Strain Rate Sensitivity Values Research Articles

Digital Image Correlation Analysis of Uniform Deformation and Necking in Solid‐State Welded Nanocrystalline Aluminum via High‐Pressure Torsion

Solid‐state welding of Al 1043 sheets was achieved via high‐pressure torsion (HPT) processing to produce bulk nanostructured Al disks. A homogeneous nanostructure without segregation was observed, with grain sizes of ∽430‐470 nm. Miniature tensile testing, coupled with the digital image correlation (DIC) technique, was employed to determine the room‐temperature tensile deformation behavior, particularly the non‐uniform behavior with necking, of the HPT‐bonded ultrafine‐grained (UFG) aluminum, comparing it with annealed coarse‐grained counterpart. The HPT‐bonded UFG Al exhibited a large fraction of post‐necking strain, which is supported by the estimated high strain rate sensitivity value of m = 0.085, suggesting the delay of local necking leading to tensile fracture. Detailed DIC analysis revealed prolonged diffuse necking, thus delaying local necking, in the HPT‐bonded UFG Al, while the annealed samples show high fractions of local necking during the non‐uniform deformation. Moreover, the DIC data illustrated that local necking predominantly occurred at a limited neck zone, maintaining a plateau strain distribution at the out‐of‐neck zone throughout necking deformation towards tensile failure for both annealed and UFG aluminum. The DIC method offers an alternative means to demonstrate the transition in necking behaviors of materials by estimating the plastic lateral contraction exponent.This article is protected by copyright. All rights reserved.

Effect of cooling rate and Nb addition on the strengthening in eutectic CoCr1.3FeMnNi0.7Nbx (0 ≤ x ≤ 0.45) high entropy alloys comprising of low stacking fault energy FCC phase

The CoCr1.3FeNi0.7MnNbx (0.3 ≤ x ≤ 0.45) eutectic high entropy alloy (EHEA) comprising of ultrafine lamellae of low stacking fault energy (SFE = 11 mJ/m2) face centered cubic (FCC) and Laves phase are synthesized at cooling rates of 2.5–10 K/s (arc melted ingots, AMIs) and 4.2 × 102 K/s (suction cast bars, SCBs). The microstructural investigation suggests that secondary dendritic arm spacing (SDAS) and interlamellar spacing are refined in SCBs due to a higher cooling rate than the AMIs. Whereas, deformation twins evolve in FCC phase due to its low SFE. The EHEAs exhibit a high yield strength of 1327 MPa for x = 0.367 SCB and a large fracture strain of 20% for x = 0.3 AMI under compression. Systematic investigation on the influence of Nb addition, microstructure refinement, and evolution of deformation twins during straining on the strengthening mechanisms reveals that the phase interface strengthening, solid solution strengthening, and twin boundary strengthening play critical roles in providing the high flow stress. The post-deformation analysis and the presence of multiple positive slopes in work hardening curves confirms that the concurrent effect of the evolution of dislocations, deformation twins, and their mutual interactions are solely responsible for the severe strain hardening at multiple stages during plastic deformation. The low values of strain rate sensitivity (0.0106–0.0111) and the activation volume (28b3-41b3) suggest that the deformation kinetics is governed by the dislocation interactions with the ultrafine lamellae interface and twin boundaries.

Mechanical softening of CuX alloys at elevated temperatures studied via high temperature scanning indentation

The thermal stability and temperature dependent hardness of ultrafine-grained Cu-alloys CuSn5 and CuZn5 after high pressure torsion are investigated using the high temperature scanning indentation (HTSI) method. Fast indentations are carried out during thermal cycling of the samples (heating-holding-cooling) to measure hardness and strain rate sensitivity as a function of temperature and time. The microstructures after each thermal cycle are investigated to characterize the coarsening behaviour of both alloys.Results show that the thermal stability of the tested alloys can be expressed in terms of several temperature regimes: A fully stable regime, a transient regime in which growth of individual grains occurs, and finally a regime in which the microstructure is fully coarsened. The onset of grain growth is accompanied by high strain rate sensitivity on the order of 0.2–0.3. Furthermore, the obtained hardness and strain rate sensitivity values are in good agreement with continuous stiffness measurement (CSM) and strain rate jump (SRJ) experiments. This highlights the applicability of the HTSI method to the characterization of the thermomechanical properties of ultrafine-grained alloys.

Hot Deformation Behavior of Inconel 718 Alloy Using Regression-based Dynamic Material Model and Applications in Ring Rolling Manufacturing

Inconel 718 nickel-based alloy is extensively used in the aerospace industry (e.g., gas turbine engine components) because of its excellent corrosion resistance and high mechanical properties at elevated temperatures. However, there is a certain limit to manufacturing the alloy through plastic deformation due to its high deformation resistance and complicated deformation behaviors. In this study, the hot deformation behavior of Inconel 718 alloy was investigated to establish how processing conditions of flow stress-strain, at strain rates from 0.001 to 10 s<sup>-1</sup>, and temperatures from 850 to 1200<sup>o</sup>C, affected dynamic recrystallization. The regression-based material model was utilized to calculate the strain-rate sensitivity, and subsequently depict the efficiency of the power dissipation and instability criterion of hot deformation. The processing map and instability criterion predicted by the developed 3<sup>rd</sup>-order polynomial regression model corresponded with the experimental results and in particular, showed a better prediction for instability regime compared to the existing discrete derivative approach. Predicting the strain-rate sensitivity values on a continuous scale with regression analysis covered the additional instability region of the high strain rate near 10 s<sup>-1</sup>. The dynamic recrystallization deformation was also characterized by microstructural analysis along with the processing map. Consequently, ring-rolled aviation parts were manufactured with the optimum processing parameters, which conform to the AMS 5663 standard (Aerospace material specifications for Inconel 718).

High Temperature Deformation of Harmonic Structure Designed CrMnFeCoNi High Entropy Alloy

The Harmonic Structure (HS) is a recently introduced concept that paves the way for engineering metallic materials to achieve superior mechanical performance. They consist of soft, coarse-grained regions surrounded in three dimensions by an interconnected network of hard, ultra-fine grained regions. In addition, from a structural materials point of view, high entropy alloys have attracted attention due to their unique mechanical properties. In the present study, the HS design was applied to a high entropy CrMnFeCoNi alloy (also called "Cantor alloy"). The HS-designed Cantor alloy was successfully fabricated by mechanical milling, which is one of the surface severe plastic deformation processes, and the subsequent sintering process. The mechanical properties of these HS and homogeneous (Homo) Cantor alloy compacts were investigated by high-temperature compression tests in the temperature range of room temperature (RT) and 1173K, under initial strain rates of 0.01 s-1, 0.001 s-1, and 0.0001 s-1. The stress-strain curves of the HS compacts showed a large initial increase in stress and then a rapid decrease with strain, while that of the Homo compact showed a gentle increase and a gradual decrease. EBSD observation of the deformed compacts revealed that the HS compacts were probably deformed not only by dynamic recrystallization, but also by grain boundary sliding during deformation. The strain rate sensitivity value m of the HS compacts was 0.541 (true strain: 0.2) at 1173 K. In other words, the HS compacts exhibited pseudo-superplastic deformation at these temperatures.

Experimental investigation of the dynamic mechanical properties of concrete under different strain rates and cyclic loading

To investigate the effects of strain rate and cyclic loading on the mechanical parameters of concrete compressive strength, residual strain, peak strain and elastic modulus, a series of cyclic loading tests over concrete were conducted by setting different numbers of cycles, magnitude factors, and cycle frequencies/periods based on the FST-1000 alpine geotechnical dynamic and static triaxial test system. The quantitative relationships between the mechanical parameters of concrete and the parameters of cyclic loading as well as the strain rate were also proposed. The results show that as the strain rate increases, the concrete compressive strength and elastic modulus increase linearly, while the residual strength and peak strain show a decreasing trend. Under cyclic loading, the compressive strength was quadratically related to each cyclic loading parameter, and the elastic modulus was quadratically related to the cyclic number and magnitude factor. The trend of the concrete compressive strength rate sensitivity undergoes a change at the specific thresholds of 0.145 for the magnitude factor and 1.93 Hz for the frequency. The minimum value of the peak strain rate sensitivity corresponds to the specific thresholds of 6023 for the number of cycles, 0.14 for the magnitude factor, and 1.38 Hz for the frequency thresholds, respectively. Additionally, the enhancement of the elastic modulus rate sensitivity occurs when the number of cycles exceeds 918 or when the magnitude factor surpasses 0.33. The results can provide a theoretical basis for the design of concrete structures and seismic engineering.

Anisotropic microstructure, nanomechanical and corrosion behavior of direct energy deposited Ti–13Nb–13Zr biomedical alloy

The present study investigates the anisotropic microstructure, nanomechanical and corrosion behavior of Ti–13Nb–13Zr biomedical alloys, which were fabricated using the direct energy deposition (DED) method. The microstructure of the as-deposited material was studied using a field-emission scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). We found anisotropic mechanical behavior using nanoindentation from strain rate sensitivity and creep tests. For the maximum load of 50 mN, the x-y plane (normal to the building direction) shows better indentation hardness and lower strain rate sensitivity value (0.014) compared to the x-z plane (0.022) (parallel to the building direction). The difference in the indentation hardness was mainly attributed to the smaller equiaxed prior-β grains and finer α’ martensitic laths on the x-y plane. In terms of creep behavior, the x-y planes show better creep strength than the x-z plane. Meanwhile, both planes show a very high creep exponent, which signifies a similar creep mechanism, i.e., dislocation based. Moreover, we further found the anisotropic corrosion behavior using electrochemical tests. Corrosion results reveal that the x-y plane is more corrosion-resistant than the x-z plane. In summary, the x-y plane of the direct energy deposited Ti–13Nb–13Zr alloy possesses better strength and corrosion resistance due to its finer microstructure.

Gradient Microstructure and Properties of Surface Mechanical Attrition–Treated AZ91D Alloy: An Effect of Colliding Balls Velocity

The surface properties of the AZ91D alloy are altered using surface mechanical attrition treatment (SMAT), a promising method of severe surface deformation, where the role of process parameters is crucial. In this study, specimens are SMATed using ≈3 and ≈10 m s−1 ball velocities (maintaining a constant percentage coverage). The SMATed specimens show higher twin density near the surface, which is reduced gradually, and twin thickness is increased with increasing depth. Further, high‐velocity balls cause more twin density and better grain refinement (≈32 nm grain size at the surface). The higher ball velocity helps form a considerably thicker gradient layer (≈3500 μm) with higher hardness (≈1.98 GPa) and compressive residual stress (≈281 MPa) within a shorter SMAT duration (≈10 min). Ball velocity also influences nanomechanical properties such as nanohardness, creep resistance, strain rate sensitivity (SRS), etc. The non‐SMATed alloy's SRS is about 0.037–0.040. The gradient microstructure affects SRS. The SRS value near the SMATed surface (where the reduced grain size plays a dominating role) is about 0.018–0.027; however, it drops suddenly to ≈0.01 (with a slight increase in depth), and subsequently, it rises with an increased distance in the SMATed layer (where twins play a dominating role).

Realizing impressive superplasticity in a low-alloyed Mg-Zn-Ca-Al-Mn alloy: The roles of grain boundary segregation and dense β-Mn particles

Achieving impressive superplasticity is an important strategy to manufacture Mg alloy products with complex shapes. In the present study, we report that an excellent superplastic deformation with elongation larger than 500% can be achieved at 623 K and 1.0 × 10−3s−1 in a Mg-1.51Zn-0.59Ca-0.59Al-0.70Mn (wt.%, ZXAM2111) alloy fabricated by equal-channel angular pressing. The superplastic deformation is mainly carried by grain boundary sliding (GBS), accompanied by a grain size growth from ∼3.0 µm to ∼6.0 µm after deformation. Before deformation, the ZXAM2111 alloy is mainly characterized by a strong co-segregation of Zn and Ca atoms at grain boundaries and uniformly distributed β-Mn particles. With deformation proceeding, the β-Mn particles further dynamically precipitate along grain boundaries that parallel the tensile axis, leading to improved resistance to grain coarsening. Although the enhanced stabilizing effects decrease the strain rate sensitivity value, the resulting impressive microstructure stability provides a cornerstone of the active operation of GBS, facilitating the achievement of superplastic deformation. The present work could provide insight into developing low-alloyed Mg alloys with high microstructure thermal stability to achieve superplasticity.

INVESTIGATION OF THE STRAIN RATE SENSITIVITY OF Mg-6Sn AND Mg-6Sn-3Y ALLOYS

This study reports the influence of the addition of Yttrium (wt.3%) on strain-rate sensitivity of Mg-6Sn alloy. The Mg-6Sn and Mg-6Sn-3Y alloys were made by using high pressure die-cast. The microstructural and X-ray diffraction results exhibited that the Sn3Y5 and MgSnY intermetallic phases were formed with addition of Y to the Mg-6Sn alloy. Furthermore, the grain structure of the Mg-6Sn alloy was changed from dendritic to globular with addition of Y. The strain-rate sensitivity value of the Mg-6Sn-3Y alloy is found higher than that of the Mg-6Sn alloy for all strain value. This result was attributed to the formation of new intermetallics (Sn3Y5 and MgSnY) and microstructure morphology (from dendritic structure to globular).

Investigation of initial metallurgical factors on the dynamic impact response and adiabatic shear bands formation of the 6061 Al alloy

The effect of multi-pass friction stir processing and initial base metal condition on the dynamic impact response of AA6061-T6 alloy at high strain rates was investigated in this study. The Split-Hopkinson Pressure Bar (SHPB) was used in dynamic experiments with strain rates ranging from 1600 to 4200/s. The results revealed that the base metal alloy's (BM) dynamic flow stress was comparable to all the friction stir processed AA6061-T6 (FSP'd) specimens as the strain rate and true strain increased. The general increase in stress response levels as strain rates and true strain increases can be attributed to strain hardening. The higher values of strain rate sensitivity in multi-pass FSP'd specimens are an indication that they are more ductile in comparison to the BM condition. The optical and electron backscattered diffraction (EBSD) analysis showed that adiabatic shear bands were formed by heterogeneous deformation. The EBSD analysis revealed that the FSP'd specimens showed more abnormal grain growth and dynamic recrystallized sites than BM. The substantial decrease in dynamic flow stress, and high strain rate sensitive values of multi-pass FSP'd specimens compared to BM for all the investigated strain rates attributed to softening effect by dynamic recrystallization (DRX), and the emergence of abnormal grain growth during friction stir process.

Microstructure and Deformation of Over-Aged Al-Zn-Mg-Cu Alloy with Fine Grains during Multiple Stress Relaxation Tests

Strong obstacles can greatly impede the motion or transmission of dislocations, which can be reflected by strain rate sensitivity and activation volume. In this study, the strain rate sensitivity and activation volume of overaged Al-Zn-Mg-Cu alloys with a grain size of 3.1 μm fabricated by powder metallurgy were measured by two different methods: a stress relaxation test and strain rate jump test. It was found that the former method gave much higher strain rate sensitivity values. After reviewing the conventional theory of stress relaxation test, it is unreasonable that the activation volume at each cycle during the stress relaxation test is a constant. At a strain rate higher than 10−5 s−1, ∂lnε˙/∂τ*, which is proportional to the activation volume in the conventional theory of the stress relaxation test, increases significantly, and nearly linearly increases with the strain rate in its logarithmic form, while at a strain rate lower than 10−5 s−1, the value of ∂lnε˙/∂τ* is nearly a constant. The grain boundary sliding mechanism was incorporated into the plastic deformation during the stress relaxation test, and the strain rate sensitivity and activation volume obtained by stress relaxation after modification agree well with that obtained by the strain rate jump test.

Rate-dependent mechanical behavior of single-, bi-, twinned-, and poly-crystals of CoCrFeNi high-entropy alloy

While considerable effort is made to understand the solid solution strengthening on the deformation behavior of high-entropy alloys (HEAs), relatively little attention is paid to the role of microstructural interfaces, especially twin boundaries (TBs), on the strain-rate sensitivity (SRS) of them. To address this, we have conducted micropillar compression experiments on single-, bi-, and twinned-crystals of CoCrFeNi HEA and compared the results with those obtained with uniaxial tensile and compression tests on polycrystalline bulk samples. Results show that SRS, as well as the yield strength and plastic flow behavior, in single crystals are orientation dependent due to the differences in the maximum Schmid factors. While the high-angle grain boundaries arrest dislocation motion, TBs allow for dislocation transmission through them, which result in distinct mechanical responses. While the bi-crystal's deformation behavior is controlled by the ‘hard’ grain, twinned crystals exhibit an ‘averaged’ response. The large diversity of the reported SRS values in face centered cubic HEAs could be due to the varying fractions and thus contributions of annealing twins in the tested samples.

Effect of Carbon Nanotubes Addition on the Formability of Al-CNT Composite

Being very strong fibers, carbon nanotubes (CNTs) are excellent choice for various metal matrix composites (MMCs). Many researchers have reported amelioration in mechanical and thermal characteristics of the MMCs driven by CNTs addition. However, the effect of multi-walled carbon nanotubes (MWCNTs) addition on the formability of the MMC is relatively less addressed area. In present study, 0.2 wt. % MWCNTs were added in aluminum matrix using induction melting technique and their effect on the various aspect of metal forming characteristics were studied. Two different type of MMC were prepared i.e., re-melting flux (RM) MMC using re-melting flux and multi-functional flux (MF) MMC using multifunctional flux.The experimental results showed that the MF Flux MMC had ultimate tensile strength 65% more than pure aluminum. An analogous behavior was seen in true stress-true strain characteristics along with necking stress and tensile toughness. Contrarily, the hardening factor, strength coefficient, necking strain and strain rate sensitivity values decreased in case of RM Flux MMC and increased for MF Flux MMC as compare to the pure aluminum. This behavior of the materials could be attributed to the process variation used for the incorporation of nanotubes in aluminum matrix.

Microstructure and mechanical characterizations of additively manufactured high oxygen-doped titanium

In the present work, an additive manufacturing method of selective laser melting (SLM) was adopted to produce high oxygen-doped commercial purity titanium (Ti-O), with raw materials of high purity Ti and rutile (TiO2) powders. Microstructure and mechanical characterizations were then applied to the materials, which indicates that the TiO2 powders decompose under the SLM process and the oxygen atoms presented as solid solutes. Uniaxial quasi-static and dynamic compressive experiments suggest that the strength of SLM Ti-O drastically increases regardless of loading rate, yet the strain rate sensitivity is quite distinguished for pure Ti and the Ti-O alloy. Namely, strain rate jump tests demonstrated positive values of strain rate sensitivity for both samples, while under dynamic loading the Ti-O alloy and pure Ti exhibit obvious strain rate softening and strain rate hardening, respectively. The difference between these two SLM samples is then found to closely relate with their microstructures. Finally, a theoretical model is proposed to elucidate the abnormal strain rate softening.

Influence of Strain Rate Sensitivity on Cube Texture Evolution in Aluminium Alloys

The influence of strain rate sensitivity on development of Cube texture and on the morphology of Cube-oriented grains is often neglected in simulations approaches. Therefore, crystal plasticity simulations and experiments were performed up to 73 pct of thickness reduction for cold rolling on Al 6016. It is found, that low values of strain rate sensitivity promote Cube grains fragmentation and avoid formation of transition bands already at 50 to 55 pct thickness reduction. High values of strain-rate sensitivity cause formation of Cube transition bands leaving thin Cube grains in the microstructure and delay their fragmentation. Other texture components are affected by changes in strain rate sensitivity as well. The Copper volume fraction in the final texture diminishes as the strain rate sensitivity decreases, while Brass and S components of the beta fiber show a moderately higher volume fraction when the strain rate sensitivity increases. The final volume fraction of Goss is highest when the strain rate sensitivity is 10−2 but low if the strain rate sensitivity is 10−3 or raises up to 10−1. Recrystallization texture components (P, Q) are not affected by strain rate sensitivity, while the invGoss fraction decreases for high values of strain rate sensitivity. The results found in cold rolling crystal plasticity simulations were compared with experimentally determined Cube distribution and texture components obtained through thickness for Al6016 rolled at 80 m/min and 600 m/min. Further crystal plasticity simulations were performed to predict the influence of strain rate sensitivity during several hot rolling conditions where activity of non-octahedral slip systems was included in the simulations. During hot rolling, high values of strain rate sensitivity contribute to Cube stabilization and promote formation of Copper texture and delay Brass and S.

Hot deformation characteristics of Ti-6Al-4V

Abstract The flow behavior and hot deformation mechanisms of Ti-6Al-4Vin both β and (α + β) phase regions have been studied by compression tests in the temperature range 750 – 1100 °C and strain rate range 0.05 –15 s –1. The flow curves displayed discontinuous yielding at temperatures above 850 °C and high strain rates, typically 5 and 15 s–1. Flow oscillations were also observed under such conditions. The relative discontinuous flow stress drop increased with increasing temperature in the (α + β) phase region and reached a steady state in the β phase region. The discontinuous yielding is attributed to the sudden increase in the mobile dislocation density occurring due to the activation of β grain boundary sources during hot deformation. The apparent activation energy estimated using the standard kinetic analysis is about 246 kJ/mol in the β region and 621 kJ/ mol in the (α + β) region while the strain rate sensitivity values are 0.34 and 0.18, respectively. The mechanism in the β region has been suggested to involve grain boundary sliding and dynamic recovery of the β phase while in the (α + β) region a transient mechanism, involving dynamic recovery followed by post deformation recrystallization, has been proposed.

The analysis of thermoelectric powder compaction mechanisms within field-activated sintering of skutterudites and Heusler alloys

The analysis of the shrinkage rate of powders, based on the power-law creep model of a porous body, was carried out in this paper to calculate the compaction parameters of CoSb3-based skutterudites and Fe2VAl-based Heusler alloys within field-activated sintering. It was indicated that this method, which had already been used for metal and ceramic powders, is applicable for thermoelectric powders. The values of strain rate sensitivity were obtained, and the corresponding powder compaction mechanisms have been defined. The main creep mechanism for skutterudites was found to be a dislocation climb, that later was replaced by grain boundary sliding, and the last sintering stage was associated with diffusional creep. The main creep mechanism for Heusler alloys was grain boundary sliding, later replaced by diffusional creep. Keywords: field-assisted sintering, numerical simulation, powders compaction, skutterudites, Heusler alloys, thermoelectrics.

Compressive behavior of CNT-reinforced aluminum matrix composites under various strain rates and temperatures

Carbon nanotubes (CNTs) have been attractively used as reinforcement in metal matrix composites (MMCs) for advanced mechanical properties. However, the effects of loading rate and temperature on such materials are still to be resolved. In the present study, aluminum-based MMCs reinforced with CNTs of different contents were investigated under various loading rates and temperatures, along with microstructural examination by electron microscopy. It is found that the strengthening capability of CNTs/Al in comparison to PBM Al decreased with the temperature increasing and loading rate decreasing. However, microstructure characterization showed that CNTs played an essential role on hindering dislocation movement and stabilizing the microstructure, thus yielding an increased strength in Al-based MMCs at temperatures up to 603 K, regardless of the loading rate. A constitutive model that takes into the effect of temperature and microstructures account is then established to illuminate the role of CNTs. In addition, experimental results showed that the CNTs-reinforced Al composites exhibited an enhanced value of strain rate sensitivity and a reduced activation volume than the Al counterpart under all the tested temperatures. It is also found that the increased temperature softened the matrix and reduced the amounts of interactions of mobile dislocations with CNTs and forest dislocations, which led to the enhanced thermal activation volume of the CNTs/Al with temperature.

Creep deformation and rupture behavior of 10Cr-3Co-2W heat-resistant steel weldments in ultra supercritical power units

Creep deformation and rupture behavior of a martensitic heat resistant steel (10Cr-3Co-2W) welded joints prepared by vacuum electron beam welding for supercritical power generation unit are investigated employing traditional uniaxial tensile creep, 898 K (625 ℃) to 948 K (675 ℃) and 105 MPa to 250 MPa, and room temperature nanoindentation on different welding micro-zones. The rupture location shifts from base metal (ductile rupture) for the shortest creep time of 74 h, to heat affected zone (brittle Type IV rupture) for the rest crept specimens. Accordingly, a softening phenomenon is observed in fine grain heat affected zone in crept specimens for longer creep times, where voids, precipitates and reduced dislocation density are found and further discussed. The extrapolating creep rupture time under various creep temperature and stress combinations and the 100,000 h creep rupture strength for different temperatures are predicted employing the Larson-Miller parameter method, which satisfies the industry service condition. Compared with tensile creep the nanoindentation creep yields faster creep rates and higher stress exponents; and the largest strain rate sensitivity value (m) in weld metal implies that weld metal has the best creep resistance while heat affected zone and base metal are more likely to appear creep rupture.