Fracture toughness of Ti-15Al-8Nb alloy under mixed mode I/III loading

The effect of mixed mode I/III loading on fracture toughness of Ti-15 at.% Al-8 at.% Nb alloy, which undergoes stress-induced martensitic transformation, was investigated for four different grain sizes. The fracture toughness under mixed mode I/III loading was found to be significantly higher than that under mode I loading in all cases. The results were explained on the basis of the stress and strain fields ahead of a mixed mode crack and its influence on the martensitic transformation zone.     

Effect of temperature on the mode I and mixed mode I/III fracture toughness of SA333 steel

The effect of temperature on tensile properties, mode I and mixed mode I/III fracture toughness of SA333 Grade 6 steel was investigated. The variation of ultimate tensile strength and strain hardening exponent with temperature as well as the appearance of serrations in the stress-strain plots indicated that dynamic strain aging regime in this steel is in the temperature range 175-300 °C at a nominal strain rate of 3 × 10⁻³ s⁻¹. Both mode I and mixed mode I/III fracture toughness values were found to exhibit a significant reduction in the DSA regime. The mixed mode I/III fracture toughness was found to be significantly lower than the mode I fracture toughness at all temperatures. However, the difference between the two toughness values was much higher prior to the onset of DSA. The results are explained on the basis of the nature of deformation fields under mode I and mixed mode I/III loading as well as the fracture mechanism prevalent in these steels at different temperatures.     

Investigation of α platelet boundaries in a near-α titanium alloy

Transmission electron microscopy (TEM) of a bimodal near-α titanium alloy revealed the existence of retained β phase layers and silicide precipitates at the α platelet boundaries inside transformed β grains. The β to α phase transformation accompanied by the precipitation of silicide resulted in the formation of a large number of dislocations at α platelet boundaries. Orientation relationships between silicide, β phase and α phase were also identified. However high-resolution TEM (HRTEM) revealed crystal mismatches between these phases generating high strains at α platelet boundaries. The strengthening effects of the platelet boundaries are discussed in terms of dislocations slip across the boundaries. The mechanism that governs the β to α phase transformation is also discussed.     

An alternative method to separate and analyse the microtextures and microstructures of primary alpha grains and transformed beta grains in near-α titanium alloy Timetal 834

An alternative technique to that recently put forward by Germain et al. [Materials Characterization 54 (2005) 216–222] to separate the orientations of primary alpha (α_p) grains from those of transformed beta (α_s) grains in the near-α titanium alloy Timetal 834 is presented. This new method involves correlating orientation image maps (OIM) obtained through electron back-scattered diffraction (EBSD) with optical images of the same area. By using optical microscopy and an appropriate etch, strong contrast between α_p and α_s is obtained enabling regions within an OIM containing the α_p and α_s to be determined using relatively straightforward image analysis. Results are presented for both high-resolution microstructure analysis and texture level EBSD datasets for material subjected to a simulated industrial thermomechanical forging process. A comparison is also made between the texture datasets for both α_p and α_s obtained using this method and that obtained using grain size distribution acquired directly from the EBSD dataset. In this case very little difference was found between the separated textures, suggesting that in the first instance a morphological/grain size approach direct from the EBSD dataset is sufficient for observing any trends in texture evolution of both α_p and α_s.     

Effect of mixed mode I/II on fracture behaviour of laminated metal matrix composites

Abstract

The effects of mixed mode loading (I/II) on the fracture toughness and fracture behaviour of both 6090/SiC/20_p-6013 diffusion bonded laminates and 2080/SiC/20_p-2080 adhesive bonded laminates tested in the crack arrester orientation were investigated. The effects of layer thickness and volume fraction ratio on the fracture behaviour under the mixed mode were also studied. The fracture behaviour under mode I/II of available similar discontinuously reinforced aluminium (DRA) materials was additionally compared to that of the laminates. The fracture behaviour of laminates under mode I/II was dependent on the volume fraction ratio and generally different from that of the monolithic and DRA. The increase in the fracture toughness of DRA by lamination with ductile layers under mode I changes somewhat under increasing load mixity, for 75/25 and 50/50 diffusion bonded laminate and 60/40 adhesive bonded laminate ABL. This results from extensive interfacial separation and delamination between the layers.     

Role of Ti<Subscript>3</Subscript>Al/silicides on tensile properties of Timetal 834 at various temperatures

Extremely fine coherent precipitates of ordered Ti₃Al and relatively coarse incoherent precipitates of S₂ silicide exist together in the near α-titanium alloy, Timetal 834, in the dual phase matrix of primary α and transformed β. In order to assess the role of these precipitates, three heat treatments viz. WQ, WQ-A and WQ-OA, were given to have no precipitates, Ti₃Al and silicide and only silicide precipitates in the respective conditions. Tensile properties in the above three heat treated conditions were determined at room temperature, 673 K and 873 K. It was observed that largely Ti₃Al precipitates were responsible for increase in the yield strength and decrease in ductility in this alloy.     

The electronic effect of N impurity in an 1 0 0 edge dislocation core system in α-iron

First-principles methods are employed to investigate the structure relaxation and the electronic structure of a N impurity in an 1 0 0 edge dislocation core (DC) system in α-iron. A 96-atom cluster model is used to simulate the local environment of N impurity in the edge dislocation. By use of the DMol method, we obtained an optimized atomic configuration for the system by calculating the forces on N impurity and its neighboring Fe atoms, and by minimizing the total energy of the cluster model. The optimization results show that the N impurity moves away from compression region to a stable position in the dilated region. By use of the discrete variational (DV) method, we calculated energetic parameters (structural energy and interatomic energy) and charge distribution. From these results, one can find that N impurity has a strong interaction with its adjacent Fe atoms in the DC system. Moreover, notable charge redistribution between the N impurity and Fe atoms indicates the formation of N impurity–Fe dislocation complex, which implies the trapping effect of DC on N impurity.     

Hydrothermal synthesis and characterization of monodisperse α-Fe2O3 nanoparticles

Monodisperse α-Fe₂O₃ nanoparticles have been successfully prepared by hydrothermal synthetic route using FeCl₃, CH₃COONa as reagents and reacted at 200 °C for 12 h. The morphology and structure of products were characterized by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results showed that the α-Fe₂O₃ nanoparticles were single-crystalline hexagonal structure and average diameters were about 80 nm. Magnetic properties have been detected by a vibrating sample magnetometer at room temperature. The nanoparticles exhibited a ferromagnetic behavior with the coercive force (Hc), saturation magnetization (Ms) and remanent magnetization (Mr) was 185.28 Oe and 0.494 emu/g, 0.077 emu/g.     

Effect of Ni-modified α-Al2O3 prepared by sol–gel and solvothermal methods on the characteristics and catalytic properties of Pd/α-Al2O3 catalysts

In the present study, Ni-modified α-Al₂O₃ with Ni/Al ratios of 0.3 and 0.5 were prepared by sol–gel and solvothermal method and then were impregnated with 0.3 wt.% Pd. Due to different crystallization mechanism of the two preparation methods used, addition of nickel during preparation of α-Al₂O₃ resulted in various species such as NiAl₂O₄, mixed phases between NiAl₂O₄ and α-Al₂O₃, and mixed phases between NiAl₂O₄ and NiO. As revealed by NH₃-temperature programmed desorption, formation of NiAl₂O₄ drastically reduced acidity of alumina, hence lower amounts of coke deposited during acetylene hydrogenation was found for the Ni-modified α-Al₂O₃ supported catalysts. For any given method, ethylene selectivity was improved in the order of Pd/Ni–Al₂O₃-0.5 > Pd/Ni–Al₂O₃-0.3 > Pd/Ni–Al₂O₃-0  Pd/α–Al₂O₃-commercial. When comparing the samples prepared by different techniques, the sol–gel-made samples showed better performances than the solvothermal-derived ones.     

Simultaneous determination of Np, Pu and Am in HNO3 solution by combining extraction, liquid scintillation counting, and α spectrometry

α Spectrometry is one of the most important and sensitive techniques for the assay of α-emitting nuclides, but various complicated procedures are often required for preparation of uniform thin plate source. Liquid scintillation counting (LSC) with pulse shape analysis (PSA) combining extraction is much simpler, more rapid and accurate technique, but the extraction processes are often very long. By combining extraction, LSC with PSA, and α spectrometry, we proposed a new approach to determine ²³⁷Np, ^238–240Pu and ²⁴¹Am. The new approach includes an extraction process of three steps for blind samples or of only one step for those samples without tailing interference with α spectra. Moreover, no complicated procedure is required for α plate source preparation because even non-uniform plate source is feasible for the new approach. The approach has taken advantages of high counting efficiency (nearly 100%) of LSC for α-radiation, high energy resolution of α spectrometry and high recovery yield of actinides by trialkylphosphine oxide (TRPO) extraction. The approach with one step extraction can be expected to determine ²³⁷Np, ^238–240Pu, ²⁴¹Am and ²⁴⁴Cm simultaneously.     

Influence of subgrain orientation on martensitic transformation in β1–Cu–Zn–Al-single crystals

In β₁–Cu–Zn–Al single crystals the course of cyclic martensititic transformation ‘β₁ parent phase↔γ′₁ martensite’ induced by tensile stress were studied with use of X-ray topography, light microscopy and etch pits. Two groups of single crystals were studied. The first one (OR) contained single crystals of subgrain boundaries parallel to the direction of elongation [001], the second one (RA) consisted of single crystals of random subgrain boundaries orientations. Single crystals from the RA group cracked after about 300 cycles of martensitic transformation; single crystals from the OR group did not crack even after 1200 cycles. In OR single crystals changes of dislocation density inside the subgrains caused by cycling occurred much more slowly than in the RA single crystals. This has been related to the dislocation movement from inside the subgrains to their boundaries.     

Doping-induced microstructural, textural and optical properties of In2Ti1−xVxO5+δ semiconductors and their role in the photocatalytic splitting of water

The physicochemical properties of V-doped indium titanates (In₂Ti_1−xV_xO_5+δ, 0.0 ≤ x ≤ 0.2) were investigated by using XPS, powder XRD, UV–vis, SEM and luminescence spectroscopy techniques. The Rietveld refinement of XRD data revealed that even though the V-containing samples were isostructural with In₂TiO₅ (orthorhombic space group Pnma), a systematic x-dependent variation was noticeable in the Ti–O bond lengths in [TiO₆] octahedral units, cell parameters and in the value of δ. XPS results confirmed the coexistence of V⁵⁺ and V⁴⁺ states, leading thereby to an enhancement in oxygen non-stoichiometry in the doped samples. A loading-dependent progressive shift from 400 to 750 nm was also observed in the onset of the absorption edge, indicating a significant narrowing of the band gap. Furthermore, the samples with higher V-content were comprised of the grain clusters having larger size and an irregular shape. The UV–vis, photoluminescence and thermoluminescence studies indicate that the doping-induced lattice defects may give rise to certain closely spaced acceptor/donor energy levels in between the band gap of host matrix. The indium titanates are found to serve as stable photocatalysts for water splitting under visible light, where oxygen was the major reaction product. The role of microstructural and morphological properties in the photocatalytic activity is discussed.