Strain Rate Sensitivity Exponent Research Articles

Dynamic deformation mechanism for distinct flow behaviors in FGH4113A superalloy with high γ′ content during isothermal compression at sub-/near-/super-solvus temperatures

This study examines the flow behavior and deformation mechanism of a FGH4113A superalloy with high γ′ content for different strain rates (0.01–1 s−1) at sub-solvus (1030, 1080 °C), near-solvus (1130 °C), and sup-solvus temperatures (1180 °C). The results indicated that the alloy exhibits four different flow behaviors: rapid softening after reaching peak stress (at 1030–1080 °C), slow softening after reaching peak stress (at 1130–1180 °C/0.1–1 s−1), steady state flow (at 1130 °C/0.01 s−1), and continuous hardening (at 1180 °C/0.01 s−1). The strain rate sensitivity exponent decreases as the flow behavior transforms from dynamic softening to continuous hardening. Moreover, the dynamic recrystallization (DRX) fraction decreases with increasing strain rate at sub-solvus temperatures, where an inverse trend was observed at near- or sup-solvus temperatures. These observations are influenced by the interplay of continuous original grain boundary migration (COBM), DRX, and grain growth. Meanwhile, the primary γ′ phase has different roles under distinct deformation conditions. At sub-solvus temperature, the primary γ′ phase facilitates DRX, grain refinement, and coordinated plastic deformation. At near-solve temperature, the primary γ′ phase predominantly contributes to coordinated plastic deformation. At the sup-solvus temperature, the γ′ phase entirely dissolves, with the grains coarsened by COBM continuously undergoing compressed, exhibiting continuous hardening.

In-situ SEM micropillar compression and nanoindentation testing of SU-8 polymer up to 1000 s−1 strain rate

In situ micropillar compression and nanoindentation were performed on photolithographically fabricated SU-8 polymer inside a scanning electron microscope (SEM), covering seven orders of strain rates from 10-3 to 103 s−1. The extracted mechanical properties – modulus, hardness, yield strength, strain rate sensitivity (SRS) exponent – were systematically compared from both microscale tests and showed excellent agreement. No change in deformation mechanism was observed at high strain rates. We report a novel experimental protocol for high strain rate nanoindentation, comprising of input profile smoothing, that minimizes resonance amplitude during unloading and allows reliable extraction of modulus and hardness using standard Oliver-Pharr analysis. To the best of our knowledge, this is the first report of mechanical properties of SU-8 beyond 1 s−1 strain rate using nanoindentation based tests.

High-temperature tensile behaviors of a bimodal-grained Mg-Mn-Ce alloy

In this work, the high-temperature tensile behaviors of an extruded Mg-0.9Mn-0.5Ce (wt%) alloy with bimodal-grained microstructure were investigated, and the tensile tests were conducted at 423 K ∼ 523 K with the strain rate ranging from 6.0 × 10−3 s−1 to 1.5 × 10−1 s−1. It was found that the alloy exhibited a maximum fracture elongation of ∼156% and a highest strain-rate sensitivity exponent of 0.36 when deformed at 473 K. The softening effects generated by continuous dynamic recrystallization, the activation of < c + a > slipping on the pyramidal planes and grain boundaries migration collaboratively contributed to the high fracture elongation. As temperature increases to 523 K, microstructure stability was rapidly decreased and the deformation ability was severely restricted owing to the growth of grains, resulting in the adequate fracture elongation and low strain-rate sensitivity exponent.

Strain hardening exponent and strain rate sensitivity exponent of dual-phase steels at quasi-static strain rates during tensile testing

Strain hardening exponent and strain rate sensitivity exponent of dual-phase steels at quasi-static strain rates during tensile testing

Effect of strain rate on the deformation of 6061-T6 aluminum alloy at cryogenic temperature

This study investigates the tensile behavior and strain rate sensitivity (SRS) of a commercial Al6061-T6 Al alloy at various temperatures (i.e., 20, −50, and −150 °C). Uniaxial tensile tests were performed at the quasi-static strain rates of 10−2, 10–3, and 10−4 s−1. A significant improvement in mechanical properties was found at the cryogenic temperature (CT), due to the enhanced work-hardening behavior during CT deformation. The SRS exponent changed from a slightly positive value at RT to a negative one as the temperature is lowered. The dislocation density was evaluated for the samples strained to 5 and 8% at 20 °C and −150 °C using X-ray diffraction to probe the strengthening mechanisms during RT and CT deformation. The dislocation density was higher for the strain rate of 10−2 s−1 at RT and 10−4 s−1 for CT. In addition, electron back scatter diffraction (EBSD) based kernel average misorientation (KAM) and coincidence site lattice (CSL) boundary distribution analysis were conducted on the initial and 8% strain-interrupted samples at both temperatures to explain the effect of temperature and strain rate on work hardening behavior. Transmission electron microscopy (TEM) was applied to investigate the dislocation structures under different strain rate and temperatures. The cryogenically deformed sample exhibited a distinctive dislocation tangling phenomenon in proximity of the grain boundary, In contrast, the room temperature deformed sample demonstrated dislocation penetration occurring at the grain boundary. The character of SRS and improved work-hardening behavior can be attributed to changing dislocation kinetics with decreasing stacking fault energy (SFE) at cryogenic temperature.

Analysis on the Key Parameters to Predict Flow Stress during Ausforming in a High-Carbon Bainitic Steel

Since flow stress is an important parameter in the processing and application of metallic materials, it is necessary to trace the flow stress during austenite deformation. Thermal compression deformation of austenite in a high-strength bainitic steel was conducted using a Gleeble-3500 thermo-mechanical simulator, within the deformation temperature range of 400 °C~900 °C. By analyzing the stress–strain curves and strain-hardening exponent, the effects of strain hardening and dynamic recovery on the dislocation density of the material during the thermal processing were considered in the present work. Based on the general form of the Kocks–Mecking–Estrin (KME) model, the effects of deformation temperature and strain on the key parameters of the model were clarified. Differing from other work which commonly terms m (strain rate sensitivity exponent) and k2 (dimensionless parameters for dynamic recovery) as constants, the current models consider the quantitative relationship between key parameters and deformation temperature and strain. The results show that m is an exponential function related to temperature and strain, which decreases with the increase in strain. Meanwhile, k2 is a temperature-dependent polynomial function that increases as the deformation temperature increases. Finally, a modified constitutive KME model was proposed to predict the austenitic plastic stress with strain. Using established m-ε and k2-T models, the predicted curves are in good agreement with the experimental measurements.

Correlation between the strain rate sensitivity exponent m and microstructure evolution during hot deformation of an as-cast TiAl alloy

The hot-deformation behavior of an as-cast TiAl alloy with a near-lamellar microstructure was investigated at a deformation temperature within the range of 1020–1200 °C and a strain rate of 0.001–1.0 s−1. A visual three-dimensional diagram of the strain rate sensitivity exponent m was developed based on hot compression data to reflect the correlation between the process parameters and microstructural evolution characteristics. The results showed that the m-value increased with an increase in temperature and a decrease in the strain rate. As the temperature increased, the growth of the m-value at a higher strain rate was faster than the growth of the m-value at a lower strain rate. Moreover, microstructure analysis at different m-values indicated a correlation between the m-value and microstructural evolution characteristics during hot deformation. The mechanism of microstructure evolution during hot deformation can be divided into three evolution modes according to different strain rate sensitivity exponent m-values. At a lower m-value, numerous γ/α2 lamellar colonies appeared similar characteristics to the initial microstructure. At a moderate m-value, broken lamellae in the microstructure formed several chain-like fragments with similar orientations. At a higher m-value, several formed equiaxed α2 and γ phases satisfied the Blackburn orientation relationship in the microstructure.

Revealing the ultra-high high-temperature compressive mechanical properties and deformation mechanism of a heterostructured AlNp/Al nanocomposite

Heterostructured materials have attracted increasing attention owing to their superior mechanical properties. In this work, the high-temperature mechanical property and deformation mechanism of a heterostructured AlNp/Al composite were revealed by isothermal compression tests. The results showed that the superior compressive strength of the AlNp/Al composite could be maintained at both room temperature and high temperature of 300–500 °C during the whole deformation process. The strong pinning effect of the dispersive AlN nanoparticle (AlNp) on the matrix grains could prevent grain boundary softening and grain growth, as well as the special layered heterogeneous structure and network structure could avoid high temperature softening. The hot deformation mechanism of the AlNp/Al composite could be affected by the strain rate and deformation temperature. It was confirmed that continuous dynamic recrystallization (CDRX) dominated the whole deformation process when deformed at low strain rates (0.0001–0.01 s−1) and high temperatures (400–500 °C); while dynamic recovery (DRV) was the main mechanism at high strain rate (0.1–1 s−1) and low temperature (300–350 °C) Furthermore, the strain rate sensitivity exponent (m) was used to assess the relationship between peak stress and deformation temperature and strain rate, which could predict the instability region of the composite. It shows that the simultaneous action of high temperatures and high strain rates should be avoided during hot deformation. This work shed light on the microstructure design of the high-strength heat-resistant aluminum matrix composite.

Systematic analysis of distinct flow characteristics and underlying microstructural evolution mechanisms of a novel fine-grained P/M nickel-based superalloy during isothermal compression

Isothermal forging (IF) is an effective method for forming difficult-to-deform materials like P/M superalloys. Understanding the isothermal compression microstructural evolution mechanism of a novel P/M superalloy provides the basis for its optimized IF planning. In this study, the isothermal compression tests of a novel fine-grained P/M nickel-based superalloy were carried out at 1000–1150 °C with strain rates of 0.001–0.01 s−1. The results indicated that the alloy exhibits three distinct flow characteristics: continuous softening after reaching the peak stress, near-steady superplastic flow, and discontinuous hardening, corresponding to different strain rate sensitivity exponent (m) values. Varied microstructural evolution mechanisms, including grain boundary sliding (GBS), dynamic recrystallization (DRX), and grain growth, are dominated in different m-value domains. Meanwhile, different roles of primary γ' play in microstructural evolution were clarified. A moderate fraction of primary γ' with 8.5%–14.2% can well coordinate the GBS and hinder excessive grain growth at a high m value domain (m > 0.4). When 0.2 < m < 0.4, the role of the primary γ' is changed to promote dislocation accumulation, accelerating the nucleation of DRXed grains. As the primary γ' is dissolved at 1150 °C, obvious grain growth was observed after compression. Work hardening effect by overgrown grains competed with DRX softening results in the discontinuous rising stress.

Effect of Mo addition on nanoindentation creep behavior of FeCoBSiNb bulk metallic glasses

Novel Fe52Co20-xB20Si4Nb4Mox (x = 0–5) bulk metallic glasses with a diameter of 2 mm are successfully prepared by vacuum suction casting and the effect of Mo addition on their nanoindentation creep behavior is studied. Through a differential scanning calorimetry and creep compliance spectral analysis, it is found that the addition of a small amount of Mo increases the enthalpies (ΔH) and free volumes of the alloys. These factors stimulate the rearrangement of the atomic structures of the alloys and induce the adjustment of their internal structure, resulting in a relatively loose atomic packing state that allows the alloys to provide a continuous response to stress during creep deformation. By analyzing the unloading and load holding sections on the load-displacement curves, it is shown that the hysteresis of deformation induces the displacement jump phenomenon and the amplitude of the jump is inversely proportional to the equivalent stress. The larger the equivalent stress, the less obvious the displacement pop-out phenomenon. The changes in strain rate sensitivity exponent (m) and hardness are also studied. Combined with a delay spectral analysis, it is found that a 5% Mo addition significantly improves the creep resistance of the alloys.

Effects of grain size and Al addition on the activation volume and strain-rate sensitivity of CoCrFeMnNi high-entropy alloy

The activation volume (V*) and strain-rate sensitivity exponent (m) of CoCrFeMnNi and Al0.5CoCrFeMnNi high entropy alloys (HEAs) with various grain sizes (ranging between 2.4 and 356 μm) were measured at different strain rates and strain levels at room temperature. As the strain rate decreased, the plastic strain decreased, but the grain size and V* increased. The enhanced solid-solution strengthening by addition of aluminum decreased V*. The Hassen plot was modified to capture the grain-size dependence of V* by considering the grain-size dependence of the dislocation density. As the plastic strain increased, the strain rate, grain size and m decreased. The behavior of m could be quantitatively predicted by using equations derived for the grain-size-dependent V*and flow stress. The difference in the grain-size dependence of m between conventional face centered cubic (FCC) metals and FCC HEAs over the nanograin size range could not be explained in terms of a relatively large Hall-Petch slope in the latter compared with the former but was explainable in terms of a substantially higher probability of activation of grain-boundary diffusion-controlled grain-boundary-sliding mechanism at nanograin sizes in the former than in the latter.

Effects of different preservation on the mechanical properties of cortical bone under quasi-static and dynamic compression.

Introduction: Mechanical properties of biological tissue are important for numerical simulations. Preservative treatments are necessary for disinfection and long-term storage when conducting biomechanical experimentation on materials. However, few studies have been focused on the effect of preservation on the mechanical properties of bone in a wide strain rate. The purpose of this study was to evaluate the influence of formalin and dehydration on the intrinsic mechanical properties of cortical bone from quasi-static to dynamic compression. Methods: Cube specimens were prepared from pig femur and divided into three groups (fresh, formalin, and dehydration). All samples underwent static and dynamic compression at a strain rate from 10-3s-1 to 103s-1. The ultimate stress, ultimate strain, elastic modulus, and strain-rate sensitivity exponent were calculated. A one-way ANOVA test was performed to determine if the preservation method showed significant differences in mechanical properties under at different strain rates. The morphology of the macroscopic and microscopic structure of bones was observed. Results: The results show that ultimate stress and ultimate strain increased as the strain rate increased, while the elastic modulus decreased. Formalin fixation and dehydration did not affect elastic modulus significantly whereas significantly increased the ultimate strain and ultimate stress. The strain-rate sensitivity exponent was the highest in the fresh group, followed by the formalin group and dehydration group. Different fracture mechanisms were observed on the fractured surface, with fresh and preserved bone tending to fracture along the oblique direction, and dried bone tending to fracture along the axial direction. Discussion: In conclusion, preservation with both formalin and dehydration showed an influence on mechanical properties. The influence of the preservation method on material properties should be fully considered in developing a numerical simulation model, especially for high strain rate simulation.

Impact design of die parameters on Severe plastic deformation during Equal channel angular pressing: An overview

Equal channel angular pressing causes less uniform deformation than simple shear, even though its not as obvious as with other metal forming procedures. Investigation is done into how internal and external factors affect the deformity inhomogeneity through Equal .channel angular pressing. Finite element analysis of plastic deformity are integrated with die corner angle and the strain harden ability of metallic workpiece. The material characteristics are significantly influenced by the type of plastic shear deformation that occurs through Equal channel angular pressing and this is primarily impacted by the die geometry, the properties of the material and the process factors. Segmenting the workpiece into a front transient zone, end transient zone, outer less sheared zone and the remaining shear deforming zone allowed researchers to examine the uneven strain distribution throughout the workpiece. The deformed geometry for the non-hardening and it was assumed that rate-insensitive materials would be largely homogeneous. In materials that are strain-rate sensitive, gaps between the upper and lower channels developed, whereas strain-hardening materials experienced the corner gap. The strain hardening and implications of strain-rate sensitivity exponent had a considerable impact on the deformation inhomogeneity. Metals having an ultrafine grain microstructure can be created by severe plastic deformation. The FE models were used to affect the process and they all took as inputs the material properties, load variation, Different velocity and boundary conditioned. For the purpose of evaluating the impact of the channel angle on the AA5083 sample, The FE analysis produced the value of strain distribution. When the channel angle was 1200, there was less strain overall, but there was also less concentrated stress in the channel corner area.

Superior ductility Mg-Mn extrusion alloys at room temperature obtained by controlling Mn content

The effect of Mn content (0.4, 0.8, 1.2 wt. %) on microstructure and mechanical properties of as-extruded dilute binary Mg-Mn alloys was studied. With the increase of Mn content from 0.4 wt. % to 1.2 wt. %, the volume fraction of dynamically recrystallized (DRXed) grains decreases from 89% to 40%, while the DRXed grain size decreases from 3.2 μm to 1.9 μm. The segregation of Mn atoms and nano α-Mn dynamical precipitates are observed at the grain boundaries of the extruded Mg-0.8Mn alloys and the amount of dynamically precipitates is increased with increasing Mn content. Tensile tests show that the Mn content significantly affects mechanical properties at room temperature (RT). The Mg-0.8 wt. % Mn alloy shows the excellent ductility of 81 % at room temperature in a strain rate of 1 · 10−3 s−1, the ductility decreases if the Mn content is lower or higher than 0.8 wt. %. The yield strength (YS) and ultimate tensile strength (UTS) increase with increasing Mn content. The large value of strain rate sensitivity exponent (m-value) and small activation volume (V*) of the as-extruded Mg-0.8Mn alloy suggest that grain boundary sliding (GBS) plays an important role in tensile plastic deformation. High volume fraction of the un-DRXed region leads to the reduced contribution of GBS to plastic deformation, decreases ductility at RT. The Atomic force microscopy (AFM) analysis indicates that the contribution of GBS to the total plastic deformation is 2.7 %, 55.1 % and 12.8 % for Mg-0.4Mn, Mg-0.8Mn and Mg-1.2Mn alloys, respectively.

Computational and experimental investigation of the strain rate sensitivity of small punch testing of the high-entropy alloy CoCrFeMnNi

The suitability of determining the strain rate sensitivity (SRS) of the CoCrFeMnNi high-entropy alloy (HEA) by small punch (SP) testing has been assessed at displacement rates ranging from 0.2 to 2 mm∙min−1. The stress was found to increase as the displacement rate was raised from 0.2 to 2 mm∙min−1, whereas the plastic strain distributions were similar in all cases. However, for a higher displacement rate of 10 mm∙min−1, the sample was found to exhibit a drop in strength and ductility attributed to casting defects. The strain-rate sensitivity exponent (m) was found to be 0.1387 whilst the Finite Element Analysis (FEA) simulations predicted a slightly smaller value of 0.1313. This latter value is closer to m = 0.091 obtained from nanoindentation strain rate jump tests since the results are insensitive to the presence of small casting defects. The relationship between the experimental and the empirically derived predicted properties from the SP tests revealed a high level of agreement for maximum stress properties. The properties predicted at 2 mm∙min−1 (R2 = 0.96) offered a stronger fit than at 0.5 mm∙min−1 (R2 = 0.92).

An inverse Hall-Petch relationship during room-temperature compression of commercially pure magnesium

The effect of grain size and strain rate on the room-temperature compression of pure magnesium was investigated. Samples with equiaxed grains and grain size ranging over 1.4–88 µm were obtained by extrusion at different temperatures. The extruded samples were compressed at strain rate between 1 × 10−2 s−1 and 1 × 10−6 s−1. In microstructures with grain size greater than 15 µm, the deformation mechanism was dominated by twinning; while in those with grain size smaller than 8 µm, the dominant deformation mechanism was dislocation slip. In addition, significant grain-boundary sliding was observed with decreasing grain size and strain rate. The Hall-Petch relationship broke down with mean grain size smaller than a critical grain size, which increased with decreased strain rate. Further, the deformation mode (compression or tension) had a negligibly small effect on the critical grain size for changes in the dominant deformation mechanisms and the Hall-Petch breakdown. The changes in deformation mechanisms with grain size were understood in terms of the strain-rate sensitivity exponent m. When the grain size decreased from 88 µm to 1.4 µm, m increased to approximately 0.3 from 0.01. The increase in m with grain refinement was consistent with enhanced grain-boundary sliding, in addition to dislocation slip and the suppression of deformation twinning. It thereby reduced the asymmetry of the tension-compression yield strength.

Nano-mechanical properties of Mo coating prepared on Invar alloy substrate by double glow plasma surface alloying

In this work, surface Mo alloying was realized on Invar alloy using double glow plasma surface alloying technology. The microstructure and phase constitution of Mo-modified coating were determined by using X-ray diffraction and scanning electron microscopy. The surface modification effect and loading strain rate (LSR) dependency of Mo coating were presented qualitatively and quantitatively via nanoindentation method based on continuous stiffness measurement technique. Results showed that the received Mo coating with a thickness of approximately 10 μm was compact and homogeneous. The phase constitution consisted of pure Mo phase. Double glow plasma surface molybdenizing improved the surface hardness and elastic modulus remarkably. Indentation response showed obvious LSR dependency. Under different LSRs, hardness and plasticity exponent increased with LSR, while elastic modulus and contact stiffness basically remained stable. Meanwhile, creep displacement and creep strain rate sensitivity exponent were positively correlated with LSR.

Nanoindentation and micro-cantilever testing for understanding cold-dwell sensitivity of heat-resistant titanium alloys

The mechanism of cold-dwell fatigue of the Ti alloys Ti-6242S and IMI 834 was investigated by measuring their local strain-rate sensitivities using nanoindentation analysis, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. In addition, microcantilever bending tests involving a load-holding-induced change in plastic strain near grain boundaries were conducted to investigate the plastic behavior of the alloys. The difference in the strain-rate sensitivity exponent between soft and hard grains was large in Ti-6242S and small in IMI 834, although slips were generated in the hard grains of both IMI 834 and Ti-6242S by activated slips in the soft grains during the microcantilever bending tests. The hardness of IMI 834 varied with its Al concentration, whereas no Al-concentration-dependent changes in hardness were observed in Ti-6242S. These results suggest that the stress redistribution was caused by differences in the concentrations of solid elements and the crystal orientation of the hard and soft grains.

Solid solution hardening effects on structure evolution and mechanical properties of nanostructured binary and high entropy alloys after high pressure torsion

Two different alloy series (Cu-X, Ni-X) have been selected to investigate the effects of solutes on the saturation grain size, the thermal stability and mechanical properties after high pressure torsion. The results of the Cu-X series indicate that the saturation grain size does not correlate with the stacking fault energy but shows good agreement with solid solution hardening according to the Labusch model. This correlation does not only hold for binaries, but also for chemically complex high entropy alloys (Ni-X) in the form of (CrMnFeCo)xNi1-x, where the Varvenne model is used to describe solid solution hardening. The alloy series exhibit a grain size in the range of 50 – 425 nm after high pressure torsion and the solutes increase the strength as well as the thermal stability of the alloys after annealing. The nanostructured alloys exhibit an enhanced strain rate sensitivity exponent, as determined from nanoindentation strain rate jump and constant contact pressure creep testing, whereas an enhanced rate sensitivity is found at low strain rates. The relatively lower rate sensitivity of the alloys as well as their higher thermal stability indicate, that defect storage and annihilation is strongly influenced by a complex interaction of solutes, dislocations and grain boundaries.

Study of strain rate sensitivity exponent and strain hardening exponent of typical titanium alloys

In this study, the flow stress-strain curves of three typical titanium alloys, cast TA15 titanium alloy (near α-type), cast Ti40 titanium alloy (full β-type) and cast TC21 titanium alloy (α + β-type), were obtained based on hot compression tests, based on which the deformation activation energy, softening fraction, strain rate sensitivity exponents and strain hardening exponents of the three typical titanium alloys were calculated under different process parameters and analyzed The variation law of each parameter was analyzed. The effect of nascent grain size on the strain rate sensitivity exponent and strain hardening exponent was analyzed by counting the dynamic recrystallization grain size of the alloy. The results show that the deformation activation energy of Ti40 titanium alloy is much higher than that of TA15 and TC21 titanium alloys during thermal deformation, mainly because the alloying element content of Ti40 titanium alloy is much higher than that of the other two. The maximum value of strain rate sensitivity exponents m is obtained at the minimum strain rate for all three titanium alloys, and the deformation temperature also has a great influence on the value of m. Combined with the microstructure of the material under different deformation conditions, it is found that the effect of strain and strain rate on the strain hardening exponents n is very significant. Moreover, grain refinement during deformation can effectively increase the strain rate sensitivity exponent and reduce the strain hardening exponent.