Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2015"

Hardened austenite steel with columnar sub-grain structure formed by laser melting

Abstract: Laser melting (LM), with a focused Nd: YAG laser beam, was used to form solid bodies from a 316L austenite stainless steel powder. The microstructure, phase content and texture of the LM stainless steel were characterized and compared with conventional 316L stainless steel. The crack-free LM samples achieved a relative density of 98.6±0.1%. The XRD pattern revealed a single phase Austenite with preferential crystallite growth along the (100) plane and an orientation degree of 0.84 on the building surface. A fine columnar sub-grain structure of size 0.5 μm was observed inside each individual large grain of single-crystal nature and with grain sizes in the range of 10–100 μm. Molybdenum was found to be enriched at the sub-grain boundaries accompanied with high dislocation concentrations. It was proposed that such a sub-grain structure is formed by the compositional fluctuation due to the slow kinetics of homogeneous alloying of large Mo atoms during rapid solidification. The local enrichment of misplaced Mo in the Austenite lattice induced a network of dislocation tangling, which would retard or even block the migration of newly formed dislocations under indentation force, turning otherwise a soft Austenite to hardened steel. In addition, local formation of spherical nano-inclusions of an amorphous chromium-containing silicate was observed. The origin and the implications of the formation of such oxide nano-inclusions were discussed.

Effect of heat treatment on the microstructure and mechanical properties of Inconel 718 processed by selective laser melting

Abstract: The microstructural and mechanical properties of Inconel 718 were determined on the specimens manufactured by selective laser melting (SLM) of prealloyed powder. High- density (99.8%) cylindrical specimens were built with four orientations (0°, 45°, 45°×45° and 90°) in relation to the building and scanning directions. Because of directional, dendritic-cellular grain growth, microstructure of the as-built specimens was characterized by columnar grains of supersaturated solid solution with internal microsegregation of Nb and Mo, demonstrated by fractions of Laves eutectic or its divorced form in interdendritic regions. Such a heterogeneous microstructure is unsuitable for direct post-process aging and makes the alloy sensitive to subsolidus liquation during rapid heating to the homogenizing temperature. In homogenized and aged condition, the alloy received a very good set of mechanical properties in comparison with the wrought material. In heat-treated condition, like in as-built condition, weak anisotropy of properties was found, manifested by lower Young's modulus, yield strength and tensile strength of the specimens extended along the build direction in comparison to the values for the other variants of the specimens. This is attributed to the fact that the grains maintained their geometric and crystallographic texture obtained during solidification.

Effect of standard heat treatment on the microstructure and mechanical properties of selective laser melting manufactured Inconel 718 superalloy

Abstract: Inconel 718 superalloy has been fabricated by selective laser melting technology (SLM). Its microstructure and mechanical properties were studied under solution+aging (SA) standard heat treatment, homogenization+solution+aging (HSA) standard heat treatment and as-fabricated conditions. Precipitated phases and microstructures were examined using OM, SEM, TEM and X-ray analysis methods. The fine dendrite structures with an average dendrite arm spacing of approximately 698 nm accompanying some interdendritic Laves phases and carbide particles can be observed in the as-fabricated materials. After standard heat treatments, dendrite microstructures are substituted by recrystallization grains, and Laves phases also dissolve into the matrix to precipitate strengthening phases and δ particles. The test values of all specimens meet Aerospace Material Specification for cast Inconel 718 alloy, and the transgranular ductile fracture mode exists for the three conditions. The strength and hardness of heat-treated SLM materials increase and are comparable with wrought Inconel 718 alloy, whereas their ductility decreases significantly compared with the as-fabricated material. This is because of the precipitation of fine γˊ and γ〞strengthening phases and needle-like δ phases. For the as-fabricated alloy, the formation of finer dislocated cellular structures that develop into a ductile dimple fracture shows excellent ductility. Due to dislocation pinning from γˊ and γ〞strengthening phases and the impediment of dislocation motion caused by the needle-like δ phases, the ductility of the SA materials decreases and causes a transgranular fracture, compared with the as-fabricated samples.

Effects of process time interval and heat treatment on the mechanical and microstructural properties of direct laser deposited 316L stainless steel

Abstract: The mechanical and microstructural properties of 316L stainless steel (SS) fabricated via Direct Laser Deposition (DLD), a laser-based additive manufacturing method, are presented and compared with those of conventionally-built counterparts. Using a Laser Engineered Net Shaping (LENS ® ) DLD system, the time interval between successive layer deposits, or inter-layer/idle time, for fabricating cylindrical specimens vertically-upward was varied by building either one or nine samples per build plate – thus increasing total assembly volume per build. Subsequently, the effect of thermal history, as well as heat treatment, on microstructural (i.e. grain size and morphology) and mechanical (i.e. tensile, compression, and microhardness) properties of DLD parts were investigated. Results indicate that the DLD 316L SS samples produced herein have a higher yield and ultimate tensile strength relative to their cast and wrought forms. Furthermore, the thermal history, microstructural evolution, and mechanical properties of DLD 316L SS are shown to be dependent on the time interval between deposits. Longer local time intervals result in higher cooling rates, leading to finer microstructures, higher/uniform strength and lower elongation to failure. In addition, porosity and less integral metallurgical bonds are found to be more prevalent in locations further upward from the build plate due to reduced laser penetration depths (e.g. previous-layer remelting decreases). Conversely, parts manufactured with shorter time intervals were found to possess a coarser microstructure, lower strength and higher elongation to failure – attributable to lower cooling rates caused by an increased bulk temperature in the part. These results may aid in future design and control of more efficient, constant-power DLD processes – especially with regard to building multiple and/or larger parts; an approach desirable for minimizing small-to-medium lot production times.

Texture, anisotropy in microstructure and mechanical properties of IN738LC alloy processed by selective laser melting (SLM)

Abstract: Nickel-based IN738LC samples were built by selective laser melting (SLM). To evaluate the anisotropic mechanical behavior of IN738LC material due to layer-wise build up, specimens were built with their cylinder axis (loading direction) oriented either parallel to the building direction, or perpendicular to the building direction and at 45° to the laser scanning direction. After building up the specimens by SLM, they were investigated either under the ‘as-built’ condition or after heat treatment and compared to IN738LC cast material. The analysis of microstructural anisotropy in SLM made IN738LC specimens was done by using EBSD, EDX and X-ray texture analysis methods, and then correlated with anisotropic material behavior observed during tensile and creep testing at room temperature and 850 °C. All SLM samples possess the same general texture, with the majority of grains forming one single component of a cube texture with one of the cubic axes parallel to the building direction, and another cubic axis parallel to the laser scanning direction. The Young׳s modulus determined during tensile testing is significantly lower parallel to the building direction than perpendicular to the building direction, with the values for cast IN738LC material in between. Creep behavior of specimens with loading parallel to the building direction is superior compared to specimens with loading axis normal to the building direction. The anisotropy of Young׳s modulus was modeled based on the single crystal elastic tensor and the measured crystallographic preferred orientations, and compares well with the data from tensile tests.

Influence of processing conditions on strut structure and compressive properties of cellular lattice structures fabricated by selective laser melting

Abstract: AlSi10Mg cellular lattice structures have been fabricated by selective laser melting (SLM) using a range of laser scanning speeds and powers. The as-fabricated strut size, morphology and internal porosity were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray microtomography (micro-CT) and correlated to the compressive properties of the structure. Strut diameter was found to increase monotonically with laser power while the porosity was largest at intermediate powers. Laser scanning speed was found to thicken the struts only at slow rates while the porosity was largest at intermediate speeds. High speed imaging showed the melt pool to be larger at high laser powers. Further the melt pool shape was found to vary cyclically over time, steadily growing before becoming increasingly instable and irregularly shaped before abruptly falling in size due to splashing of molten materials and the process repeating. Upon compressive loading, lattice deformation was homogeneous prior to the peak stress before falling sharply due to the creation of a (one strut wide) shear band at around 45° to the compression axis. The specific yield strength expressed as the yield stress/(yield stress of the aluminium × relative density) is not independent of processing conditions, suggesting that further improvements in properties can be achieved by process optimisation. Lattice struts failed near nodes by a mixture of ductile and brittle fracture.

Microstructure and tensile properties of bulk nanostructured aluminum/graphene composites prepared via cryomilling

Abstract: In order to develop high strength metal–matrix composites with acceptable ductility, bulk nanostructured aluminum–matrix composites reinforced with graphene nanoflakes were fabricated by cryomilling and hot extrusion processes. Microstructure and mechanical properties were characterized and determined using transmission electron microscopy, electron dispersion spectroscopy, as well as static tensile tests. The results show that, with an addition of only 0.5 wt% graphene nanoflakes, the bulk nanostructured aluminum/graphene composite exhibited increased strength and unsubdued ductility over pure aluminum. Besides, the mechanical properties of the composites with higher content of graphene nanoflakes were also measured and investigated. Above 1.0 wt% of graphene nanoflakes, however, this strengthening effect sharply dropped due to the clustering of graphene nanoflakes. Furthermore, the optimal addition of graphene nanoflakes into the nanocrystalline aluminum matrix was calculated and discussed.

Effects of AL addition on microstructure and mechanical properties of AlxCoCrFeNi High-entropy alloy

Abstract: The effects of Al on microstructure and mechanical properties of AlxCoCrFeNi (x=0.1, 0.75 and 1.5) high-entropy alloys were systematically studied by using various characterization methods. It was found that the crystalline structure of AlxCoCrFeNi high-entropy alloy varies markedly with Al content, which changes from the initial single face-centered cubic (fcc) to fcc plus ordered body-centered cubic (bcc) structure (B2) and then to a duplex bcc structure (A2+B2) as the Al content is increased. The chemical composition analysis reveals that Al primarily partitions to B2 phase, suggesting Al is a stabilizer of B2 structure. With increasing Al content, more Ni and Al partition to the B2 phase due to the very negative mixing enthalpy of Ni and Al, and another phase enriched in Cr and Fe transforms from fcc to disordered bcc. Nano indentation measurements show that the hardness of AlxCoCrFeNi high-entropy alloy increases with Al content, accompanied by the decrease of ductility. The stability of single-phase solid solution in AlxCoCrFeNi HEAs is deduced from various criteria. Combined with the experiment results of other similar HEA systems, such as AlxCoCrFeNiCu, the effects of Al addition on the microstructure of AlxCoCrFeNi HEAs are discussed based on the Gibbs free energy of all competing phases and the fundamental properties of constituent elements. The aim of current study is to provide experimental evidence to establish a correlation between the microstructure and mechanical properties to search for high-entropy alloys with higher performances.

Microstructure and mechanical properties of aluminium alloy cellular lattice structures manufactured by direct metal laser sintering

Abstract: This study thoroughly investigated the microstructure and mechanical properties of AlSi10Mg periodic cellular lattice structures with a wide range of volume fractions (5–20%) and unit cell sizes (3–7 mm) fabricated via direct metal laser sintering (DMLS). It was found that the arc-shaped melt pools are overlapping with each other and comprising near fully dense struts (relative densities≥99%) of the as-built lattice structures. The melt pools of the struts are characterized with very fine cellular-dendritic microstructure. Two distinctive zones in the melt pool can be distinguished: the boundary of melt pool possesses the coarse cellular/dendritic microstructure with the cell size or dendrite arm spacing ranging of 2–4 µm, while the interior of melt pool exhibits the much finer cellular microstructure consisting of the 400–700 nm cells mainly filled with the α-Al matrix and some embedded rod-type Si-phases, and the network boundaries predominantly generated by the aggregates of approximately 20 nm Si particles. Both compression strength and microhardness decrease with the increase in the unit cell size when the volume fraction is fixed. This is mainly because the thinner struts of the smaller unit cell size lattice structures were cooled faster by their surroundings and then exhibit a higher cooling rate, leading to finer microstructure. The compression strength increases with increasing the volume fraction, and an equation based on the Gibson–Ashby model is established to estimate the compression strength of DMLS-produced AlSi10Mg gyroid cellular lattice structures with the 3 mm unit cell size.

Comparative study of the microstructures and mechanical properties of direct laser fabricated and arc-melted AlxCoCrFeNi high entropy alloys

Abstract: High entropy alloys (HEA) are a relatively new metal alloy system that have promising potential in high temperature applications. These multi-component alloys are typically produced by arc-melting, requiring several remelts to achieve chemical homogeneity. Direct laser fabrication (DLF) is a rapid prototyping technique, which produces complex components from alloy powder by selectively melting micron-sized powder in successive layers. However, studies of the fabrication of complex alloys from simple elemental powder blends are sparse. In this study, DLF was employed to fabricate bulk samples of three alloys based on the Al x CoCrFeNi HEA system, where x was 0.3, 0.6 and 0.85 M fraction of Al. This produced FCC, FCC/BCC and BCC crystal structures, respectively. Corresponding alloys were also produced by arc-melting, and all microstructures were characterised and compared longitudinal and transverse to the build/solidification direction by x-ray diffraction, glow discharge optical emission spectroscopy and scanning electron microscopy (EDX and EBSD). Strong similarities were observed between the single phase FCC and BCC alloys produced by both techniques, however the FCC/BCC structures differed significantly. This has been attributed to a difference in the solidification rate and thermal gradient in the melt pool between the two different techniques. Room temperature compression testing showed very similar mechanical behaviour and properties for the two different processing routes. DLF was concluded to be a successful technique to manufacture bulk HEA׳s.

Mechanical Behavior of Porous Commercially Pure Ti And Ti–TiB Composite Materials Manufactured By Selective Laser Melting

Abstract: Commercially pure titanium (CP-Ti) and Ti–TiB composite parts with three different porosity levels (i.e. 10%, 17% and 37%) were produced by selective laser melting (SLM). Scanning electron microscopy (SEM) investigations show that martensitic (α′) microstructure exists in SLM-processed CP-Ti parts, whilst SLM-processed Ti–TiB composites present needle-shape TiB particles distributed in α-Ti matrix. Mechanical properties of these porous samples decrease with porosity level increasing. The yield strength and elastic modulus of porous CP-Ti parts range 113–350 MPa and 13–68 GPa respectively, which are much lower than those for porous Ti–TiB counterparts (234–767 MPa and 25–84 GPa respectively) mainly due to the strengthening effect induced by TiB particles in Ti–TiB samples. Compression stress–strain curves of 37% porous CP-Ti parts show a typical three-stage behavior of ductile porous metals. Also, the elastic moduli of both 37% porous CP-Ti and Ti–TiB samples are similar to that of human bone. SEM investigations of the porous CP-Ti samples after compression testing show that no crack presents until 50% compressive strain and most of deformation is absorbed by porous areas. In contrast, μ-CT investigations indicate that all porous Ti–TiB samples fail at early stages of compression testing due to cracks resulting from insufficient ductility of struts of porous areas, because they are not able to accommodate high strains of the deformation at high strengths.

An improved X-ray diffraction analysis method to characterize dislocation density in lath martensitic structures

Abstract: An improved X-ray diffraction line profile analysis method is developed to determine dislocation density of lath martensitic steels. This method combines the modified Warren–Averbach (MWA) and the modified Williamson–Hall (MWH) methods. The developed method is robust and leads to unique values for the dislocation density, the effective outer cut-off radius of the dislocations ( R e ) and the dislocations distribution parameter ( M ). Dislocation structures of lath martensite in a steel, in the as-quenched condition as well as in tempered conditions, are characterized by using the proposed method. The calculated dislocation density is compared with the values obtained from the MWH method by considering a constant value for M . It was found that both methods provide dislocation densities in the range of the values calculated from the dislocation strengthening component of the yield strength.

Tensile ductility of an AlCoCrFeNi multi-phase high-entropy alloy through hot isostatic pressing (HIP) and homogenization

Abstract: The microstructure and phase composition of an AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast (AlCoCrFeNi-AC, AC represents as-cast) and homogenized (AlCoCrFeNi-HP, HP signifies hot isostatic pressed and homogenized) conditions. The AlCoCrFeNi-AC ally has a dendritric structure in the consisting primarily of a nano-lamellar mixture of A2 (disordered body-centered-cubic (BCC)) and B2 (ordered BCC) phases, formed by an eutectic reaction. The homogenization heat treatment, consisting of hot isostatic pressed for 1 h at 1100 °C, 207 MPa and annealing at 1150 °C for 50 h, resulted in an increase in the volume fraction of the A1 phase and formation of a Sigma (σ) phase. Tensile properties in as-cast and homogenized conditions are reported at 700 °C. The ultimate tensile strength was virtually unaffected by heat treatment, and was 396±4 MPa at 700 °C. However, homogenization produced a noticeable increase in ductility. The AlCoCrFeNi-AC alloy showed a tensile elongation of only 1.0%, while after the heat-treatment, the elongation of AlCoCrFeNi-HP was 11.7%. Thermodynamic modeling of non-equilibrium and equilibrium phase diagrams for the AlCoCrFeNi HEA gave good agreement with the experimental observations of the phase contents in the AlCoCrFeNi-AC and AlCoCrFeNi-HP. The reasons for the improvement of ductility after the heat treatment and the crack initiation subjected to tensile loading were discussed.

On the room temperature deformation mechanisms of a TiZrHfNbTa refractory high-entropy alloy

Abstract: The deformation micro-mechanisms of an as-cast equimolar refractory high-entropy alloy composed of Ti, Zr, Hf, Nb and Ta are analyzed by monotonic and relaxation compression tests coupled to transmission electron microscopy observations The evolution of the work hardening with plastic strain displays 3 stages After a sharp decrease until a plastic strain of approximately 3%, work hardening stabilizes at 1300±50 MPa and finally decreases again The measured apparent activation volumes V a p p * slightly evolve with plastic strain and decrease from approximately 50b3 to 30b3 These values are coherent with a Peierls mechanism due to strong intrinsic lattice friction In addition, the measured activation volumes correlate well with TEM observations, which give evidence that dislocation glide is controlled – in the first stages – by the movement of screw dislocations The deformation is rapidly localized in bands in which dislocation dipoles, loops and tangles are induced at higher plastic strains

TiB2 reinforced aluminum based in situ composites fabricated by stir casting

Abstract: In this study, a new technique involving mechanical stirring at the salts/aluminum interface was developed to fabricate TiB 2 particulate reinforced aluminum based in situ composites with improved particle distribution. Processing parameters in terms of stirring intensity, stirring duration and stirring start time were optimized according to the microstructure and mechanical properties evaluation. The results show that, the first and last 15 min of the entire 60 min holding are of prime importance to the particle distribution of the final composites. When applying 180 rpm (revolutions per minute) stirring at the salts/aluminum interface in these two intervals, a more uniform microstructure can be achieved and the Al-4 wt% TiB 2 composite thus produced exhibits superior mechanical performance. Synchrotron radiation X-ray computed tomography (SR-CT) was used to give a full-scale imaging of the particle distribution. From the SR-CT results, the in situ Al– x TiB 2 composites ( x =1, 4 and 7, all in wt%) fabricated by the present technique are characterized by fine and clean TiB 2 particles distributed uniformly throughout the Al matrix. These composites not only have higher yield strength ( σ 0.2 ) and ultimate tensile strength (UTS), but also exhibit superior ductility, with respect to the Al–TiB 2 composites fabricated by the conventional process. The σ 0.2 and UTS of the Al–7TiB 2 composite in the present work, are 260% and 180% higher than those of the matrix. A combined mechanism was also presented to interpret the improvements in yield strength of the composites as influenced by their microstructures and processing history. The predicted values are in good agreement with the experimental results, strongly supporting the strengthening mechanism we proposed. Fractography reveals that the composites thus fabricated, follow ductile fracture mechanism in spite of the presence of stiff reinforcements.

Tensile properties of low-stacking fault energy high-entropy alloys

Abstract: An equiatomic NiFeCrCoMn alloy, two non-equiatomic NiFeCrCoMn alloys optimized for low stacking fault energy, and an equiatomic NiFeCrCo alloy were produced by arc melting. Samples were homogenized, cold rolled, and annealed at temperatures between 575 and 1100 °C. Samples annealed at a moderate temperature near their recrystallization temperature (625–675 °C) and 1100 °C were cut into flat tensile samples and tested at a strain rate of 7.3×10−4 s−1. Equiatomic NiFeCrCo had the highest ductility and toughness after annealing at both temperatures, followed by Ni18.5Fe18.5Cr18.5Co26Mn18.5. Ni14Fe20Cr26Co20Mn20 exhibited poor thermal stability, forming σ-phase intermetallics at temperatures below 1100 °C. Observation of the fracture surfaces suggested that the high performance of NiFeCrCo might be due to the absence of oxide particles that form in the Mn-containing alloys. The strain-hardening rate and exponent were calculated from the results, showing a large deviation from typical behavior and significant grain size dependence.

High temperature deformation behavior and dynamic recrystallization in CoCrFeNiMn high entropy alloy

Abstract: Microstructure evolution in high-entropy alloy CoCrFeNiMn during uniaxial compression to a height reduction of true strain of ≈1.4 in the temperature interval 600–1100 °C was studied. Although some differences was observed in the mechanical behavior of the alloy and the activation energy of deformation in warm (below 800 °C) and hot (above 800 °C) temperature intervals, microstructure evolution at all studied temperatures was found to be accompanied by discontinuous dynamic recrystallization (dDRX). During hot deformation recrystallization was primary associated with nucleation of new grains on the initial grain boundaries, while in the warm interval dDRX was mainly observed in shear bands. The volume fraction of the recrystallized structure was respectively 0.085 and 0.95 at 600 and 1000 °C and the recrystallized grain size was found to be 0.2 and 40.4 µm for 600 and 1100 °C, respectively.

Non-equiatomic high entropy alloys: Approach towards rapid alloy screening and property-oriented design

Abstract: The high entropy alloy (HEA) concept has triggered a renewed interest in alloy design, even though some aspects of the underlying thermodynamic concepts are still under debate This study addresses the short-comings of this alloy design strategy with the aim to open up new directions of HEA research targeting specifically non-equiatomic yet massively alloyed compositions We propose that a wide range of massive single phase solid solutions could be designed by including non-equiatomic variants It is demonstrated by introducing a set of novel non-equiatomic multi-component CoCrFeMnNi alloys produced by metallurgical rapid alloy prototyping Despite the reduced configurational entropy, detailed characterization of these materials reveals a strong resemblance to the well-studied equiatomic single phase HEA: The microstructure of these novel alloys exhibits a random distribution of alloying elements (confirmed by Energy-Dispersive Spectroscopy and Atom Probe Tomography) in a single face-centered-cubic phase (confirmed by X-ray Diffraction and Electron Backscatter Diffraction), which deforms through planar slip (confirmed by Electron-Channeling Contrast Imaging) and leads to excellent ductility (confirmed by uniaxial tensile tests) This approach widens the field of HEAs to non-equiatomic multi-component alloys since the concept enables to tailor the stacking fault energy and associated transformation phenomena which act as main mechanisms to design useful strain hardening behavior

Effects of retained austenite volume fraction, morphology, and carbon content on strength and ductility of nanostructured TRIP-assisted steels

Abstract: With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C–0.30Si–1.76Mn–1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100 MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800 MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.

Grain-size dependent mechanical behavior of nanocrystalline metals

Abstract: Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 106 s−1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d>1 μm, 1 μm

Effect of martensite morphology and volume fraction on strain hardening and fracture behavior of martensite–ferrite dual phase steel

Abstract: Two different morphologies of martensite in dual phase (DP) steel were obtained using two different processing routes. In one case, intermediate quenching (IQ) was adapted, where DP steel was water-quenched to obtain martensite phase, followed by inter-critical annealing. In the second case, the steel was cold rolled, followed by inter-critical annealing (CR-IA). For IQ and CR-IA steels, the inter-critical temperatures varied from 750 °C to 850 °C to obtain different volume fractions of martensite. An understanding of structure–property was obtained using a combination of scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. It was observed that fibrous martensite presented in IQ samples, gradually transformed to blocky martensite with increase in inter-critical temperature, resembling the CR-IA steels. The fibrous martensite encouraged martensite cracking, however, the martensite cracking was dramatically decreased in the IQ samples with increase in martensite fraction. The strain hardening behavior studied using the differential C – J model indicated multistage depending on the fraction of martensite. The low volume fraction of martensite in the DP steel provided high ductility–toughness combination and improved strain hardening ability due to the presence of soft ferrite phase in DP steel. Fibrous martensite in DP steel resulted in less strain hardening than blocky martensite, prior to exceeding a threshold volume fraction. The threshold value was significantly smaller for DP steel with blocky martensite.

Enhanced tensile properties of magnesium composites reinforced with graphene nanoplatelets

Abstract: The aim of this studys is to fabricate magnesium reinforced metal matrix composites using graphene nanoplatelets (GNPs) via powder metallurgy processing in order to enhance room temperature mechanical properties. The microstructural evaluation and mechanical behaviors of composite powders and extruded bulk materials were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometer and mechanical tests. The uniform dispersion and large specific surface area per volume of GNPs embedded in magnesium matrix led to increament in microhardness, tensile strength and fracture strains of the composites. For example, when employing the pure magnesium reinforced with 0.30 wt% GNPs, the increase of Young׳s modulus, yield strength, and failure strain of extruded nanocomposite was +131%, +49.5% and +74.2% respectively, compared to those of extruded materials with no GNPs additive. Additionally, mechanical properties of synthesized composites were compared with previously reported Mg–CNTs composites. It was found that GNPs outperform CNTs due their high specific surface area.

Texture, local misorientation, grain boundary and recrystallization fraction in pipeline steels related to hydrogen induced cracking

Abstract: In the present study, API X60 and X60SS pipeline steels were cathodically charged by hydrogen for 8 h using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate. After charging, SEM observations showed that the hydrogen induced cracking (HIC) appeared at the center of cross section in the X60 specimen. However, HIC did not appear in the X60SS steel. Therefore, electron backscatter diffraction (EBSD) technique was used to analyze the center of cross section of as-received X60SS, X60 and HIC tested X60 specimens. The results showed that the HIC crack not only can propagate through 〈100〉||ND oriented grains but also its growth may happen in various orientations. In HIC tested X60 specimen, an accumulation of low angle grain boundaries around the crack path documented that full recrystallization was not achieved during hot rolling. Kernel Average Misorientaion (KAM) histogram illustrated that the deformation is more concentrated in as-received and HIC tested X60 specimens rather than in as-received X60SS specimen. Moreover, the concentration of coincidence site lattice (CSL) boundary in HIC tested X60 specimen was very low compared with other samples. The recrystallization area fraction in X60SS steel was very high. This high amount of recrystallization fraction with no stored energy is one of the main reasons for high HIC resistance of this steel to HIC. The orientation distribution function (ODF) of the recrystallized, substructured and deformed fractions in as-received X60SS and X60 steel showed relative close orientations in both as-received specimens.

Processing and properties of topologically optimised biomedical Ti-24Nb-4Zr-8Sn scaffolds manufactured by selective laser melting

Abstract: This study investigated the effect of the processing parameters on the quality and mechanical properties of a biomedical titanium alloy (Ti–24Nb–4Zr–8Sn) scaffolds fabricated by selective laser melting. Optimal manufacturing parameters were then determined through analysing the pores distribution, geometrical accuracy and the mechanical properties of the produced components. The evaporation of tin during the process is thought to be the main cause of pore generation at higher incident energy densities. Using the optimal processing conditions, the strength of the scaffold reached 51 MPa at a scaffold density of 3 and a high solid strut relative density of ~99.3%. Fracture surface analysis found that the main reason for strut early failure was the weaknesses of struts caused by the presence of pores as well the thickness of strut and internal unmelted powders. **Co-corresponding author.

Determination of Johnson cook material and failure model constants and numerical modelling of Charpy impact test of armour steel

Abstract: The behaviour of typical armour steel material under large strains, high strain rates and elevated temperatures needs to be investigated to analyse and reliably predict its response to various types of dynamic loading like impact. An empirical constitutive relation developed by Johnson and Cook (J–C) is widely used to capture strain rate sensitivity of the metals. A failure model proposed by Johnson and Cook is used to model the damage evolution and predict failure in many engineering materials. In this work, model constants of J–C constitutive relation and damage parameters of J–C failure model for a typical armour steel material have been determined experimentally from four types of uniaxial tensile test. Some modifications in the J–C damage model have been suggested and Finite Element simulation of three different tensile tests on armour steel specimens under dynamic strain rate (10 −1 s −1 ), high triaxiality and elevated temperature respectively has been done in ABAQUS platform using the modified J–C failure model as user material sub-routine. The simulation results are validated by the experimental data. Thereafter, a moderately high strain rate event viz. Charpy impact test on armour steel specimen has been simulated using J–C material and failure models with the same material parameters. Reasonable agreement between the simulation and experimental results has been achieved.

Characterization of hot-pressed graphene reinforced ZrB2 -SiC composite

Abstract: In this paper, the hot pressing of monolithic ZrB2 ceramic (Z), ZrB2–25 vol% SiC composite (ZS), as well as 5 wt% graphene reinforced ZrB2–25 vol% SiC composite (ZSG) is investigated. The hot pressing at 1850 1C for 60 min under a uniaxial pressure of 20 MPa resulted in a near fully-dense ZSG composite (499% relative density). In addition, the influence of graphene reinforcement on the sintering process, microstructure, and mechanical properties (Vickers hardness and fracture toughness) of ZrB2–SiC composite is discussed. It was disclosed that the grain growth of the ZrB2 matrix was effectively stopped by SiC particles and graphene nano-platelets. The fracture toughness of ZSG composite (6.4 MPa m 1/2 ) was strongly enhanced by incorporating the mentioned reinforcements into the ZrB2 matrix, which is higher than that of Z ceramic (1.8 MPa m 1/2 ) and ZS composite (4.3 MPa m 1/2 ). After the hot pressing process, the fractographical outcomes revealed that some graphene nano-platelets were kept in the composite microstructure, apart from SiC grains, which lead to the toughening of the composite through graphene nano-platelets wrapping and pull out, crack deflection, and crack bridging. & 2014 Elsevier B.V. All rights reserved.

Hot deformation behavior, dynamic recrystallization, and physically-based constitutive modeling of plain carbon steels

Abstract: The high-temperature deformation behaviors of low and medium carbon steels with respectively 0.06 and 0.5 wt% C were investigated under strain rate and temperature ranges of 10−4–10−1 s−1 and of 900–1100 °C. Three types of dynamic recrystallization (DRX) flow behaviors were identified, namely single peak, multiple transient steady state (MTSS), and cyclic behaviors. The normalized critical stress (and strain) for the low and medium carbon steels were about 0.846 (0.531) and 0.879 (0.537), respectively. For both steels, the apparent deformation activation energy and the power of the hyperbolic sine law were found to be near the lattice self-diffusion activation energy of austenite (270 kJ/mol) and 4.5, respectively. As a result, it was concluded that the flow stress of plain carbon steels in hot deformation is mainly controlled by dislocation climb during their intragranular motion, and based on physically-based constitutive analysis, it was found that carbon has a slight effect on the hot flow stress of plain carbon steels. The significance of the approach used in this work was shown to be its reliance on the theoretical analysis based on the deformation mechanisms, which makes the comparison more reliable.

Fabrication of metal matrix composites by friction stir processing with different Particles and processing parameters

Abstract: Friction stir processing has been employed to produce metal matrix composites by incorporating reinforcement particles in an Al 5059 matrix. The fabrication method involved repeated friction stir passes on a groove which contained powder reinforcements, with different processing parameters and tool geometries used for each pass. Various particles with sizes from 130 nm to 4.3 μm, and different process parameters were examined to obtain a uniform distribution of particles within the stir zone. Mechanical properties (i.e. tensile and microhardness) of the Al 5059 matrix MMCs reinforced with Al2O3, SiC, and B4C with particle sizes of 130, 250, and 35 nm respectively were compared. Tensile tests showed 11%, 20%, and 38% increases in yield strength compared to the matrix alloy for composites containing nano-scale Al2O3, SiC, and B4C, respectively. When 4.3 μm Al2O3 particles were employed, higher volume fractions could be achieved which resulted in a 32% increase in yield strength compared to the base metal. The average microhardness value within the stir zone increased from 85 HV in the base material to a maximum of 170 HV in the B4C-reinforced composite. Nano-scale particles seem to be more effective to increase hardness by increasing the particle fraction in the produced composites.

The effect of location on the microstructure and mechanical properties of titanium aluminides produced by additive layer manufacturing using in-situ alloying and gas tungsten arc welding

Abstract: An innovative and low cost additive layer manufacturing (ALM) process is used to produce γ-TiAl based alloy wall components. Gas tungsten arc welding (GTAW) provides the heat source for this new approach, combined with in-situ alloying through separate feeding of commercially pure Ti and Al wires into the weld pool. This paper investigates the morphology, microstructure and mechanical properties of the additively manufactured TiAl material, and how these are affected by the location within the manufactured component. The typical additively layer manufactured morphology exhibits epitaxial growth of columnar grains and several layer bands. The fabricated γ-TiAl based alloy consists of comparatively large α 2 grains in the near-substrate region, fully lamellar colonies with various sizes and interdendritic γ structure in the intermediate layer bands, followed by fine dendrites and interdendritic γ phases in the top region. Microhardness measurements and tensile testing results indicated relatively homogeneous mechanical characteristics throughout the deposited material. The exception to this homogeneity occurs in the near-substrate region immediately adjacent to the pure Ti substrate used in these experiments, where the alloying process is not as well controlled as in the higher regions. The tensile properties are also different for the vertical (build) direction and horizontal (travel) direction because of the differing microstructure in each direction. The microstructure variation and strengthening mechanisms resulting from the new manufacturing approach are analysed in detail. The results demonstrate the potential to produce full density titanium aluminide components directly using the new additive layer manufacturing method.

Dissimilar laser welding of AISI 316L stainless steel to Ti6–Al4–6V alloy via pure vanadium interlayer

Abstract: Successful continuous laser joining of AISI 316L stainless steel with Ti6Al4V titanium alloy through pure vanadium interlayer has been performed. Three welding configurations were tested: one-pass welding involving all three materials and two pass and double spot welding involving creation of two melted zones separated by remaining solid vanadium. For the most relevant welds, the investigation of microstructure, phase content and mechanical properties has been carried out. In case of formation of a single melted zone, the insertion of steel elements into V-based solid solution embrittles the weld. In case of creation of two separated melted zones, the mechanical resistance of the junction is determined by annealing of remaining vanadium interlayer, which can be witnessed by observing the increase of grain size and decrease of UTS. The two pass configuration allows attain highest mechanical resistance: 367 MPa or 92% of UTS of annealed vanadium. Double spot configuration produces excessive heat supply to vanadium interlayer, which results in important decrease of tensile strength down to 72% of UTS of annealed vanadium. It was found that undesirable σ phase which forms between Fe and V is not created during the laser welding process because of high cooling rates. However, the zones whose composition corresponds to σ homogeneity range are crack-susceptible, so the best choice is to reduce the V content in steel/vanadium melted zone below σ phase formation limit. In the same time, the proportion between V and Ti in Ti6Al4V/vanadium melted zones does not influence mechanical properties as these elements form ideal solid solution.