the concentration of Zr is 0.015. The improvement of CGHAZ toughness is obtained when the heat input is 80–100 kJ cm^− 1. The possible reasons about the effects of Zr on the toughness of CGHAZ, including Zr containing inclusions, precipitates and microstructure characteristics, were also discussed.