Materials Science and Technology

Fatigue cracks in titanium alloys are often found to initiate at faceted alpha grains. In the very high cycle fatigue regime, crack initiation tends to shift from the surface towards the interior of the material, and more initiation facets can be found on the fracture surface. In this study, fatigue tests were performed on drawn and heat-treated Ti–6Al–4V wires. Only a few samples fractured due to interior initiation. The facets at the initiation sites of these samples were not flat, but had markings on the nano-scale, and were highly inclined. A possible explanation for these aspects is the crystallographic texture of the wire, and a reflection is made on the suggested mechanisms of facet formation.

This paper is part of a Themed Issue on Euromech 570: Interface-dominated materials.

In this paper, a multiscale numerical method is presented with the aim to model the response of a Ti6Al4V sheet under explosive forming. The numerical modelling focuses on the accurate definition of the plastic deformation of the Ti6Al4V specimens based on the texture of the material. The viscoplastic self-consistent polycrystal model code (VPSC) is used as a link between macroscopic response and the underlying microstructure taking into account the viscoplastic deformation of the specimen. Comparison is made with experimental results. The Cazacu–Barlat (CB06) material model is used because of its capability to describe the yielding asymmetry between tension and compression and to take into account the anisotropic behaviour of the sheet. The study focuses on the evaluation of the material model parameters and on how these affect the structural response of the Ti6Al4V specimens under explosive loading.

This paper is part of a Themed Issue on Euromech 570: Interface-dominated materials.

Depth-dependent hardness variation of dimethylamine borane-reduced electroless Ni–5 wt-%B deposits has been examined using the nanoindentation technique. The deposits were characterised using ICP-OES, FESEM, XRD and DSC for evaluating the composition, morphology, structure and phase transformation behaviour, respectively. Coatings were also analysed for hardness and wear resistance. The surface of the as-plated deposit exhibits a typical nodular morphology. DSC traces show the presence of a single exothermic peak at 313°C conforming to its phase transformation. X-ray diffraction pattern of as-prepared deposit contains a mixture of amorphous and sharp microcrystalline nickel peaks. Heat-treated coating exhibits improved hardness and wear resistance. Depth-dependent nanohardness profile of as-deposited film neither obeys Nix–Gao nor the Lam–Chong model of indentation.

FeCrNiCoB _x high-entropy alloy coatings were prepared by laser cladding, and the effects of boron addition on the microstructure, hardness and corrosion resistance of the coatings were studied. Results showed that the coatings comprised a simple FCC solid solution with boride precipitation. When 0.5 ≤ x ≤ 1.0, (Cr, Fe)₂B with an orthorhombic structure was found in the coatings, and the hardness and corrosion resistance of the coatings were enhanced by increasing boron content. As x approached 1.25, the borides changed from orthorhombic (Cr, Fe)₂B to tetragonal (Fe, Cr)₂B, which deteriorated the corrosion resistance of the coatings. Stacking faults found in (Cr, Fe)₂B may be caused by the phase transformation of borides.

In the present work, W–Bi–S-tridoped TiO₂ nanoparticles were synthesised by a simple sol–gel method. The structure and morphology of as-prepared W–Bi–S-tridoped TiO₂ were characterised by using X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy and UV–Vis diffuse reflectance spectrum. It was found that the W–Bi–S-tridoped TiO₂ nanoparticles were well crystallised with a small size distribution. The absorption edge of TiO₂ was extended into visible-light region obviously after being doped with W, Bi and S. The photocatalytic degradation of methylene blue was used as a probe reaction to evaluate the efficiency of W, Bi and S doping. The W–Bi–S-tridoped TiO₂ exhibited the best photocatalytic activity, compared with TiO₂, S-TiO₂, W–S–TiO₂ and Bi–S–TiO₂. The mechanism that enhanced photocatalytic activity might be attributed to the synergistic effect of W, Bi and S.

Addition of alloying elements usually moves eutectoid point to left whilst aluminium enhances eutectoid carbon content. The carbon expands lattice parameters of austenite. High aluminium addition in steels led to volume expansion at pearlite-to-austenite transformation since specific volume of austenite is larger than that of mother phases, which is proposed to determine the volume expansion or contraction. It has been proven by calculations based on thermodynamics as well as accurate experimental observations of dilatometry study.

In the present work, crack networks with white etching areas (WEAs) in cross-sections of bearings were investigated by a complementary use of SEM and TEM with the focus on the use of orientation contrast imaging and electron backscatter diffraction (EBSD). Orientation contrast imaging was used for the first time to give detailed insight into the microstructure of WEA. A significant difference between Nital-etched and polished WEA samples was observed. It was revealed that WEAs are composed of different areas with varying grain sizes. As a result of secondary transformation, needle-shaped grains were observed within WEAs. Using EBSD analysis, evidence was obtained that WEA formation and accompanying crack growth are without relation microstructural features. In addition, an inhomogeneous chemical structure of WEA as a result of carbide dissolution is revealed by analytical investigations.

The molybdenum–iron–boron–chromium claddings with different Mo/B atomic ratios were produced on Q235 steel using argon arc welding. The microstructure and crystalline phases were studied by optical microscopy, scanning electron microscopy and XRD. In addition, the formation mechanism of hard phase was investigated by thermodynamic calculations and phase diagrams. The results showed that the claddings were composed of the Mo₂FeB₂, M₃B₂ (M: Mo, Fe and Cr) hard phases and σ-FeMoCr solid solution. In addition, calculated results revealed that the M₃B₂, MB and σ-FeMoCr were successively precipitated from the melting pool. Moreover, the maximum microhardness value of the cladding was about 1600 HV_0.5. Wearing test indicated that claddings of lower Mo/B ratios have better wear resistance.

The influences of deformation degrees on the microstructure evolution of GCr15 steel in cold ring rolling and following heat treatment (quenching and temper) are investigated in this paper. As the deformation is uneven, microstructure at the outer surface, middle place and inner surface on the cross-section of the ring is selected for the study. The microstructure is characterised by scanning electron microscopy, optical microscopy and X-ray diffraction. The transformation is studied with differential scanning calorimetry, and the hardness is examined by Vickers hardness tester. The results show that the deformation is severe on the outer surface but slight on the middle place and inner surface. A large degree of deformation leads to the acceleration of carbide dissolution, the grain refinement and the increase of hardness. In addition, the deformation results in the decrease of transformation temperature according to the differential scanning calorimetry data.

The purpose of the research reported here was to establish how plug shape influences the size and porosity of hollow shells produced by Mannesmann piercing of an ingot. Ingots were cast from 6060 aluminium alloy and then pierced with a solid plug, a plug with a cavity or a hollow plug. Computer simulation of these processes was carried out using ProCast and QForm software and showed that a hollow plug should produce hollow shells with minimal porosity. However, the results of the experimental piercing revealed that the hollow plug in fact produced hollow shells with higher porosity than that of shells produced using either of the other two plugs. This appears to be consequence of the hollow plug losing stability.

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.

Roll forming is one of the fastest developing forming technologies for martensitic ultra-high-strength steels in the automotive industry. As the automotive industry continues to demand materials exhibiting higher strength in addition to improved formability, development of novel martensitic ultra-high-strength steels for roll forming technologies is a priority topic in the steel industry. Six such novel steels were developed, exhibiting ultimate tensile strength from 1500 to 2000 MPa, total elongation from 4% to 11%, bend ratio from 5t to 2t and hole-expansion capacity from 18% to 80%. It was concluded that the novel steels exhibit very attractive properties for roll forming technologies and provide low cost down-gauging opportunities with improved crashworthiness for the automotive industry.