High-temperature Deformation Process Research Articles

Characterization of high-temperature deformation behavior and processing map of TB17 titanium alloy

Isothermal compression testing of a new high-strength TB17 titanium alloy was investigated on a Gleeble 3800 thermo-mechanical simulator over a temperature range of 775 °C–905 °C and a strain rate range of 0.001 s−1 to 10 s−1 with height reductions of 25%, 50% and 70%. Optical microscopy (OM), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) were employed to investigate the effect of deformation parameters on the microstructure and the deformation mechanism. The hot deformation behavior of TB17 titanium alloy is characterized by the analysis of the strain-stress curve, the strain rate sensitivity exponent and processing maps. The results show that the flow stress decreases with the increase of the deformation temperature and decrease of the strain rate. The strain rate sensitivity exponent m remains constant in α+β phase field deformation and increases sharply in β phase field. The TB17 titanium alloy has a narrow deformation field and the optimized deformation parameters is found at the temperature of 885 °C and the strain rate of 0.001 s−1 at height reduction of 70%. The microstructure in the α+β phase field deformation is characteristic of refined DRX α phase. While in the β phase, the morphology is characteristic of refined β grains. The increase of strain supported the occurrence of DRX at lower strain rates. The morphology is characterized by flow localization in instability domain.

Grain boundary sliding associated with low strain rate at 1000 °C in recrystallized ODS ferritic steel

The high-temperature deformation process of the recrystallized 16CrODS ferritic steel was investigated at 1000°C for the stress loading perpendicular to the elongated grain structure. The strain rate was varied in the range from 1.0×10−2 to 1.0×10−5s−1. At the strain rate over 1.0×10−4s−1, deformation is dominated by the conventional dislocation creep. Decreasing strain rate from 1.0×10−4s−1, grain boundary sliding becomes prominent. Accommodation process for the localized stress induced by grain boundary sliding could be dislocation creep at 1.0×10−4s−1, and by diffusional creep at 1.0×10−5s−1 or less. These were verified through the observation of void formation and localized strain accumulation by KAM map.

Effect of extension [formula omitted] twins on texture evolution at elevated temperature deformation accompanied by dynamic recrystallization

High temperature deformation processing of magnesium and its alloys is often accompanied by dynamic recrystallization (DRX). Deformation twinning is one of the main deformation mechanisms in HCP metals, but very few works are available in literature (experimental or modelling) which investigate the effect of the deformation twinning on dynamic recrystallization. The current study provides insights regarding the deformation mechanisms of magnesium alloy during compression test at elevated temperatures. The role of the extension twins in DRX is examined using a previously developed dynamic recrystallization model based on the crystal plasticity finite element approach. In order to consider extension twins, an EBSD IPF map with reoriented twinning elements is used as input. An entire element is reoriented into twin orientation and DRX proceeds within the new microstructure. This way, twins can serve as a nucleation site because they provide high angle grain boundaries. The results of compression simulations for extruded AZ31 Mg alloy are presented and validated with the experimental data available in the literature. The results show twinning when present affects the resultant texture as most of the new nucleating grains are clos eto the twin matrix interface and have twin orientation.

Effect of titanium on the rheological properties of MoSi2-based materials prepared by SHS

We have studied the moldability of molybdenum disilicide-based materials using free self-propagating high-temperature synthesis (SHS) compression, a method that combines combustion and high-temperature deformation processes. As a moldability criterion, we used strain. The strain of the synthesized materials has been determined experimentally as a function of the time delay before high-temperature deformation, compaction pressure, heating temperature, and metallic binder content of the starting mixture. To improve the plasticity of the synthesized material, 1–2 wt % titanium was added to the starting mixture as a metallic binder.

Optimization of thermal processing parameters of Ti555211 alloy using processing maps based on Murty criterion

Isothermal compression testing of Ti555211 titanium alloys was carried out at deformation temperatures from 750 to 950 °C in 50 °C intervals with a strain rate of 0.001–1.000 s−1. The high-temperature deformation behavior of the Ti555211 alloy was characterized by analysis of stress–strain behavior, kinetics and processing maps. A constitutive equation was formulated to describe the flow stress as a function of deformation temperature and strain rate, and the calculated apparent activation energies are found to be 454.50 and 207.52 kJ·mol−1 in the α + β-phase and β-phase regions, respectively. A processing map based on the Murty instability criterion was developed at a strain of 0.7. The maps exhibit two domains of peak efficiency from 750 to 950 °C. A ~60 % peak efficiency occurs at 800–850 °C/0.001–0.010 s−1. The other peak efficiency of ~60 % occurs at ≥950 °C/0.001–0.010 s−1, which can be considered to be the optimum condition for high-temperature working of this alloy. However, at strain rates of higher than 1.000 s−1 and deformation temperatures of 750 and 950 °C, clear process flow lines and bands of flow localization occur in the high-temperature deformation process, which should be avoided in Ti555211 alloy hot processing. The mechanism in stability domain and instability domain was also discussed.

Dynamic precipitation of nickel-based superalloys undergoing severe deformation below the solvus temperature

Abstract The authors performed uniaxial compression tests of nickel-based superalloys: single crystal CMSX–4, also precipitation hardened; Inconel 718 and X750, at temperatures below the γ′ solvus, in order to study the effect of temperature and strain rate on their flow stress and microstructural development. On the basis of the obtained flow stress values, the activation energy of a high-temperature deformation process was estimated. Microstructural observations of the deformed samples at high temperatures, previously solution heat treated and aged CMSX–4 and Inconel alloys revealed non-uniform deformation effects. Distribution of either molybdenum- or niobium-rich carbides was found to be affected by localized flow within the investigated strain range at relatively low deformation temperatures, 720–850 °C. Microstructural examination of the alloys also showed that shear banding and cavity growth were responsible for the decrease in flow stress and a specimen fracture at larger strains.

Structural effects of high-temperature plastic deformation process on martensite plate morphology in a Fe-Mn-Si-Cr SMA

An Fe–Mn–Si–Cr shape memory alloy (SMA) was induction melted, in a cold crucible furnace, cast into cylindrical moulds and swaged by means of two hot working processes. From repeatedly hot forged and hot rolled billets, different specimens were prepared with longitudinal and transversal orientations with respect to main plastic deformation direction. After corresponding metallographic preparation, structural analyses comprising optical (OM), scanning electron (SEM) and atomic force (AFM) microscopy observations were performed with the aim to emphasise the structural effects, of the two above mentioned plastic deformation processes on martensite plate morphology, size and distribution. The paper reports the results recorded by OM, SEM and AFM and corroborates martensite plate surface relief with high–temperature plastic deformation process. The results show that martensite plates were shorter and deeper in hot forged specimens as compared to hot rolled ones, an effect which is atypical for Fe–Mn–Si–Cr SMAs.

Mechanical Aspects of Plastic Deformation of Nickel Based Superalloy

Variations of a true stress vs. true strain illustrate behaviour of materials during plastic deformation. Stress-strain relationship is generally evaluated by a torsion, compression and tensile tests. Results of these tests provide crucial information pertaining to the stress values which are necessary to run deformation process at specified temperature and cooling rate. Uniaxial compression tests at temperatures below the γ solvus were conducted on nickel based superalloy CMSX-4, to study the effect of temperature and strain rate on its flow stress. On the basis of received flow stress values activation energy of a high-temperature deformation process was estimated. Mathematical dependences (σpl -T i σpl - ἐ) and compression data were used to determine material constants. These constants allow to derive a formula that describes the relationship between strain rate, deformation temperature and true stress.

Hydrogen induced softening mechanism in near alpha titanium alloy

Hydrogen induced softening mechanism in near alpha titanium alloy Ti600 (Ti-6.0Al-2.5Sn-4.0Zr-0.5Mo-0.4Si-0.1Y) is studied in this paper. The results show that, the hydrogenated alloy has a lower flow stress than the unhydrogenated alloy at the same temperature, and 0.3wt.% is the optimum hydrogen content for Ti600 deformed at 850°C. Compared with unhydrogenated Ti600 deformed at 950°C, deformation resistance decreases by one third. A new method based on thermogravimetry analysis for studying dehydrogenation process of titanium alloys is proposed. The results demonstrate that with the addition of 0.3wt.% hydrogen the phase transformation temperature decreases by around 100°C. The reasons of hydrogen-induced softening are studied by XRD and TEM methods, which are attributed to the effects of hydrogen on the phase transformation, dislocation mobility, dynamic recovery and recrystallization during high temperature deformation process.

Investigation of high temperature deformation process of aluminium based alloys using response surface methodology

The influence of flow properties of aluminium alloys (Al – 5Si, Al –4Mg,RR58) obtained from aluminium powder is discussed. The constitutive equations relating flow-stress, temperature, density, strain, aspect ratio have been analysed in order to establish structure – property correlations. These response variables have generally been investigated by means of hot compression tests with the help of statistical analysis. In statistical techniques, response surface methodology has been used for this investigation. It is a collection of mathematical and statistical techniques that is useful for the analysis of problems in which the response is influenced by several variables. A mathematical model has been developed to compute the flow stress which combines the above mentioned process parameters. The main and interaction effects of the process parameters have also been studied using ANOVA techniques. The proposed mathematical model has been validated by the experimental results available from earlier investigations. The proposed model can be applied to predict the magnitude of flow stress during metal working of aluminium alloys. The results show that flow stress is more sensitive to strain and temperature than to other parameters.

High-temperature deformation behavior and processing map of 7050 aluminum alloy re]20101008

The high-temperature deformation behavior and processing map of 7050 aluminum alloy were investigated by tensile tests conducted at various temperatures (340, 380, 420, and 460 °C) with various strain rates of 10−4, 10−3, 10−2, and 0.1 s−1. The results show that the instability region with a peak power dissipation efficiency of 100 % occurs at the low deformation temperature region of 340 °C to 380 °C and high strain rates (>10−3 s−1). The 7050 aluminum alloy exhibited a continuous dynamic recrystallization domain with power dissipation efficiency of 35% to 60 % in the deformation temperature range of 410 °C to 460 °C and the strain rate range of 10−4–10−3 s−1. The domain with a power dissipation efficiency of 35 % to 50 % occurring at high deformation temperatures and strain rates was interpreted to represent dynamic recovery. Dynamic recovery and continuous dynamic recrystallization provide chosen domains for excellent hot workability.

Investigation of high temperature deformation process of aluminium based alloys using response surface methodology

The influence of flow properties of aluminium alloys (Al – 5Si, Al –4Mg,RR58) obtained from aluminium powder is discussed. The constitutive equations relating flow-stress, temperature, density, strain, aspect ratio have been analysed in order to establish structure – property correlations. These response variables have generally been investigated by means of hot compression tests with the help of statistical analysis. In statistical techniques, response surface methodology has been used for this investigation. It is a collection of mathematical and statistical techniques that is useful for the analysis of problems in which the response is influenced by several variables. A mathematical model has been developed to compute the flow stress which combines the above mentioned process parameters. The main and interaction effects of the process parameters have also been studied using ANOVA techniques. The proposed mathematical model has been validated by the experimental results available from earlier investigations. The proposed model can be applied to predict the magnitude of flow stress during metal working of aluminium alloys. The results show that flow stress is more sensitive to strain and temperature than to other parameters.

High-temperature deformation processing maps for a NiTiCu shape memory alloy

Abstract

High Temperature Deformation Mechanical Model and Processing Map of Ti-6Al-2Zr-1Mo-1V Alloy

Based on the high temperature compression simulation experiments, the mechanical behavior of Ti-6AI-2Zr-1Mo-1V alloy were studied over the range of temperature from 850°C to 1100°C , strain rate from 0.01 to 10s-1. The results show that the flow stress reduces with temperature increasing at the constant strain rate and increases rapidly with strain rate increasing at the constant temperature. The stress index n and deformation activation energy Q is respectively 7.0874 and 610.463 kJ/mol at 850-950°C . While at 950-1100°C , n is 4.7324 and Q is 238.030 kJ/mol. From the obtained processing map, it is found that two unstable regions present at the lower temperature or higher strain rate and two optimum regions in hot deformation process. The unstable zones are 850-950°C , 0.001-0.008s-1of strain rate, and 940-1030°C , 2-10s-1of strain rate respectively. In the optimum zone with the temperature range of 1060-1100°C , strain rate of 0.05-0.65s-1, and the peak efficiency of power dissipation of about 0.42; while in the other zone with the deformation temperature 890-940°C , strain rate of 0.06-0.18s-1, and the peak efficiency of power dissipation of about 0.33.

Effect of Nb on Austenite Recrystallization in High Temperature Deformation Process

By analyzing the stepped test pieces with optical microscope, the influence of the technological parameters including rolling temperature and reduction on the austenite recrystallization fraction of steel with Nb content of 0.08% deformed at high temperature processing was studied, and the deformed austenite recrystallization figures of the test steel were obtained. The result indicates that when the reduction is 60%, the critical temperature of complete recrystallization is 1070 °C and the borderline temperature of non-recrystallization is 900 °C. In contrast with general HSLA steel, the recrystallization and non-recrystallization temperatures are respectively 120 °C and 100 °C higher than those of ordinary HSLA, so the steel with high content of Nb has better hot processing performance because Nb has great function of holding back recrystallization.

High temperature deformation processing maps for boron modified Ti–6Al–4V alloys

The alloy, Ti–6Al–4V is an α + β Ti alloy that has large prior β grain size (∼2 mm) in the as cast state. Minor addition of B (about 0.1 wt.%) to it refines the grain size significantly as well as produces in-situ TiB needles. The role played by these microstructural modifications on high temperature deformation processing maps of B-modified Ti64 alloys is examined in this paper. Power dissipation efficiency and instability maps have been generated within the temperature range of 750–1000 °C and strain rate range of 10 −3–10 +1 s −1. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate four distinct deformation domains within the range of experimental conditions examined, with the combination of 900–1000 °C and 10 −3–10 −2 s −1 being the optimum for hot working. In that zone, dynamic globularization of α laths is the principle deformation mechanism. The marked reduction in the prior β grain size, achieved with the addition of B, does not appear to alter this domain markedly. The other domains, with negative values of instability parameter, show undesirable microstructural features such as extensive kinking/bending of α laths and breaking of β laths for Ti64–0.0B as well as generation of voids and cracks in the matrix and TiB needles in the B-modified alloys.

Composition and structure of nitrogen-containing dispersoids in trimodal aluminum metal–matrix composites

Trimodal aluminum (Al) metal–matrix-composites (MMCs), consisting of B4C particulates, a nanocrystalline Al (NC-Al) phase, and a coarse-grain Al phase (CG-Al), has been fabricated. These MMCs exhibits extremely high compressive strength and tailorable ductility. Excellent thermal stability of NC-Al grains and high strength has been attributed partially to the nitrogen present within the trimodal Al MMCs, which is introduced during the cryomilling process in liquid nitrogen. This paper describes an investigation into the concentration and constituents of nitrogen within the trimodal Al MMCs. The structure of nitrogen-containing dispersoids was examined by analytical transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) was employed to determine the total concentration of nitrogen. The nitrogen concentration increased linearly with an increase in cryomilling time up to 24 h. Both crystalline and amorphous aluminum nitrides with very fine size, down to 5 nm, as dispersoids, have been observed by analytical TEM. Correlations between the cryomilling time, nitrogen concentration, NC-Al grain size, and composite hardness are presented and discussed. The presence of nitrogen as nitride-dispersoids can contribute to the outstanding mechanical properties of trimodal Al MMCs by inhibiting NC-Al grain growth during the high temperature consolidation and deformation process, and by dispersion-strengthening.

Creep behavior of hot extruded Al-Al 2O 3 nanocomposite powder

A commercial gas-atomized aluminum powder was mechanically milled in a planetary ball mill under an argon atmosphere for 12 h to produce alumina dispersion strengthened aluminum powder. Transmission electron microscopy (TEM) revealed that about 2 vol.% alumina particles with an average size of 100 nm were distributed in the aluminum matrix. The nanocomposite powder was canned in an aluminum container, vacuum de-gassed, and hot extruded at 723 K at an extrusion ratio of 16:1. The creep behavior of the extruded billet in the direction of extrusion was examined at a constant applied load ranging from 10 to 40 MPa at temperatures of 648, 673 and 723 K. A threshold creep-stress was noticed which indicates that detachment of dislocations from the Al 2O 3 nanoparticles occurred during the high-temperature deformation process. The threshold stress was found to be temperature dependent as it decreased from 8.3 MPa at 648 K to 2.7 MPa at 723 K. The stress exponent index also decreased from 8 to 3, revealing a change in the creep mechanism by increasing the testing temperature. The creep behavior is explained according to the invariant substructure model and dislocation glide process.

High Temperature Deformation Mechanisms and Processing Map for Hot Working of Cast-Homogenized Mg-3Sn-2Ca Alloy

The hot working behavior of Mg-3Sn-2Ca alloy has been investigated in the temperature range 300–500 oC and strain rate range 0.0003–10 s-1, with a view to evaluate the mechanisms and optimum parameters of hot working. For this purpose, a processing map has been developed on the basis of the flow stress data obtained from compression tests. The stress-strain curves exhibited steady state behavior at strain rates lower than 0.01 s-1 and at temperatures higher than 350 oC and flow softening occurred at higher strain rates. The processing map exhibited two dynamic recrystallization domains in the temperature and strain rate ranges: (1) 300–420 oC and 0.0003–0.003 s-1, and (2) 420–500 oC and 0.003–1.0 s-1, the latter one being useful for commercial hot working. Kinetic analysis yielded apparent activation energy values of 161 and 175 kJ/mole in domains (1) and (2) respectively. These values are higher than that for self-diffusion in magnesium suggesting that the large volume fraction of intermetallic particles CaMgSn present in the matrix generates considerable back stress. The processing map reveals a wide regime of flow instability which gets reduced with increase in temperature or decrease in strain rate.

Constitutive model for high temperature deformation of titanium alloys using internal state variables

The internal state variable approach nowadays is more and more used to describe the deformation behavior in all of the metallic materials. In this paper, firstly the dislocation density rate and the grain growth rate varying with the processing parameters (deformation temperature, strain rate and strain) are established using the dislocation density rate as an internal state variable. Secondly the flow stress model in high temperature deformation process is analyzed for each phase of titanium alloys, in which the flow stress contains a thermal stress and an athermal stress. A Kock–Mecking model is adopted to describe the thermally activated stress, and an athermal stress model is established using two-parameter internal state variables. Finally, a constitutive model coupling the grain size, volume fraction and dislocation density is developed based on the microstructure and crystal plasticity models. And, the material constants in present model may be identified by a genetic algorithm (GA)-based objective optimization technique. Applying the constitutive model to the isothermal compression of Ti–6Al–4V titanium alloy in the deformation temperature ranging from 1093 to1303 K and the strain rate ranging from 0.001 to 10.0 s −1, the 20 material constants in those models are identified with the help of experimental flow stress and grain size of prior α phase in the isothermal compression of Ti–6Al–4V titanium alloy. The relative difference between the predicted and experimental flow stress is 6.13%, and those of the sampled and the non-sampled grain size are 6.19% and 7.94%, respectively. It can be concluded that the constitutive model with high prediction precision can be used to describe the high temperature deformation behavior of titanium alloys.