Peak Flow Stress Research Articles

High-temperature compressive properties and microstructural evolution of in situ reacted Nano-SiC whisker/ Ti6Al4V composites manufactured by selective laser melting

A V-shaped stirring mixing method was utilised to prepare SiCw/Ti6Al4V composite powder, followed by selective laser melting (SLM) to fabricate SiCw/Ti6Al4V matrix composites. Then, the thermal compression behaviour of the Ti6Al4V-SiCw composites and Ti6Al4V alloy were analysed at a deformation of 60%, temperature of 900 °C, and strain rate of 1 s−1. The flow stress of Ti6Al4V-SiCw composites was always higher than that of Ti6Al4V alloys throughout the deformation stage and the high-temperature compressive resistance improved mainly due to the in-situ generation of TiC reinforcing phases during the forming process leading to grain refinement and dislocation reinforcement, thereby enhancing the composite's high-temperature compressive resistance. The composites showed flow softening behaviour after the peak of flow stress during hot pressing attributed to the presence of nanoscale TiC to enhance the composite recrystallisation. Moreover, microstructural analysis after hot pressing showed that the bonding between the TiC reinforcing phase in the composites and the matrix was still strong, and the matrix remains intact without any visible cracks. These results revealed that the in-situ introduction of reinforcing phases increased the dislocation density of the composites and refined the grain size ,which is the main reason for the improvement of the mechanical properties of the composites.

Modelling of hot deformation of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N

The deformation behaviour of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N in the temperature range 1000–1250 °C and strain rates in the range 0.1–10 s–1 has been studied. It is shown that the flow stresses drop with increasing temperature and decreasing strain rate in accordance with the change of the Ziner–Hollomon parameter of the temperature-velocity deformation regime. From the analysis of peak flow stresses, the value of the effective activation energy of hot deformation (Q = 490.3 kJ/mol) required for the calculation of the Ziner–Hollomon parameter is determined. An expression for the peak flow stress in the form of a hyperbolic function of the Ziner–Hollomon parameter was obtained. A comparative evaluation of hot plasticity of austenitic non-magnetic steel by finding the strain value corresponding to the appearance of the first macrocracks on the surface of the specimen is carried out.

Modelling of hot deformation of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N

The deformation behaviour of cast austenitic corrosion-resistant steel type 20Mn–18Cr–3Ni–2Mo–0.4N in the temperature range 1000–1250 °C and strain rates in the range 0.1–10 s–1 has been studied. It is shown that the flow stresses drop with increasing temperature and decreasing strain rate in accordance with the change of the Ziner–Hollomon parameter of the temperature-velocity deformation regime. From the analysis of peak flow stresses, the value of the effective activation energy of hot deformation (Q = 490.3 kJ/mol) required for the calculation of the Ziner–Hollomon parameter is determined. An expression for the peak flow stress in the form of a hyperbolic function of the Ziner–Hollomon parameter was obtained. A comparative evaluation of hot plasticity of austenitic non-magnetic steel by finding the strain value corresponding to the appearance of the first macrocracks on the surface of the specimen is carried out.

Flow stress behavior, constitutive modeling, processing map and microstructure evolution in hot deformation of Fe-7.1Al-0.7Mn-0.4C-0.3Nb alloy

The automotive industry has experienced rapid growth, with lightweight materials becoming a significant research focus. Among these materials, low-density Fe–Mn–Al–C alloys show promise for this kind of application. To optimize the hot workability parameters of these alloys for practical applications, it is essential to explore the hot deformation behavior of these alloys. In this study, we investigated the hot deformation behavior of the low-density alloy Fe-0.77Mn-7.10Al-0.45C-0.31Nb using hot plane-strain compression tests for several deformation temperatures (850, 950, 1050, and 1150 °C) and nominal strain rates (0.01, 1, and 10 s−1). Unusual flow stress curves were observed for low (0.01 s−1) and intermediate (1 s−1) strain rates, with discontinuous yielding and multiple stress peaks. This high-temperature behavior is associated with non-homogeneous strain partitioning in duplex microstructure (δ-ferrite + γ). Using the Zenner-Hollomon constitutive modeling approach, we estimated the thermal activation energy for hot deformation as 297 kJ/mol from the experimental peak flow stresses. Additionally, processing maps and microstructure correlation analysis revealed that the alloy exhibits good hot workability in the 975–1075 °C temperature range under low and intermediate strain rates (0.01–0.3 s−1). Under these processing conditions, the δ-ferrite microstructure is submitted to dynamic recovery, while the austenitic phase undergoes dynamic recrystallization.

(FeCoNi)75Cu25-xSix high entropy alloys prepared by mechanical alloying and spark plasma sintering: Microstructure, deformation behavior and dynamic recrystallization kinetics

The present research aimed to produce (FeCoNi)75Cu25-xSix (x = 0, 5, 10, 15, 20, 25 at.%) high-entropy alloys (HEAs) using mechanical alloying (MA) and spark plasma sintering (SPS). The HEAs containing 0 to 25 at.% of Si are designated as Si0, Si5, Si10, Si15, Si20, and Si25 alloys. The microstructure, phase evolution, and deformation behavior of the HEAs were investigated before and after SPS. Compression tests were conducted on SPSed alloys at room temperature and 800 °C. XRD examination revealed a single-phase FCC structure in Si0 alloy, with Si5 and Si10 alloys consisting of distinct FCC phases. Si15 alloy contained BCC phase and intermetallic compound Ni74Si26. Si20 alloy had BCC and two intermetallics Ni74Si26 and FeSi, while Si25 alloy consisted of intermetallic Ni31Si12 and BCC phases. For Si0, Si5, Si10, and Si15 alloys, cold pressing resulted in ductile failure; for Si20 and Si25 alloys, it caused brittle failure. The Si20 alloy had the highest strength at 1520 MPa, while the Si25 alloy had the lowest strength at 810 MPa. Dynamic recrystallization (DRX) occurs at 800 °C in all alloys with a peak in flow stress followed by a gradual decrease to a steady state stress. Stress-strain curves at 800 °C were analyzed using DRX kinetics, and the Avrami exponent n was between 1 and 2 for all alloys.

Effects of recrystallization and element diffusion behavior on interfacial bonding quality and mechanical properties of aluminum laminated composites

For aluminum laminated composites (ALCs), interlayer bonding strength plays a crucial role in its performance. In this work, 7050/Al/7A52 and 7A62/Al/7A52 ALC were studied to explore the effects of recrystallization and element diffusion on interfacial bonding quality and mechanical properties. The results show that attributed to the deformation localization occurs at the 7A52/pure Al interface, continuous dynamic recrystallization (CDRX), discontinuous dynamic recrystallization (DDRX) and geometric dynamic recrystallization (GDRX) coexist, while only DDRX exists at the bonding interfaces of 7050/pure Al and 7A62/pure Al. Among them, 7A62/pure Al interface has the most obvious element diffusion behavior among the three kinds of interfaces. Notably, recrystallization would have a higher effect on improving bonding quality, which causes the 7A52/pure Al interface of the composites more difficult to delaminate during the tensile test. Besides, the enhancement of element diffusion depth at the 7A62/pure Al interface promotes the bonding strength of 7A62/Al/7A52 ALC to increase by 21.0% compared with that of 7050/Al/7A52 ALC. After the dynamic impact test, the peak flow stress and energy absorption value of 7A62/Al/7A52 ALC reached 567.1 MPa and 72.2 MJ/m3, and the interface remains intact. However, due to the existence of huge strength deviation in the 7A62/pure Al interface, it is more prone to fail during the tensile process, resulting in a decrease in the elongation of the composites.

Study on Deformation Behavior and Mechanical Properties of 42CrMo High-Strength Steel with Multi-Station Warm Upsetting

In order to find out the deformation behavior and mechanical properties of 42CrMo steel under warm upsetting conditions, the Gleeble-3500 thermal simulation testing machine was used to carry out a warm upsetting physical simulation experiment on 42CrMo steel. By controlling deformation temperature, strain rate, and constant temperature deformation pass, the microstructure evolution rule under different warm upsetting conditions was analyzed, and its hardness value was measured. Then, the simulation experiment is carried out based on the Deform-3D finite element platform. The results show that, with the increase in deformation temperature, 42CrMo steel has a temperature rise softening effect, which significantly reduces the peak value of rheological stress. At 650 °C, the maximum peak value of rheological stress is only 45.3% of that of cold upsetting deformation at room temperature, and the stress-strain curve tends to be gentle at the plastic deformation stage, which is the most suitable temperature for warm upsetting deformation. The maximum peak flow stress of 42CrMo steel increases with the increase in strain rate, but the number of deformation channels has little influence on the stress-strain curve. The warm, upsetting deformation can refine the internal grain structure significantly, and the grain refinement mechanism is mechanical crushing. When the temperature is slightly higher, the broken grain will recover, and the grain size will grow. During the process of warm upsetting, the strain rate has a great influence on the microhardness of the sample. The deformation pass has little influence on the hardness, and the hardness increases slightly with the increase in the deformation pass. Through the Deform-3D simulation, the correlation coefficient R and the average absolute relative error (AARE) between the simulation value and the experimental value were calculated, and the correlation coefficient R-value was 0.9948, and the average absolute relative error (AARE) was 2.05%, indicating that the simulation can accurately reflect the relationship between displacement and applied load.

Microstructure, deformation behavior, and dynamic recrystallization kinetics of FeNiMnCu-based high entropy alloys prepared by mechanical alloying and spark plasma sintering

In the present research, FeNiMnCuX (X: Co, Cr, Mo, Ti, W) high entropy alloys (HEAs) were successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). SPS transformed the base and Co-containing alloys into FCC structures, Cr- containing alloys changed into a mix of FCC and BCC structures, while other alloys formed a combination of FCC/BCC structure and an intermetallic compound. The base alloy and alloys with Co, and Cr additions exhibited plastic deformation in cold compression tests, whereas other alloys exhibited brittle behavior. During hot compression tests, dynamic recrystallization occurred with a peak in flow stress followed by a gradual decrease to a steady state stress in alloys with Co, Mo, Ti and W additions. The highest peak stresses were observed at 800 °C for Ti- and W- containing alloys. The dynamic recrystallization and fractional softening kinetics are explained by the Avrami kinetic model. An Avrami exponent around 1 was obtained based on the kinetics of dynamic recrystallization for the alloys doped with Co, Mo, Ti, and W.

High-temperature compression deformation behavior of the high Nb-containing TiAl alloys doped with Sn

In this work, the high-temperature compression behavior of high Nb-containing TiAl–Sn alloys was investigated at deformed temperatures of 700 °C–1000 °C and strain rates of 0.002 s−1–0.2 s−1. The true stress-strain curves, microstructures, and constitutive equations of TiAl-xSn (x = 0, 0.5, 1, 1.5) alloys under different deformation conditions were systematically compared. Sn addition can increase the flow stress of TiAl alloys at the same deformation temperature, which is caused by the solid solution strengthening effect. At 800 °C/0.002 s−1, the peak flow stress of TiAl–1Sn alloy increases by 180 MPa compared with TiAl–0Sn alloy of 600 MPa. Below the peak stress, the flow stress increases with the increase of strain. Above the peak stress, the flow stress first decreases and then gradually trends to a steady state as the increase of deformation. Furthermore, the Sn addition decreases the brittle-ductile transition temperature of TiAl alloys. The TiAl-xSn (x = 0.5, 1, 1.5) alloys displays obvious flow softening behavior at 700 °C while TiAl–0Sn alloy fractures before reaching the target deformation. The relationship between of the flow stress, strain rate, and deformation temperature for TiAl–0Sn and TiAl–1Sn alloys is suitable for the hyperbolic sinusoidal equation. The dominant softening mechanism of hot deformation for TiAl-xSn (x = 0, 1) alloys is dynamic recrystallization (DRX). The activation energies Q of hot deformation for TiAl–0Sn and TiAl–1Sn alloys are 311 kJ mol−1 and 429 kJ mol−1 at condition of 700–1000 °C/0.002–0.2 s−1, indicating that the TiAl–1Sn alloy exhibits a better stability under the same application condition.

Differences in mechanical properties and high temperature deformation behavior between TiBw/Ti65 and TiBw/TA15 composites with network structure

In this paper, the differences in mechanical properties and high temperature (HT) deformation behavior between TiBw/Ti65 and TiBw/TA15 composites with network structure were studied. The alloy element distribution and microstructure of TiBw/Ti65 and TiBw/TA15 composites were characterized by the electron probe micro analyzer and transmission electron microscopy. The microstructure evolution during HT deformation was analyzed by electron back scatter diffraction. The mechanical properties of two composites were tested at room temperature and 600–750 °C. Arrhenius and artificial neural network models were developed to predict the peak stress and flow stress, respectively. The results show that the multiple alloying elements and silicide in TiBw/Ti65 refined the grains. TiBw/Ti65 composites had higher deformation activation energy and higher strength under the same deformation conditions. Based on the analysis of differences in microstructure, the strengthening mechanisms of TiBw/Ti65 compared to TiBw/TA15 was mainly from fine grain strengthening, dislocation density strengthening, and Orowan strengthening of silicide. The initial recrystallization temperatures for TiBw/TA15 and TiBw/Ti65 were 700 °C and 750 °C with strain rate of 0.001 s−1, respectively. The above results show that TiBw/Ti65 had better HT deformation resistance.

Hot compression behavior and recrystallization mechanism of 1.5 wt% Ti3AlC2 ceramic-enhanced Mo alloys with two-dimensional layers

Improving the high temperature strength of Mo alloys is a key problem to expand the application range of Mo alloys. In this study, the Mo-1.5 wt%Ti3AlC2 alloy was designed by introducing new two-dimensional (2D)-layered ceramic particles Ti3AlC2 into Mo. The peak flow stress of the Mo-1.5 wt%Ti3AlC2 alloy reaches 308 MPa at 1200 °C. The diffusion activation energy Q of the Mo-1.5 wt%Ti3AlC2 alloy is as high as 533.38 kJ/mol, which is higher than that of pure Mo (480 kJ/mol). Sintering in the reducing atmosphere of hydrogen at high temperature, the Ti3AlC2 consumes a large amount of energy to form AlTi3 and Ti5O9, which leads to an increase in the diffusion activation energy. We have established the constitutive equation of the Mo-1.5 wt%Ti3AlC2 alloy, and studied the microstructure evolution and deformation mechanism under hot compression. The results show that the stress index of n1 > 3 indicates that the plastic deformation mechanism was a dislocation mechanism. With the addition of Ti3AlC2, a large number of dislocations were produced in the alloy at the early stage of hot deformation. With increasing temperature, the dislocation entanglement gradually cancels out, resulting in the decrease of dislocation density. With the increase of temperature and deformation rate, the dynamic recrystallization (DRX) mechanism of the Mo-1.5 wt%Ti3AlC2 alloy was mainly continuous dynamic recrystallization (CDRX) at 1200 °C/0.01 s−1 and discontinuous dynamic recrystallization (DDRX) at 1600 °C/10 s−1. This work provides a new research direction for designing high performance Mo alloys.

Advancements in processing of Ni-based superalloys by microstructure engineering via discontinuous γ′ break-down

Wrought γ′-strengthened Ni-based superalloys are critical materials for gas turbine disks in aircraft and power-generation applications. However, their high inherent strength makes their hot formability challenging, and this is often related to intergranular cracking. We propose that this type of crack formation may be mitigated by grain boundary serrations which has not yet been demonstrated in superalloys. Here, we introduce engineered grain boundary serrations in a Ni-based superalloy via discontinuous γ′ precipitation during slow cooling after super-solvus solutionising. We compare the formability of this microstructure to a counterpart with globular γ′ and straight grain boundaries, during sub-solvus compression at 950–1130 °C. We demonstrate that the microstructure with discontinuous γ′ shows excellent cracking resistance, while the one with globular γ′ develops severe intergranular cracks during compression ≤1100 °C. The discontinuous γ′ microstructure exhibits up to 26% lower peak flow stresses and recrystallises at temperatures ≤1100 °C, compared to the globular γ′ microstructure where recrystallisation does not start until 1130 °C. Compression of the discontinuous γ′ microstructure at 1000–1050 °C yields recrystallised fractions of >75 vol.% and results in a duplex γ-γ′ microstructure with grain diameters <4 μm. We argue that its significantly improved formability results from these engineered serrated grain boundaries, the coarsened γ′ morphology, energy absorption by the break-down of discontinuous γ′, and the resulting development of a fine-grained duplex γ-γ′ microstructure. Integrating such discontinuous γ′ precipitation into advanced manufacturing routes will facilitate or, in some cases, even enable manufacturing of parts for aircraft turbines and gas engines at lower temperatures and forces.

Influence of B4C Particle Size on the Microstructure and Mechanical Properties of B4C/Al Composites Fabricated by Pressureless Infiltration

To investigate the effect of B4C particle size on the microstructure and mechanical properties of B4C/Al composites, and to provide theoretical guidance for the subsequent thermal processing of composites, B4C/Al composites with varying B4C particle sizes (0.2 µm, 0.5 µm, 1 µm, 10 µm) were fabricated using pressureless infiltration. The microstructure of the composites was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the mechanical properties were analyzed by hardness test, three-point bending and high temperature compression. The results indicated that Al3BC and AlB2 were the primary interfacial reaction products in B4C/Al composites, and interface reaction could be alleviated with increasing particle size. B4C/Al composites with larger B4C particle sizes exhibited a relatively uniform and discrete distribution of B4C, while those with smaller B4C particle sizes showed agglomeration of B4C. The Vickers hardness and peak flow stress of B4C/Al composites gradually decreased with the increase of B4C particle size, while the bending strength, flexural modulus, and fracture toughness tended to increase. In addition, when B4C particle size was 10 µm, the composites displayed optimal comprehensive performance with the lowest peak flow stress (150 MPa) and the highest fracture toughness (12.75 MPa·m1/2).

Constitutive modeling for elevated temperature flow behavior of a novel Cr-Si alloyed hot stamping steel

The flow behavior of a novel Cr-Si alloyed hot stamping steel (Cr-Si steel) at elevated temperature was investigated via isothermal compression tests on a Gleeble-3500 thermomechanical simulator with a temperature range of 600 ∼ 900 °C and a strain rate range of 0.1 ∼ 10 s−1. Subsequently, the Arrhenius-type constitutive model, comprising strain compensation, was established in accordance with the friction and adiabatic heating corrected stress-strain curves. Furthermore, the predictability and prediction accuracy of the constitutive model were verified. The results reveal that at a constant strain rate, the flow stress of the Cr-Si steel initially increases as the strain increases before tending to stabilize, owning to the combined effects of work hardening and dynamic recovery. The peak flow stresses decrease as the temperatures increase and the strain rates decrease. The constitutive model can accurately predict the elevated temperature constitutive relationship of the Cr-Si steel during the hot stamping process.

Effects of process parameters and loading direction on the impact strength of additively manufactured 18%Ni-M350 maraging steel under dynamic impact loading

The microstructural and damage evolution of 18%Ni M350 maraging steel fabricated by laser-based directed energy deposition (DED) process under dynamic impact loading are investigated in this study. The influence of additive manufacturing (AM) processing parameters, including laser power, powder feed rate, and energy area density (EAD) are discussed. The mechanical testing of the additively manufactured alloy was done using the split Hopkinson pressure bar (SHPB) system. The dynamic impact test was conducted using strain rates ranging from 100 to 4000 s−1. The test was conducted on cylindrical specimens (7 mm diameter and 8 mm long) with axes parallel to the build direction (Z-axis) and perpendicular (X- and Y-axis) to the build direction to evaluate the effects of loading directions on mechanical responses and microstructural evolution in the alloy under the impact loading conditions. The test results revealed that the materials processed using the lowest EAD parameters exhibit the greatest impact strength under dynamic impact loading for all directions. Specimens impacted along the build direction (Z-axis) exhibit a continuous drop in peak flow stress once the strain rates exceeded 2300 s−1 indicating the greatest susceptibility to thermal softening at high strain rates. Strain localization leading to the formation of adiabatic shear bands (ASB) was observed in some specimens. Cracks developed along the ASBs, while some AM-process-related defects, such as the lack-of-fusion, interacted with adiabatic shear bands, promoting cracking inside the shear bands. Despite possessing the highest impact strength, the materials produced by the lowest EAD parameters are the most susceptible to the development of adiabatic shear bands leading to cracking and fracture.

On the Hot Deformation of a Fe-Al-Ta Iron Aluminide Prepared via Laser Powder Bed Fusion

In the present work, a combined process of laser powder bed fusion (LPBF) and hot working in terms of microstructure refinement was investigated for Fe-25Al-1.5Ta alloy samples. Uniaxial compression tests were carried out parallel and perpendicular to the building direction (BD) at 1000 °C, where BCC A2-phase was stable, at a strain rate of 0.0013 s−1. The true stress–true strain curves indicated a broad flow stress peak followed by a slight decrease, which is typical for dynamic recrystallization (DRX) of conventional BCC metals such as ferritic iron. A negligible dependence in the flow stress behavior on the compression direction was observed. DRX initiated at a stress of 18.7 MPa for the sample compressed parallel to the BD, corresponding to a true strain of 0.011, and at 18.1 MPa for the samples compressed normal to the BD, which corresponded to a true strain of 0.010. The microstructural investigations by electron backscatter diffraction (EBSD) showed that the relatively coarse and elongated grains of the as-LPBF builds were significantly refined after hot working. The microstructure of the compressed samples mainly consisted deformed grains. These were fragmented by sub-grains bounded by low-angle boundaries independent of the compression axis, indicating the occurrence of dynamic recovery (DRV) during hot working. In addition, a few equiaxed, small grains were observed in the pre-existing grain boundaries, which formed due to DRX. Most pores in the as-LPBF builds were closed after hot compression, particularly in the central region of the deformed specimens where the compressive stress state is dominant. In summary, hot compression reveals a practical thermomechanical post-processing treatment for Fe-Al-Ta iron aluminides built by LPBF. The hot working refines the epitaxially elongated microstructure of the as-LPBF builds by DRV/DRX and reduces the porosity.

Effects of Melt Hydrogenation on the Microstructure Evolution and Hot Deformation Behavior of TiBw/Ti-6Al-4V Composites

In this study, Ti-6Al-4V matrix composites reinforced with TiB ceramic whiskers were in situ synthesized and hydrogenated using the melt hydrogenation technique (MHT). The effects of MHT on the microstructure evolution and hot compression behavior of the composites were investigated by optical microscopy (OM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Hot compression tests were performed at strain rates of 0.1/s, 0.01/s, and 0.001/s and temperatures of 800 °C, 850 °C, and 900 °C; the hot workability of composites significantly improved after hydrogenation, for example, the 900 °C peak flow stress of hydrogenated composites (43 MPa) decreased by 53.76% compared with that of unhydrogenated ones (93 MPa) at a strain rate of 0.01/s. Microstructural observations show that MHT can effectively facilitate the dispersion of TiB whiskers and induce the α/β lath refinement of the matrix in our as-cast hydrogenated composite. During hot compression, MHT effectively promoted the as-cast composite microstructure refinement, accelerated the dynamic recrystallization (DRX) generation, and reduced the stress concentration at the interface between the reinforcement and matrix; in turn, the hydrogenated composites presented low peak stress during hot compression.

On the Uniaxial Compression Testing of Metallic Alloys at High Strain Rates: An Assessment of DEFORM-3D Simulation

In this study, the challenges associated with conducting high-strain rate uniaxial compression testing experiments are highlighted. To address these challenges, DEFORM-3D simulation was explored as an alternative approach to experimental testing. Previously established constitutive constants obtained from experimental low strain rate uniaxial compression testing of three titanium (α + β) alloys were used as input codes. From the results, the peak flow stress values obtained from the DEFORM-3D simulation were close to the values obtained experimentally at low (0.1 to 10/s) and high (20 and 50/s) strain rates. For the alloys considered in this study, a discrepancy of ~20% in the peak flow stress was obtained at a 10/s strain rate. The difference in peak flow stress for strain rates less than 10/s or higher (20 and 50/s) is within acceptable limits. The limitations of using DEFORM-3D simulations for high strain rate uniaxial compression testing are highlighted.

Substructure evolution in two phases based constitutive model for hot deformation of TC18 in α + β phase region

The α + β dual phase titanium alloys are key structural materials in aviation and aerospace industries, and the complicated flow behavior of these titanium alloys during hot deformation requires to establish a constitutive model incorporating physical mechanism for optimizing processing parameters and designing forming tools. This work aims to establish a constitutive model incorporating physical mechanism for hot deformation of TC18 in α + β phase region. Firstly, the flow behavior and microstructure evolution for hot deformation of TC18 in α + β phase region are characterized. The TC18 shows significant strain hardening rate and negative strain hardening exponent around and after peak flow stress, respectively. After peak flow stress, Dynamic Recovery (DRV) mechanism dominates the evolution of α and β phases according to the results of substructure evolution. Then, the internal state variables method is applied to establish a constitutive model incorporating physical mechanism for hot deformation of dual phase titanium alloys. The variation of dislocation density during the hot deformation of titanium alloys is modeled by considering the accumulation of dislocation due to the impediment to dislocation movement by substructure obstacles and the annihilation of dislocation due to the dynamic restoration effect. The interaction between dislocations, the subgrain boundaries and the grain/phase boundaries obstruct the dislocation movement in the α phase, and the first two obstructs the dislocation movement in the β phase during the hot deformation of TC18. The dislocation annihilation process in the α and β phases during the hot deformation of TC18 is dominated by DRV. Finally, the substructure evolution in the two phases based constitutive model for hot deformation of TC18 in α + β phase region is presented. This model is well applied to predict the flow stress and quantitively analyze the role of DRV effect in the evolution of α and β phases during the hot deformation of TC18.

Flow Behavior and Microstructure of Hot-Worked Fe-30.9Mn-4.9Al-4.5Cr-0.4C and Fe-21.3Mn-7.6Al-4.3Cr-1C Low-Density Stainless Steels

Two as-cast low-density steels grades (austenite-based duplex Fe-30.9Mn-4.9Al-4.5Cr-0.4C and austenitic Fe-21.3Mn-7.6Al-4.3Cr-1C) with an initial dendritic microstructure were subjected to hot working conditions to understand the influence of deformation parameters on flow behavior and microstructural evolution. The alloys were produced using electric arc melting, and their phase constituents were determined using optical microscopy and X-ray diffraction analysis. This was then corroborated with the phase predicted from Thermo-Calc simulation. The as-cast alloys were machined to 10 × 10 × 7 mm specimen configurations for rectangular axial testing on the Gleeble 3500 thermomechanical simulator. The samples were deformed to a total strain of 0.5 at different deformation temperatures (800, 900, and 1000 °C) and strain rates (0.1 and 5 s−1). Thereafter, a hardness test was conducted on the deformed samples, and post-deformed microstructures were analyzed using optical and scanning electron microscopes. The results showed that the alloys’ dendritic structures were effectively transformed at temperatures below 1000 °C regardless of the strain rate. At all deformation conditions, the peak flow stress of Fe-21.3Mn-7.6Al-4.3Cr-1C alloy was at least 50% higher than that of Fe-30.9Mn-4.9Al-4.5Cr-0.4C alloy owing to the higher carbon content in the austenitic low-density stainless steel. The hardness of all the deformed samples was superior to that of the as-cast samples, which indicates microstructural reconstitution and grain refinement in the alloys. Dynamic recrystallization, dynamic globularization, and dynamic recovery influenced the softening process and the microstructural changes observed in the alloys under different deformation conditions.