Influence Of Initial Microstructure Research Articles

Effect of Initial Microstructure on the Temperature Dependence of the Flow Stress and Deformation Microstructure under Uniaxial Compression of Ti-407

In this study, the influence of initial microstructure and deformation temperature on the flow stress behavior and microstructural evolution of TIMETAL®407 (Ti-407) alloy are investigated. For this purpose, compression cylinders were β-annealed at 940 °C and then cooled to room temperature using furnace cooling, static air, and water quenching to promote three initial microstructures with different α lath thicknesses. The annealed cylinders were compressed isothermally in the range of 750 °C to 910 °C at a constant crosshead speed of 0.05 mm/s up to an engineering strain of −0.8. The resulting stress–strain curves are discussed in terms of the morphology and distribution of the α and β phases. It was found that flow stress is inversely proportional to deformation temperature for all initial microstructures. At the lowest temperatures, compressive yield strength was higher in water-quenched and air-cooled samples than in furnace-cooled specimens, suggesting that the acicular α-phase morphology obtained by rapid cooling could enhance mechanical strength by hindering dislocation motion. Two high-temperature flow regimes were determined based on the shape of the flow stress curves, indicating microstructural changes occurring during deformation. At higher temperatures, the effect of the initial microstructure is negligible as the primary α phase is transformed to the β phase at around 850 °C irrespective of the initial α-lath thickness.

Influence of initial microstructure on surface integrity of ultrasonic impacting strengthened 34CrNi3MoA steel crankshaft

Influence of initial microstructure on surface integrity of ultrasonic impacting strengthened 34CrNi3MoA steel crankshaft

Influence of initial microstructure on reaustenitization behavior in low alloy steel by in-situ high-temperature EBSD characterization

In-situ high-temperature EBSD characterization was carried out to directly observe reaustenitization in a low alloy steel with different initial microstructures. For lath bainite, globular austenite mainly nucleated at prior austenite grain boundaries, and grew gradually to form uniform small grains from 780 to 860˚C. For granular bainite, martensite-austenite constituents accelerated acicular austenite formation via martensitic reversion transformation. Then, acicular grains coalesced. Finally, recrystallization occurred due to high dislocation density in martensitic revered austenite.

Towards Multi-Scale Modeling of Zirconium Alloys Recrystallization and Application to Thermo-Mechanical Processes Optimization

Zirconium alloys are used in the nuclear industry due to their low neutron capture cross-section and resistance to corrosion, irradiation and creep. The microstructure of the nuclear fuel components evolves during the manufacturing route and can impact the subsequent processes or the final properties. Thus, numerical modeling of thermo-mechanical manufacturing processes is of interest to understand and master these microstructure evolutions.Numerical modeling of thermo-mechanical manufacturing processes with FORGE® NxT software is applied. These models provide the thermo-mechanical history of the material at each integration point of the finite element (FE) mesh, which can be used to assess locally the continuous dynamic and post-dynamic recrystallization during hot extrusion.Mean-field models were developed in Python and integrated into FORGE® NxT software, to quantify the microstructure evolution at the macro-scale of the component. Full-field models (DIGIMU® software1) were also developed for considering microstructural heterogeneities and the influence of initial microstructure at the mesoscopic scale while improving the mean-field equations by homogenization.After validation based on experimental results, these two recrystallization models provide complementary information to optimize the process parameters at the macro-scale and to better understand mesoscopic scale phenomena, such as:• At the macro-scale: influence of hot extrusion parameters on the continuous dynamic and post-dynamic recrystallization of Zircaloy-4.• At the meso-scale: influence of the initial microstructure on the recrystallization phenomena with improved precision. Indeed, the topology of the microstructure is predicted and not only the mean values/distributions of the state variables.

Investigation on the formation of grain boundary serrations in additively manufactured superalloy Haynes 230

Solid-solution and carbide-strengthened superalloys such as Haynes 230 are the materials of choice for the hot-section components of gas turbines, e.g., combustion cans and transition ducts. Under severe thermal conditions, to which those parts are exposed, creep strength is a crucial property of the related materials during their lifetime. Recently, the introduction of serrated grain boundaries in Haynes 230 has been intensively studied [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015); L. Jiang, R. Hu, H. Kou, J. Li, G. Bai, and H. Fu, “The effect of M23C6 carbides on the formation of grain boundary serrations in a wrought Ni-based superalloy,” Mater. Sci. Eng. A 536, 37–44 (2012)], and nearly a triplication of the time to creep failure at high temperature and low stress conditions has been observed [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015)]. The aim of this paper is to achieve serrated grain boundaries in Haynes 230 through an appropriate thermal process chain including the intrinsic heat treatments of the laser metal deposition (LMD) process, subsequent hot isostatic pressing and suitable heat treatments. The formation of serrations is a relatively new technique for Haynes 230 (i.e., first paper in 2012), and similar alloys and thus serrations have only been introduced in conventionally cast or wrought alloys so far. Optical and scanning electron microscopies are employed in this work to investigate the created microstructures, whose grain and carbide structure is finer compared to the recently studied conventionally processed alloys. Within the LMD samples, serrations were already found on almost all of the observed grain boundaries even in the as-build condition. This result was rather unexpected, as literature reports slow-cooling to be responsible for the formation of serrations, while fast-cooling is prevalent in LMD. Some authors associated the formation of serrations to the precipitation of M23C6-carbides at the grain boundaries during slow cooling conditions [L. Jiang, R. Hu, H. Kou, J. Li, G. Bai, and H. Fu, “The effect of M23C6 carbides on the formation of grain boundary serrations in a wrought Ni-based superalloy,” Mater. Sci. Eng. A 536, 37–44 (2012)]. The lower density of carbides along grain boundaries in the as-build state, however, makes this mechanism seem unlikely. Other authors attributed the emergence of serrations to a phenomenon similar to the faceting mechanism [J. G. Yoon, H. W. Jeong, Y. S. Yoo, and H. U. Hong, “Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C,” Mater. Charact. 101, 49–57 (2015)]. It can be said that no uniform theory for the emergence of grain boundary serrations exists as of now. The electron backscatter diffraction (EBSD) investigations performed in this work indicated a correlation between serrated grain boundary segments, the {111}-directions of the crystal lattice, and possibly segregations along dendritic subgrain boundaries for a two-dimensional case. Serial sectioning in combination with EBSD analysis confirmed an agreement between the three-dimensional orientation of serrated grain boundary segments and the {111}-direction of adjacent grains. Hence, a mechanism different from the ones described in previous works is proposed for the formation of grain boundary serrations in the additively manufactured Haynes 230 alloy.

Influence of initial microstructure on the microstructure evolution and mechanical properties of 1.0C-1.5Cr steel in the laser surface quenching

Laser surface quenching was conducted on the 1.0C-1.5Cr steel with different initial microstructures. The effects of initial microstructure on the cementite dissolution and grain size were studied, and the distributions of temperature and stress were simulated. The results indicated that the thickness of hardened layer was larger when the initial microstructure is martensite, which is mainly because the martensite-to-austenite transformation is displacive transformation during laser heating. Within hardened layer, the volume fraction of cementite (VFC) increases with the increase of depth. For the initial microstructure composed of ferrite matrix and cementite, the refining of cementite will facilitate its dissolution, and within hardened layer, mean diameter of cementite (MDC) decreases first and then increases with the increase of depth. For the initial microstructure composed of martensite matrix and cementite, the MDC changes slightly at deeper position of hardened layer. Tensile stress will form in the heat affect zone, and due to the high sensitivity to tensile stress, the impact resistance and bending properties of spheroidized microstructure are worse after laser surface quenching. Under identical laser parameters, the impact absorbed energy of spheroidized microstructure is about 50% of tempered sorbite and 60% of martensitic microstructure, and its bending strength is about 60% of tempered sorbite, and 40% of martensitic microstructure. The impact absorbed energy can be increased by at least 26% through preheating at 160 °C. In addition, the refining of initial martensitic microstructure will enhance bending resistance.

Influence of Initial Microstructure on Manufacturing Processes with Thermal Loads accompanied by Hardening

Abstract The presented investigation focuses on the correlation between concrete physical loads and resulting modifications. The physical and in a wider sense chemical loads are called internal material loads. The correlation between the generated internal material loads and the modifications is called Process Signature. This contribution presents some Process Signature components for thermal dominated processes with phase transformation and accompanying hardening. The selection of the Process Signature components considers the residual stress state at the surface and the positions of the zero crossings of the residual stress curve. Based on Finite Element simulation and experimental work the external thermal loading of both processes were characterized by the resulting maximal temperature and temperature gradient, which correlates with the change of the residual stress state. During the CIRP CSI conference 2018 in Tianjin the authors presented results for the mentioned Process Signature components for a ferrite/perlite state. The actual work focuses on the question, how the above described Process Signature components depend on the initial microstructure of the steel. For this purpose the phase kinetics of different initial quenched and tempered states are investigated for fast heating processes up to 5000 K/s. Additionally the mechanical materials properties of these material states will be presented.

High-Stress Compressive Creep Behavior of Ti-6Al-4V ELI Alloys with Different Microstructures

Influence of initial microstructure of Ti-6Al-4V ELI alloys on their compressive creep behavior at ambient temperature was investigated with applying compression stresses from 695 to 1092 MPa The experimental results show that the basketweave alloys have better compressive creep resistances than those duplex ones. The constitutive equations in steady-state compressive creeps of duplex or basketweave structure are calculated to be =2.77×10-15(σ-710)2.1 and =2.36×10-14(σ-740)1.7 by fitting the linear regression creep curves after uniaxial compression tests. The noticeable compressive creep strains occur when the applied compression stresses are higher than the threshold stresses, i.e. 710 MPa for duplex Ti-6Al-4V ELI alloys and 740 MPa for basketweave alloys. Microstructural analysis indicates that the creep deformation of Ti-6Al-4V ELI alloys at ambient temperature is mainly controlled by dislocation slip. The creep behavior of Ti-6Al-4V ELI alloy with duplex microstructure is controlled by dislocation slip, like slip dislocations with a-type Burgers vector sliding on the basal or prismatic planes and a few c+a type dislocation sliding on the pyramidal planes. While creep mechanism for basketweave ones is dislocation glide controlled by c+a type Burgers vector sliding on the pyramidal planes and a-type sliding on the basal or prismatic planes.

Influence of initial microstructure and grain size on transformation of bainite to austenite in large size forgings

The kinetics of austenite formation in the surface and center regions of a 40 t forged ingot of a high-strength medium-carbon low-alloy steel was studied using high-resolution dilatometry. The starting microstructures from the surface or center regions had different proportions of bainite and residual austenite as well as different prior austenite grain sizes. Two heating rates representing the actual heating rates in the surface (5 °C s−1) and center regions (0.5 °C s−1) of large size forged blocks were utilized. Dilatometric curves revealed only one transformation step of austenite formation at both heating rates independent of grain size or proportion of phases. Optical microscopy, field emission gun scanning electron microscopy and X-ray diffraction were used to study microstructure evolution and confirm the results obtained by dilatometry. The kinetic parameters for austenite formation were determined from the dilatometry data by Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The JMAK coefficients were determined for each condition of the investigated steels. The calculations indicated that the nucleation and growth of austenite in the surface region were accelerated more than 10,000 times due to a significantly smaller average prior austenite grain size, stability of initial retained austenite, and accumulation of coarse carbides at the surface. The results were discussed in the framework of classical nucleation and growth theories using the kinetic parameters for austenite formation.

Numerical and experimental investigation on influence of initial microstructure on GTA-welded age-hardened AA2024

Influence of gas tungsten arc welding on the thermal responses, residual stress distribution and final mechanical properties of the naturally age-hardened (T4) and artificially age-hardened (T6) AA2024 plates was studied. The results showed that age hardening and over-aging occurred in various sections of the heat-affected zone (HAZ) of T4 sample, depending on their locations, while in the HAZ of T6 sample, over-aging is the governing phenomenon. Also, subsequent natural aging occurred in HAZ, weld metal (WM) and partially melted material (PMM) of both weldments. This recent phenomenon is more obvious in the T4 sample in comparison with the T6 one. In both microstructures, weld metal is the weakest part of the weldments and base metal is the strongest section. Furthermore, a double-peak longitudinal residual stress distribution was detected in both temper designations due to the local plastic deformation of the plates. In addition, although the initial microstructure showed a trivial effect on the width of the tensile region, it changed the maximum value of longitudinal residual stress of about 20 MPa. It could be concluded that welding has less negative effects on AA2024-T4 samples in comparison with T6 ones, especially over time, when natural age hardening occurs.

Effect of initial microstructure on induction heating of A319 aluminium alloy

Induction heating finds applications in metal manufacturing processes where high energy efficiency and short heating time are required. Since heat is directly induced within a workpiece by circulating eddy currents and hysteresis, the high energy efficiency of induction heating would be obtained with high resistivity and magnetic materials. In this work, influence of initial microstructure on induction heating of A319 aluminium alloy was investigated in order to understand the induction heating phenomenon of non-magnetic materials. Billets with dendritic and globular grain structure were heated to different temperatures in a simple helical wound coil using an induction furnace. Heat induced in the billets was measured using a calorimetry method. Noticeably higher heat content was found in the dendritic grain structure billets with all investigated induction heating temperatures. Possible reasons of this difference in heating behavior of the differently initial microstructure billets were discussed

Influence of Initial Microstructure on the Strengthening Effect of Extruded Mg–8Sn–4Zn–2Al Alloys

The Mg–8Sn–4Zn–2Al (TZA842, in wt%) alloys with different initial microstructure (as-cast-AC and homogenization treatment-HT) subjected to hot extrusion. Also, the strengthening responses to AC and HT for the extruded TZA842 alloys were reported. The results revealed that the alloy subjected to HT shows finer grain size, more homogenous microstructure and weaker basal texture than those of counterpart subjected to AC. In addition, compared with TZA842-AC alloy, precipitates were finer and uniformly dispersed in TZA842-HT owing to the utilization of HT. Moreover, the TZA842-HT alloy showed higher yield strength of 200 MPa, ultimate tensile strength of 290 MPa and elongation (EL) of 17.9% than those of TZA842-AC, which was mainly attributed to the combined effects of grain boundary strengthening, precipitation strengthening, solid solution strengthening and weak texture. Strengthening mechanism for both alloys was discussed in detail.

Influence of Initial Microstructure on Microstructural Stability and Mechanical Behavior of Cryorolled A356 Alloy Subjected to Annealing

In the present work, various heat treatment cycles are imposed on an A356 aluminum alloy to develop two different base microstructures, one with supersaturated aluminum matrix and the other with coarse and uniformly distributed precipitates. Both the developed materials are subjected to cryorolling and then isochronally annealed for 1 hour at various temperatures ranging from 373 K to 673 K (100 °C to 400 °C). The overall strength is maximum for the cryorolled material with supersaturated base microstructure due to the dominant dislocation strengthening and precipitation strengthening mechanisms. However, the cryorolled material with precipitated base microstructure is found to have superior microstructural stability with retained ultrafine-grained (UFG) microstructure up to the annealing temperature of 473 K (200 °C) due to effective grain boundary pinning by the precipitates. Also, the material retains about 85 pct of as-cryorolled strength with enhanced ductility even after annealing at 473 K (200 °C). A detailed investigation on the microstructural evolution of the material at various annealing temperatures along with their mechanical behavior is presented in this article.

Comparative Tensile Flow and Work-Hardening Behavior of 9 Pct Chromium Ferritic-Martensitic Steels in the Framework of the Estrin–Mecking Internal-Variable Approach

The comparative tensile flow and work-hardening behavior of P9 steel in two different product forms, normalized and tempered plate and thick section tube plate forging, and P91 steel were investigated in the framework of the dislocation dynamics based Estrin–Mecking (E–M) one-internal-variable approach. The analysis indicated that the flow behavior of P9 and P91 steels was adequately described by the E–M approach in a wide range of temperatures. It was suggested that dislocation dense martensite lath/cell boundaries and precipitates together act as effective barriers to dislocation motion in P9 and P91 steels. At room and intermediate temperatures, the evolution of the internal-state variable, i.e., the dislocation density with plastic strain, exhibited insignificant variations with respect to temperature. At high temperatures, a rapid evolution of dislocation density with plastic strain toward saturation with increasing temperature was observed. The softer P9 steel tube plate forging exhibited higher work hardening in terms of larger gains in the dislocation density and flow stress contribution from dislocations than the P9 steel plate and P91 steel at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). The evaluation of activation energy suggests that the deformation is controlled by cross-slip of dislocations at room and intermediate temperatures, and climb of dislocations at high temperatures. The relative influence of initial microstructure on flow and work-hardening parameters associated with the E–M approach was discussed in the three temperature regimes displayed by P9 and P91 steels.

Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C

The effect of initial microstructure on creep properties at 900°C has been investigated in a solid-solution-strengthened Haynes 230 superalloy. The proprietary direct aging combined with controlled cooling rate led successfully to the transition of serrated grain boundaries. The serrated grain boundaries were found to have plane normal terminated by low-index planes. The lamellar carbides formed by discontinuous precipitation process became more significant in the serrated specimen, as the aging temperature to which the specimen was slow-cooled from 1280°C decreased. The fine intragranular carbides which precipitate during initial stage of creep are responsible for the strong hardening effect by pinning the dislocations in the standard specimen. The serrated grain boundaries with stable planar M23C6 carbides enhanced highly creep ductility due to their high resistance to cavitation cracking. However, the serrated specimens exhibited a higher creep rate, which is attributed to the promotion of intragranular deformation due to the lack of fine intragranular carbides, as well as cavitation at the lamellar carbides. Based on the relationships between microstructural factors and creep behaviors at 900°C, the optimal microstructure is discussed.

Influence of initial microstructure on thermomechanical fatigue behavior of Cu films on substrates

During a switch event in a power semiconductor device temperature changes of up to 300K can occur in the Cu layer. Repeated switching operations causes cyclic thermal cycling which may finally lead to thermomechanical fatigue with severe microstructural changes. In this study, the influence of the starting microstructure and film thickness (600nm and 5000nm) on thermomechanical fatigue was investigated for epitaxial and polycrystalline Cu films for up to 1000 thermal cycles. Severe surface roughening and a texture change (crystal rotation) are detected during thermal cycling for the polycrystalline Cu films, while the epitaxial films maintain their microstructure. Controlling the initial microstructure of a Cu layer in a device exposed to cyclic thermomechanical straining is a route to delay surface damage.

In situ synchrotron X-ray diffraction and dilatometric study of austenite formation in a multi-component steel: Influence of initial microstructure and heating rate

The formation of austenite during both slow and fast heating (0.25–100°Cs−1) was investigated for different microstructures of a selected low-alloy steel. With the simultaneous use of dilatometry and high-energy X-ray diffraction, it was possible to follow not only the global progress of the austenitization, but also the individual evolutions of each phase (ferrite, cementite and retained austenite if present in the initial microstructure). The results confirm earlier published data regarding the ease of austenitization of different initial microstructures (ferrite–pearlite, bainite and tempered martensite). More importantly, two stages were clearly identified, corresponding to the simultaneous transformation of ferrite and cementite, followed by the progressive disappearance of the remaining ferrite. While this is well known for ferrite–pearlite microstructures, it is not yet documented for bainite and tempered martensite. Microstructure evolution calculations based on a diffusion-controlled mechanism helped rationalize the differences observed between the three initial microstructures. In addition, they also strongly suggested the existence of a critical carbide size beyond which the second austenitization phase would correspond to carbide dissolution instead of ferrite transformation.

Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel (1.2% C in mass percent), initially quenched or spheroidized, using a Gleeble-3500 system. The true stress is observed to decrease with increasing temperature and decreasing strain rate. The true stress of the initially quenched steel is lower than that of the initially spheroidized steel at high deformation temperature (700 °C) and low deformation strain rate (0. 001 s−1). The value of the deformation activation energy (Q) of the initially quenched steel (331. 56 kJ/mol) is higher than that of the initially spheroidized steel (297. 94 kJ/mol). The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel. The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel. The fragmentation of lamellar cementites is the spheroidizing mechanism of the cementites in the initially quenched steel. Results of transmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.

Tensile Work Hardening Behavior of Thin-Section Plate and Thick-Section Tubeplate Forging of 9Cr-1Mo Steel in the Framework of One-Internal-Variable Kocks–Mecking Approach

Comparative tensile flow and work hardening behavior of normalized and tempered plate and quenched and tempered tubeplate forgings of 9Cr-1Mo steel have been examined in the framework of one-internal-variable Kocks–Mecking approach at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). Detailed analysis in terms of the variations of instantaneous work hardening rate, θ (θ = dσ/dep = dσp/dep, where σ is the true stress, σp is the plastic flow stress component, and ep is the true plastic strain) with σ and σp indicated two-stage work hardening behavior, and three distinct temperature regimes in the variations of work hardening parameters, θ − σ and θ − σp, with temperature. The influence of initial microstructures associated with different product forms of the steel is reflected in the systematic variations in work hardening parameters at temperatures ranging from 300 K to 873 K (27 °C to 600 °C). Tubeplate forging exhibited improved work hardening characteristics in terms of higher plastic component of flow stress because of microstructural softening than that of the plate material in the steel.

The influence of initial microstructure and temperature on the deformation behavior of AZ91 magnesium alloy

The deformation behavior of AZ91 magnesium alloy has been investigated using uniaxial compression tests at a temperature range of 100–300°C. The different processing routes including homogenization treatment, hot rolling and annealing have been employed to study the effect of initial microstructure on the compressive mechanical response of the AZ91 alloy. The results show that the hot-rolled material presents an enhanced compressive workability at temperatures as low as 100°C. The experimental alloy exhibit dynamic recrystallization during compression in any of the initial microstructures. The maximum and minimum DRX (dynamic recrystallization) fraction has been obtained in hot-rolled and homogenized conditions, respectively. The recrystallized fraction increases with raising the temperature. In addition the effect of initial microstructure on the peak stress diminishes with increasing temperature while its effect on the peak strain remains remarkable. The softening fraction has been increased with temperature, where a pronounced effect has been recorded in the case of homogenized (un-rolled) material.