Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2010"

Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD

Abstract: We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD). We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods are demonstrated to calculate the GND density from the EBSD data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall GND density is shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

The effect of grain size and grain orientation on deformation twinning in a Fe-22 wt.% Mn-0.6 wt.% C TWIP steel

Abstract: We investigate the effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3 μm and 50 μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall–Petch-type relation for the twinning stress using the same Hall–Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 〈1 1 1〉//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid's law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains.

Micro-alloying Mg with Y, Ce, Gd and La for texture modification-a comparative study

Abstract: A comparative study of the effectiveness of Y, La, Ce and Gd as texture modifiers during the extrusion of magnesium-based alloys has been carried out. It was found that La, Ce and Gd are all effective texture modifiers, being able to produce the “rare earth” texture at the low alloying levels of 300, 400 and 600 ppm respectively. Y was not as effective as the other three elements in modifying the texture, and at no concentration studied did this element form a typical “rare earth” texture. It is proposed that a strong interaction of solutes with dislocations and grain boundaries is responsible for the significant impact rare earth additions have on the extruded grain size and texture at very low alloying levels.

Effect of rare earth elements on the microstructure and texture development in magnesium-manganese alloys during extrusion

Abstract: Single additions of the rare earth (RE) elements cerium, yttrium or neodymium have been made to magnesium–manganese alloys in order to investigate their influence on the microstructure and texture formed during indirect extrusion and the resulting mechanical properties. Whereas the binary Mg–Mn alloy M1 exhibits a 〈10.0〉 or 〈10.0〉–〈11.0〉 fibre texture depending on the extrusion rate, the RE-containing alloys exhibit weaker recrystallisation textures and the formation of a new texture component. The preferential growth of grains having 〈11.0〉 parallel to the extrusion direction was hindered in these alloys. For the rare earth elements used in this work it appears that Nd is a much stronger texture modifier compared to Ce or Y in Mg–Mn alloys. The weaker texture leads to increased ductility, lower yield and ultimate stresses, but a decrease in the asymmetric yield behaviour of the extruded bars.

Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels

Abstract: Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4m) and ultrafine grained (1.2m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4m) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

Stacking fault energy and plastic deformation of fully austenitic high manganese steels: Effect of Al addition

Abstract: Dependence of the dislocation glide mode and mechanical twinning on the stacking fault energy (SFE) in fully austenitic high manganese steels was investigated. Fully austenitic Fe–22Mn–xAl–0.6C (x = 0, 3, and 6) steels with the SFE in the range of 20–50 mJ/m2 were tensile tested at room temperature, and their deformed microstructures were examined at the different strain levels by optical microscopy and transmission electron microscopy. Deformation of all steels was dominated by planar glide before occurrence of mechanical twinning, and its tendency became more evident with increasing the SFE. No dislocation cell formation associated with wavy glide was observed in any steels up to failure. Dominance of planar glide regardless of the SFE is to be attributed to the glide plane softening phenomenon associated with short range ordering in the solid solution state of the present steels. Regarding mechanical twinning, the higher the SFE is, the higher the stress for mechanical twinning becomes. However, in the present steels, mechanical twinning was observed at the stresses lower than those predicted by the previous model in which the partial dislocation separation is considered to be a function of not only the SFE but also the applied stress. An analysis revealed that, of the various dislocation–defect interactions in the solid solution alloy, the Fisher interaction tied to short range ordering is qualitatively shown to lower the critical stress for mechanical twinning.

A modified Johnson-Cook model for tensile behaviors of typical high-strength alloy steel

Abstract: The uniaxial tensile tests were conducted with the initial strain rates range of (0.0001–0.01) s−1 and the temperature range of (1123–1373) K for typical high-strength alloy steel. Based on the experimental results, the modified Johnson–Cook model, which considers the coupled effects of strain, strain rate and deformation temperature, was proposed to describe the tensile behaviors of the studied alloy steel. Results show that the stress–strain values predicted by the proposed model well agree with experimental ones, which confirmed that the modified Johnson–Cook model can give an accurate and precise estimate of the flow stress for the studied typical high-strength alloy steel.

Compressive strength and microstructure of carbon nanotubes–fly ash cement composites

Abstract: In this work, carbon nanotubes of 0.5 and 1% by weight were added for the first time in a fly ash cement system to produce carbon nanotubes–fly ash composites in the form of pastes and mortars. Compressive strengths of the composites were then investigated. It was found that the use of carbon nanotubes resulted in higher strength of fly ash mortars. The highest strength obtained for 20% fly ash cement mortars was found at 1% carbon nanotubes where the compressive strength at 28 days was 51.8 MPa. This benefit can clearly be seen in fly ash cement with fly ash of 20% where the importance of the addition of carbon nanotubes means that the relative strength to that of Portland cement became almost 100% at 28 days. In addition, scanning electron micrographs also showed that good interaction between carbon nanotubes and the fly ash cement matrix is seen with carbon nanotubes acting as a filler resulting in a denser microstructure and higher strength when compared to the reference fly ash mix without CNTs.

Crashworthiness design for functionally graded foam-filled thin-walled structures

Abstract: Foam-filled thin-wall structures have exhibited significant advantages in light weight and high energy absorption and been widely applied in automotive, aerospace, transportation and defence industries. Unlike existing uniform foam materials, this paper introduces functionally graded foam (FGF) fillers to fill thin-walled structures, aiming to improve crashworthiness. In this novel structure, the foam density varies throughout the depth in a certain gradient. Numerical simulations showed that gradient exponential parameter m that controls the variation of foam density has significant effect on system crashworthiness. In this study, the single and multiobjective particle swarm optimization methods are used to seek for optimal gradient, where response surface models are established to formulate specific energy absorption and peak crushing force. The results yielded from the optimizations indicate that the FGF material is superior to its uniform counterparts in overall crashworthiness. The data has considerable implication in design of FGF materials for optimizing structural crashworthiness.

Enhanced mechanical properties of friction stir welded dissimilar Al–Cu joint by intermetallic compounds

Abstract: Aluminum and copper plates were successfully friction stir welded by offsetting the tool to the aluminum side, producing excellent metallurgical bonding on the Al-Cu interface with the formation of a thin, continuous and uniform Al-Cu intermetallic compound (IMC) layer. Furthermore, many IMC particles were generated in the nugget zone, forming a composite structure. Tensile tests indicated that the FSW joint failed in the heat-affected zone of the aluminum side with the Al-Cu interface bonding strength being higher than 210 MPa. (C) 2010 Elsevier B.V. All rights reserved.

Strengthening mechanisms in an Al–Mg alloy

Abstract: Recent improvements in strength and ductility of 5083 aluminum alloys have been obtained through the development of complex microstructures containing either reduced grain sizes (ultra-fine and nano-grained materials), grain size distributions (bimodal microstructures), particle reinforcements, or combinations of the above. Optimization of such microstructures requires an understanding of the conventional, coarse-grained basis alloy. We present here a complete experimental data set for conventional Al-5083 H-131, with primary alloying element Mg (4.77 wt%) and secondary element Mn (0.68 wt%) in compression over a range of strain rates (10−4–6000 s−1) at room temperature. The various strengthening mechanisms in Al-5083 are explored, including solute strengthening, precipitate hardening, strain hardening, strain rate hardening, and strengthening due to dislocation sub-structures. Previous experiments found in the literature on Al–Mg binary alloys allow us to calculate the solute strengthening due to Mg in solid solution, and TEM analysis provides information about precipitate hardening and dislocation cell structures. A basic strength model including these strengthening mechanisms is suggested.

Flow stress analysis of TWIP steel via the XRD measurement of dislocation density

Abstract: In this study, the rate of dislocation accumulation in the tensile strained twinning induced plasticity (TWIP) steel was calculated via the X-ray diffraction (XRD) measurements and compared with other fcc metals and alloys. The results indicated that the XRD technique is an alternative method to estimate the dislocation density. Moreover, flow stress analysis of Fe–31Mn–3Al–3Si TWIP steel with the grain size of about 18 μm indicated that, beside a direct effect of the dislocation interactions on the flow stress, another strengthening mechanism is also required to describe the flow behavior. For this reason, the strengthening contribution due to the formation of mechanical twins was considered as a reduction of dislocation mean free path. Interestingly, the estimated flow stress equation consisting of the strengthening effects of both dislocation interactions and dynamic microstructure refinement due to mechanical twinning (i.e., the dynamic Hall–Petch effect) are in good agreement with the experimental data and equation proposed by Ludwigson for low SFE materials.

Local plastic strain evolution in a high strength dual-phase steel

Abstract: The evolution of local plastic deformation in a dual-phase (DP) steel has been studied using Digital Image Correlation (DIC) and in-situ tensile testing inside a scanning electron microscope. Tests were performed using specially designed samples to study the initiation and evolution of damage in DP1000 steel by measuring the strains at the scale of the microstructure. Micrographs have been analysed using DIC at different stages throughout a tensile test to measure local strain distributions within the ferrite–martensite microstructure. The results show progressive localisation of deformation into bands orientated at 45° with respect to the loading direction. Strain magnitudes are higher in the ferrite phase with local values reaching up to 120%. Several mechanisms for damage initiation are identified and related to the local strains in this steel. The procedure used and the results obtained in this work may help the development of models aimed at predicting the properties of new generation automotive steels.

Characterization of Ti–6Al–4V open cellular foams fabricated by additive manufacturing using electron beam melting

Abstract: Ti–6Al–4V open cellular foams were fabricated by additive manufacturing using electron beam melting (EBM). Foam models were developed from CT-scans of aluminum open cellular foams and embedded in CAD for EBM. These foams were fabricated with solid cell structures as well as hollow cell structures and exhibit tailorable stiffness and strength. The strength in proportion to the measured microindentation hardness is as much as 40% higher for hollow cell (wall) structures in contrast to solid, fully dense EBM fabricated components. Plots of relative stiffness versus relative density were in good agreement with the Gibson–Ashby model for open cellular foam materials. Stiffness or Young's modulus values measured using a resonant frequency-damping analysis technique were found to vary inversely with porosity especially for solid cell wall, open cellular structure foams. These foams exhibit the potential for novel biomedical, aeronautics, and automotive applications.

Investigation of particle size and amount of alumina on microstructure and mechanical properties of al matrix composite made by powder metallurgy

Abstract: Al matrix composite is well known, in which Al2O3 is the most widely used reinforcement. The aim of this study is to investigate the effect of alumina particle size and its amount on the relative density, hardness, microstructure, wear resistance, yield and compressive strength and elongation in Al–Al2O3 composites. To this end, the amount of 0–20 wt.% alumina with average particle sizes 48, 12 and 3 μm was used along with pure aluminum of average particle size of 30 μm. Powder metallurgy is a method used in the fabrication of this composite in which the powders were mixed using a planetary ball mill. Consolidation was conducted by axial pressing at 440 MPa. Sintering procedure was done at 550 °C for 45 min. The results indicated that as the alumina particle size is reduced, density raises at first, then, declines. Moreover, as the alumina particle size decreases, hardness, yield strength, compressive strength and elongation increase and factors such as wear resistance, microstructure grain size and distribution homogeneity in matrix decreases. For instance, as the alumina particle size gets smaller from 48 to 3 μm at 10 wt.% alumina, hardness rises from 50 to 70 BHN, compressive strength improves from 168 to 307 MPa and wear rate rises from 0.0289 to 0.0341 mm3/m. On the other hand, as the amount of alumina increases, hardness and wear resistance increase and relative density and elongation is decreased. However, compressive and yield strength rise at first, then drop. For example, if the amount of alumina with 12 μm particle size increases from 5 to 10 wt.%, hardness increases from 47 to 62 BHN and compressive strength rises from 190 to 273 MPa. Nevertheless, erosion rate after 300 m decreases from 0.0447 to 0.0311 mm3/m.

Microstructure and properties of age-hardenable AlxCrFe1.5MnNi0.5 alloys

Abstract: Two promising high-entropy alloys (HEAs) Al x CrFe 1.5 MnNi 0.5 ( x = 0.3 and 0.5) were designed from Al–Co–Cr–Cu–Fe–Ni alloys by substituting Mn for expensive Co and excluding Cu to avoid Cu segregation. Microstructures and properties were investigated and compared at different states: as-cast, as-homogenized, as-rolled and as-aged states. Al 0.3 CrFe 1.5 MnNi 0.5 alloy in the as-cast, as-homogenized and as-rolled states has a dual-phase structure of BCC phase and FCC phase, in which Al, Ni-rich precipitates of B2-type BCC structure disperse in the BCC phase. Al 0.5 CrFe 1.5 MnNi 0.5 alloy in the corresponding states has a matrix of BCC phase in which Cr-rich particles of BCC structure and Al, Ni-rich precipitates of B2-type BCC structure disperse. These three BCC phases have the same lattice constant. Both alloys are workable and show a hardness range of Hv 300–500 in the as-cast, as-forged, as-homogenized and as-rolled states. Al 0.5 CrFe 1.5 MnNi 0.5 alloy has a higher hardness level than Al 0.3 CrFe 1.5 MnNi 0.5 one because of its full BCC phase. Both alloys thus can be used as structural parts requiring stronger strength. Both alloys display a significant high-temperature age-hardening phenomenon. As-cast Al 0.3 CrFe 1.5 MnNi 0.5 alloy can attain the highest hardness, Hv 850, at 600 °C for 100 h, and Al 0.5 CrFe 1.5 MnNi 0.5 can get even higher hardness, Hv 890. The aging hardening is resulted from the formation of ρ phase (Cr 5 Fe 6 Mn 8 -like phase). Prior rolling on the alloys before aging could significantly enhance the age-hardening rate and hardness level due to introduced defects. Al 0.5 CrFe 1.5 MnNi 0.5 alloy exhibits excellent oxidation resistance up to 800 °C, which is better than Al 0.5 CrFe 1.5 MnNi 0.5 alloy. Combining this merit with its high softening resistance and wear resistance as compared to commercial alloys Al 0.5 CrFe 1.5 MnNi 0.5 alloy has the potential for high-temperature structural applications.

Evolution of microstructure and mechanical properties in naturally aged 7050 and 7075 Al friction stir welds

Abstract: The microstructural and mechanical property evolution of friction stir welded 7050-T7651 and 7075-T651 Al alloys were examined as a function of room temperature (natural) aging for up to 67,920 h. During the range of aging times studied, transverse tensile strengths continuously increased, and are still increasing, with improvements of 24% and 29% measured for the 7050-T7651 and 7075-T651 Al alloy friction stir welds, respectively. Microstructural evolution within the weld nugget and heat-affected zone was evaluated with both transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Formation of a high volume fraction of GP(II) zones produced a majority of the strength improvement within the weld nugget and HAZ regions. The rational for the microstructural changes are discussed in light of the mechanical properties.

Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening

Abstract: The aim of this paper was to address the effects of multiple laser peening and laser peening intensity used in laser shock peening (LSP) on the residual stress, micro-hardness and three-point bending fatigue performance of Ti–6Al–4V alloy. The multiple laser peening was accomplished by using the successive laser shocks at the same spot and the laser peening intensity was changed through changing the number of overlapped laser spots. The microstructure, which was characterized by highly tangled and dense dislocation arrangements due to high strain rate, can be found near the surface of the laser-peened specimen. By comparing with the as-received specimen, high micro-hardness and compressive residual stress were introduced at the surface of the laser-peened specimen. With increasing the number of overlapped laser spots, the fatigue life of the laser-peened specimen increased, reached a local maximum and then decreased. The specimen treated by using three overlapped laser spots exhibited the highest fatigue life. When the number of overlapped laser spots was kept to be three, the LSP treatments with one single laser shock and two successive laser shocks respectively provided a 22.2% and 41.7% increase in the fatigue strength as compared with the as-received specimens.

Mechanisms of joint and microstructure formation in high power ultrasonic spot welding 6111 aluminium automotive sheet

Abstract: Resistance spot welding (RSW) is difficult to apply to aluminium automotive alloys. High power ultrasonic spot welding (HP-USW) is a new alternative method which is extremely efficient, using ∼2% of the energy of RSW. However, to date there have been few studies of the mechanisms of bond formation and the material interactions that take place with this process. Here, we report on a detailed investigation where we have used X-ray tomography, high resolution SEM, and EBSD, and dissimilar alloy welds, to track the interface position and characterise the stages of weld formation, and microstructure evolution, as a function of welding energy. Under optimum conditions high quality welds are produced, showing few defects. Welding proceeds by the development and spread of microwelds, until extensive plastic deformation occurs within the weld zone, where the temperature reaches ∼380 °C. The origin of the weld interface ‘flow features’ characteristic of HP-USW are discussed.

Microstructures and compressive properties of multicomponent AlCoCrFeNiMox alloys

Abstract: Multicomponent AlCoCrFeNiMo(x) (x values in molar ratio, x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys were prepared using a well-developed copper mould casting. The effects of Mo element on the structure and properties of AlCoCrFeNi alloy were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). It was found that Mo addition took significant effects on the structure and properties of AlCoCrFeNi alloy. Mo(0.1) alloy, similar to Mo(0) alloy, was of single BCC solid solution structure. When Mo content was more than 0.1, the alloy exhibited a typical laminar eutectic structure. The alloy strength was improved obviously, but the ductility of alloy was lowered at the same time. The maximum yield strength reached 2757 MPa when Mo content was 0.5, and the maximum compressive fracture strength reached 3208 MPa when Mo content was 0.3. The strengthening effect and mechanism of Mo addition on AlCoCrFeNi alloy were discussed from different aspects. (C) 2010 Elsevier B.V. All rights reserved.

Precipitation behavior of the ß phase in Al-5083

Abstract: The formation and growth of the magnesium-rich s phase, Al3Mg2, in Al-5083 has been studied by transmission electron microscopy (TEM). The s phase forms heterogeneously both at grain boundaries, and on preexisting particles which are enriched in manganese. The s precipitates at grain boundaries, containing a high density of very fine faults, thicken faster than the rate estimated assuming volume diffusion controlled growth of planar interfaces. It is suggested that pipe diffusion through dislocations contributes to the enhanced growth rate.

Processing and properties of nanocrystalline CuNiCoZnAlTi high entropy alloys by mechanical alloying

Abstract: The present study describes the synthesis of nanocrystalline equiatomic CuNiCoZnAlTi high entropy alloy by mechanical alloying and characterized by XRD, SEM and TEM. The CuNiCoZnAlTi high entropy alloy is mainly composed of BCC solid solution with crystallite size less than 10 nm in as milled condition. The alloy is thermally stable at elevated temperature about 800 °C as it retained its nanostructure. This alloy powder was consolidated using vacuum hot press at 800 °C with 30 MPa pressure to a density of 99.95%. The hardness and compressive strength of the high entropy alloy are found to be 7.55 and 2.36 GPa, respectively. Superior strength of high entropy alloy is attributed to solid solution strengthening and its nanocrystalline nature.

Manufacturing of high-strength aluminum/alumina composite by accumulative roll bonding

Abstract: The ARB process used as a technique in this study provides an effective alternative method for manufacturing high-strength aluminum/alumina composites. The microstructural evolution and mechanical properties of the aluminum/15 vol.% alumina composite are reported. The composite shows an excellent alumina particle distribution in the matrix. It is found that by increasing the number of ARB cycles, not only does elongation increase in the composites produced but also the tensile strength of the Al/15 vol.% Al2O3 composite improves by 4 times compared to that of the annealed aluminum used as the original raw material. Fracture surfaces after tensile tests are observed by scanning electron microscopy (SEM) to investigate the failure mode. Observations reveal that the failure mode in both ARB-processed composites and monolithic aluminum is of the shear ductile rupture type.

Evolution of dislocation density, size of subgrains and MX-type precipitates in a P91 steel during creep and during thermal ageing at 600 °C for more than 100,000 h

Abstract: There are rather few quantitative data on the microstructure of the 9-12%Cr heat resistant steels after long-term creep. This paper presents results of the quantitative measurement of the size of MX precipitates, subgrain size and dislocation density in a P91 steel that had been creep tested for 113,431 h at 600 ◦ C. The same measurements were conducted in the same P91 steel in the as received conditions. Transmission electron microscopy investigations were conducted using thin foils and revealed a decrease in dislocation density and an increase in subgrain size after creep exposure. MX carbonitrides are very stable during thermal and creep exposure of P91 steel at 600 ◦ C up to 113,431 h. Electron Backscatter Diffraction (EBSD) investigations also revealed a significant change in the substructure of the steel after creep exposure.

Intermetallic compounds in Al 6016/IF-steel friction stir spot welds

Abstract: The joining of a 1.2 mm thickness Al 6016 to a 2 mm thickness IF-steel has been performed by friction stir spot welding (FSSW). The intermetallic compounds (IMC) have been identified at the interface Al 6016/IF-steel and quantified as a function of the rotational speed and tool penetration. TEM observations indicated the presence of tangles of elliptical intermetallic compounds. FeAl 3 , Fe 2 Al 5 and FeAl 2 were identified depending on welding conditions. The influence of IMC on tensile shear strength has been established. An IMC layer seems necessary to improve the weld strength, but if the layer is too thick, cracks initiate and propagate easily through the hard IMC tangles.

Microstructure and mechanical properties of the Mg/Al laminated composite fabricated by accumulative roll bonding (ARB)

Abstract: The Mg/Al laminated composite was fabricated by the accumulative roll bonding (ARB) using the pure magnesium and Al5052 alloy at 400 °C. Tensile properties along rolling direction and the transverse direction and the microhardness were evaluated at the ambient temperature. The tensile strength of the laminated Mg/Al composite along both directions increased gradually till two ARB cycles, but then decreased after the third ARB cycles. Optical microscopy and scanning electron microscopy (SEM) were utilized to reveal the microstructure evolution and the failure mechanism. Grain refinement of Mg layers was not obvious during the ARB process due to the high temperature and interval reheating. The obvious crack at the coarse intermetallic compounds and rupture of the Al layer after the third cycle led to the premature failure of the sample along the rolling direction during the tensile test.

Characterization of microstructure obtained by quenching and partitioning process in low alloy martensitic steel

Abstract: Microstructure of low alloy martensitic steel treated by quenching and partitioning (QP the fresh martensite is formed at the final quenching step and looks like ‘blocky’ type phase with size about 0.2–3 μm, and the retained austenite is mainly located on the packet boundary and initial austenite grain boundary. The measured volume fractions and carbon contents of these phases were slightly different from those predicted by the constrained paraequilibrium (CPE) model proposed by Speer, which was interpreted by the effects of the different grain sizes of the untransformed austenite after first quenching. Mechanical properties of steels processed by Q&P assume much higher strength and ductility than those processed by Q&T. It is concluded that Q&P process is a promising approach to control the multiphase structure with hard matrix and a ductile retained austenite, which gives an excellent combination of strength and ductility.

Compressive behavior of pervious concretes and a quantification of the influence of random pore structure features

Abstract: Properties of a random porous material such as pervious concrete are strongly dependent on its pore structure features, porosity being an important one among them. This study deals with developing an understanding of the material structure–compressive response relationships in pervious concretes. Several pervious concrete mixtures with different pore structure features are proportioned and subjected to static compression tests. The pore structure features such as pore area fractions, pore sizes, mean free spacing of the pores, specific surface area, and the three-dimensional pore distribution density are extracted using image analysis methods. The compressive stress–strain response of pervious concretes, a model to predict the stress–strain response, and its relationship to several of the pore structure features are outlined. Larger aggregate sizes and increase in paste volume fractions are observed to result in increased compressive strengths. The compressive response is found to be influenced by the pore sizes, their distributions and spacing. A statistical model is used to relate the compressive strength to the relevant pore structure features, which is then used as a base model in a Monte-Carlo simulation to evaluate the sensitivity of the predicted compressive strength to the model terms.

Sub-zero treatments of AISI D2 steel: Part I. Microstructure and hardness

Abstract: Microstructure, hardness and wear behavior of AISI D2 steel subjected to varied sub-zero treatments have been examined with reference to conventional heat treatment. Part I of this work presents the variations of microstructure and hardness, whereas part II deals with the wear behavior. The sub-zero treatments studied are cold treatment, shallow cryogenic treatment and deep cryogenic treatment. The developed microstructures have been characterized by XRD, optical microscopy and SEM examinations coupled with EDX and image analyses. Macrohardness and microhardness of the specimens have been evaluated by Vickers indentation technique. The obtained results infer that (i) retained austenite content is reduced by cold treatment, but is almost completely eliminated by both shallow and deep cryogenic treatments, (ii) the sub-zero treatments modify the precipitation behavior of secondary carbides; lower the temperature of sub-zero treatment higher is the degree of modification, (iii) the deep cryogenic treatment refines the secondary carbides, increases their amount and population density, and leads to more uniform distribution, and (iv) bulk hardness increases marginally but apparent hardness of the matrix improves considerably by deep cryogenic treatment.