Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2013"

Microstructure and tensile properties of selectively laser-melted and of HIPed laser-melted Ti–6Al–4V

Abstract: Ti–6Al–4V samples have been prepared by selective laser melting (SLM) with varied processing conditions. Some of the samples were stress-relieved or hot isostatically pressed (HIPed). The microstructures of all samples were characterised using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) and the tensile properties measured before and after HIPing. It was found that the porosity level generally decreased with increase of laser power and laser scanning speed. Horizontally built samples were found to have a higher level of porosity than vertically built samples. The as-fabricated microstructure was dominated by columnar grains and martensites. HIPing closed the majority of the pores and also fully transformed the martensite into α and β phases. The as-fabricated microstructure exhibits very high tensile strengths but poor ductility with elongation generally smaller than 10%. The horizontally built samples show even lower elongation than vertically built samples. HIPing considerably improved ductility but led to a reduction in strength. With HIPing, the SLMed samples were found to show tensile properties comparable with those thermomechanically processed and annealed samples.

Nanostructured aluminium alloys produced by severe plastic deformation: New horizons in development

Abstract: In recent years, much progress has been made in the studies of nanostructured Al alloys for advanced structural and functional use associated both with the development of novel routes for the fabrication of bulk nanostructured materials using severe plastic deformation (SPD) techniques and with investigation of fundamental mechanisms leading to improved properties. This review paper discusses new concepts and principles in application of SPD processing to fabricate bulk nanostructured Al alloys with advanced properties. Special emphasis is placed on the relationship between microstructural features, mechanical, chemical, and physical properties, as well as the innovation potential of the SPD-produced nanostructured Al alloys.

Mechanical properties of low-density, refractory multi-principal element alloys of the Cr–Nb–Ti–V–Zr system

Abstract: Room temperature and elevated temperature mechanical properties of four multi-principal element alloys, NbTiVZr, NbTiV 2 Zr, CrNbTiZr and CrNbTiVZr, are reported. The alloys were prepared by vacuum arc melting followed by hot isostatic pressing and homogenization. Disordered BCC solid solution phases are the major phases in these alloys. The Cr-containing alloys additionally contain an ordered FCC Laves phase. The NbTiVZr and NbTiV 2 Zr alloys showed good compressive ductility at all studied temperatures while the Cr-containing alloys showed brittle-to-ductile transition occurring somewhere between 298 and 873 K. Strong work hardening was observed in the NbTiVZr and NbTiV 2 Zr alloys during deformation at room temperature. The alloys had yield strengths of 1105 MPa and 918 MPa, respectively, and their strength continuously increased, exceeding 2000 MPa after ∼40% compression strain. The CrNbTiZr and CrNbTiVZr alloys showed high yield strength (1260 MPa and 1298 MPa, respectively) but low ductility (6% and 3% compression strain) at room temperature. Strain softening and steady state flow were typical during compression deformation of these alloys at temperatures above 873 K. In these conditions, the alloys survived 50% compression strain without fracture and their yield strength continuously decreased with an increase in temperature. During deformation at 1273 K, the NbTiVZr, NbTiV 2 Zr, CrNbTIZr, and CrNbTiVZr alloys showed yield strengths of 58 MPa, 72 MPa, 115 MPa and 259 MPa, respectively.

The study of the laser parameters and environment variables effect on mechanical properties of high compact parts elaborated by selective laser melting 316L powder

Abstract: In this work, a systematic analysis of the main parameters for the selective laser melting (SLM) of a commercial stainless steel 316L powder was conducted to improve the mechanical properties and dimensional accuracy of the fabricated parts. First, the effects of the processing parameters, such as the laser beam scanning velocity, laser power, substrate condition and thickness of the powder layer, on the formation of single tracks for achieving a continuous melting and densification of the material were analysed. Then, the influence of the environmental conditions (gas nature) and of the preheating temperature on the density and dimensional accuracy of the parts was considered. The microstructural features of the SLM SS 316L parts were carefully observed to elucidate the melting-solidification mechanism and the thermal history, which are the basis of the manufacturing process. Finally, the mechanical properties of the corresponding material were also determined.

Effects of processing on microstructure and mechanical properties of a titanium alloy (Ti–6Al–4V) fabricated using electron beam melting (EBM), Part 2: Energy input, orientation, and location

Abstract: Selective electron beam melting (EBM) is a layer-by-layer additive manufacturing technique that shows great promise for fabrication of medical devices and aerospace components. Before its potential can be fully realized, however, a comprehensive understanding of processing-microstructure-properties relationships is necessary. Titanium alloy (Ti–6Al–4V) parts were built in a newly developed, unique geometry to allow accurate investigation of the following intra-build processing parameters: energy input, orientation, and location. Microstructure evaluation (qualitative prior-β grain size, quantitative α lath thickness), tensile testing, and Vickers microhardness were performed for each specimen. For a wide range of energy input (speed factor 30–40), small differences in mechanical properties (2% change in ultimate tensile strength (UTS) and 3% change in yield strength (YS)) were measured. Vertically built parts were found to have no difference in UTS or YS compared to horizontally built parts, but the percent elongation at break (% EL) was 30% lower. The difference in % EL was attributed to a different orientation of the tensile axis for horizontal and vertical parts compared to the elongated prior-β grain and microstructural texture direction in EBM Ti–6Al–4V. Orientation within the x – y plane as well as location were found to have less than 3% effect on mechanical properties, and it is possible a second order effect of thermal mass contributed to these results.

Effects of processing on microstructure and mechanical properties of a titanium alloy (Ti–6Al–4V) fabricated using electron beam melting (EBM), part 1: Distance from build plate and part size

Abstract: Selective electron beam melting (EBM) is a layer-by-layer additive manufacturing technique that shows great promise for fabrication of medical devices and aerospace components. Before its potential can be fully realized, however, a comprehensive understanding of processing–microstructure–properties relationships is necessary. Titanium alloy (Ti–6Al–4V) parts were built in a geometry developed to allow investigation of the following two intra-build processing parameters: distance from the build plate and part size. Microstructure evaluation (qualitative prior-β grain size, quantitative α lath thickness), tensile testing, and Vickers microhardness were performed for each specimen. Microstructure and mechanical properties, including microhardness, were not found to vary as a function of distance from the build plate, which was hypothesized to be influenced by the build plate preheating associated with the EBM process. Part size, however, was found to influence ultimate tensile strength (UTS) and yield strength (YS) by less than 2% over the size range investigated. A second order effect of thermal mass might also have influenced these results. Differences were observed between the EBM Ti–6Al–4V microstructure of this work and the expected acicular or Widmanstatten microstructure normally achieved through annealing above the β transus. Therefore, a different relationship between α lath thickness and mechanical properties might be expected.

Effect of iron on the microstructure and mechanical property of Al–Mg–Si–Mn and Al–Mg–Si diecast alloys

Abstract: This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2012 Elsevier B.V.

Inconel 939 processed by selective laser melting: Effect of microstructure and temperature on the mechanical properties under static and cyclic loading

Abstract: Nickel-based superalloys, such as Inconel 939, are a long-established construction material for high-temperature applications and profound knowledge of the mechanical properties for this alloy produced by conventional techniques exists. However, many applications demand for highly complex geometries, e.g. in order to optimize the cooling capability of thermally loaded parts. Thus, additive manufacturing (AM) techniques have recently attracted substantial interest as they provide for an increased freedom of design. However, the microstructural features after AM processing are different from those after conventional processing. Thus, further research is vital for understanding the microstructure-processing relationship and its impact on the resulting mechanical properties. The aim of the present study was to investigate Inconel 939 processed by selective laser melting (SLM) and to reveal the differences to the conventional cast alloy. Thorough examinations were conducted using electron backscatter diffraction, transmission electron microscopy, optical microscopy and mechanical testing. It is demonstrated that the microstructure of the SLM-material is highly influenced by the heat flux during layer-wise manufacturing and consequently anisotropic microstructural features prevail. An epitaxial grain growth accounts for strong bonding between the single layers resulting in good mechanical properties already in the as-built condition. A heat treatment following SLM leads to microstructural features different to those obtained after the same heat treatment of the cast alloy. Still, the mechanical performance of the latter is met underlining the potential of this technique for producing complex parts for high temperature applications.

A comparative study of pulsed Nd:YAG laser welding and TIG welding of thin Ti6Al4V titanium alloy plate

Abstract: This paper reports on a study aiming at comparing properties of the Ti6Al4V titanium alloy joints between pulsed Nd:YAG laser welding and traditional fusion welding. To achieve the research purpose, Ti6Al4V titanium alloy plates with a thickness of 0.8 mm were welded using pulsed Nd:YAG laser beam welding (LBW) and gas tungsten arc welding (TIG), respectively. Residual distortions, weld geometry, microstructure and mechanical properties of the joints produced with LBW and TIG welding were compared. During the tensile test, with the aid of a high speed infrared camera, evolution of the plastic strain within tensile specimens corresponding to LBW and TIG welding were recorded and analyzed. Compared with the TIG, the welded joint by LBW has the characters of small overall residual distortion, fine microstructure, narrow heat-affected zone (HAZ), high Vickers hardness. LBW welding method can produce joints with higher strength and ductility. It can be concluded that Pulsed Nd:YAG laser welding is much more suitable for welding the thin Ti6Al4V titanium alloy plate than TIG welding.

Basal and non-basal dislocation slip in Mg-Y

Abstract: The activation of non-basal slip systems is of high importance for the ductility in hcp Mg and its alloys. In particular, for Mg–Y alloys where a higher activation of pyramidal dislocation slip causes an increased ductility detailed characterization of the activated slip systems is essential to understand and describe plasticity in these alloys. In this study a detailed analysis of the activated dislocations and slip systems via post-mortem TEM and SEM-EBSD based slip band analysis in 3% deformed Mg–3 wt% Y is presented. The analysis reveals a substantial activity of pyramidal dislocations with different Burgers vectors. The obtained dislocation densities and active slip systems are discussed with respect to atomistic simulations of non-basal dislocations in hcp Mg.

Static mechanical behaviours of stainless steel micro-lattice structures under different loading conditions

Abstract: Recently, some published studies have shown that laser melting method can be an impressive way for periodic metallic micro-cellular structures with the satisfying requirements to be built. In this respect, this study deeply focuses on examining the experimental static behaviours of stainless steel micro-lattice structures manufactured with the laser melting method for both different cell topologies and loading scenarios such as compression, shear, tension and combined loadings. The results show that the two major parameters characterising the mechanical behaviours of these structures are the relative density and cell topology, which govern the altitude of stress–strain curve and collapse mechanism. Also, pure shear behaviours are unpredictable and have no repeatability due to the complex and highly imperfect surfaces of micro-struts and cells. On the contrary, the compression tests show a large repeatability and stability.

Development of 3rd generation AHSS with medium Mn content alloying compositions

Abstract: In this paper, four different steel compositions, centered on Mn as the main alloying element, are designated as candidates for Third Generation AHSS grades. The design of these steels is based on controlling the deformation behavior of the retained austenite. Thus, heat treatment process parameters are determined in order to obtain different amounts and morphologies of retained austenite. The evolution of the microstructure, during processing as well as deformation, is characterized by using optical, electron microscopy techniques and mechanical tests. The effect of alloy composition and processing parameters on the deformation mechanisms of these steels is discussed.

General relation between tensile strength and fatigue strength of metallic materials

Abstract: With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength sigma(w) and tensile strength sigma(b) of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290 MPa to 2130 MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula sigma(w)=sigma(b)(C-P.sigma(b)) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength sigma(max)(w) at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials. (c) 2012 Elsevier B.V. All rights reserved.

An electron backscattered diffraction study on the dynamic recrystallization behavior of a nickel–chromium alloy (800H) during hot deformation

Abstract: The objective of the study described here is to evaluate the effect of temperature, strain rate, and strain on the microstructure of dynamically recrystallized nickel–chromium alloy (800H) subjected to hot compression over a wide range of strain rates. The microstructural evolution was studied by electron backscattered diffraction (EBSD) and the effect of adiabatic heating on hot deformation was analyzed to correct the flow curves at high strains. The grain orientation spread (GOS) approach was used to distinguish the dynamic recrystallization (DRX) grains from the deformed matrix. The nucleation mechanism of DRX and the role of Σ 3 n CSL boundaries during DRX were explored. Additionally, the influence of carbides on the DRX behavior was studied within the temperature of 850–950 °C. The results indicated that the DRX can be stimulated by adiabatic heating and strong dislocation–dislocation interaction occurring with increase in the strain rate in the range of 1–30 s −1 . The threshold value of GOS (1.2°) separated the DRX grains from the deformed matrix. The recrystallized grains nucleated at pre-existing grain boundaries by extensive bulging associated with grain fragmentation. The Σ 3 n CSL boundaries play an important role in DRX and they can be generated through interaction among them after the initiation of DRX. The precipitation of Cr 23 C 6 and Ti(C, N) at the parent grain boundary could restrain or even inhibit the occurrence of DRX in the temperature range of 850–950 °C.

Modeling bending of α-titanium with embedded polycrystal plasticity in implicit finite elements

Abstract: An accurate description of the mechanical response of α-titanium requires consideration of mechanical anisotropy. In this work we adapt a polycrystal self-consistent model embedded in finite elements to simulate deformation of textured α-titanium under quasi-static conditions at room temperature. Monotonic tensile and compressive macroscopic stress–strain curves, electron backscattered diffraction and neutron diffraction data are used to calibrate and validate the model. We show that the model captures with great accuracy the anisotropic strain hardening and texture evolution in the material. Comparisons between predictions and experimental data allow us to elucidate the role that the different plastic deformation mechanisms play in determining microstructure and texture evolution. The polycrystal model, embedded in an implicit finite element code, is then used to simulate geometrical changes in bending experiments of α-titanium bars. These predictions, together with results of a macroscopic orthotropic elasto-plastic model that accounts for evolving anisotropy, are compared with the experiments. Both models accurately capture the experimentally observed upward shift of the neutral axis as well as the rigidity of the material response along hard-to-deform crystallographic direction.

Effects of cerium addition on the microstructure, mechanical properties and hot workability of ZK60 alloy

Abstract: The effects of cerium (Ce) addition on the microstructure and mechanical properties of ZK60 alloy were investigated using SEM, EBSD, and TEM and by performing tensile tests of indirect-extruded ZK60 alloys with 0.5, 1.0, and 1.5 wt% Ce contents. The variation of hot workability due to Ce addition was also investigated by establishing processing maps of these alloys. The results revealed that Ce addition had an obvious influence, reducing the average grain size and weakening the basal fiber texture of the as-extruded ZK60 alloys; these changes were attributed to the promotion of dynamic recrystallization (DRX) by particle stimulated nucleation (PSN) at Mg–Zn–Ce particles. The yield and tensile strengths were improved by the Ce addition, while the elongation was decreased due to the hard Mg–Zn–Ce particles. It was also found that the hot workability improves up to the addition of 1.0 wt% Ce and then deteriorates.

Correlation between 2D and 3D flow curve modelling of DP steels using a microstructure-based RVE approach

Abstract: A microstructure-based approach by means of representative volume elements (RVEs) is employed to evaluate the flow curve of DP steels using virtual tensile tests. Microstructures with different martensite fractions and morphologies are studied in two- and three-dimensional approaches. Micro sections of DP microstructures with various amounts of martensite have been converted to 2D RVEs, while 3D RVEs were constructed statistically with randomly distributed phases. A dislocation-based model is used to describe the flow curve of each ferrite and martensite phase separately as a function of carbon partitioning and microstructural features. Numerical tensile tests of RVE were carried out using the ABAQUS/Standard code to predict the flow behaviour of DP steels. It is observed that 2D plane strain modelling gives an underpredicted flow curve for DP steels, while the 3D modelling gives a quantitatively reasonable description of flow curve in comparison to the experimental data. In this work, a von Mises stress correlation factor σ3D/σ2D has been identified to compare the predicted flow curves of these two dimensionalities showing a third order polynomial relation with respect to martensite fraction and a second order polynomial relation with respect to equivalent plastic strain, respectively. The quantification of this polynomial correlation factor is performed based on laboratory-annealed DP600 chemistry with varying martensite content and it is validated for industrially produced DP qualities with various chemistry, strength level and martensite fraction.

Graphene NanoPlatelets reinforced tantalum carbide consolidated by spark plasma sintering

Abstract: Graphene NanoPlatelets (GNP) reinforced tantalum carbide composites are synthesized by spark plasma sintering (SPS) at processing conditions of 1850 °C and 80–100 MPa. The GNP addition enhances the densification of TaC–GNP composites to 99% theoretical density, while reducing the grain size by over 60% through grain wrapping mechanism. Survival and structure retention of GNP is confirmed through scanning electron microscopy and micro-Raman spectroscopy. Nanoindentation and high load (20–30 N) microindentation are utilized to evaluate elastic modulus and hardness. GNP improves fracture toughness of TaC by up to 99% through toughening mechanisms such as GNP bending, sheet sliding, cracking bridging, and crack deflection.

Shear band-related recrystallization and grain growth in two rolled magnesium-rare earth alloys

Abstract: Two binary alloys, Mg–1Ce and Mg–1Gd (wt%), were subjected to large strain rolling and different annealing treatments. The onset of recrystallization in the shear banded microstructure was delayed to 300 °C in both alloys. Mg–1Ce showed virtually no potential for rolling texture modification during annealing at various temperatures, retaining the deformation texture, whereas Mg–1Gd revealed important deformation/recrystallization texture transition producing significant texture modification. This was attributed to favorable growth behavior. Correlations between deformation, annealing conditions and material composition were established.

Evolution of microstructure, macrotexture and mechanical properties of commercially pure Ti during ECAP-conform processing and drawing

Abstract: Long-length ultrafine-grained (UFG) Ti rods are produced by equal-channel angular pressing via the conform scheme (ECAP-C) at 200 °C, which is followed by drawing at 200 °C. The evolution of microstructure, macrotexture, and mechanical properties (yield strength, ultimate tensile strength, failure stress, uniform elongation, elongation to failure) of pure Ti during this thermo-mechanical processing is studied. Special attention is also paid to the effect of microstructure on the mechanical behavior of the material after macrolocalization of plastic flow. The number of ECAP-C passes varies in the range of 1–10. The microstructure is more refined with increasing number of ECAP-C passes. Formation of homogeneous microstructure with a grain/subgrain size of 200 nm and its saturation after 6 ECAP-C passes are observed. Strength properties increase with increasing number of ECAP passes and saturate after 6 ECAP-C passes to a yield strength of 973 MPa, an ultimate tensile strength of 1035 MPa, and a true failure stress of 1400 MPa (from 625, 750, and 1150 MPa in the as-received condition). The true strain at failure failure decreases after ECAP-C processing. The reduction of area and true strain to failure values do not decrease after ECAP-C processing. The sample after 6 ECAP-C passes is subjected to drawing at 200¯C resulting in reduction of a grain/subgrain size to 150 nm, formation of (10 1 ¯ 0) fiber texture with respect to the rod axis, and further increase of the yield strength up to 1190 MPa, the ultimate tensile strength up to 1230 MPa and the true failure stress up to 1600 MPa. It is demonstrated that UFG CP Ti has low resistance to macrolocalization of plastic deformation and high resistance to crack formation after necking.

Microstructure, tensile and toughness properties after quenching and partitioning treatments of a medium-carbon steel

Abstract: The effect of different microstructures on the tensile and toughness properties of a low alloy medium carbon steel (0.28C–1.4Si–0.67Mn–1.49Cr–0.56Mo wt%) was investigated, comparing the properties obtained after the application of selected quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments. After Q&T the strength–toughness combination was the lowest, whereas the best combination was achieved by Q&P, as a result of the carbon depletion of the martensite and the high stabilization of the austenite. Nonetheless, the presence of islands of martensite/austenite (MA) constituents after Q&P treatments prevented the achievement of toughness levels comparable to the ones currently obtainable with other steels and heat treatments.

Effect of graphene nanosheets reinforcement on the performance of SnAgCu lead-free solder

Abstract: Varying weight fractions of graphene nanosheets were successfully incorporated into lead-free solder using the powder metallurgy route. The effects of graphene nanosheets on the physical, thermal, and mechanical properties of a SnAgCu solder alloy were investigated. With the increasing addition of graphene nanosheets, the nanocomposite solders showed a corresponding improvement in their wetting property but an insignificant change in their melting point. The thermomechanical analysis showed that the presence of graphene nanosheets can effectively decrease the coefficient of thermal expansion (CTE) of the nanocomposites. Furthermore, an improvement in UTS and a decrease in ductility were recorded with the addition of graphene nanosheets. The reinforcing mechanism of the graphene nanosheets on various properties obtained was also analyzed in this study.

Evolution of activation energy during hot deformation of AA7150 aluminum alloy

Abstract: The hot deformation behavior of a homogenized AA7150 aluminum alloy was studied in compression tests conducted at various temperatures (573–723 K) and strain rates (0.001–10 s −1 ). The flow stress behavior and microstructural evolution were observed during the hot deformation process. A revised Sellars’ constitutive equation was proposed, which considered the effects of the deformation temperature and strain rate on the material variables and which provided an accurate estimate of the hot deformation behavior of the AA7150 aluminum alloy. The results revealed that the activation energy for the hot deformation of the AA7150 aluminum alloy is not a constant value but rather varies as a function of the deformation conditions. The activation energy for hot deformation decreased with increasing deformation temperature and strain rate. The peak flow stresses under various deformation conditions were predicted by a revised constitutive equation and correlated with the experimental data with excellent accuracy.

The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr–9Mn–Ni–Cu stainless steel

Abstract: In order to improve understanding on the behavior of ultrafine-grained austenitic stainless steels during deformation, the influence of the austenite grain size and microstructure on the strain-induced martensite transformation was investigated in an austenitic 15Cr–9Mn–Ni–Cu (Type 204Cu) stainless steel. By different reversion treatments of the 60% cold-rolled sheet, varying grain sizes from ultrafine (0.5 μm), micron-scale (1.5 μm), fine (4 μm) to coarse (18 μm) were obtained. Some microstructures also contained a mixture of ultrafine or micron-scale and coarse initially cold-worked austenite grains. Samples were tested in tensile loading and deformation structures were analyzed after 2%, 10% and 20% engineering strains by means of martensite content measurements, scanning electron microscope together with a electron backscatter diffraction device and transmission electron microscope. The results showed that the martensite nucleation sites and the rate of transformation vary. In ultrafine grains strain-induced α′-martensite nucleates at grain boundaries and twins, whereas in coarser grains as well as in coarse-grained retained austenite, α′-martensite formation occurs at shear bands, sometimes via e-martensite. The transformation rate of strain-induced α′-martensite decreases with decreasing grain size to 1.5 μm. However, the rate is fastest in the microstructure containing a mixture of ultrafine and retained cold-worked austenite grains. There the ultrafine grains transform quite readily to martensite similarly as the coarse retained austenite grains, where the previous cold-worked microstructure is still partly remaining.

Evolution of microstructures and mechanical properties during dissimilar electron beam welding of titanium alloy to stainless steel via copper interlayer

Abstract: The influence of operational parameters on the local phase composition and mechanical stability of the electron beam welds between titanium alloy and AISI 316L austenitic stainless steel with a copper foil as an intermediate layer has been studied. It was shown that two types of weld morphologies could be obtained depending on beam offset from the center line. Beam shift toward the titanium alloy side results in formation of a large amount of the brittle TiFe2 phase, which is located at the steel/melted zone interface and leads to reducing the mechanical resistance of the weld. Beam shift toward the steel side inhibits the melting of titanium alloy and, so, the formation of brittle intermetallics at the titanium alloy/melted zone interface. Mechanical stability of the obtained junctions was shown to depend on the thickness of this intermetallic layer. The fracture zone of the weld was found to be a mixture of TiCu (3–42 wt%), TiCu1−xFex (x=0.72–0.84) (22–68 wt%) and TiCu1−xFex (x=0.09–0.034) (0–22 wt%). In order to achieve the maximal ultimate tensile strength (350 MPa), the diffusion path length of Ti in the melted zone should be equal to 40–80 µm.

Effect of welding heat input on microstructures and toughness in simulated CGHAZ of V-N high strength steel

Abstract: For the purpose of obtaining the appropriate heat input in the simulated weld CGHAZ of the hot-rolled V–N microalloyed high strength S-lean steel, the microstructural evolution, hardness, and toughness subjected to four different heat inputs were investigated. The results indicate that the hardness decreases with increase in the heat input, while the toughness first increases and then decreases. Moderate heat input is optimum, and the microstructure is fine polygonal ferrite, granular bainite, and acicular ferrite with dispersive nano-scale V(C,N) precipitates. The hardness is well-matched with that of the base metal. Moreover, the occurrence of energy dissipating micromechanisms (ductile dimples, tear ridges) contributes to the maximum total impact energy. The detrimental effect of the free N atoms on the toughness can be partly remedied by optimizing the microstructural type, fraction, morphologies, and crystallographic characteristics. The potency of V(C,N) precipitates on intragranular ferrite nucleation without MnS assistance under different heat inputs was discussed.

Inconsistent effects of mechanical stability of retained austenite on ductility and toughness of transformation-induced plasticity steels

Abstract: The tensile and impact properties of two low-carbon bainitic transformation-induced plasticity (TRIP) steels, one being Al-free and the other being Al-containing, have been investigated. The two steels are featured by the existence of different sizes and shapes of retained austenite showing different mechanical stabilities. Compared with the Al-containing steel, the Al-free steel shows significantly improved tensile strength, uniform strain and total elongation, but exhibits much decreased impact toughness. These results demonstrate conflicting effects of strain-induced martensitic transformation of retained austenite on tensile ductility and impact toughness of TRIP steels. It is indicated that a wide distribution of size and shape of retained austenite is favorable for sustained high strain hardening rate and increased uniform tensile strain; however, thin films of retained austenite, which is mechanically more stable, tends to enhance impact toughness.

Precipitation hardening of 2024-T3 aluminum alloy during creep aging

Abstract: Exposure of Al–Cu–Mg alloys to an elastic loading, either for “creep age forming” or other manufacturing processes at relatively high temperature, will lead to microstructural changes in materials. Effects of the external stress and creep aging temperature on the hardness and precipitation process in a typical Al–Cu–Mg alloy (2024-T3 aluminum alloy) were studied by uniaxial tensile creep experiments. Also, the effects of aging temperature on the age-hardening curves of 2024-T3 aluminum alloy were studied by stress-free-aged experiments. It is found that the precipitation process is very sensitive to the external stress and aging temperature. Temperatures of 423, 448, and 473 K are validated as the “under aging”, “peak aging” and “over aging” temperatures, respectively. An external stress can accelerate the precipitation hardening (also called age hardening) of 2024-T3 aluminum alloy. The large external stress and high aging temperature easily make the preferential precipitation process as SSS →GPB→ S ″ → S ′ → S , and S phase (Al 2 CuMg) is the main precipitate under the experimental conditions. With the increase of the external stress and creep aging temperature, S phase easily grows up and coarsens. Meanwhile, the creep aging treatment results in the discontinuous distribution of precipitation phases on grain boundaries, which can improve the corrosion-resistance of 2024-T3 aluminum alloy.

Micro-tension behaviour of lath martensite structures of carbon steel

Abstract: A micro-tension testing technique was used to investigate the deformation behaviour of lath martensite structures with several types of boundaries in carbon steel. The martensite structures exhibited sufficient necking strains and ductile fractures, whereas the uniform strain was limited owing to a lack of strain-hardening ability despite the increased flow stress. The yield stress of the lath martensite structures strongly depended on the in-lath-plane orientation. The critical resolved shear stress of the in-lath-plane slip systems was considerably lower than that of the out-of-lath-plane slip systems. This finding suggests that the block boundaries are an effective grain boundary for impeding dislocation gliding. Plastic deformation transfer was restricted by the packet boundaries, which greatly rotated the crystallographic orientation of the in-lath-plane slip systems between neighbouring martensite variants.

Interfacial microstructure and mechanical property of Ti6Al4V/A6061 dissimilar joint by direct laser brazing without filler metal and groove

Abstract: Laser brazing of Ti6Al4V and A6061-T6 alloys with 2 mm thickness was conducted by focusing laser beam on aluminum alloy side, and the effect of laser offset distance on microstructure and mechanical properties of the dissimilar butt joint was investigated. Laser offset has a great influence on the thickness of interfacial intermetallic compound (IMC) layer and the mechanical property of joint. The thickness of interfacial IMC layer is less than 500 nm, and the average tensile strength of the joint reaches 64% of aluminum base material strength, when suitable welding conditions are used. The interfacial IMC is TiAl 3 . The formation of interfacial IMC layer and its effect on mechanical property of the joint are discussed in the present study.