Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2009"
TL;DR: In this article, a subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese steels with carbon contents of 0 to 1.2
Abstract: A subregular solution thermodynamic model was used to calculate the stacking fault energies (SFEs) of high-manganese (10 to 35 wt pct) steels with carbon contents of 0 to 1.2 wt pct. Based on these calculations, composition-dependent diagrams were developed showing the regions of different SFE values for the mentioned composition range. These diagrams were called SFE maps. In addition, variations in the SFE maps were observed through increasing the temperature, aluminum content, and austenite grain size. These changes were seen either as an increasing trend of SFE caused by raising the temperature and aluminum content, or as a decreasing behavior caused by increasing the grain size. The SFE value of 20 mJ/m2 within these diagrams was introduced as the upper limit for the strain-induced martensite formation. The variations in this limit caused by increasing the temperature and aluminum content were mathematically evaluated to find out the minimum amount of manganese that was required to avoid the martensitic transformation. By introducing the isocarbon and isomanganese diagrams of the SFE, it was seen that both temperature and aluminum had a greater effect on the SFE when added to the steels with the lower manganese contents. Moreover, by adding more aluminum to the composition of the high-manganese steels, its influence on the SFE decreased continuously.
579 citations
TL;DR: In this paper, a model that considers solid-solution strengthening, Hall-Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed.
Abstract: Polycrystalline γ-γ′ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall–Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Cross-slip-induced hardening of the Ni3Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the γ′ solvus temperature benefit from a strong Hall–Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing.
412 citations
TL;DR: In this paper, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct, and failure modes of the tensile specimens were analyzed with fractography.
Abstract: Laser net shape manufacturing (LNSM) is a laser cladding/deposition based technology, which can fabricate and repair near-net-shape high-performance components directly from metal powders. Characterizing mechanical properties of the laser net shape manufactured components is prerequisite to the applications of LNSM in aircraft engine industrial productions. Nickel-based superalloys such as INCONEL 718 are the most commonly used metal materials in aircraft engine high-performance components. In this study, the laser deposition process is optimized through a set of designed experiments to reduce the porosity to less than 0.03 pct. It is found that the use of plasma rotating electrode processed (PREP) powder and a high energy input level greater than 80 J/mm are necessary conditions to minimize the porosity. Material microstructure and tensile properties of laser-deposited INCONEL 718 are studied and compared under heat treatment conditions of as deposited, direct aged, solution treatment and aging (STA), and full homogenization followed by STA. Tensile test results showed that the direct age heat treatment produces the highest tensile strength equivalent to the wrought material, which is followed by the STA-treated and the homogenization-treated tensile strengths, while the ductility exhibits the reverse trend. Finally, failure modes of the tensile specimens were analyzed with fractography.
393 citations
TL;DR: In this paper, the physical origin of fatigue crack initiation in ductile metals is discussed from a historical perspective, and the cyclic slip irreversibilities in a microstructural sense that occur not only at the surface but also in the bulk at the dislocation scale are assessed.
Abstract: In this article, the physical origin of fatigue crack initiation in ductile metals is discussed from a historical perspective. The main focus is to assess those cyclic slip irreversibilities in a microstructural sense that occur not only at the surface but also in the bulk at the dislocation scale and to show how they contribute to surface fatigue damage. The evolution of early fatigue damage, as evidenced experimentally in the last decades, is reviewed. The phenomenon of cyclic strain localization in persistent slip bands (PSBs) and models of the formation of extrusions, intrusions, and microcracks are discussed in detail. The predictions of these models are compared with experimental evidence obtained on mono- and polycrystalline face-centered-cubic (fcc) metals. In addition, examples of the evolution of fatigue damage in selected fcc solid solution alloys and precipitation-hardened alloys and in body-centered-cubic (bcc) metals are analyzed. Where possible, the cyclic slip irreversibilities p, defined as the fraction of plastic shear strain that is microstructurally irreversible, have been estimated quantitatively. Broadly speaking, p has been found to vary over orders of magnitude (0 < p < 1), being almost negligible at low loading amplitudes (high fatigue lives) and substantial at larger loading amplitudes (low fatigue lives).
205 citations
TL;DR: In this article, a novel processing route of cold rolling and reversion annealing for enhanced mechanical properties has been investigated in metastable 17Cr-7Ni-type austenitic stainless steels.
Abstract: A novel processing route of cold rolling and reversion annealing for enhanced mechanical properties has been investigated in metastable 17Cr-7Ni-type austenitic stainless steels, i.e., commercial grades AISI 301LN and AISI 301, and in some experimental heats. The investigation was essentially aimed at studying the possibility of processing nano/submicron-grained structure in these steels and to rationalize the possible effects of alloying elements on the reversion mechanisms. The steels were cold rolled to various reductions between 45 and 78 pct to induce the formation of martensite, and subsequently annealed between 600 °C to 1000 °C for short annealing times (mostly 1 to 100 seconds). Microstructure examinations of the reversion-annealed 301LN steel revealed that an ultrafine-grained austenitic structure was formed by the diffusional transformation mechanism within a short holding time above 700 °C, even after the lowest cold-rolling reduction. In contrast, in 301 steel and experimental heats, the shear type of transformation occurred at temperatures above 650 °C, but fine austenite grains were only formed by recrystallization at higher temperatures or longer holding times, e.g., at 900 °C/100 s. An attempt has been made to determine the reversion mechanisms in various steels by modifying the criteria governing the Gibbs free energy change during the martensite-austenite reversion in Cr-Ni alloys. The room temperature (RT)-tensile property evaluation showed that excellent combinations of yield or tensile strength and elongation are possible to achieve, depending mainly on annealing conditions both in the 301LN and 301 steels, but the experimental heats were too unstable for high ductility. Ultrafine grain size of austenite contributed to this in 301LN and shear-transformed high-dislocated austenite in 301. Upon reversion annealing, the reversion mechanism did not affect the texture. The texture of the reverted fine-grained austenite is very strong compared to the typical texture of commercially cold-rolled and annealed 301LN steel.
199 citations
TL;DR: In this article, a commercial dual-phase steel in sheet form and comprised of ferrite, martensite, and bainite was subjected to uniaxial tension up to fracture and the damage characteristics were studied through extensive quantitative metallography and scanning electron microscope (SEM) observations of polished sections and fracture surfaces of failed specimens.
Abstract: Commercial dual-phase (DP) steel in sheet form and comprised of ferrite, martensite, and bainite was subjected to uniaxial tension up to fracture. The damage characteristics were studied through extensive quantitative metallography and scanning electron microscope (SEM) observations of polished sections and fracture surfaces of failed specimens. The observed void nucleation mechanisms include nucleation at the martensite/ferrite interface or triple junction (most predominant), nucleation due to the cracking of martensite particles, and nucleation at the inclusions. The void characteristics in terms of area fraction, void density, void size ranges, and void orientations were analyzed as a function of thickness strain from various regions of the different uniaxial tensile test specimens taken to fracture. The damage analysis suggests that the void nucleation occurs during the entire deformation process with an almost constant rate and this rate reduces before fracture. A nucleation strain of 0.15 has been estimated for this material.
186 citations
TL;DR: In this article, the quenching and partitioning of a low-carbon steel containing 1.1-wt pct aluminum by heat treatments consisting of partial austenitization at 900 −°C and subsequent rapid cooling to a quench temperature in the range between 125 −C and 175 −C, followed by an isothermal treatment (partitioning step) at 250 −C for different times.
Abstract: The “quenching and partitioning” (Q&P) process has been studied in a low-carbon steel containing 1.1 wt pct aluminum by heat treatments consisting of partial austenitization at 900 °C and subsequent rapid cooling to a quenching temperature in the range between 125 °C and 175 °C, followed by an isothermal treatment (partitioning step) at 250 °C and 350 °C for different times. Characterization by means of optical and scanning electron microscopy, electron backscattered diffraction (EBSD), magnetization measurements, and X-ray diffraction (XRD) has shown a multiphase microstructure formed by intercritical ferrite, epitaxial ferrite, retained austenite, bainite, and martensite after different stages of tempering. A considerable amount of retained austenite has been obtained in the specimens partitioned at 350 °C for 100 seconds. Experimental results have been interpreted based on concepts of the martensite tempering, bainite transformation, and kinetics calculations of the carbon partitioning from martensite to austenite.
167 citations
TL;DR: In this paper, a model for the estimation of matrix/precipitate interfacial energies is developed, which takes into account atomic bindings over an arbitrary number of neighboring shells and accounts for general, multicomponent solid solutions.
Abstract: In this article, a model for the estimation of matrix/precipitate interfacial energies is developed. The classic nearest-neighbor broken-bond (NNBB) model is taken as a basis and further developed, to (1) take into account atomic bindings over an arbitrary number of neighboring shells and (2) account for general, multicomponent solid solutions. The model is sufficiently simple and yet reliable for providing estimates of interfacial energies in applications in complex, time-consuming computer simulations of a microstructure/precipitate evolution in which more sophisticated approaches cannot be used. As an example, the model is applied to randomly oriented interfaces in fcc and bcc crystal structures. It is shown that both kinds of crystal interfaces, fcc and bcc, exhibit roughly the same mean interfacial energies, as long as a sufficient number of nearest-neighbor shells is taken into account. A comparison with published experimental and theoretical data on interfacial energies shows good agreement.
147 citations
TL;DR: In this article, the authors rationalized the origin of room-temperature ductility in terms of strain accommodation mechanisms of reduction of glide plane spacing in Taylor lattice (TL) formation at low strains and TL rotation forming domain boundaries (DBs) and microbands (MBs) at high strains.
Abstract: Fully austenitic Fe-28Mn-10Al-1.0C steel with high stacking fault energy exhibited exceptionally high uniform elongations (85 to 100 pct) and total elongations (100 to 110 pct) at room temperature. The origin of such exceptional room-temperature ductility was rationalized in terms of strain accommodation mechanisms of reduction of glide plane spacing in Taylor lattice (TL) formation at low strains and TL rotation forming domain boundaries (DBs) and microbands (MBs) at high strains.
146 citations
TL;DR: In this article, X-ray diffraction (XRD) analysis clearly reveals that a pressure-induced phase transformation occurs from α phase to ω phase during high pressure torsion (HPT) processing when the applied pressure is more than 4 GPa and the straining facilitates this phase transformation.
Abstract: Pure Ti (99.4 pct) is processed by high-pressure torsion (HPT) at applied pressures in a wide range of 1.2 to 40 giga-pascals (GPa) for equivalent strain up to ~200. X-ray diffraction (XRD) analysis clearly reveals that a pressure-induced phase transformation occurs from α phase to ω phase during HPT processing when the applied pressure is more than ~4 GPa and the straining facilitates this phase transformation. The hardness and the tensile strength increase, but the ductility decreases by the phase transformation. Hardness measurements demonstrate that all values obtained at each pressure fall on a single curve when they are plotted as a function of equivalent strain. The hardness increases with an increase in the equivalent strain at an early stage of straining and saturates to a constant level, where the hardness remains unchanged with further straining. It is shown that the saturation level as well as the onset of the saturation depends on the applied pressure.
141 citations
TL;DR: In this paper, the microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within the dynamic recrystallization (DRX) regime using hot torsion testing.
Abstract: The microstructure and crystallographic texture development in an austenitic Ni-30 pct Fe model alloy was investigated within the dynamic recrystallization (DRX) regime using hot torsion testing. The prominent DRX nucleation mechanism was strain-induced grain boundary migration accompanied by the formation of large-angle sub-boundaries and annealing twins. The increase in DRX volume fraction occurred through the formation of multiple twinning chains. With increasing strain, the pre-existing Σ3 twin boundaries became gradually converted to general boundaries capable of acting as potent DRX nucleation sites. The texture characteristics of deformed grains resulted from the preferred consumption of high Taylor factor components by new recrystallized grains. Similarly, the texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during the DRX process. The substructure of deformed grains was characterized by “organized,” banded subgrain arrangements, while that of the DRX grains displayed “random,” more equiaxed subgrain/cell configurations.
TL;DR: The High Performance Corrosion-Resistant Materials (HPCRM) Program as discussed by the authors was sponsored by the Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) and the U.S. Department of Energy (DOE) Office of Civilian and Radioactive Waste Management (OCRWM).
Abstract: An overview of the High-Performance Corrosion-Resistant Materials (HPCRM) Program, which was cosponsored by the Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) and the U.S. Department of Energy (DOE) Office of Civilian and Radioactive Waste Management (OCRWM), is discussed. Programmatic investigations have included a broad range of topics: alloy design and composition, materials synthesis, thermal stability, corrosion resistance, environmental cracking, mechanical properties, damage tolerance, radiation effects, and important potential applications. Amorphous alloys identified as SAM2X5 (Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4) and SAM1651 (Fe48Mo14Cr15Y2C15B6) have been produced as meltspun ribbons (MSRs), dropcast ingots, and thermal-spray coatings. Chromium (Cr), molybdenum (Mo), and tungsten (W) additions provided corrosion resistance, while boron (B) enabled glass formation. Earlier electrochemical studies of MSRs and ingots of these amorphous alloys demonstrated outstanding passive film stability. More recently, thermal-spray coatings of these amorphous alloys have been made and subjected to long-term salt-fog and immersion tests; good corrosion resistance has been observed during salt-fog testing. Corrosion rates were measured in situ with linear polarization, while the open-circuit corrosion potentials (OCPs) were simultaneously monitored; reasonably good performance was observed. The sensitivity of these measurements to electrolyte composition and temperature was determined. The high boron content of this particular amorphous metal makes this amorphous alloy an effective neutron absorber and suitable for criticality-control applications. In general, the corrosion resistance of such iron-based amorphous metals is maintained at operating temperatures up to the glass transition temperature. These materials are much harder than conventional stainless steel and Ni-based materials, and are proving to have excellent wear properties, sufficient to warrant their use in earth excavation, drilling, and tunnel-boring applications. Large areas have been successfully coated with these materials, with thicknesses of approximately 1 cm. The observed corrosion resistance may enable applications of importance in industries such as oil and gas production, refining, nuclear power generation, shipping, etc.
TL;DR: In this paper, a dimensionless Niyama criterion is proposed to predict the amount of shrinkage porosity that forms during solidification of metal alloy castings, without the need to know the threshold value below which porosity forms; such threshold values are generally unknown and alloy dependent.
Abstract: A method is presented to use a dimensionless form of the well-known Niyama criterion to directly predict the amount of shrinkage porosity that forms during solidification of metal alloy castings. The main advancement offered by this method is that it avoids the need to know the threshold Niyama value below which shrinkage porosity forms; such threshold values are generally unknown and alloy dependent. The dimensionless criterion accounts for both the local thermal conditions (as in the original Niyama criterion) and the properties and solidification characteristics of the alloy. Once a dimensionless Niyama criterion value is obtained from casting simulation results, the corresponding shrinkage pore volume fraction can be determined knowing only the solid fraction-temperature curve and the total solidification shrinkage of the alloy. Curves providing the shrinkage pore volume percentage as a function of the dimensionless Niyama criterion are given for WCB steel, aluminum alloy A356, and magnesium alloy AZ91D. The present method is used in a general-purpose casting simulation software package to predict shrinkage porosity in three-dimensional (3-D) castings. Comparisons between simulated and experimental shrinkage porosity results for a WCB steel plate casting demonstrate that this method can reasonably predict shrinkage. Additional simulations for magnesium alloy AZ91D illustrate that this method is applicable to a wide variety of alloys and casting conditions.
TL;DR: In this paper, the hot compression behavior of a 17-4 PH stainless steel (AISI 630) was investigated at temperatures of 950 °C to 1150 °C and strain rates of 10−3 to 10 s−1.
Abstract: The hot compression behavior of a 17-4 PH stainless steel (AISI 630) has been investigated at temperatures of 950 °C to 1150 °C and strain rates of 10−3 to 10 s−1. Glass powder in the Rastegaev reservoirs of the specimen was used as a lubricant material. A step-by-step procedure for data analysis in the hot compression test was given. The work hardening rate analysis was performed to reveal if dynamic recrystallization (DRX) occurred. Many samples exhibited typical DRX stress-strain curves with a single peak stress followed by a gradual fall toward the steady-state stress. At low Zener–Hollomon (Z) parameter, this material showed a new DRX flow behavior, which was called multiple transient steady state (MTSS). At high Z, as a result of adiabatic deformation heating, a drop in flow stress was observed. The general constitutive equations were used to determine the hot working constants of this material. Moreover, after a critical discussion, the deformation activation energy of 17-4 PH stainless steel was determined as 337 kJ/mol.
TL;DR: In this paper, the supersonic impact of a copper microparticle onto a Cu 11000 surface was investigated using a focussed ion beam/scanning electron microscope (FIB/SEM).
Abstract: The supersonic impact of a copper microparticle onto a Cu 11000 surface was investigated. The particle was dissected and imaged using a focussed ion beam/scanning electron microscope (FIB/SEM). This was compared with transmission electron microscopy (TEM) of a thick cold spray deposit of copper. Significant refinement occurred in both cases due to severe plastic deformation (SPD) at high strain rates. In the single-particle case, this was much more pronounced in the highly deformed region, closer to the particle-substrate interface. In the bulk deposit, a highly nonhomogeneous microstructure resulted. The appearance of nanosized grains and microtwins was an indication of the large strains imposed during impact.
TL;DR: In this article, a model for the room-temperature dynamic strain aging of high-manganese FeMnC and FeAlC twinning-induced plasticity (TWIP) steel was proposed.
Abstract: High-manganese FeMnC and FeMnAlC austenitic twinning-induced plasticity (TWIP) steel exhibits excellent strain-hardening properties due to the gradual reduction of the mean free path for dislocations glide resulting from deformation twinning. Serrated stress-strain curves are often obtained when this type of steel is tested in a uniaxial tensile test. This phenomenon is due to dynamic strain aging (DSA). It is related to the occurrence of localized Portevin–LeChatelier (PLC) deformation bands. The properties of the PLC bands were accurately determined for a FeMnAlC TWIP steel using a combination of high-sensitivity infrared (IR) thermographic imaging and optical strain analysis carried out in situ during tensile deformation. Strain rate jump tests were conducted at room temperature to measure the instantaneous and steady-state strain rate sensitivity as a function of true stress and true strain. Negative values of the steady-state strain rate sensitivity were measured in both upward and downward jump tests. These measurements explain why FeMnC and FeMnAlC TWIP steels have a limited postuniform elongation. A model for the room-temperature DSA of high-Mn austenitic TWIP steel containing C in solid solution is proposed.
TL;DR: In this paper, the microstructural evolution during hot-compression deformation of the biomedical Co-29Cr-6Mo (weight percent) alloy without the addition of Ni was examined.
Abstract: In order to examine the microstructural evolution during hot-compression deformation of the biomedical Co-29Cr-6Mo (weight percent) alloy without the addition of Ni, hot-compression tests have been conducted at deformation temperatures ranging from 1050 °C to 1200 °C at various strain rates of 10−3 to 10 s−1. The grain refinement due to dynamic recrystallization (DRX) was identified under all deformation conditions by means of field-emission scanning electron microscopy/electron backscattered diffraction (FESEM/EBSD) and transmission electron microscopy (TEM) observations. Although the DRX grain size (d) of the deformed specimens considerably decreased with an increasing Zener–Hollomon (Z) parameter at strain rates ranging from 10−3 to 0.1 s−1, a grain size coarser than that predicted from the d-Z relation was obtained at strain rates of 1.0 and 10 s−1. An ultrafine-grained microstructure with a grain size of approximately 0.6 μm was obtained under deformation at 1050 °C at 0.1 s−1, from an initial grain size of 40 μm. The grain refinement to a submicron scale of biomedical Co-Cr-Mo alloys has been achieved with hot deformation by ~60 pct due to DRX, in which the bulging mechanism is not operative. The ultrafine grains obtained due to DRX without bulging is closely related to the considerably low stacking-fault energy (SFE) of the Co-Cr-Mo alloy at deformation temperatures.
TL;DR: In this paper, a microalloyed boron and aluminum precoated steel, which has been isothermally deformed under uniaxial tensile tests, was investigated at temperatures between 873 and 1223 K, using a fixed strain rate value of 0.08 s−1.
Abstract: The strains, transformation temperatures, microstructure, and microhardness of a microalloyed boron and aluminum precoated steel, which has been isothermally deformed under uniaxial tensile tests, have been investigated at temperatures between 873 and 1223 K, using a fixed strain rate value of 0.08 s−1. The effect of each factor, such as temperature and strain value, has been later valued considering the shift generated on the continuous cooling transformation (CCT) diagram. The experimental results consist of the starting temperatures that occur for each transformation, the microhardness values, and the obtained microstructure at the end of each thermomechanical treatment. All the thermomechanical treatments were performed using the thermomechanical simulator Gleeble 1500. The results showed that increasing hot prestrain (HPS) values generate, at the same cooling rate, lower hardness values; this means that the increasing of HPS generates a shift of the CCT diagram toward a lower starting time for each transformation. Therefore, high values of hot deformations during the hot stamping process require a strict control of the cooling process in order to ensure cooling rate values that allow maintaining good mechanical component characteristics. This phenomenon is amplified when the prestrain occurs at lower temperatures, and thus, it is very sensitive to the temperature level.
TL;DR: In this article, the authors investigated the creep behavior of the cast AZ91 magnesium alloy by impression testing under constant punching stress in the range 100 to 650 MPa, corresponding to 0.007 ≤ σimp/G ≤ 0.044.
Abstract: The creep behavior of the cast AZ91 magnesium alloy was investigated by impression testing. The tests were carried out under constant punching stress in the range 100 to 650 MPa, corresponding to 0.007 ≤ σimp/G ≤ 0.044, at temperatures in the range 425 to 570 K. Assuming a power-law relationship between the impression velocity and stress, depending on the testing temperature, stress exponents of 4.2 to 6.0 were obtained. When the experimental creep rates were normalized to the grain size and effective diffusion coefficient, a stress exponent of approximately 5 was obtained, which is in complete agreement with stress exponents determined by the conventional creep testing of the same material reported in the literature. Calculation of the activation energy showed a slight decrease in the activation energy with increasing stress such that the creep-activation energy of 122.9 kJ/mol at σimp/G = 0.020 decreases to 94.0 kJ/mol at σimp/G = 0.040. Based on the obtained stress exponents and activation energy data, it is proposed that dislocation climb is the controlling creep mechanism. However, due to the decreasing trend of creep-activation energy with stress, it is suggested that two parallel mechanisms of lattice and pipe-diffusion-controlled dislocation climb are competing. To elucidate the contribution of each mechanism to the overall creep deformation, the creep rates were calculated based on the effective activation energy. This yielded a criterion that showed that, in the high-stress regimes, the experimental activation energies fall in the range in which the operative creep mechanism is dislocation climb controlled by dislocation pipe diffusion. In the low-stress regime, however, the lattice-diffusion dislocation climb is dominant.
TL;DR: In this paper, the flow behavior and microstructural evolution during subtransus isothermal forging of Ti-5Al-5Mo-5V-3Cr has been investigated for two different starting microstructures and analysis has incorporated previously published results.
Abstract: High-strength metastable β alloys, for example, Ti-5Al-5Mo-5V-3Cr, have replaced steel as the material of choice for large components, such as the main truck beam on the latest generation of airframes. The production of these components is carried out by hot near-net-shape forging, during which process variable control is essential to achieve the desired microstructural condition and subsequent mechanical properties. The flow behavior and microstructural evolution during subtransus isothermal forging of Ti-5Al-5Mo-5V-3Cr has been investigated for two different starting microstructures and analysis has incorporated previously published results. The flow behavior, irrespective of initial microstructural condition, is found to be very similar at strains ≥0.35. It is thought that this is due to a common microstructural state being reached, where dynamic recovery of the β phase is the dominating deformation mechanism. At strains <0.35, the flow behavior is believed to be dominated by the morphology and volume fraction of the α phase. Small globular α particles are thought to have little effect on the flow behavior, while the observed flow softening is directly linked to the fragmentation of acicular α precipitates.
TL;DR: In this paper, the authors studied the effect of alloy composition and processing on actuator stability during thermomechanical cycling and found that functional fatigue of binary NiTi and ternary NiTiCu (with 5, 75, and 10 at pct Cu) shape memory actuators results in an accumulation of irreversible deformation in martensite and austenite.
Abstract: The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 75, and 10 at pct Cu) shape memory (SM) spring actuators We study how the alloy composition and processing affect the actuator stability during thermomechanical cycling Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures We present phenomenological equations that quantify both phenomena We show that cyclic actuator stability can be improved by using precycling, subjecting the material to cold work, and adding copper Adding copper is more attractive than cold work, because it improves cyclic stability without sacrificing the exploitable actuator stroke Copper reduces the width of the thermal hysteresis and improves geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent and the product phase There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable deformation associated with thermomechanical cycling We interpret this finding on the basis of a scenario in which dislocations are created during the phase transformations that remain in the microstructure during subsequent cycling These dislocations facilitate the formation of martensite (increasing martensite start (M S ) temperatures) and account for the accumulation of irreversible strain in martensite and austenite
TL;DR: In this paper, the authors studied the micromechanical behavior of dual-phase alloys using the in-situ high-energy X-ray diffraction (HEXRD) technique and established a method to separate the stress and strain in the ferrite and martensite during loading.
Abstract: The direct measurement of the stress or strain partitioning during deformation in the materials, consisting of two phases with the same crystallographic structure and different microstructures, is still difficult so far. This is due to the fact that no effective characterization tool is available with the ability to distinguish the local strain and stress at microscale level. In this article, we studied the micromechanical behavior of ferrite/martensite dual-phase (DP) alloys using the in-situ high-energy X-ray diffraction (HEXRD) technique. We established a new method to separate the stress and strain in the ferrite and martensite during loading. Although the ferrite and martensite exhibit the same crystal structure with similar lattice parameters, the dependence of (200) lattice strains on the applied stress is obviously different for each phase. A visco-plastic self-consistent (VPSC) model, which can simulate the micromechanical behavior of two-phase materials, was used to construct the respective constitutive laws for both phases from the experimental lattice strains and to fit the macro-stress-strain curve. The material parameters for each phase extracted from our experiments and simulations could be used for designing other DP alloys and optimizing some complex industrial processes.
TL;DR: In this paper, a porosity-based crack initiation model was developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei, and crack initiation was taken to occur when stable pores formed within the coherent dendrite region, depending upon hydrogen content.
Abstract: In the present work, mechanisms are proposed for solidification crack initiation and growth in aluminum alloy 6060 arc welds. Calculations for an interdendritic liquid pressure drop, made using the Rappaz–Drezet–Gremaud (RDG) model, demonstrate that cavitation as a liquid fracture mechanism is not likely to occur except at elevated levels of hydrogen content. Instead, a porosity-based crack initiation model has been developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei. Crack initiation is taken to occur when stable pores form within the coherent dendrite region, depending upon hydrogen content. Following initiation, crack growth is modeled using a mass balance approach, controlled by local strain rate conditions. The critical grain boundary liquid deformation rate needed for solidification crack growth has been determined for a weld made with a 16 pct 4043 filler addition, based upon the local strain rate measurement and a simplified strain rate partitioning model. Combined models show that hydrogen and strain rate control crack initiation and growth, respectively. A hypothetical hydrogen strain rate map is presented, defining conceptually the combined conditions needed for cracking and porosity.
TL;DR: A detailed qualitative and quantitative examination of the microstructure and mechanical properties of three different classes of DP600 and DP450 dual-phase (DP) steels was carried out.
Abstract: A detailed qualitative and quantitative examination of the microstructure and mechanical properties of three different classes of DP600 and DP450 dual-phase (DP) steels was carried out. The tested DP steels are characterized by different alloying elements: aluminum, boron, and phosphorus. Among them, aluminum DP steels showed the lowest percentages of hard phases, while phosphorus DP steels exhibited the highest resistance values. The Hollomon, Pickering, Crussard–Jaoul (CJ), and Bergstrom models were used to reproduce the strain hardening behavior of DP steels. Relationships that correlate the fitting parameters with the chemical composition and the thermal cycle parameters were found, and the predictive abilities of different models were evaluated. The Pickering equation, among the tested models, is the best one in the reproduction of the experimental stress-strain data.
TL;DR: In this paper, the laser-engineered net shaping (LENS) process is implemented to fabricate net-shaped Fe-based Fe-B-Cr-C-Mn-Mo-W-Zr metallic glass (MG) components.
Abstract: In this article, the laser-engineered net shaping (LENS) process is implemented to fabricate net-shaped Fe-based Fe-B-Cr-C-Mn-Mo-W-Zr metallic glass (MG) components. The glass-forming ability (GFA), glass transition, crystallization behavior, and mechanical properties of the glassy alloy are analyzed to provide fundamental insights into the underlying physical mechanisms. The microstructures of various LENS-processed component geometries are characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results reveal that the as-processed microstructure consists of nanocrystalline α-Fe particles embedded in an amorphous matrix. An amorphous microstructure is observed in deposited layers that are located near the substrate. From a microstructure standpoint, the fraction of crystalline phases increases with the increasing number of deposited layers, effectively resulting in the formation of a functionally graded microstructure with in-situ-precipitated particles in an MG matrix. The microhardness of LENS-processed Fe-based MG components has a high value of 9.52 GPa.
TL;DR: In this article, the microstructural and linear strain changes of martensitic medium-carbon steel were investigated during continuous heating to 600 °C at different rates using dilatometry and transmission electron microscopy (TEM).
Abstract: The microstructural and linear strain changes of martensitic medium-carbon steel were investigated during continuous heating to 600 °C at different rates using dilatometry and transmission electron microscopy (TEM). The precipitation of transition e-carbides between 70 °C to 240 °C and the precipitation of cementite between 200 °C to 450 °C were observed depending on the heating rate. The measured strain changes during tempering stages 1 and 3 were converted to the fraction of tempered martensite based on the theoretical iron atomic volume change between martensite and tempered martensite. The presegregation amount of carbon before tempering was calculated to be about 0.16 wt pct by comparing the theoretical strain change with the measured strain change. Tempering kinetic models were developed using the tempered martensite fractions converted from the measured strain changes during tempering stages 1 and 3. The kinetics models exhibit a good correlation with the experimentally measured tempering kinetics.
TL;DR: In this article, the effects of the martensite phase on the failure mode and ductility of dual-phase (DP) steels were investigated using a micromechanics-based finite element method.
Abstract: The effects of the mechanical properties of the martensite phase on the failure mode and ductility of dual-phase (DP) steels are investigated using a micromechanics-based finite element method. Actual microstructures of DP steels obtained from scanning electron microscopy (SEM) are used as representative volume elements (RVEs) in the finite element calculations. Ductile failure of the RVE is predicted as plastic strain localization during the deformation process. Systematic computations are conducted on the RVE to quantitatively evaluate the influence of the martensite mechanical properties and volume fraction on the macroscopic mechanical properties of DP steels. These properties include the ultimate tensile strength (UTS), ultimate ductility, and failure modes. The computational results show that, as the strength and volume fraction of the martensite phase increase, the UTS of DP steels increases, but the UTS strain and failure strain decrease. In addition, shear-dominant failure modes usually develop for DP steels with lower martensite strengths, whereas split failure modes typically develop for DP steels with higher martensite strengths. The methodology and data presented in this article can be used to tailor DP steel design for its intended purposes and desired properties.
TL;DR: In this paper, the progress of martensite formation in plain carbon steels Fe-0.46C, Fe- 0.66C, and Fe 0.80C has been investigated by dilatometry.
Abstract: The progress of martensite formation in plain carbon steels Fe-0.46C, Fe-0.66C, and Fe-0.80C has been investigated by dilatometry. It is demonstrated that carbon enrichment of the remaining austenite due to intercritical annealing of Fe-0.46C and Fe-0.66C does not only depress the start temperature for martensite, but also slows the progress of the transformation with temperature compared to full austenitization. In contrast, such a change of kinetics is not observed when the remaining austenite of lean-Si steel Fe-0.80C is stabilized due to a partial transformation to bainite, which suggests that the stabilization is not of a chemical but of a mechanical nature. The growth of bainite and martensite is accompanied by a shape change at the microstructural scale, which leads to plastic deformation and thus strengthening of the surrounding austenite. Based on this stabilizing mechanism, the athermal transformation kinetics is rationalized by balancing the increase in driving force corresponding to a temperature decrease with the increase in strain energy required for the formation of martensite in the strengthened remaining austenite.
TL;DR: In this paper, a small amount of SiC nanoparticles is used to influence the solidification of magnesium-zinc alloys, resulting in strong, ductile, and castable materials.
Abstract: Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesium-zinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn2 phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn2 formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.
TL;DR: In this paper, laser-ablated platinum nanoparticles on the surface of a polycrystalline metal (Rene 88DT) were used to obtain the local strain behavior from an in-situ scanning electron microscope tensile experiment at room temperature.
Abstract: Digital image correlation of laser-ablated platinum nanoparticles on the surface of a polycrystalline metal (nickel-based superalloy Rene 88DT) was used to obtain the local strain behavior from an in-situ scanning electron microscope tensile experiment at room temperature. By fusing this information with crystallographic orientations from electron backscatter diffraction (EBSD), a subsequent analysis shows that the average maximum shear strain tends to increase with increasing Schmid factor. Additionally, the range of the extreme values for the maximum shear strain also increases closer to the grain boundary, signifying that grain boundaries and triple junctions accumulate plasticity at strains just beyond yield in polycrystalline Rene 88DT. In-situ experiments illuminating microstructure-property relationships of this ilk may be important for understanding damage nucleation in polycrystalline metals at high temperatures.