Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2011"

General relationship between strength and hardness

Abstract: [Zhang, P.; Li, S. X.; Zhang, Z. F.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China.;Zhang, ZF (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China;zhfzhang@imr.ac.cn

Graphene–aluminum nanocomposites

Abstract: Composites of graphene platelets and powdered aluminum were made using ball milling, hot isostatic pressing and extrusion. The mechanical properties and microstructure were studied using hardness and tensile tests, as well as electron microscopy, X-ray diffraction and differential scanning calorimetry. Compared to the pure aluminum and multi-walled carbon nanotube composites, the graphene–aluminum composite showed decreased strength and hardness. This is explained in the context of enhanced aluminum carbide formation with the graphene filler.

Microstructure and mechanical properties of Al-Al2O3 micro and nano composites fabricated by stir casting

Abstract: Aluminum matrix composites (AMCs) reinforced with micro and nano-sized Al2O3 particles are widely used for high performance applications such as automotive, military, aerospace and electricity industries because of their improved physical and mechanical properties. In this study, in order to improve the wettability and distribution of reinforcement particles within the matrix, a novel three step mixing method was used. The process included heat treatment of micro and nano Al2O3 particles, injection of heat-treated particles within the molten A356 aluminum alloy by inert argon gas and stirring the melt at different speeds. The influence of various processing parameters such as heat treatment of particles, injection process, stirring speed, reinforcement particle size and weight percentage of reinforcement particles on the microstructure and mechanical properties of composites was investigated. The matrix grain size, morphology and distribution of Al2O3 nanoparticles were recognized by scanning electron microscopy (SEM), optical microscope (OM) equipped with image analyzer, energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Also, the hardness and compression strength of samples was investigated. The results showed the poor incorporation of nano particles in the aluminum melt prepared by the common condition. However, the use of heat-treated particles, injection of particles and the stirring system improved the wettability and distribution of the nano particles within the aluminum melt. In addition, it was revealed that the amount of hardness, compressive strength and porosity increased as weight percentage of nano Al2O3 particles increased.

Micro-CT-based improvement of geometrical and mechanical controllability of selective laser melted Ti6Al4V porous structures

Abstract: Despite the fact that additive manufacturing (AM) techniques allow to manufacture complex porous parts with a controlled architecture, differences can occur between designed and as-produced morphological properties. Therefore this study aimed at optimizing the robustness and controllability of the production of porous Ti6Al4V structures using selective laser melting (SLM) by reducing the mismatch between designed and as-produced morphological and mechanical properties in two runs. In the first run, porous Ti6Al4V structures with different pore sizes were designed, manufactured by SLM, analyzed by microfocus X-ray computed tomography (micro-CT) image analysis and compared to the original design. The comparison was based on the following morphological parameters: pore size, strut thickness, porosity, surface area and structure volume. Integration of the mismatch between designed and measured properties into a second run enabled a decrease of the mismatch. For example, for the average pore size the mismatch decreased from 45% to 5%. The demonstrated protocol is furthermore applicable to other 3D structures, properties and production techniques, powder metallurgy, titanium alloys, porous materials, mechanical characterization, tomography.

Sheet texture modification in magnesium-based alloys by selective rare earth alloying

Abstract: The current study examines the influence of select rare earth elements; Gd, Nd, Ce, La and mischmetal (MM) on the sheet texture modification during warm rolling and annealing of a ZEK100 magnesium alloy, and the resulting formability and anisotropy during subsequent tensile testing at room temperature. It was found that all the investigated RE elements led to weak sheet textures and hence promoted enhanced ductility and reduced anisotropy over conventional Mg sheet. Gd was of a particular interest because it gave rise to a desired Mg sheet texture despite its coarsest grain size resulting in promising mechanical properties. It is suggested that solute related effects on the grain boundary migration and the relative strengths of different deformation mechanisms are responsible for altering the common concepts of recrystallization and grain growth during annealing, and the activation scenarios of slip and twinning during deformation.

Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy

Abstract: A new refractory alloy, Nb 20 Cr 20 Mo 10 Ta 10 Ti 20 Zr 20 , was produced by vacuum arc melting. To close shrinkage porosity, it was hot isostatically pressed (HIPd) at T = 1723 K and P = 207 MPa for 3 h. In both as-solidified and HIPd conditions, the alloy contained three phases: two body centered cubic (BCC1 and BCC2) and one face centered cubic (FCC). The BCC1 phase was enriched with Nb, Mo and Ta and depleted with Zr and Cr, and its lattice parameter after HIP was a = 324.76 ± 0.16 pm. The BCC2 phase was enriched with Zr and Ti and considerably depleted with Mo, Cr and Ta, and its lattice parameter after HIP was estimated to be a = 341.0 ± 1.0 pm. The FCC phase was highly enriched with Cr and it was identified as a Laves C15 phase, (Zr,Ta)(Cr,Mo,Nb) 2 , with the lattice parameter a = 733.38 ± 0.18 pm. The volume fractions of the BCC1, BCC2 and FCC phases were 67%, 16% and 17%, respectively. The alloy density and Vickers microhardness were ρ = 8.23 ± 0.01 g/cm 3 and H v = 5288 ± 71 MPa. The alloy had compression yield strength of 1595 MPa at 296 K, 983 MPa at 1073 K, 546 MPa at 1273 K and 171 MPa at 1473 K. During deformation at 296 K and 1073 K, the alloy showed a mixture of ductile and brittle fracture after plastic compression strain of ∼5–6%. No macroscopic fracture was observed after 50% compression strain at 1273 K and 1473 K. Phase transformations and particle coarsening considerably accelerated by the plastic deformation occurred in the temperature range of 1073–1473 K.

Effect of friction stir welding parameters on the microstructure and mechanical properties of the dissimilar Al-Cu joints

Abstract: Butt joints of 1060 aluminum alloy and commercially pure copper were produced by friction stir welding (FSW) and the effect of welding parameters on surface morphology, interface microstructure and mechanical properties was investigated. The experimental results revealed that sound defect-free joints could be obtained under larger pin offsets when the hard Cu plate was fixed at the advancing side. Good tensile properties were achieved at higher rotation rates and proper pin offsets of 2 and 2.5 mm; further, the joint produced at 600 rpm with a pin offset of 2 mm could be bended to 180 degrees without fracture. The mechanical properties of the FSW Al-Cu joints were related closely to the interface microstructure between the Al matrix and Cu bulk. A thin, uniform and continuous intermetallic compound (IMC) layer at the Al-Cu butted interface was necessary for achieving sound FSW Al-Cu joints. Stacking layered structure developed at the Al-Cu interface under higher rotation rates, and crack initiated easily in this case, resulting in the poor mechanical properties. (C) 2011 Elsevier B.V. All rights reserved.

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Abstract: Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.

Investigation of mechanical properties of Cu/SiC composite fabricated by FSP: Effect of SiC particles’ size and volume fraction

Abstract: In this experiment, copper-base composites reinforced with 30 nm and 5 μm SiC particles are fabricated on the surface of a purecopper sheetvia friction stir processing (FSP). Microstructure, mechanical properties and wear resistance of friction stir processed (FSPed) materials are investigated as a function of volume fraction of SiC particles. Results show that, applying FSP, without SiC particles, increases the percent elongation significantly (more than 2.5 times) and decreases copper's strength. Adding micro- and nano-sized SiC particles decreases the tensile strength and percent elongation. Increasing the volume fraction or decreasing the reinforcing particle size enhances the tensile strength and wear resistance and lowers the percent elongation.

On the superior hot hardness and softening resistance of AlCoCrxFeMo0.5Ni high-entropy alloys

Abstract: Effects of Cr content on microstructures and hot hardness of AlCoCrxFeMo0.5Ni high-entropy alloys (x = 0–2.0) were investigated. The cast microstructure of AlCoCrxFeMo0.5Ni consists of B2 and σ phases, both being multi-element solid solutions. Increasing Cr content increases the volume fraction of σ phase and causes the matrix phase of the dendrite to change from B2 phase to σ phase. The alloy hardness increases from Hv 601 at x = 0 to Hv 867 at x = 2.0 as a result of increasing amount of hard σ phase. A phase diagram for the AlCoCrxFeMo0.5Ni alloy system is constructed based on SEM, HTXRD and DTA analyses, providing useful information for understanding and designing high-entropy alloys. All the AlCoCrxFeMo0.5Ni alloys possess higher hot-hardness level than that of Ni-based superalloys, In 718 and In 718 H, from room temperature to 1273 K. Cr-1.5 and Cr-2.0 alloys exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K and have respective hardness values, Hv 374 and Hv 450 at 1273 K, being much higher than those, around Hv 127, of In 718 and In 718 H. The mechanism of larger strengthening and softening resistance is related with B2 and σ phases, both having multi-principal-element effect. The AlCoCrxFeMo0.5Ni alloy system has a potential in high-temperature applications.

Effect of microstructure on retained austenite stability and work hardening of TRIP steels

Abstract: Retained austenite is a metastable phase in transformation induced plasticity (TRIP) steels that transforms into martensite under local stress and strain. This transformation improves sheet formability, allowing this class of higher strength steels to be used for applications such as automotive structural components. The current work studies two distinct TRIP steel microstructures, i.e. equiaxed versus lamellar, and how microstructure affects the austenite transformation during uniaxial tensile loading. Different heat treatments were employed to obtain the two microstructures, and the bainite hold times of the treatments were varied to change the volume fraction of retained austenite. Based on uniaxial tensile response and magnetic saturation measurements, the bainite hold time of 100 s was determined to produce the best results in terms of largest strain at the ultimate tensile strength and highest volume fraction of retained austenite. The work hardening of the samples with a 100 s bainite hold was evaluated by calculating the instantaneous n value as a function of strain. It was found that the lamellar microstructure has a lower maximum instantaneous n value than the equiaxed microstructure, but has higher work hardening values for strain levels greater than 0.05 and up to the ultimate tensile strength. This difference in work hardening behaviour corresponds directly to the transformation rate of retained austenite in the two microstructures. The slower rate of transformation in the lamellar microstructure allows for work hardening to persist at high strains where the transformation effect has already been exhausted in the equiaxed microstructure. The different rates of transformation can be attributed to the location, carbon content, and size of the retained austenite grains in the respective TRIP microstructures.

Hot deformation behavior of a medium carbon microalloyed steel

Abstract: The hot deformation behavior of a medium carbon microalloyed steel was studied using the hot compression flow curves corresponding to the temperature range of 850–1150 °C under strain rates from 0.0001 to 3 s −1 . A step-by-step procedure for data analysis in hot deformation was also given. The work hardening rate versus stress curves were used to reveal if dynamic recrystallization (DRX) occurred. The application of constitutive equations to determine the hot working constants of this material was critically discussed. Furthermore, the effect of Zener–Hollomon parameter ( Z ) on the characteristic points of flow curves was studied using the power law relation. The deformation activation energy of this steel was determined as 394 kJ/mol and the normalized critical stress and strain for initiation of DRX were found to be 0.89 and 0.62, respectively. Some behaviors were also compared to other steels.

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Abstract: The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

Hierarchical microstructure of explosive joints: Example of titanium to steel cladding

Abstract: The microstructure of explosive cladding joints formed among parallel Ti and steel plates was examined by electron microscopy. The bonding interface and the bulk materials around it form pronounced hierarchical microstructures. This hierarchy is characterized by the following features: at the mesoscopic scale of the hierarchy a wavy course of the interface characterizes the interface zone. This microstructure level is formed by heavy plastic shear waves (wavelength ≈ 0.5 mm) which expand within the two metal plates during the explosion parallel to the bonding interface. At the micro-scale range, intermetallic inclusions (size ≈ 100–200 μm) are formed just behind the wave crests on the steel side as a result of partial melting. Electron diffraction revealed FeTi and metastable Fe 9.64 Ti 0.36 . Most of the observed phases do not appear in the equilibrium Fe–Ti phase diagram. These intermetallic inclusions are often accompanied by micro-cracks of similar dimension. At the smallest hierarchy level we observe a reaction layer of about 100–300 nm thickness consisting of nano-sized grains formed along the entire bonding interface. Within that complex hierarchical micro- and nanostructure, the mesoscopic regime, more precisely the type and brittleness of the intermetallic zones, seems to play the dominant role for the mechanical behavior of the entire compound.

On the feasibility of friction spot joining in magnesium/fiber-reinforced polymer composite hybrid structures

Abstract: In the present study, the feasibility of the friction spot joining technique on magnesium AZ31–O/glass fiber and carbon fiber reinforced poly(phenylene sulfide) joints is addressed. The thermo-mechanical phenomena associated with the friction spot joining process promoted metallurgical and polymer physical–chemical transformations. These effects resulted in grain refinement by dynamic recrystallization and changes in local (microhardness) and global strength (lap shear). Friction spot lap joints with elevated mechanical performance (20–28 MPa) were produced without surface pre-treatment. This preliminary investigation has successfully shown that friction spot joining is an alternative technology for producing hybrid polymer–metal structures.

Woven hybrid composites: Tensile and flexural properties of oil palm-woven jute fibres based epoxy composites

Abstract: In this research, tensile and flexural performance of tri layer oil palm empty fruit bunches (EFB)/woven jute (Jw) fibre reinforced epoxy hybrid composites subjected to layering pattern has been experimentally investigated. Sandwich composites were fabricated by hand lay-up technique in a mould and cured with 105 °C temperatures for 1 h by using hot press. Pure EFB and woven jute composites were also fabricate for comparison purpose. Results showed that tensile and flexural properties of pure EFB composite can be improved by hybridization with woven jute fibre as extreme woven jute fibre mat. It was found that tensile and flexural properties of hybrid composite is higher than that of EFB composite but less than woven jute composite. Statistical analysis of composites done by ANOVA-one way, it showed significant differences between the results obtained. The fracture surface morphology of the tensile samples of the hybrid composites was performed by using scanning electron microscopy.

Assessing the suitability of three Australian fly ashes as an aluminosilicate source for geopolymers in high temperature applications

Abstract: Fly ash characteristics cannot be assumed to be constant between power stations as they are highly dependent on the coal source and burning conditions. It is critical to understand the characteristics of fly ash in order to produce geopolymers suitable for high temperature applications. We report on the characterisation of fly ash from three Australian power stations in terms of elemental composition, phase composition, particle size, density and morphology. Geopolymers were synthesised from each of the fly ashes using sodium silicate and sodium aluminate solutions to achieve a range of Si:Al compositional ratios. Mechanical properties of geopolymer binders are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. It was found that the twenty eight day strength of geopolymers is largely dependent on the sub 20 μm size fraction of the fly ash. Strength loss after high temperature exposure was found to be dependent on the concentration of iron in the fly ash precursor and the Si:Al ratio of the geopolymer mixture.

Effect of friction stir processing (FSP) on microstructure and properties of Al–TiC in situ composite

Abstract: Aluminium based in situ composites have many advantages over their conventional counterparts. However, a major problem in such composites is the segregation of the in situ formed particles at the grain boundaries. In this study, it has been shown for the first time that friction stir processing (FSP) can be used effectively to homogenise the particle distribution in Al based in situ composites. An Al-5 wt.% TiC composite was processed in situ using a mixture of K 2 TiF 6 and graphite powders in aluminium melt. Friction stir processing was employed on the as-cast composite to uniformly distribute the TiC particles in the Al matrix. The composite was subjected to single and double pass FSP and its effect on the microstructure and properties was evaluated. A single pass of FSP was enough to break the particle segregation from the grain boundaries and improve the distribution. Two passes of FSP resulted in complete homogenization and elimination of casting defects. The grain size was also refined after each FSP pass. This led to significant improvement in the mechanical properties. The novel feature of the composite is that while the strength and hardness improved substantially after FSP, the ductility was not compromised.

Influence of intermetallic phases and Kirkendall-porosity on the mechanical properties of joints between steel and aluminium alloys

Abstract: The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5 wt.%) and Al–5 wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200–600 ◦ C for 9–64 min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the phase (Al5Fe2) as the dominant component after annealing at 450 ◦ C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al–5 wt.% Si is controlled by the thickness of the phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers.

Study on laser welding-brazing of zinc coated steel to aluminum alloy with a zinc based filler

Abstract: Laser welding–brazing (LWB) technique with CW Nd:YAG laser was used for lap joining of zinc coated steel (DP600) with aluminum alloy (AA6016) using a filler wire composed of 85% Zn and 15% Al. LWB was performed with varying laser power, brazing speed, and wire feed speed. The microstructure and composition analyses of the brazed joints were examined using SEM and EDS while the mechanical properties were measured in the form of micro-hardness and tensile strength. The thickness of reaction layers formed along the steel–seam interface was in the range of 3–23 μm, and it varied with the brazing speed. The average micro-hardness value of this layer was 348 HV, compared to 150 HV in brazed seam and 230 HV in steel matrix. It has been found that joints produced with heat inputs between 60 and 110 J/mm exhibited higher mechanical resistance of about 220 MPa and the failures occurred away from the joints on aluminum side. The corresponding brazing speeds are between 0.5 and 0.8 m/min and the thickness of the layer produced ranged between 8 and 12 μm.

In situ characterization of the deformation and failure behavior of non-stochastic porous structures processed by selective laser melting

Abstract: Cellular materials are promising candidates for load adapted light-weight structures Direct manufacturing (DM) tools are effective methods to produce non-stochastic structures Many DM studies currently focus on optimization of the geometric nature of the structures obtained The literature available so far reports on the mechanical properties but local deformation mechanisms are not taken into account In order to fill this gap, the current study addresses the deformation behavior of a lattice structure produced by selective laser melting (SLM) on the local scale by means of a comprehensive experimental in situ approach, including electron backscatter diffraction, scanning electron microscopy and digital image correlation SLM-processed as well as heat treated lattice structures made from TiAl6V4 alloy were employed for mechanical testing It is demonstrated that the current approach provides means to understand the microstructure-mechanical property–local deformation relationship to allow for optimization of load adapted lattice structures

Effects of heat treatments on the microstructure and mechanical properties of a 6061 aluminium alloy

Abstract: This paper describes the mechanical behavior of the 6061-T6 aluminium alloy at room temperature for various previous thermal histories representative of an electron beam welding. A fast-heating device has been designed to control and apply thermal loadings on tensile specimens. Tensile tests show that the yield stress at ambient temperature decreases if the maximum temperature reached increases or if the heating rate decreases. This variation of the mechanical properties is the result of microstructural changes which have been observed by Transmission Electron Microscopy (TEM).

Particle effects on recrystallization in magnesium?manganese alloys: Particle pinning

Abstract: Dispersoid particles are widely used in wrought aluminium alloys to control grain structure during thermomechanical processing. The aim of this work was to investigate whether this approach could be utilized in wrought magnesium alloys to obtain better control of recrystallization. A binary magnesium–manganese alloy was heat treated to produce a fine dispersion of manganese precipitates. The effect of this dispersion on dynamic and static recrystallization during channel die deformation (at a slow strain rate), hot rolling, and annealing was studied and compared with that of an alloy free of fine particles. It was found that the presence of particles did not suppress dynamic recrystallization during channel die deformation. Fine particles did lead to a much reduced recrystallized fraction after hot rolling, attributed to a retardation of static recrystallization kinetics. Although the presence of pinning particles greatly slowed recrystallization kinetics on annealing, for no conditions studied was it possible to prevent recrystallization of the as-deformed structure using particles.

Recrystallization of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation. Part I: Dynamic recrystallization

Abstract: The metadynamic recrystallization (MDRX) behavior of 30Cr2Ni4MoV ultra-super-critical (USC) rotor steel during hot deformation was investigated based on the first part of this study, in which the evolution of the dynamically recrystallized structure was studied in detail. Compression tests were performed using double hit schedules at temperatures of 970–1250 °C, strain rates of 0.001–0.1 s−1 and inter-pass time of 1–100 s. Based on the experimental results, the kinetic equations and grain size model were established. Results show that the effects of deformation parameters, including forming temperature and strain rate, on MDRX softening fractions and austenite grain size in the two-pass hot deformed 30Cr2Ni4MoV steel are significant. Results also reveal that the pre-strain (beyond the peak strain) has little influence on the MDRX behaviors in 30Cr2Ni4MoV steel. Comparisons between the experimental and the predicted results were carried out. A good agreement between the experimental and the predicted results was obtained, which verified the developed models.

Work hardening associated with ɛ-martensitic transformation, deformation twinning and dynamic strain aging in Fe–17Mn–0.6C and Fe–17Mn–0.8C TWIP steels

Abstract: The tensile properties of carbon-containing twinning induced plasticity (TWIP) steels and their temperature dependence were investigated. Two steels with carbon concentrations of 0.6% and 0.8% (w/w) were tensile-tested at 173, 223, 273, 294, and 373 K. Three deformation modes were observed during tensile testing: ɛ -martensitic transformation, deformation twinning, and dynamic strain aging. The characteristic deformation mode that contributed to the work hardening rates changed with the deformation temperature and chemical compositions. The work hardening rate in the carbon-containing TWIP steels increased according to the deformation modes in the following order: ɛ -martensitic transformation > deformation twinning > dynamic strain aging.

Microstructure and mechanical properties of Fe–0.2C–5Mn steel processed by ART-annealing

Abstract: Microstructure and mechanical properties of medium manganese steel (Fe–0.2C–5Mn) processed by annealing at 650 °C with annealing time up to 12 h after accelerated cooling were studied. It was found that the martensite structure was gradually transformed into a superfine ferrite and austenite duplex structure by austenite reverted transformation during annealing process. The annealing heat treatment results in a large volume fraction of austenite (34%) and an excellent combination of ultimate tensile strength (∼1000 MPa) and total elongation (>40%) at room temperature. The ultrahigh tensile strength and improved ductility of present steel were mainly attributed to the enhanced phase transformation induced plasticity due to the large fractioned austenite.

Dislocation structure evolution and its effects on cyclic deformation response of AISI 316L stainless steel

Abstract: The cyclic deformation response of an austenitic stainless steel is characterised in terms of its cyclic peak tensile stress properties by three stages of behaviour: a hardening stage followed by a softening stage, and finally a stable stress response stage. A series of tests have been performed and interrupted at selected numbers of cycles in the different stages of mechanical response. At each interruption point, specimens have been examined by transmission electron microscopy (TEM) with different beam directions by means of the tilting function in order to investigate the formation and the development of dislocation structures from the as-received condition until the end of fatigue life. A new 3D representation of dislocation structure evolution during cyclic loading is proposed on the basis of the microstructural observations. The 3D representation provides a deeper insight into the development of dislocation structures in AISI 316L during low cycle fatigue loading at room temperature. By investigating the dislocation evolution, the study shows that the hardening response is mainly associated with an increase of total dislocation density, whereas the softening stage is a result of the formation of dislocation-free regions. Further development of the dislocation structure into a cellular structure is responsible for the stable stress response stage.

A new effect of retained austenite on ductility enhancement in high-strength quenching–partitioning–tempering martensitic steel

Abstract: A high-strength martensitic steel treated by a quenching–partitioning–tempering process is presented to examine the effect of retained austenite on ductility enhancement in martensitic steels. Results from X-ray diffraction line profile analysis (XLPA) indicate that the average dislocation density in martensite during uniform deformation is lower than before deformation, which effectively intensifies the deformation ability. The average dislocation density in retained austenite rapidly increases with increased strain and exceeds that in martensite. Based on the XLPA results, a new effect of austenite on the ductility enhancement is proposed: the austenite phase can continuously absorb ample dislocations from neighbouring martensite laths. This effect is indirectly verified by transmission electron microscopy.

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

Abstract: Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.

A comparative study on Arrhenius-type constitutive model and artificial neural network model to predict high-temperature deformation behaviour in Aermet100 steel

Abstract: For predicting high-temperature deformation behaviour in Aermet100 steel, the experimental stress–strain data from isothermal hot compression tests on a Gleeble-3800 thermo-mechanical simulator, in a wide range of temperatures (1073–1473 K) and strain rates (0.01–50 s−1), were employed to develop the Arrhenius-type constitutive model and artificial neural network (ANN) model, and their predictability for high-temperature deformation behaviour of Aermet100 steel was further evaluated. The predictability of two models was quantified in terms of correlation coefficient (R) and average absolute relative error (AARE). The R and AARE for the Arrhenius-type constitutive model were found to be 0.9861 and 7.62% respectively, while the R and AARE for the feed-forward back-propagation ANN model are 0.9995 and 2.58% respectively. The breakdown of the Arrhenius-type constitutive model at the instability regimes (i.e. at 1073 K and 1173 K in 0.1, 1, 10 and 50 s−1, and at 1373 K in 50 s−1) is possibly due to that physical mechanisms in the instability regimes, where microstructure exhibits cracking, shear bands and twin kink bands, are far different from that of the stability regimes where dynamic recovery and recrystallization occur. But the feed-forward back-propagation ANN model can accurately track the experimental data across the whole hot working domain, which indicates it has good capacity to model the complex high-temperature deformation behaviour of materials associated with various interconnecting metallurgical phenomena like work hardening, dynamic recovery, dynamic recrystallization, flow instability, etc.