Activation Energy For Plastic Flow Research Articles

Hot deformation behavior of NiTiHf alloy under compression: Effect of deformation heating on flow softening

High-temperature shape memory alloys are suitable materials for substituting hydraulic elements in the aerospace industry due to light weight and high reactive stresses developing during thermoelastic martensitic transformation and acceptable recoverable strain. These properties may be improved by plastic deformation; however, the plastic deformation of NiTiHf alloys along with structure investigation has yet to be studied systematically. Here, the hot deformation behavior of a NiTiHf alloy with hafnium content >20 at.% has been investigated, and the processing map has been elaborated according to the dynamic material model (DMM) based on the Prasad instability criterion. The deformation heating effect has been shown to affect the flow curves and accounts for profound softening during deformation due to quasi-adiabatic deformation mode at high strain rates. NiTiHf alloys have a much higher activation energy of plastic flow, and the instability region of plastic flow is much larger compared to binary NiTi alloys. Microstructural observations have revealed that in the instability regions, oxide bands and crack nucleation occur, whereas outside this region, such defects are not observed. Electron-back scatted diffraction has demonstrated that at high temperatures and slow strain rates, discontinuous dynamic recrystallization occurs resulting in the neckless type structure, while at high strain rates, this phenomenon has not been observed. Based on the obtained results, the preferable deformation conditions are established to be 850–1000 °C and 0.003–0.05 s−1.

Deformation Mechanisms and Processing Maps for High Entropy Alloys (Presentation of Processing Maps in Terms of Zener-Hollomon Parameter): Review.

In this review paper, the hot compressive deformation mechanisms and processing maps of high-entropy alloys (HEAs) with different chemical compositions and crystal structures are analyzed. The stress exponent (n1) values measured from the series of compression tests for the HEAs performed at different temperatures and strain rates are distributed between 3 and 35, and they are most populated between 3 and 7. Power law breakdown (PLB) is found to typically occur at T/Tm ≤ 0.6 (where T is the testing temperature and Tm is the melting temperature). In AlxCrMnFeCoNi (x = 0-1) and AlxCrFeCoNi (x = 0-1) HEAs, n1 tends to decrease as the concentration of Al increases, suggesting that Al acts as a solute atom that exerts a drag force on dislocation slip motion at high temperatures. The values of activation energy for plastic flow (Qc) for the HEAs are most populated in the range between 300 and 400 kJ/mol. These values are close to the activation energy of the tracer diffusivity of elements in the HEAs ranging between 240 and 408 kJ/mol. The power dissipation efficiency η of the HEAs is shown to follow a single equation, which is uniquely related to n1. Flow instability for the HEAs is shown to occur near n1 = 7, implying that the onset of flow instability occurs at the transition from power law creep to PLB. Processing maps for the HEAs are demonstrated to be represented by plotting η as a function of the Zener-Hollomon parameter (Z = expQcRT, where R is the gas constant). Flow stability prevails at Z ≤ 1012 s-1, while flow instability does at Z ≥ 3 × 1014 s-1.

The hot compressive deformation behavior of cast Mg-Gd-Y-Zn-Zr alloys with and without LPSO phase in their initial microstructures

Samples of Mg-8.2Gd-3.8Y-1.1Zn-0.4Zr alloy with and without an intragranular lamellae-shaped long period stacking ordered (LPSO) phase were prepared through heat treatment and a series of hot compression tests on these materials were conducted to examine and evaluate the influence of LPSO on the hot compressive deformation behavior and deformation mechanisms at a given alloy composition. The values of activation energy for plastic flow (Q c ) of the solution treated (without LPSO phase) and annealed alloys (with intragranular LPSO phase) were larger than that for pure Mg, indicating that the presence of a high amount of rare earth (RE) elements and LPSO in the Mg matrix significantly increases Q c . The Q c value of the annealed alloy was larger than that of the solution treated alloy at all the strain levels (223.3 vs. 195.5 kJ/mol in average) and the largest difference in Q c between the two alloys was recorded at the smallest strain of 0.1 where precipitation of LPSO during deformation was limited in the solution treated alloy. These observations imply that the formation of LPSO phase out of the RE-rich solid solution matrix during deformation increases Q c , but the increment is not so large. Analysis of the hot compressive data of the alloys with LPSO phase and the alloys with RE-rich solid solution matrix in literatures indicates the similarity of the effect of the LPSO and RE-rich solid solution matrix phases on Q c and high-temperature strength.

Hot Deformation Behaviour and Processing Map of Cast Alloy 825

Alloy 825 is a nickel-based alloy that is commonly used in applications where both high strength and corrosion resistance are required, such as tanks in the chemical, food and petrochemical industries and oil and gas pipelines. Components made from Alloy 825 are often manufactured using hot deformation. However, there is no systematic study to optimise the processing conditions reported in literature. In this study, a processing map for as-cast Alloy 825 is established to maximise the power dissipation efficiency of hot deformation in the temperature range of 950 to 1250 °C at an interval of 50 °C and strain rate range of 0.01, {text{s}}^{ - 1} to 10.0, {text{s}}^{ - 1} to a true strain of 0.7 using a Gleeble-3500 thermomechanical simulator. The processing conditions are also correlated to the Vickers hardness of the final material, which is also characterised using optical microscopy and scanning electron microscopy, including electron backscattered diffraction. The true stress-true strain curves exhibit peak stresses followed by softening due to occurrence of dynamic recrystallization. The activation energy for plastic flow in the temperature range tested is approximately 450,{text{ kJ mol}}^{ - 1}, and the value of the stress exponent in the (hyperbolic sine-based) constitutive equation, n = 5.0, suggests that the rate-limiting mechanism of deformation is dislocation climb. Increasing deformation temperature led to a lower Vickers hardness in the deformed material, due to increased dynamic recrystallization. Raising the strain rate led to an increase in Vickers hardness in the deformed material due to increased work hardening. The maximum power dissipation efficiency is over 35%, obtained for deformation in the temperature range 1100-1250 °C and a strain rate of 0.01, {text{s}}^{ - 1}-0.1, {text{s}}^{ - 1}. These are the optimum conditions for hot working.

On the activation energy for plastic flow during tension test at room temperature and the anisotropy behaviour of Ti-6Al-4V alloy

The movement of dislocations inside of crystals during the tension test at constant crosshead velocity is related to activation energy for plastic flow in spatially extended polycrystalline systems. In this work, the main objectives are to establish the influence of the slip systems for α hexagonal compact packaging (hcp) titanium phase on the activation energy for plastic flow and the anisotropy influence on the plastic flow behaviour of Ti-6Al-4V alloy in sheet form under tension test at constant crosshead velocity of 1 mm/min in three in-plane orientations, namely in the rolling 0° (RD), 45° (DD) and transverse (TD) directions, respectively. By applying the quantum mechanics and relativistic model (QM-RM) proposed by Muñoz-Andrade, it is shown that for a <c+a> pyramidal type slip with a Burgers vector b=0.553 nm, the activation energy barrier for plastic flow at maximum stress in uniaxial tension in the sheet rolling direction, Q=115.115 kJ/mol is lower in comparison with Q=116,629 kJ/mol obtained by the <a> basal type slip with a Burgers vector b=0.295nm. These results are in accordance with the reduced cross-slip activation energy barrier for cross-slip onto pyramidal planes in an a hexagonal crystalline structure with compact packaging (hcp) titanium phase. Additionally, it was shown that transverse direction (TD) of Ti-6Al-4V alloy in sheet form has developed the maximum stress in uniaxial tension test and ductility.

Examination of high-temperature mechanisms and behavior under compression and processing maps of pure copper

The hot compression behavior of pure copper (commercial grade) was studied in a temperature range between 843 and 993K and in a strain rate range between 10−3 and 10s−1. The activation energy for plastic flow was determined to be 215kJ/mol, which is similar to the activation energy for lattice diffusion in copper (=197kJ/mol). The stress exponent values associated with the plastic flow were 7–9, which are larger than the theoretical values associated with lattice-diffusion controlled dislocation climb creep (4.5–5). A comparison of the current data with the data obtained from the creep tests performed on the pure copper at low strain rates by other investigators in a similar temperature range revealed that the most of the data studied in this study belong to the regime where power-law breakdown occurs. Processing map constructed at a strain where steady-state plastic flow was observed showed that the power dissipation efficiency was low (≤21%) in the entire experimental ranges of strain rate and temperature. Discontinuous dynamic recrystallization (DDRX) associated with a single peak or multiple peaks in the stress-strain curves occurred during the first stage of plastic deformation. Dynamic recovery occurred after the DDRX activity ended, leading to the formation of a subgrains with low-and intermediate angle grain boundaries within the recrystallized grains with high-angle grain boundaries. Due to the formation of extensive substructure within DDRX grains, the fractions of high-angle grain boundaries measured after the compressive deformation were low (≤30%).

Grain size and temperature effect on the tensile behavior and deformation mechanisms of non-equiatomic Fe41Mn25Ni24Co8Cr2 high entropy alloy

The effect of the grain size on the tensile properties and deformation mechanisms of a nonequiatomic Fe41Mn25Ni24Co8Cr2 high-entropy alloy was studied in the temperature range between 298 and 1173 K by preparing the samples with three different grain sizes through severe plastic deformation and subsequent annealing: ultrafine (sub)grain size (≤0.5 μm), 8.1 μm and 590.2 μm. In the temperature between 298 and 773 K, the material with the large grain size of 590.2 μm exhibited the largest tensile ductility (57 %–82 %) due to its high strain hardening associated with mechanical twinning, but it exhibited the lowest strength due to its large grain size. The material with the ultrafine (sub)grain size exhibited the lowest tensile ductility (3 %–7 %) due to a greatly reduced strain hardening ability after severe plastic deformation, but it exhibited the highest strength due to the dislocation strengthening and grain refinement strengthening. At tensile testing at temperatures above 973 K, recrystallization occurred in the material with the ultrafine (sub)grains during the sample heating and holding stage, leading to the formation of fine and equiaxed grains with the sizes of 6.8-13.5 μm. The deformation behavior of the Fe41Mn25Ni24Co8Cr2 with different grain sizes in the high temperature range between 973 and 1173 K, where pseudosteady-state flow was attained in the stress-strain curves, could be explained by considering the simultaneous contribution of grain boundary sliding and dislocation-climb creep to total plastic flow. The activation energies for plastic flow for the materials with different grain sizes were similar as ∼199 kJ/mol. In predicting the deformation mechanism, it was important to consider the change in grain size by rapid grain growth or recrystallization during the sample heating and holding stage because grain boundary sliding is a grain-size-dependent deformation mechanism. The sample with the ultrafine (sub)grains exhibited the large tensile elongations of 30 %–85 % due to its high strain rate sensitivity, m (0.1-0.5) at temperatures of 973–1173 K. The material with the large grain size of 590.2 μm exhibited the very small elongations of 0.2 %–8 % due to its small m values (0.1-0.2) and occurrence of brittle intergranular fracture at the early stage of plastic deformation.

Comparative study of the hot processing behavior in advanced Ni-based superalloys for use in A-USC applications

Three Ni-based superalloys, Haynes 214, Haynes 230 and Inconel 740H, have been analyzed for their mechanical behavior under conventional industrial hot-processing conditions. These and other comparable alloys are expected to be used in sCO2 heat exchanger cycles, as proposed in the Advanced Ultra-Supercritical (A-USC) program, due to their unique combination of mechanical, corrosion and creep properties. However, these alloys are difficult to form, specifically for sCO2 components, such as header pipes and manifolds, requiring optimal forming conditions. In this study, hot-deformation testing was conducted using a Gleeble 3500, to simulate temperatures and strain rates expected during industrial forming operations. The effect of using as-received material on the deformation behavior was analyzed, emulating microstructures under real conditions more accurately. The flow-stress curves for all three alloys showed the occurrence of dynamic recrystallization (DRX) with some signs of dynamic recovery, for the slowest strain rates and higher temperatures. Additionally, the influence of secondary phases on the hot deformation behavior was analyzed. The M6C-carbide in Haynes 230 showed the strongest effect, promoting recrystallization necklacing and inhibiting DRX growth. The activation energies for plastic flow were 246, 302 and 217 kJ mol−1, for Haynes 214, Haynes 230 and Inconel 740H, respectively. Hot-processing maps were generated for all alloys and suggested specific domains for optimal workability under the tested conditions. For Haynes 214, that is ε˙≤ 0.1/s and T ≥ 1050 ∘ C. For Haynes 230, it is 1050 ∘ C ≤ T ≤ 1150 ∘ C and 0.01/s ≤ε˙≤ 0.5/s. For Inconel 740H, it is 1010 ∘ C ≤ T ≤ 1075 ∘ C and 0.01/s ≤ε˙≤ 0.1/s.

The effect of Al to high-temperature deformation mechanisms and processing maps of Al0.5CoCrFeMnNi high entropy alloy

The high-temperature deformation mechanism and processing maps of cast Al0.5CoCrFeMnNi that comprises face centered cubic (FCC) and body centered cubic (BCC) phases (where BCC phase is a minor phase) were studied at temperatures in the range of 1023–1323 K and at strain rates in the range of 10−3 to 101 s−1. During hot compressive deformation, the hard BCC phase provided nucleation sites for dynamically recrystallized (DRXed) grains in the soft FCC matrix through particle stimulated nucleation (PSN). The PSN-induced DRX occurred by continuous dynamic recrystallization (CDRX) and CDRX accelerated as temperature increased. Due to the effect of PSN, the fraction of DRXed grains was notably higher and the size of the DRXed grain size was considerably smaller in Al0.5CoCrFeMnNi than those in the equiatomic CoCrFeMnNi with a single FCC phase. The effect of the BCC phase on the fraction of DRXed grains was pronounced even at the high strain rate of 10 s-1 in the temperature range between 1173 and 1323 K, which is advantageous for the practical use of cast Al0.5CoCrFeMnNi in the hot-working industry. Solute drag creep appeared at low strain rates and at high temperatures as the rate-controlling deformation mechanism due to the presence of Al solutes in the FCC matrix phase. The activation energy for plastic flow associated with solute drag creep was estimated to be 251 kJ/mol. This value most likely represents the activation energy for the diffusivity of aluminum solutes in Al0.5CoCrFeMnNi. The solute drag creep behavior could be predicted by the Weertman model, suggesting that Al0.5CoCrFeMnNi behaves similar to a typical class I type solid solution metal alloy. By adding aluminum to CoCrFeMnNi, the hot workability can be greatly improved according to the processing maps. The power dissipation efficiency notably increases, and the flow instability regime decreases in size.

High-temperature deformation mechanisms and processing maps of equiatomic CoCrFeMnNi high-entropy alloy

The hot compressive deformation mechanism and processing maps of the equiatomic FCC CoCrFeMnNi high-entropy alloy (HEA) were studied in the temperature range between 1023 and 1323 K and in the strain rate range between 10−3 and 10 s−1. At high strain rates above 1 s-1, strain hardening was dominant even at the very high temperature of 0.84Tm, which may be attributed to the sluggish diffusion coefficient and low stacking fault energy of the CoCrFeMnNi HEA, leading to suppression of dynamic recovery. According to the processing maps, the best condition for hot working was near 10−3 s−1 at 1323 K. Power-law breakdown and unstable flow occurred at low temperatures and high strain rates where the strain hardening was pronounced. The activation energy for plastic flow measured in the power-law creep regime when considering the dependence of elastic modulus on temperature was 312.2 kJ/mol; this value is close to the activation energy for the weighted diffusion coefficient calculated by weighting the contribution of each element in the CoCrFeMnNi HEA (284 kJ/mol). The size and fraction of the dynamically recrystallized grains increased as the strain rate decreased and the temperature increased, as in conventional metals. Both discontinuous dynamic recrystallization and continuous dynamic recrystallization (CDRX) occurred. CDRX became more distinct as the temperature increased. The deformation mechanism and behavior of the CoCrFeMnNi HEA were very similar to those of FCC pure metals in terms of the stress exponent and the effect of the stacking fault energy and diffusivity on the creep rates.

Microstructure and Hot Deformation Behavior of Twin Roll Cast Mg-2Zn-1Al-0.3Ca Alloy.

In the present work, the microstructure, texture, mechanical properties as well as hot deformation behavior of a Mg-2Zn-1Al-0.3Ca sheet manufactured by twin roll casting were investigated. The twin roll cast state reveals a dendritic microstructure with intermetallic compounds predominantly located in the interdendritic areas. The twin roll cast samples were annealed at 420 °C for 2 h followed by plane strain compression tests in order to study the hardening and softening behavior. Annealing treatment leads to the formation of a grain structure, consisting of equiaxed grains with an average diameter of approximately 19 µm. The twin roll cast state reveals a typical basal texture and the annealed state shows a weakened texture, by spreading basal poles along the transverse direction. The twin roll cast Mg-2Zn-1Al-0.3Ca alloy offers a good ultimate tensile strength of 240 MPa. The course of the flow curves indicate that dynamic recrystallization occurs during hot deformation. For the validity range from 250 °C to 450 °C as well as equivalent logarithmic strain rates from 0.01 s−1 to 10 s−1 calculated model coefficients are shown. The average activation energy for plastic flow of the twin roll cast and annealed Mg-2Zn-1Al-0.3Ca alloy amounts to 180.5 kJ/mol. The processing map reveals one domain with flow instability at temperatures above 370 °C and strain rates ranging from 3 s−1 to 10 s−1. Under these forming conditions, intergranular cracks arose and grew along the grain boundaries.

Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-Y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized (DRXed) grains, and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller, higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in magnesium. Compared with the cast alloys, the onset of the power law breakdown (PLB) occurred at larger Zener-Holloman (Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.

A Comparison of the Plastic-Flow Response of a Powder-Metallurgy Nickel-Base Superalloy Under Nominally-Isothermal and Transient-Heating Hot-Working Conditions

The flow-stress behavior at hot-working temperatures and strain rates of the powder-metallurgy superalloy LSHR was determined under nominally-isothermal and transient-heating conditions. Two conventional methods, compression of right-circular cylinders and torsion of thin-walled tubes, were used for isothermal tests. A direct-resistance-heating technique utilizing torsion of round-bar specimens in a Gleeble® machine was applied for both isothermal and transient-heating conditions. When expressed in terms of effective stress and strain, baseline data determined by the two conventional methods showed good agreement. With the aid of a flow-localization analysis to assess the confounding influence of axial (and radial) temperature gradients on deformation uniformity, the flow stresses determined from nominally-isothermal Gleeble® torsion tests were shown to be broadly similar to those from the conventional tests. With regard to transient phenomena, Gleeble® tests were also useful in quantifying the effect of rapid heating and short soak time on the observed higher flow stress associated with a metastable microstructure. The present work also introduces two new test techniques using direct-resistance-heated torsion specimens. One involves continuous heating under constant-torque conditions, and the other comprises testing an individual specimen at a series of temperatures and strain rates. Using a single specimen, the former method enabled the determination of the apparent activation energy for plastic flow, which was similar to that determined from the series of isothermal tests; the latter provided a low-cost, high-throughput approach to quantify the flow behavior.

The effect of 0.5 wt.% Ca addition on the hot compressive characteristics and processing maps of the cast and extruded Mg–3Al–1Zn alloys

The hot compressive characteristics and processing maps of the extruded 0.5 wt.% Ca–AZ31 (Ca–AZ31) alloy and the extruded AZ31 alloy were examined and compared in the temperature range between 473 and 673 K and in the strain rate range between 10−3 and 10 s−1. The extruded Ca–AZ31 alloy had a smaller grain size (10.64 vs. 23.44 μm) and a more homogeneous microstructure than the extruded AZ31 alloy due to the presence of thermally stable Al2Ca particles in the former. The addition of 0.5 wt.% Ca increased the ignition temperature of AZ31 by 270.5 K. While the added Ca did not affect the activation energy for plastic flow, it slightly decreased the stress exponent and increased the flow stress level. Dislocation climb creep associated with stress exponents of 5–7 dominated the plastic flow of the extruded AZ31 and extruded Ca–AZ31 alloys. The power-law breakdown did not appear in either alloy under the given testing conditions. In comparison with the extruded AZ31 alloy, the extruded Ca–AZ31 alloy exhibited a poor hot workability at low temperatures below 523 K. However, there was an improved hot workability at higher temperatures due to the presence of the Al2Ca particles that promoted dynamic recrystallization (DRX) and retarded coarsening of the DRXed grains. Compared with the extruded Ca–AZ31 alloy, the cast Ca–AZ31 alloy with a much larger grain size (86.2 μm) and a coarser secondary phase structure exhibited a poor hot workability at high strain rates (10−1–100 s−1) over the entire testing temperature range due to the occurrence of unstable flow.

Effect of Ca and CaO on the microstructure and hot compressive deformation behavior of Mg–9.5Zn–2.0Y alloy

In this study, the effect of adding 0.3wt% Ca to a Mg–9.5Zn–2.0Y alloy with icosahedral (I) phase (Mg3Zn6Y) was examined. The Ca alloying was done by two different routes, either through a master alloy produced with Mg and CaO (which is convenient from the points of view of safety and cost) or by direct addition of powdered Ca to the molten alloy. The small addition of Ca (CaO) effectively increased the ignition temperature of the alloys (by about 90K), which is promising for fire suppression and safe handling with the molten alloys, but it led to the formation of Ca2Mg5Zn13 phase at grain boundaries or interdendritic interfaces, causing partial melting and intergranular cracking during compressive deformation near ∼615K. As a result, the upper limit of hot workability temperature decreased to 598K, which is lower than that of the Mg–9.5Zn–2.0Y alloy by 125K. The cast Mg–9.5Zn–2.0Y–0.3Ca alloys prepared using the two different routes exhibited high similarity in ignition temperature, chemical composition, microstructure, hot compressive behaviors and processing maps, indicating that the use of CaO is as effective as the use of Ca in producing the same quality of Ca containing Mg alloys. The flow stresses during hot compressive deformation and the activation energy for plastic flow remained unaffected by the Ca (CaO) alloying, but decreased the strain rate sensitivity at high strain rates over 10−3s−1 due to the initiation of either power-law breakdown or partial melting by adiabatic heating. Because of this, the Ca containing alloy lost hot workability at high strain rates by comparison to the Ca-free alloy.

The Activation Energy for Plastic Flow in Spatially Extended Polycrystalline Systems during Tension Test

The target of this work is to obtain the activation energy for plastic flow in spatially extended polycrystalline systems during tension test with constant crosshead velocity. For the reason that, during a tension test at constant crosshead velocity in spatially extended polycrystalline systems, the strain rate is reduced period-by-period and these thermo mechanical conditions could change the amplitude of vibration of the atoms and their frequency due to the amount of vibration energy absorbed and related with the variety of disorders occurring during such irreversible deformation process. In this framework, the activation energy for plastic flow in spatially extended polycrystalline systems is obtained by applying the theoretical mathematical model of quantum mechanics and relativistic cosmic micromechanics connection proposed by Munoz-Andrade.

High‐Temperature Plastic Behavior of TZP–Ni Cermets

The creep behavior of tetragonal zirconia TZP–Ni cermets with metal contents below, close to, and above the percolation limit has been studied. Compressive creep tests were performed on as‐received materials and samples in which the metal phase was chemically removed (ceramic skeletons). The stress exponent and the activation energy for plastic flow are independent of the nickel content and decrease continuously on increasing the stress and/or the temperature; skeleton structures display the same trend, suggesting that creep is controlled by the zirconia matrix. The steady‐state constitutive equation for high‐purity monolithic zirconia applies to the cermets when the stress is corrected with the porosity and volume fraction of percolated nickel.

Creep Behaviour of Fe–Mn Binary Alloys

Tensile creep behaviour of fine-grained Fe–Mn binary alloys containing0.42–1.21 wt.% Mn has been investigated in the temperature range fromroom temperature to 475 K under 10–50 MPa. Tensile tests are carriedout with a constant cross-head speed under uniaxial load at a strainrate 10−4s−1. Stress exponent and activation energy aredetermined to clarify deformation mechanism. The obtained variation ofsteady state creep rate with respect to the applied stress for Fe–Mnbinary alloys exhibits two distinct regimes at about 20 MPa, indicatinga possible change in creep mechanism. The average stress exponent isapproximately 2.2, which is a characteristic of grain boundary slidingin the alloys. The activation energy for plastic flow varies from 135to 92 kJ/mol, depending on the Mn content.

High Temperature Mechanical Behavior of a 30%Ni-19%Cr Steel

The deformation behavior at high temperature of a Type 800 Incoloy alloy reinforced by TiC particles was investigated by tensile and torsion tests at temperatures ranging from 800 to 1 200°C. The as-received material exhibited a microstructure of coarse equiaxic grains with subgrains decorated by small TiC particles. The material showed an activation energy for plastic flow of 400 kJ/mol similar to the activation energy for lattice diffusion in austenitic stainless steels. The stress exponents varied strongly with testing conditions. After testing at temperatures up to 1 000°C, the initial austenitic grains were elongated in the tensile direction, but the subgrain structure did not change with stress. In contrast, a microstructural refinement was observed after tensile testing at 1 100°C, which was associated with a process of dynamic recrystallization that occurs during deformation. At this temperature the deformation behavior of the material can be described by a slip creep mechanism. At the rest of the test temperatures the controlling mechanism is that of constant-structure slip creep.

Superplastic Behavior and Activation Energies for Plastic Flow in Cu–Ag Mocrocomposites

No abstracts.