Search

Oxygen and nitrogen have been both known to have a strong hardening effect on Ti and its alloys, while also imposing a serious embrittlement effect. In the present work, Ti samples with non-uniformly dispersed oxygen and nitrogen solid solutions were produced using high purity Ti via powder metallurgy (PM) methods. The experimental results suggested that, when the solutes are non-uniformly distributed, the high solute content region can serve as strengthening particles as that in metal matrix composites. In light of this, we propose a design of novel structure in high oxygen/nitrogen Ti materials to achieve both improved strengths and ductility. The highlight of the structure is a design of strain gradient at the matrix-particle interface, which can mitigate the strain compatibility that commonly reported in general MMCs as the crack initiator. Instead of using ceramics as reinforcements, here we propose to use the matrix phase itself that hardened by high oxygen/nitrogen solutes as reinforcing particles. [ABSTRACT FROM AUTHOR]

Abstract: Many previous studies have presented results on the relaxation of residual stress in a welded component as a result of postweld heat treatment. Techniques such as neutron diffraction and deep hole drilling have been used to measure the residual stress after the heat treatment and compare this with the residual stress for the component in the as-welded condition. The work described in this paper is novel: neutron diffraction is used to measure the relaxation of residual stress continuously as the heat treatment is being carried out. Residual stresses are measured in a butt-welded ferritic steel pipe as the pipe is heat treated to 650°C and then cooled to room temperature. The results identify those parts of the heat treatment that lead to significant stress relaxation and the mechanisms responsible for this relaxation. The techniques developed during this work allow future heat treatments to be optimised to achieve the low levels of residual stress in welded components. [Copyright &y& Elsevier]

Abstract: The present work considers the role of thermo-mechanical history on the generation and relaxation of residual stresses, typical of those encountered in Type 316H austenitic stainless steel thick section weldments. A series of thermo-mechanical pre-treatments have been developed and applied to simulate the critical microstructures observed within the heat affected zone of the thick section parent material. The through thickness distributions of the residual macro-stresses in cylindrical specimens have been measured by neutron diffraction and then the rates of the relaxation are shown to be a function of microstructure. The susceptibility to intergranular brittle fracture at a temperature of −196°C is shown to be a function of M23C6 carbide precipitates and phosphorous segregation at the grain boundaries. Finally, the link of the present study to the understanding of the reheat cracking is briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: The constitutive behavior of metallic materials with numerous microvoids is investigated. The representative volume element of the materials is idealized as an incompressible ellipsoidal cell with a single ellipsoidal void at its center. Through the analysis of the microscopic velocity and strain fields of the void-cell model and making use of a nonclassical elastoplastic constitutive relation, the microscopic stresses of the ellipsoidal void-cell model are obtained, and it is further transformed to macroscopic stresses with the principle of energy coincidence. Combining the evolution of void growth with that of void nucleation, the void evolution rule is obtained. The obtained expression of macroscopic stress and the void evolution rule are embedded in a nonclassical elastoplastic constitutive description obtained from a simple mechanical model incorporating damage. The corresponding numerical algorithm and the finite element approach are developed and applied to the analysis of the elastoplastic response and the porosity of PD3 steel. The computed results show satisfactory agreement with experiments. [Copyright &y& Elsevier]

Abstract: Insect cuticles as a natural biocomposite include many favorable microstructures which have been refined over centuries and endow the cuticles excellent mechanical and physical properties, such as light weight, high strength and toughness, etc. The various microstructures of a Scarabaei cuticle are investigated with a scanning electronic microscope and reported in this paper. It is found that the cuticle is a kind of fiber-reinforced biocomposite composed of chitin-fiber layers and sclerous protein matrixes. Different chitin-fiber layers have different orientations, composed of crossed and helicoidal structures at different location. In the helicoidal structure, each fiber layer rotates with an almost fixed angle against its neighboring layer. The maximum pullout energy of the helicoidal structure is analyzed based on the representative model of the structure. The result shows that the pullout energy of the helicoidal structure is markedly larger than that of the conventional 0°-structure. A biomimetic composite with the observed helicoidal structure is designed and fabricated. A comparative test shows that the fracture toughness of the biomimetic composite is markedly larger than that of the 0°-layer composite. [Copyright &y& Elsevier]

In this study, the origin of the high tensile ductility (>10% strain to fracture) of a new class of high-strength (yield strength >500 MPa) aluminium (Al) metal matrix composites (Al MMCs) co-reinforced by ex-situ carbon nanotubes (CNTs) and in-situ γ−Al 2 O 3 nanoparticles was investigated. The microstructures of these unique Al MMCs prior to and post tensile testing were characterized in detail using high resolution transmission electron microscopy, in order to understand their response to tensile deformation. The pathways to the in-situ formation of γ−Al 2 O 3 nanoparticles were identified and attributed to the high energy ball milling process, and the subsequent spark plasma sintering, which converted the amorphous Al-oxide into γ−Al 2 O 3 nanoparticles. The in-situ γ−Al 2 O 3 nanoparticles and Al matrix exhibited a semi-coherent interface, which led to a markedly increased dislocation density in the matrix around the nanoparticles during tensile deformation. These dislocations were precursors to microvoid formation and consequent dimples on further deformation. The co-operation of CNTs and γ−Al 2 O 3 nanoparticles resulted in a long strain-softening stage and therefore excellent tensile ductility. The results suggested that the exploitation of hybrid reinforcement by ex-situ CNTs and in-situ nanoparticles can enable the fabrication of high-performance metal matrix composites. [ABSTRACT FROM AUTHOR]

In the present work, a powder rolling process was utilized in the fabrication of fine grained pure Mg via powder metallurgy. Mg flakes were obtained after each rolling process, and broken into pieces for further rolling or sintering. Mg samples of experiencing 0, 5 and 10 rolling passes were obtained following spark plasma sintering (SPS) and hot extrusion. Microstructural results from electron backscatter diffraction (EBSD) revealed that, without experiencing powder rolling, the specimen contained a great number of residual tensile twins; in contrast, after powder rolling the specimen showed uniform and equiaxed grain structures. In addition, the average grain size was measured to be around 9.2, 2.9 and 2.1 µm for the samples subjected to 0, 5 and 10 rolling passes. The powder rolled specimens were found superior in mechanical properties. Post-loading microstructure examinations were also performed for the samples and a discussion regarding the relationship between their mechanical behavior and microstructures was provided. [ABSTRACT FROM AUTHOR]

Transient liquid phase (TLP) diffusion bonding of a Ti 3 Al-based alloy and TiAl intermetallics was firstly conducted with Ti–15Cu–15Ni (wt%) interlayer. Ti 2 Al layer, α 2 -Ti 3 Al band, Ti-rich phase, Ti 2 Ni and Ti 2 Cu compounds were formed within the joint and the joint showed a low shear strength of 74–79 MPa. For the newly designed Ti–Zr–Cu–Ni–Co system filler alloy, the bonded joint mainly consisted of Ti-rich phase, Ti 2 Al layer, (Ti, Zr) 2 Al phase and (Ti, Zr) 3 Al dissolved with Cu, Ni and Co. The joints bonded at 1253 K for 600 s with a pressure of 2 MPa presented the maximum shear strength of 435 MPa at room temperature, and the strength of 234 MPa was maintained at 873 K. CuZr and Zr 2 Ni phases rather than Ti 2 Ni and Ti 2 Cu compounds were formed within the latter joint. The formation of the CuZr and Zr 2 Ni phases as well as the plastic α-Ti phase increased the joint ductility to a great extent. [ABSTRACT FROM AUTHOR]

Aluminum composites reinforced with carbon-based nano-particles or fibers have been widely studied. Yet, the rate dependence of their properties has been barely reported. In the present study, CNTs-reinforced Al composites with CNTs of two aspect ratios were produced by different powder metallurgy methods, followed by spark plasma sintering and hot extrusion. The mechanical properties and the underlying mechanisms of CNTs-reinforced Al composites at various loading rates were studied, with the aim of exploring the role of CNTs on strengthening, esp. on rate-dependent properties. The mechanical experiments revealed that the addition of CNTs not only increased the strength but also the strain rate sensitivity in comparison with pure Al. It was found that, under dynamic loading, the materials showed an increased strength and elongation-to-failure simultaneously, due to improved strain hardening rates. A careful analysis suggested that the long-range back stress produced at the CNTs-Al interfaces and the geometrically necessary dislocations accumulated due to strain gradient along the interface, mainly contributed to the strain hardening of CNTs/Al composites. A novel microstructure-based model using microscopically non-uniform dispersion of CNTs was proposed to account for the mechanical properties with better prediction than the traditional models. The results might shed some light on understanding metal matrix composites reinforced with nano-particles or fibers. [ABSTRACT FROM AUTHOR]

Homogeneous graphene dispersion and suitable interface bonding are two key challenges to realize the high strengthening potential of graphene in metal matrix composites (MMCs). In this study, graphene nanosheets (GNSs) with deposited nickel (Ni) nanoparticles were synthesized to simultaneously overcome the two challenges. Ni-coated-GNS-reinforced aluminum (Al) MMCs were fabricated by graphene synthesis, Ni decoration and subsequent powder metallurgy. It was revealed that Ni-coated GNSs exhibited noticeably enhanced strengthening effect compared with graphene reinforcements in most previous studies. With the addition of 1.5 wt% Ni-coated GNSs, the composites exhibited 132% higher strength than that of unreinforced aluminum. The enhancement in Vickers hardness and Young's modulus further confirmed the remarkable strengthening effect. The high strength and high Young's modulus were examined by strengthening models, and a comparison between experimental and theoretical values was reached. The main strengthening mechanisms in the Ni-coated GNSs/Al composites were clarified as dislocation-related ones. [ABSTRACT FROM AUTHOR]

Abstract It is generally recognised that electron beam melted (EBM) Ti-6Al-4V alloys exhibit a microstructural gradient along the build direction, but there have been some inconsistent experimental observations and debate as to the origin and magnitude of this effect. Here we present an unambiguous evaluation of this microstructural gradient and associated mechanical property along the EBM build direction on purpose-built round bar RB samples with build height of 380 mm and rectangular plate RP samples with build height of 120 mm. Columnar prior β grain width was found to increase (from 86 ± 38 to 154 ± 56 µm in RB and from 79 ± 34 to 122 ± 56 µm in RP samples) with the build height and the similar increase was also observed for α lath width (from 0.58 ± 24 to 0.87 ± 33 µm in RB and from 1.50 ± 45 to 1.80 ± 49 µm in RP samples). These observations can be attributed to the thermal gradient in the powder bed that produced a cooling rate gradient along the build height. The measured α lath width variation along the build height followed a log-normal distribution. The graded microstructure resulted in a decrease in micro-hardness which correlated very well with the mean α lath width by following a Hall-Petch relation. [ABSTRACT FROM AUTHOR]

Effect of coherent L1 2 nanoprecipitates on the tensile behavior of a novel-designed Al 0.2 CrFeCoNi 2 Cu 0.2 high-entropy alloy is investigated in this study. Different heat treatments after cold rolling were used to achieve the alloys with two different microstructures. One is composed of a fcc matrix and coherent L1 2 (Ni,Cu) 3 Al nanoprecipitates after annealing at 700 ° C for 20 h; the other owns a fcc single-phase structure after annealing at 800 °C for 1 h as reference. Furthermore, the two alloys have nearly identical average grain sizes (~4.5 µm), and thus the exclusive effect of L1 2 nanoprecipitates could be identified by directly comparing their tensile behaviors. It is shown that the presence of L1 2 nanoprecipitates results in increases of 259 MPa and 316 MPa in the yield strength and ultimate tensile strength, respectively, accompanied by the maintenance of a high elongation (30.4%). This excellent strengthening effect is explained by the interaction between dislocations and L1 2 nanoprecipitates during the tensile straining process. Dislocation shearing of the L1 2 nanoprecipitates, which is evidently observed and also verified by the yield strength increment calculation, leads to predominant planar dislocation glide and the formation of crystallographically aligned slip bands as the main deformation mechanism, eventually resulting in a considerable work-hardening capacity enhancement with almost none harm to the elongation. [ABSTRACT FROM AUTHOR]

Dynamic recrystallization and strengthening-toughening effects were investigated in the processed near- Ti-xSi alloy by hot extrusion. The yielding and ultimate tensile strengths were enhanced by 138.5 MPa and 118.7 MPa in the hot-extruded Ti-Si alloy compared with pure Ti. The double GSS effects (gradient silicon solutes - gradient shearing strain) led to the bimodal microstructure (21.8 µm deformed strips and 5.2 µm recrystallized grains) and grain refinement by dragging the recrystallization nucleation and grains growth during extrusion. The yielding strength enhancement was resulted from multi-factor Si solid solution, grain refinement and oxygen solid solution. Multi-factor Si solid solution was found to be the dominant mechanism, which was a summation of size misfit, modulus mismatch, strain hardening, narrow slip band and texture strengthening. Obvious strain hardening and late necking occurrence would be positive in obtaining the excellent ultimate tensile strength. Ductility was maintained at 22.9% from strain compatibility and cracks suppression. This study may provide a new understanding on the relationship among the mechanical properties, microstructure and processing of Ti-xSi alloy. [ABSTRACT FROM AUTHOR]

Twinning mechanism of powder metallurgy Ti-Si alloy was investigated by interrupted in-situ tensile tests. Extension twins {10−12}<10-1-1> in the fine-grained Ti-Si alloy were found in the uniform deformation period, but no twinning in the coarse pure Ti. Three deformation twinning nucleation mechanisms were proposed: i) local stress concentration by neighboured slip incompatibility, ii) slip/twin oriented relationship in the parent grain and iii) slip/twin transfer by high Luster-Morris oriented relationship. The interior back-stress state, grains rotation and dislocations pile-up drove the occurrence of detwinning phenomenon. Silicon-facilitation twinning verified the hypothesis that the substitutional Si solutes affected the core structures and thus the mobility of screw dislocations. Enhanced driving force and decreased energy barrier of nucleation in the micro/atomic scale were further proposed in the twinning activation. It was expected to deepen the understanding of twinning/detwinning behaviors and supply direct evidences building immature twinning mechanism. In-depth understanding about the relationship among the processing, mechanical properties and microstructure of Ti alloy was facilitated in the present work. [ABSTRACT FROM AUTHOR]

Nanometer-sized grains (around 50–100 nm) were generated in the surface layer (0–30 μm) of an Mg–3Gd alloy by means of surface mechanical attrition treatment (SMAT) at room temperature. The deformation process and the formation mechanism of nano grains were investigated by using transmission electron microscopy. The results show that twinning dominates the initial stage of the plastic deformation when the dislocation slips are obstructed. And till all the coarse grains are divided into substructures by twin–twin interactions and twin-dislocation arrays intersections, dislocation slips and stacking faults begin to play an important role in impelling subgrains to nanograins by lattice rotating through dislocation arrays slipping. Dynamic rotation recrystallization is the primarily formation mechanism of the nanocrystalline of Mg–3Gd alloy by SMAT, which is distinct from the migration recrystallization normally observed in severe plastic deformation process. [ABSTRACT FROM AUTHOR]

Role of oxidation and recrystallisation on very-high-cycle fatigue of columnar-grained DZ125 and single-crystal DD6 superalloys are investigated. With the temperature increase from 850 to 1000 °C for DZ125 while from 1000 to 1100 °C for DD6, the Mode-I cracking tendency increases due to internal oxide penetration. Recrystallisation can occur for both the electrolytically and mechanically polished surface conditions, controlled primarily by temperature and test duration. Its presence can lead to strain localisation at subsurface, but the surface recrystallisation does not necessarily determine the oxide penetration. By comparison with the single-crystal DD6 that shows the pure Stage I at 1000 °C, the columnar-grained DZ125 exhibits the distinctive Stage I cracking followed by Mode I propagation. This fracture characteristics in DZ125 can be attributed to the combined effect of the high-angle grain boundaries and far-field stress. The microstructure-related factor plays a vital role when the crack length is small, whereas the role of stress becomes predominant when the crack length is comparable to a couple of grains, ultimately leading to the shift from the crystallographic Stage I to Mode I propagation. [Display omitted] • Very-high-cycle fatigue of columnar-grained and single-crystal superalloys at 850, 1000 and 1100 °C • High-angle grain boundaries assisted crack path deflection of columnar-grained superalloy • Oxidation assisted Mode I vs. Stage I cracking tendency increasing • Surface recrystallisation does not necessarily determine the oxide penetration • Factors affecting the occurrence of recrystallisation [ABSTRACT FROM AUTHOR]

Abstract: High-energy synchrotron X-ray diffraction technique was used to in-situ characterize microstructure, lattice strain, and phase transition behavior of a Ni47Ti44Nb9 shape memory alloy. Phase transformation kinetics and deformation mechanisms were studied under a uniaxial tension at three testing temperatures, i.e., −70°C, 25°C, and 150°C. At a testing temperature of −70°C, a complicated phase transformation with four stages of micromechanical deformation was identified which is associated with changes of martensite substructures. At room temperature of 25°C, there was no stress-induced selection process of variants of B19′ phases observed. Whereas at a testing temperature of 150°C, there was no any phase transformation observed. It is verified that β-Nb phase, an effective stabilizer for the austenite, delays the process of martensitic transformation and relaxes the strain energy without strengthening the matrix. This new finding is important to understand the relationship between the micromechanical deformation behavior and phase transformations in the Ni47Ti44Nb9 SMA. [Copyright &y& Elsevier]

To enhance the process stability and densification, semi-melt step has been introduced when fabricating the TiC/high Nb–TiAl nanocomposite via electron beam melting. The homogenous TiAl matrix microstructure with dispersed nano-scale carbides was realised. During the EBM melt, most TiC nanoparticles dissolved and Ti 2 AlC formed with near-spherical and rod-like shapes. The particles had an influence on solidification behaviour and the subsequent microstructural degradation. High Nb–TiAl nanocomposites with 1.2 wt% TiC addition exhibited a duplex microstructure with dispersed carbides, while a nearly lamellar microstructure (carbide-free) was found in samples with 0.6 and 0.8 wt% TiC. Furthermore, a lower scanning speed resulted in higher relative density, greater Al loss, increased α 2 -phase but reduced carbide fractions. The microhardness of 433 ± 10 HV 0.2 , ultimate tensile strength of 657 ± 155 MPa and fracture toughness of 8.1 ± 0.1 MPa√m in 1.2 wt% TiC/high Nb–TiAl nanocomposite processed by EBM are very promising. In addition, the compressive yield strength of 1085 ± 55 MPa, fracture strength of 2698 ± 34 MPa and strain to fracture of 26.1 ± 1.0%, are superior to those processed by conventional means. The strengthening and toughening mechanisms have been interpreted on the basis of crack path observation. [Display omitted] • Electron beam melted TiC/high Nb–TiAl nanocomposite. • Semi-melt step helps to achieve good process stability and densification. • A homogeneous TiAl matrix microstructure with dispersed nano-scale carbides leading to promising mechanical properties. • The evolution of nano-TiC, solidification microstructure and degradation have been elucidated. [ABSTRACT FROM AUTHOR]

Selective electron beam melting (SEBM) was used to process crack-free Ni 3 Al-based IC21 alloy (low density superalloy) containing ~85% γ′-volume fraction. There are distinct differences between dendrites and inter-dendritic regions with the presence of coarse γ+γ′ eutectic and secondary solidification microconstituents (Cr and Mo-rich) in the latter. The pronounced inter-dendritic eutectic regions suggest that a significant elemental partitioning between the liquid and solid occurred during the SEBM. The terminal liquid is trapped at boundaries between dendrites and grains, as evidenced by the liquid films on cracked surfaces. In contrast to extensive studies indicating the segregation of Zr and B, we show unambiguously the segregation of Si to low melting point liquid films and thereby enhancing the susceptibility to solidification cracking in IC21 produced by SEBM. The tensile specimens extracted from the crack-free IC21 samples exhibit superior properties at room temperature (RT) and 1000 °C. The RT deformation mechanism is characterised by cutting γ′-phase with two paired dislocations and antiphase boundaries in between. At 1000 °C tensile deformation, the well-developed γ/γ′ interfacial dislocation networks are in good agreement with their promising high-temperature performance (σ y = 518 ± 10 MPa, σ UTS = 560 ± 16 MPa, 20.5% for ductility). [ABSTRACT FROM AUTHOR]

The microscopic and macroscopic material strengthening mechanisms encountered in brazed joints were experimentally investigated and quantified in this work. Microstructural contribution towards the overall strength of a stainless steel brazed joint was evaluated by conducting multi-scale microstructural characterisations. Theoretical evaluation of the collected microstructural data suggests a Cu–Mn solid-solution strengthening enhancement of ~200 MPa towards the overall joint strength. The mechanical constraint effect was considered as the macroscopic strengthening mechanism, as revealed by comparing two identical joints but with their interfaces orientated at 90° and 45°with respect to the applied load. Bridgman necking criteria was applied to derive the longitudinal flow stress for the 90° joint configuration. The discrepancy (~330 MPa) between the calculated and experimentally determined strengths can thus be concluded as the contribution of mechanical constraint. In addition, the pile-up of geometrically necessary dislocations (GNDs) was observed at the base-filler metal interface for the 90° joint, but a homogeneous GND distribution was revealed for the 45° one. This observation indicates that GNDs were introduced to accommodate deformation incompatibility imposed by the mechanical constraint. This finding is thus considered as an experimental (microscopic) evidence for strain inhomogeneity due to the presence of mechanical constraint. [ABSTRACT FROM AUTHOR]