Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2012"

Grain boundaries in ultrafine grained materials processed by severe plastic deformation and related phenomena

Abstract: Grain boundaries in ultrafine grained (UFG) materials processed by severe plastic deformation (SPD) are often called “non-equilibrium” grain boundaries. Such boundaries are characterized by excess grain boundary energy, presence of long range elastic stresses and enhanced free volumes. These features and related phenomena (diffusion, segregation, etc.) have been the object of intense studies and the obtained results provide convincing evidence of the importance of a non-equilibrium state of high angle grain boundaries for UFG materials with unusual properties. The aims of the present paper are first to give a short overview of this research field and then to consider tangled, yet unclear issues and outline the ways of oncoming studies. A special emphasis is given on the specific structure of grain boundaries in ultrafine grained materials processed by SPD, on grain boundary segregation, on interfacial mixing linked to heterophase boundaries and on grain boundary diffusion. The connection between these unique features and the mechanical properties or the thermal stability of the ultrafine grained alloys is also discussed.

Comparison between current models for the strength of particulate-reinforced metal matrix nanocomposites with emphasis on consideration of Hall–Petch effect

Abstract: There are several modeling methods available for predicting the strength of metal matrix nanocomposites Among these, Zhang and Chen approach and Clyne method predict closer results to experimental values However, in Zhang and Chen method, the effect of Hall–Petch strengthening is neglected which results in less precision of the results In this study, the effects of all active strengthening mechanisms including Hall–Petch, Orowan, CTE mismatch and load bearing are considered, by using Clyne approach The results are in good agreement with experimental data and more precise than those from other models Furthermore, studies on different strengthening mechanisms showed that Hall–Petch strengthening mechanism is the most important factor, which should not be neglected even in micro-scale grain size

Twinning and martensite in a 304 austenitic stainless steel

Abstract: The microstructure characteristics and deformation behavior of 304L stainless steel during tensile deformation at two different strain rates have been investigated by means of interrupted tensile tests, electron-backscatter-diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The volume fractions of transformed martensite and deformation twins at different stages of the deformation process were measured using X-ray diffraction method and TEM observations. It is found that the volume fraction of martensite monotonically increases with increasing strain but decreases with increasing strain rate. On the other hand, the volume fraction of twins increases with increasing strain for strain level less than 57%. Beyond that, the volume fraction of twins decreases with increasing strain. Careful TEM observations show that stacking faults (SFs) and twins preferentially occur before the nucleation of martensite. Meanwhile, both ɛ-martensite and α′-martensite are observed in the deformation microstructures, indicating the co-existence of stress-induced-transformation and strain-induced-transformation. We also discussed the effects of twinning and martensite transformation on work-hardening as well as the relationship between stacking faults, twinning and martensite transformation.

Effect of Nb addition on the microstructure and properties of AlCoCrFeNi high-entropy alloy

Abstract: The microstructures and properties of the AlCoCrFeNbxNi high-entropy alloys (HEAs) were investigated. Two phases were found in the prepared AlCoCrFeNbxNi HEAs: one is body-centered-cubic (BCC) solid solution phase; the other is the Laves phase of (CoCr)Nb type. The microstructures of the alloy series vary from hypoeutectic to hypereutectic, and the compressive yield strength and Vickers hardness have an approximately linear increase with increasing Nb content. The residual magnetization (Mr) reaches a maximum for AlCoCrFeNb0.1Ni alloy, which is 6.106 emu/g. The factor of Ω, which is defined as entropy of mixing times 1000 over enthalpy of mixing, well predicts the phase formation for the multicomponents alloys.

Alloying behavior in multi-component AlCoCrCuFe and NiCoCrCuFe high entropy alloys

Abstract: Multi-component high entropy alloys (HEAs) are observed to form simple solid solutions in contrary to general perception that complex compounds may form in such multi-component equi-atomic alloys. In the present study, alloying behavior was investigated using XRD in AlCoCrCuFe and NiCoCrCuFe equi-atomic high entropy alloys synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). Simple FCC and BCC phases evolved after MA, while Cu-rich FCC and sigma (σ) phases evolved along with FCC and BCC phases after SPS. Further, NiCoCuFe, NiCoCrFe and NiCoFe equi-atomic alloys were investigated to confirm the formation of Cu-rich FCC, and σ phases. The hardness was observed to be 770 ± 10 HV for AlCoCrCuFe and 400 ± 10 HV for NiCoCrCuFe. Phase evolution after MA and SPS indicate that configurational entropy is not sufficient enough to suppress the formation of Cu-rich FCC, and σ phases, and enthalpy of mixing appears to play an important role in determining the phase formation in high entropy alloys after sintering.

Microstructural and mechanical approaches of the selective laser melting process applied to a nickel-base superalloy

Abstract: This article aims at presenting the Nimonic 263 as-processed microstructure of the selective laser melting which is an innovative process. Because the melting pool is small and the scanning speed of the laser beam is relatively high, the as-processed microstructure is out-of-equilibrium and very typical to additive manufacturing processes. To match the industrial requirement, the microstructures are modified through heat treatments in order to either produce precipitation hardening or relieve the thermal stresses. Tensile tests at room temperature give rise to high mechanical properties close or above those presented by Wang et al. [1]. However, it is noted a strong anisotropy as a function of the building direction of the samples because of the columnar grain growth.

Tensile properties of an AlCrCuNiFeCo high-entropy alloy in as-cast and wrought conditions

Abstract: Extensive multistep forging at 950 °C was applied to the cast AlCuCrFeNiCo high-entropy alloy to transform the cast coarse dendritic structure into a fine equiaxed duplex structure consisting of the mixture of BCC and FCC phases, with the average grain/particle size of ∼1.5 ± 0.9 μm. Tensile properties of the alloy in the as-cast and forged conditions were determined in the temperature range of 20–1000 °C. The hot forged alloy was stronger and more ductile during testing at room temperature, than the as-cast alloy. The yield stress (YS), ultimate tensile strength (UTS), and tensile ductility ( δ ) of the forged condition were 1040 MPa, 1170 MPa, and 1%, respectively, against 790 MPa, 790 MPa and 0.2% for the as-cast condition. In both conditions, the alloy showed brittle to ductile transition (BDT), with a noticeable increase in the tensile ductility within a narrow temperature range. In the as-cast condition, this transition occurred between 700 and 800 °C, while in the forged condition, it was observed between 600 and 700 °C. With an increase in the testing temperature above the BDT, a continuous decrease in tensile flow stress and an increase in tensile ductility were observed. In the temperature range of 800–1000 °C, the forged alloy showed superplastic behavior. The tensile elongation was above 400% and reached 860% at 1000 °C.

Morphology, microstructure, and hardness of titanium (Ti-6Al-4V) blocks deposited by wire-feed additive layer manufacturing (ALM)

Abstract: Additive layer manufacturing offers a potential for time and cost savings, especially for aerospace components made from costly titanium alloys. In this paper, the morphology, microstructure, chemical composition, and hardness of additive manufactured Ti-6Al-4V blocks are investigated and discussed. Blocks (7 beads wide, 7 layers high) were deposited using Ti-6Al-4V wire and a Nd:YAG laser. Two different sets of parameters are used and three different post heat treatment conditions (as-built, 600 °C/4 h, 1200 °C/2 h) are investigated. The experiments reveal elementary properties of additive manufactured Ti-6Al-4V material in correlation to process parameters and heat treatments, which are discussed comprehensively.

Microstructures and mechanical properties of multiprincipal component CoCrFeNiTix alloys

Abstract: The purpose of this study is to investigate the effects of Ti addition on the microstructures and mechanical properties of multiprincipal component CoCrFeNiTi x ( x values in molar ratio, x =0, 0.3, and 0.5) alloys. The CoCrFeNi quaternary alloy displayed a crystalline structure constructed by a simple face-centered cubic solid solution, whereas a plate-like structure consisting of a mixture of (Ni, Ti)-rich R phase and (Cr, Fe)-rich σ phase was observed within the face-centered cubic matrix of a CoCrFeNiTi 0.3 alloy. In a CoCrFeNiTi 0.5 alloy, an face-centered cubic matrix, a (Ti, Co)-rich Laves phase, and R+σ mixed phases were discovered. The compressive strength of the alloys rose by approximately 75% after the addition of Ti. Alloys with high levels of Ti content had high yield stress values and low ductility values. The solid-solution strengthening of the face-centered cubic matrix and the secondary-phase hardening were the two main factors that strengthened the alloy. The CoCrFeNiTi 0.3 alloy exhibited a compressive strength of 1529 MPa and a fracture strain of 0.60; this indicates that this material shows potential for the development of a ductile, high-strength alloy.

Effects of calcium on texture and mechanical properties of hot-extruded Mg–Zn–Ca alloys

Abstract: The effects of calcium on microstructure, texture and mechanical properties of hot-extruded Mg–Zn–Ca alloys were studied. The results showed that calcium elements can weaken the strong basal textures and refined the grain size of extruded Mg–Zn–Ca alloys. The weakening of extrusion textures with increasing content of Ca is related to the particle stimulated nucleation of recrystallization (PSN) and the particle retarded the growth of the dynamic recrystallization grains. The increase of the tensile elongation is attributing to the weakened extrusion textures and refined grains.

Effect of nano-SiO2 and nano-TiO2 addition on the rheological behavior and the hardened properties of cement mortars

Abstract: This paper reports on the use of nano-SiO2 (nS) and nano-TiO2 (nT) in cement pastes and mortars. Samples with 0–3 wt.% nS, 0–12 wt.% nT and 0.5 water/binder weight ratio were prepared. Rheological and flow table measurements were carried out. In addition, the design of experiments was applied to validate the results found. The temperature of hydration and compressive strength with 28 days was also determined. In general, mortars exhibited noticeable differences in the rheological behavior, but less evident in temperature of hydration and compressive strength. The values of torque, yield stress and plastic viscosity of mortars with nanoadditives increased significantly, reducing the open testing time in rheology tests. Meanwhile, the flow table values reduced. In addition, spread on table and initial yield stress exhibited a power correlation, while the spread on table and plastic viscosity did not show any special relationship. The results of kinetics of hydration followed the same tendency found by rheology, in which samples with higher amounts of nS and nT showed remarkable changes in relation to the samples without nanoadditives. Mechanical properties were not significantly affected by nanoparticles in the range considered in this work.

Ultra high-strength Mg–Gd–Y–Zn–Zr alloy sheets processed by large-strain hot rolling and ageing

Abstract: Ultra high-strength Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr alloy sheet was prepared by large-strain hot rolling and subsequent ageing process. The sheet exhibits excellent tensile properties at ambient temperature with ultimate tensile strength of 517 MPa, 0.2% proof stress of 426 MPa and elongation to failure of 4.5%. The notable improvement in strength is attributed to the dense distribution of the fine precipitates inside the grains, scattered precipitates at the grain boundaries, bimodal grain size distribution with fine recrystallized grains and large deformed grains with intense basal texture.

EBSD study of a hot deformed austenitic stainless steel

Abstract: The microstructural evolution of a 304 H austenitic stainless steel subjected to hot compression was studied by the electron backscattered diffraction (EBSD) technique. Detailed data about the boundaries, coincidence site lattice (CSL) relationships and grain size were acquired from the orientation imaging microscopy (OIM) maps. It was found that twins play an important role in the nucleation and growth of dynamic recrystallization (DRX) during hot deformation. Moreover, the conventional discontinuous DRX (DDRX) was found to be in charge of grain refinement reached under the testing conditions studied. Furthermore, the recrystallized fraction (X) was determined from the grain average misorientation (GAM) distribution based on the threshold value of 1.55°. The frequency of high angle boundaries showed a direct relationship with X. A time exponent of 1.11 was determined from Avrami analysis, which was related to the observed single-peak behavior in the stress–strain flow curves.

A constitutive model of twinning and detwinning for hexagonal close packed polycrystals

Abstract: A new constitutive model to describe twinning and detwinning for polycrystalline materials with the hexagonal close packed (HCP) crystallographic structure is developed and implemented in the recently developed elastic viscoplastic self-consistent (EVPSC) polycrystal model. The new model is then applied to magnesium alloy Mg–3 Al–1 Zn (AZ31B) sheet under cyclic loadings and strain path changes. It is demonstrated that the new twinning model is able to capture key features associated with twinning and detwinning observed experimentally.

Thermogravimetric behavior of natural fibers reinforced polymer composites—An overview

Abstract: Natural fibers obtained from plants, known as lignocellulosic fibers are environmentally friendly alternatives for synthetic fiber, as polymer composite reinforcement. Applications of natural fiber composites are expanding in many engineering areas, from civil construction to automobile manufacturing. In recent years, a considerable number of scientific and technological works, including review papers, were dedicated to the characterization and properties of natural fibers and their composites. The mechanical behavior and the fracture characteristics are usually the most investigated and reviewed themes for the purpose of comparison to corresponding polymer composites reinforced with synthetic fibers, mainly fiberglass. The thermal behavior is also of practical interest for conditions associated with temperatures above the ambient, as in fire damage, curing or process involving heating procedures. In fact, several works also assessed distinct thermal responses, particularly in terms of thermogravimetric properties of natural fiber polymer composites. As no general review was conducted so far on the thermogravimetric (TG) behavior of these materials, this article presents an overview limited to temperature effects related to the loss of mass by means of TG analysis and the related derivative, DTG, for different polymer composites reinforced with the most common and relevant lignocellulosic fibers.

Evolution of microstructure and mechanical properties of medium Mn steels during double annealing

Abstract: A double annealing process was applied to cold rolled medium Mn steel. The evolution of both microstructure and mechanical properties during the second annealing were analysed. Austenite reverted transformation (ART) was observed during intercritical annealing. It was shown that a complex ultra-fine microstructure composed of three phases (retained austenite/martensite/ferrite) was formed and two types of morphologies were detected (lath-like and polygonal). Furthermore, a high volume fraction of retained austenite (22%), which was stabilized at room temperature, was the origin of a TRIP effect. A good balance between strength and ductility can be achieved by optimizing the heat treatment. The various results are discussed and some mechanisms are proposed to explain the observations.

A study of the heterogeneity of plastic deformation in IF steel by EBSD

Abstract: The objective of this experimental study is to recognize the roles of several quantities like grain size and orientation distributions on the development of plastic heterogeneities. The measurements are performed on an interstitial free (IF) steel by Electron Back Scattered Diffraction (EBSD) at different states of deformation (from 0% to 17% tensile deformation). For each level of deformation, EBSD maps are performed before and after the deformation on exactly the same area. Several parameters as the Grain Orientation Spread (GOS), the Grain Orientation Spread over the grain Diameter (GOS/D) and the Geometrically Necessary Dislocation (GND) densities can thus be determined for different subpopulations of grains ranked as a function of individual grains sizes to follow the evolution of the deformed-induced microstructure. It appears that none of these grain scale measures are deciding and that grain neighborhood interactions play an important role.

Effect of crystal orientation on the mechanical properties and strain hardening behavior of magnesium alloy AZ31 during uniaxial compression

Abstract: The effect of initial texture on the mechanical properties and strain hardening behavior of AZ31 magnesium alloy has been investigated. Cylindrical specimens of extruded and hot rolled AZ31 are compressed along different directions, with the compression axis (C) perpendicular or parallel to the extrusion direction (ED) or the sheet normal direction (ND), referred to as C⊥ED, C//ED, C⊥ND and C//ND specimen, respectively. The compression tests are conducted at room temperature with a strain rate of 0.01 s−1. The results indicate that the yield strength and the strain hardening rate are highly anisotropic with respect to the initial texture. The significant yield behavior can be induced by only a small volume of twins. When the initial grain orientations are unfavorable for { 10 1 ¯ 2 } twinning, the strain hardening rate decreases gradually. When the initial grain orientations are favorable for { 10 1 ¯ 2 } twinning, the strain hardening behavior exhibits three distinct stages. The main contribution to increasing strain hardening rate (corresponding to the stage II) results from texture strengthening, which rotates grain orientations into hard orientations by the { 10 1 ¯ 2 } twinning. The length of the stage II is predominantly related to the volume fraction of grains which are favorable for { 10 1 ¯ 2 } twinning.

Investigation of laser shock peening effects on residual stress state and fatigue performance of titanium alloys

Abstract: Laser shock peening can potentially enhance fatigue life of titanium components by inducing compressive residual stresses in surface layers much deeper than caused by traditional shot peening (SP). In the present study, the high cycle fatigue (HCF) performance of α Ti-alloy Ti–2.5Cu, (α + β) Ti-alloy TIMETAL 54M and the metastable β Ti-alloy TIMETAL LCB was investigated after laser shock peening without coating (LPwC). The fatigue results were interpreted by examining the changes of surface morphology, microhardness and residual stress generated in the surface layer. Furthermore, thermal stability of residual stresses in aged Ti–2.5Cu, as an example, was evaluated after annealing LPwC-treated material at various elevated temperatures and exposure times by applying a Zener–Wert–Avrami approach. The depth profiles of residual stresses were obtained by means of synchrotron X-ray diffraction or by incremental hole drilling method. Results revealed that the HCF performance of Ti–2.5Cu and TIMETAL LCB was markedly improved after LPwC, while it was deteriorated in TIMETAL 54M. Compared to LPwC, better 10 7 fatigue strength of Ti–2.5Cu was obtained after ball-burnishing (BB). Moreover, LPwC-induced residual stresses are thermally more stable than shot peening-induced ones.

Creep properties of different γ′-strengthened Co-base superalloys

Abstract: The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ′-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes188. The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Ni-base superalloy IN100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850 °C only a slight tendency for directional coarsening occurs, while at 950 °C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950 °C.

Tensile behaviour of a nanocrystalline bainitic steel containing 3 wt% silicon

Abstract: Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125–350 °C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the B S – M S temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2–1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ∼3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural features.

Friction-stir dissimilar welding of aluminium alloy to high strength steels: Mechanical properties and their relation to microstructure

Abstract: The use of light-weight materials for industrial applications is a driving force for the development of joining techniques. Friction stir welding (FSW) inspired joints of dissimilar materials because it does not involve bulk melting of the basic components. Here, two different grades of high strength steel (HSS), with different microstructures and strengths, were joined to AA6181-T4 Al alloy by FSW. The purpose of this study is to clarify the influence of the distinct HSS base material on the joint efficiency. The joints were produced using the same welding parameter/setup and characterised regarding microstructure and mechanical properties. Both joints could be produced without any defects. Microstructure investigations reveal similar microstructure developments in both joints, although there are differences e.g. in the size and amount of detached steel particles in the aluminium alloy (heat and thermomechanical affected zone). The weld strengths are similar, showing that the joint efficiency depends foremost on the mechanical properties of the heat and the thermomechanical affected zone of the aluminium alloy.

The flow behavior modeling of cast A356 aluminum alloy at elevated temperatures considering the effect of strain

Abstract: The flow stress behavior of cast A356 aluminum alloy has been studied by a set of isothermal hot compression tests. The compression tests were carried out in the temperature range of 400–540 °C and strain rates of 0.001, 0.01 and 0.1 s −1 up to a true strain of 0.6. The effects of temperature and strain rate on deformation behavior were represented by Zener–Hollomon parameter in an exponent type equation. Employing an Arrhenius-type constitutive equation, the influence of strain has been incorporated by considering the related materials’ constants as functions of strain. The accuracy of the developed constitutive equations has been evaluated using standard statistical parameters such as correlation coefficient and average absolute relative error. The results indicate that the strain-dependent constitutive equation can lead to a good agreement between the calculated and measured flow stresses in the relevant temperature range.

Effects of thermal cycling on microstructure and properties in Nitinol

Abstract: The effects of thermal cycling through the martensite–austenite transformation were investigated in NiTi shape memory alloys with DSC and TEM. Thermal cycling caused a ∼25 K decrease in Ms with a concomitant increase in dislocation density from ∼1012 m−2 to 5 × 1014 m−2 after 100 thermal cycles. Thermodynamic analysis is consistent with increasing elastic strain energy and irreversible frictional energy with cycling. The transformation-induced dislocations were determined to be shear loops with 〈0 1 0〉A{1 0 1}A slip system, corresponding to the twinning direction and plane in martensite. It is speculated that the loops form during the movement of the martensite interface and that repeated interfacial movement tends to create bands that consist of highly tangled sessile dislocations. These dislocation bands form along ( 0.756 ¯ , 0.383 ¯ , 0.192 ) A , which is 2.3° from the accepted ( 0.889 ¯ , 0.404 ¯ , 0.215 ) A lattice invariant plane. Furthermore, plastically deformed austenitic Nitinol exhibits slip on 〈0 1 0〉A{1 0 1}A slip system and forms {1 1 0} shear bands with several variants of the dislocations within a given region.

Effect of large strain cold rolling and subsequent annealing on microstructure and mechanical properties of an austenitic stainless steel

Abstract: The microstructural evolution of an S304H steel during bar rolling to a strain of 4 and subsequent annealing as well as its effect on the mechanical properties were investigated. The cold working was accompanied by a strain-induced martensitic transformation, leading to the development of lamellar-type microstructure consisting of highly elongated austenite/ferrite subgrains with a mean transverse size of approximately 50 nm; the austenite volume fraction was approximately 0.35. This material exhibited a yield strength above 2000 MPa. The subsequent annealing resulted in grain coarsening following the ferrite → austenite reversion, which led to almost full austenitization at temperatures above 700 °C. The formation of the austenite/ferrite lamellar structure that mixed with separate equiaxed grains occurred after annealing at temperatures of T ≤ 700 °C. The grain coarsening was accompanied by a degradation in strength, although the yield strength of above 1000 MPa remained after 2 h of annealing at 700 °C. The discontinuous recrystallization of austenite resulted in the development of a relatively coarse-grained microstructure at T ≥ 800 °C.

On the relationship between work hardening and twinning rate in TWIP steels

Abstract: High-manganese austenitic TWIP steels exhibit very high strength and elongation before necking. The peculiarity of these steels is that mechanical twins form during straining due to their low stacking fault energy (SFE). These twins are usually thought to have a huge impact on the outstanding properties of the materials, either by bringing about a dynamic Hall & Petch effect and/or a composite effect. In this study, the appearance of mechanical twins during tensile straining is investigated for a Fe-20%Mn-1.2%C TWIP steel. The twinning rate was estimated by means of point counting analysis on EBSD micrographs at different strain levels. The reliability of this method is first thoroughly discussed. It is then shown that there exists a first order relationship between this twinning rate and the work hardening rate. © 2012 Elsevier B.V..

The effect of high strain rate deformation on intermetallic reaction during ultrasonic welding aluminium to magnesium

Abstract: High power ultrasonic spot welding (USW) is a low heat input solid-state joining process that may offer a solution for welding difficult dissimilar-material couples, like magnesium (Mg) to aluminium (Al) for automotive body applications. However, the high strain rate dynamic deformation in USW has been claimed to accelerate inter-diffusion rates in dissimilar joints. The interfacial reaction between Al, AA6111, and Mg AZ31 alloys has been studied as a function of welding energy. For the optimum welding condition of 600 J (0.4 s) the reaction layer thickness was already ∼5 μm thick. Intermetallic reaction centres were found to nucleate within microwelds at the interface at very short welding times and spread and grow rapidly to form a continuous layer, composed of two sub-layers of Al 12 Mg 17 and Al 3 Mg 2 . Interface liquation was also found for longer welding times at temperatures below the recognised lowest eutectic reaction temperature in the Al–Mg binary system. Modelling has been used to show that the solid state reaction kinetics were over twice the rate expected from parabolic growth predictions made using rate constants obtained under static test conditions. The reasons for this discrepancy and the depressed melting reaction are discussed.

The kinetics of dynamic recrystallization of 42CrMo steel

Abstract: The dynamic recrystallization behavior in 42CrMo steel was investigated by hot compression tests. The effects of deformation temperature, strain rate, and initial austenite grain size on the dynamic recrystallization behavior were discussed. Based on the experimental results, the kinetic equations for the dynamic recrystallization behavior of 42CrMo steel were proposed. Results indicate that the effects of the deformation temperature, strain rate and initial austenitic grain size on the dynamic recrystallization behavior in 42CrMo are significant. The dynamic recrystallization in 42CrMo steel easily occurs at high deformation temperature, low strain rate and fine initial austenitic grain. A good agreement between the experimental and predicted results shows that the proposed kinetic equations can give an accurate estimate of the dynamic recrystallization behavior in hot deformed 42CrMo steel.

Continuous cooling precipitation diagrams of Al-Mg-Si alloys

Abstract: The temperature- and time-dependent precipitation behaviour of Al-based 6060, 6063, 6005A and 6082 alloys at different cooling rates after solution annealing has been investigated. The continuous cooling precipitation diagrams of these alloys were recorded by differential scanning calorimetry. The cooling rate was varied over five orders of magnitude (0.05–20,000 K/min). Cooling-rate-dependent precipitate formation was analysed by light microscopy, scanning and transmission electron microscopy. Cooling-rate-dependent hardness was tested after artificial ageing. Over an appropriate range of cooling rates all alloys show similar precipitation behaviour. At least two precipitation reactions were observed in different temperature ranges. The high-temperature reactions correspond to the precipitation of the equilibrium phase Mg 2 Si, and the low-temperature reactions correspond to the precipitation of precursor phases such as β′ and B′. The precipitation kinetics depends on the alloy composition. Maximum hardness values are to find as long as the materials were cooled faster than alloy specific critical cooling rate, which increases with increasing alloy content.

Fabrication of aluminum-alumina metal matrix composites via cold gas dynamic spraying at low pressure followed by friction stir processing

Abstract: Cold gas dynamic spraying at low pressure (1 MPa gage or 150 psig) was used to fabricate Al–Al2O3 metal matrix composite (MMC) coatings onto 6061 Al alloy. The powder contained Al powder admixed with −10 μm Al2O3 in fractions up to 90 wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and volume fraction of reinforcing particles in the coatings. The coatings were then friction-stir processed (FSP) at tool rotation speeds of 894 or 1723 RPM using a flat cylindrical tool. The Al2O3 content and hardness of the final MMC coatings increased with increasing fractions of Al2O3 particles in the feedstock powder, resulting in a maximum Al2O3 content of 48 wt.% and a hardness of 85 HV of the as-sprayed coating when 90 wt.% Al2O3 was used in the feed powder blend. After FSP, the hardness of the MMC increased to a maximum of 137 HV. The as-sprayed coatings contained Al2O3 particles that were segregated between the Al particles, and FSP was effective in dispersing these Al2O3 particles and decreasing their mean free path. It was suggested that this re-distribution and Al2O3 particle size refinement during FSP improved the hardness of the MMC coatings.