Decrease In Strain Rate Sensitivity Research Articles

Dynamic Behavior of Additively Manufactured FeCoCrNi High Entropy Alloy

Additively manufactured face-centered-cubic high entropy alloys have a combination of high strength and good ductility, and are promising impact-resistant structural materials. However, the dynamic behavior of additively manufactured face-centered-cubic high entropy alloys is seldomly reported. In this study, FeCoCrNi high entropy alloy was fabricated, using the laser beam powder bed fusion technique, and dynamic tests were performed by means of a Split Hopkinson Pressure Bar. The high entropy alloy showed a more excellent combination of yield stress and toughness at high strain rates, than previously reported alloys. This was attributed to the dislocation cell structure of the additively manufactured FeCoCrNi HEA, which provided high local stress concentration, leading to the formation of microbands and deformation twins. The high entropy alloy showed higher strain rate sensitivity than the cast counterpart, at both quasi-static and strain rates over 3000 s−1. Interestingly, the yield stress kept stable at a strain rate from 1000 s−1 to 3000 s−1, showed a steep decrease of strain rate sensitivity and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.

The anterior cruciate ligament in murine post-traumatic osteoarthritis: markers and mechanics

BackgroundKnee joint injuries, common in athletes, have a high risk of developing post-traumatic osteoarthritis (PTOA). Ligaments, matrix-rich connective tissues, play important mechanical functions stabilising the knee joint, and yet their role post-trauma is not understood. Recent studies have shown that ligament extracellular matrix structure is compromised in the early stages of spontaneous osteoarthritis (OA) and PTOA, but it remains unclear how ligament matrix pathology affects ligament mechanical function. In this study, we aim to investigate both structural and mechanical changes in the anterior cruciate ligament (ACL) in a mouse model of knee trauma.MethodsKnee joints were analysed following non-invasive mechanical loading in male C57BL/6 J mice (10-week-old). Knee joints were analysed for joint space mineralisation to evaluate OA progression, and the ACLs were assessed with histology and mechanical testing.ResultsJoints with PTOA had a 33–46% increase in joint space mineralisation, indicating OA progression. Post-trauma ACLs exhibited extracellular matrix modifications, including COL2 and proteoglycan deposition. Additional changes included cells expressing chondrogenic markers (SOX9 and RUNX2) expanding from the ACL tibial enthesis to the mid-substance. Viscoelastic and mechanical changes in the ACLs from post-trauma knee joints included a 20–21% decrease in tangent modulus at 2 MPa of stress, a decrease in strain rate sensitivity at higher strain rates and an increase in relaxation during stress-relaxation, but no changes to hysteresis and ultimate load to failure were observed.ConclusionsThese results demonstrate that ACL pathology and viscoelastic function are compromised in the post-trauma knee joint and reveal an important role of viscoelastic mechanical properties for ligament and potentially knee joint health.

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.

Anomalous high strain rate compressive behavior of additively manufactured copper micropillars

Microscale dynamic testing is vital to the understanding of material behavior at application relevant strain rates. However, despite two decades of intense micromechanics research, the testing of microscale metals has been largely limited to quasi-static strain rates. Here we report the dynamic compression testing of pristine 3D printed copper micropillars at strain rates from ∼0.001 s−1 to ∼500 s−1. It was identified that microcrystalline copper micropillars deform in a single-shear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to ∼0.1 s−1, suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity by two orders of magnitude and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.

Redox reaction in a Mg/Nb2O5 nanocomposite processed by high-pressure torsion

Severe plastic deformation by high-pressure torsion can induce solid-state reactions, including redox reactions, which can affect the mechanical properties of metal matrix composites and hybrids. The present paper shows evidence of a redox reaction between magnesium and nanostructured niobium pentoxide. These chemical reactions release metallic niobium and magnesium oxide. Accordingly, aglomerations of nanocrystalline magnesium oxide is found in the magnesium matrix and a supersaturated solid solution is formed with niobium. Hardness and indentation creep tests show a minor increase in strength and decrease in strain rate sensitivity in the processed material, compared to data for pure magnesium in the literature.

Deformation and tribological behavior of ductile refractory high-entropy alloys

Refractory high-entropy alloys have attracted widespread attention as potential structural materials for high temperature applications. However, there is limited understanding of their high temperature deformation behavior and tribological response. Here, the temperature dependence of strain rate sensitivity and wear behavior was evaluated for two refractory high-entropy alloys, namely HfNbTiZr and HfNbTaTiZr. HfNbTaTiZr showed higher hardness, strength, and lower strain rate sensitivity compared to HfNbTiZr. The extrinsic strain rate sensitivity of both alloys obtained from micro-pillar compression was higher compared to the intrinsic values from nanoindentation due to the larger surface to volume ratio in micro-pillars leading to annihilation of dislocations and their easier movement. The decrease in strain rate sensitivity with increase in temperature was attributed to activation of both edge and screw dislocations as well as phonon drag controlled screw dislocation motion at high temperatures. Wear rate for both alloys increased with increase in sliding cycle frequency due to frictional heating. Wear rate also increased with increase in temperature from 298 K to 423 K and then decreased with further increase in temperature to 573 K. The underlying mechanism changed from abrasive wear at room temperature to extensive ploughing with fine wear debris at intermediate temperatures and oxidative wear at the higher temperatures.

Mechanical mixing of Mg and Zn using high-pressure torsion

High-pressure torsion (HPT) processing has proved to be a powerful tool to consolidate metallic particles and fabricate nanostructured metal-matrix composites with a wide range of compositions. In this study, HPT was used to fabricate different Mg-Zn composites at room temperature, and the evolution of microstructure and mechanical properties were analyzed by x-ray diffraction, scanning and transmission electron microscopy, dynamic hardness and Vickers microhardness tests. The results show that ultrafine-grained microstructures were achieved in all composites compositions. Smaller grain sizes are observed in Mg-rich phases near areas with significant segregations of Zn. The Mg-rich phase appears to retain less deformation than the Zn-rich phase. Bending and vortex phenomena, that are usually reported in microscale for materials mixed by HPT, are observed in nanoscale in this work. There is evidence of the MgZn2 intermetallic phase with a morphology that varies with composition. Higher levels of deformation imposed by HPT leads to an increase in hardening and a decrease in strain-rate sensitivity which are attributed to the tendency to form intermetallics and Zn segregations that prevent grain boundary sliding. Moreover, a model is proposed to explain the mixing of phases in microscale and its relation to the evolution of mechanical properties.

Creep behaviors of Ta-alloyed Cuzr-Based metallic glass composite

In the current work, the tensile creep behaviors of a (Cu0.5Zr0.5)92Ta8 metallic glass composite were investigated. The precipitates and the metallic glass matrix in the as-spun ribbons have nearly same compositions. The addition of Ta element enhances the thermal stability of binary Cu50Zr50 metallic glass. The apparent activation energies of tensile creep increase with the applied stress, and the values of strain rate sensitivity decrease from 0.39 to 0.16 with the increasing applied temperatures. The correlationship of activation energy and activation volume suggests that the largest activation energy corresponds to the smallest activation volume, and vice versa. The values of strain rate sensitivity are noticeably smaller than reported values, which is related to the softening of (Cu0.5Zr0.5)92Ta8 ribbons with the increasing creep temperatures.

Effect of strain and strain rate on the development of deformation heterogeneity during tensile deformation of a solution annealed 304 LN austenitic stainless steel: An EBSD study

Evolution of the microstructure and geometrically necessary dislocation (GND) structure was studied during tensile deformation of a solution annealed 304 LN austenitic stainless steel. Microstructures of the steel at varying engineering strains and strain rates (i.e., 1x10-4 s-1, 1x10-3 s-1 and 1x10-2 s-1) were analyzed using electron back scatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) at ambient temperature. EBSD was used to quantify the evolution of the GND structure and the martensite formed during tensile straining to reveal the deformation mechanisms in the presence of microstructural heterogeneities. The average GND density and the amount of deformation induced martensite increased with increasing plastic strain at a faster rate than expected, following a concave up pattern that accelerated GND formation as strain increased. ECCI was used to examine the dislocation storage arrangements with plastic strain. Various strain rates were imposed on the steel specimens and the results show that the average GND density increased only slightly with increasing engineering strain rate. This substantiates the small decrease in strain rate sensitivity of the steel that was observed as the plastic strain increased. Therefore, the steel is tolerant to a change in the strain rate during forming. This study provides an understanding for the way in which the plastic deformation behavior of the steel is influenced by the evolution of GND density in the presence of microstructural heterogeneities, and by deformation induced martensitic transformations.

Geometrically necessary dislocations induced size effect in the torsional stress relaxation behavior of thin metallic wires

Torsional stress relaxation experiments on thin copper wires are performed. A size effect is observed. That is a faster relaxation deformation arises in a thinner wire. As the wire diameter increases, the activation volume increases but the strain rate sensitivity decreases. A greater effect of geometrically necessary dislocations on the relaxation deformation accounts for the size effect. In a thinner wire, geometrically necessary dislocations undergo fewer obstacles to reach the wire center and then neutralize, causing a more rapid evolution of mobile dislocations.

Strain-rate sensitivity of die-cast magnesium-aluminium based alloys

The strain-rate effect in die-cast magnesium-aluminium based alloys under quasi-static strain rates ranging from 10−6 to 10−1s−1 was investigated. The strain-rate sensitivity was shown to decrease with increasing aluminium solute level in the matrix. Microstructural examination by electron backscattered diffraction (EBSD) revealed that deformation twinning is more active in the alloys with lower strain-rate sensitivity. It is suggested that the decrease in strain-rate sensitivity with increasing aluminium solute level is likely due to dynamic strain ageing from the interaction between aluminium solute and dislocations. The correlation between strain-rate sensitivity and ductility in AE44 is briefly discussed.

Solid solution effects on hardness and strain rate sensitivity of nanocrystalline NiFe alloy

Solid solution effects on the hardness and strain rate sensitivity of nanocrystalline (NC) alloys were studied using electro-deposited Ni and NiFe thin films. Nanoindentation testing showed enlarged hardness with increasing Fe content, and grain size reduction was proposed to be the main mechanism underlying such strengthening. Strain rate sensitivity was found to be effectively reduced as Fe content was increased. Compared with pure NC Ni, this behavior was not expected as NC NiFe alloy possesses much smaller grain size, thus should exhibit higher strain rate sensitivity. As a consequence of grain boundary pinning via Fe solute, the changeover in dominant deformation mechanism from intergranular to intragranular plasticity was proposed to interpret the unexpected decrease in strain rate sensitivity with increasing Fe content.

Hot Forming of Mg AZ31B Alloy Sheet Processed by Warm Rolling

Mg alloy AZ31B is of interest for hot forming because it can achieve a superplastic response at high temperatures and slow strain rates. As temperature decreases and forming rate increases, its strain-rate sensitivity decreases and significant plastic anisotropy can arise. These effects are the result of a transition in deformation mechanisms from grain-boundary-sliding (GBS) to dislocation-climb (DC) creep. However, sheet production using warm rolling can produce a material with a smaller grain size and weaker basal texture. These microstructural changes promote GBS creep and decrease the degree of anisotropy under DC creep. Microstructural and tensile data are presented to show these effects at 350 and 450C through comparisons to a similar material having a more usual microstructure.

Role of Si modification on the compressive flow behavior of Al–Si based alloy

The flow characteristics of a near-eutectic heat-treated Al–Si based cast alloy have been examined in compression at strain rates varying from 3×10−4 to 102s−1 and at three different temperatures, i.e., room temperature (RT), 100°C and 200°C. The dependence of flow behavior on modification is examined by testing the alloy in both the unmodified and modified conditions. Modification has strong influence on strain rate sensitivity (SRS), strength and work hardening behavior of the alloy. The strength of the alloy is found to increase with increase in strain rate for both the conditions. The increase is more rapid above the strain rate of 10−1s−1 for the unmodified alloy at all the temperatures. This rapid increase is observed at 1s−1 at RT and 100°C, and at 10−2s−1 at 200°C for the modified alloy. The thermally dependent process of the Al matrix is rate controlling in the unmodified alloy. On the other hand, the thermally dependent process of both Al matrix and Si particles are rate controlling, which is responsible for the higher strain rate sensitivity (SRS) in the modified alloy. The unmodified alloy exhibits a larger work hardening rate than the modified alloy during the initial stages of straining due to fiber loading of unmodified Si particles. However, the hardening rate decreases sharply at higher strains for the unmodified alloy due to a higher rate of Si particle fracture. Thermal softening is observed for both alloys at 200°C due to precipitate coarsening, which leads to a decrease in SRS at higher temperatures. Stress simulations by microstructure based finite element method support the experimentally observed particle and matrix fracture behavior. Negative SRS and serrated flow are observed at lower strain rate regime (3×10−4 to 10−2s−1) at RT and 100°C, in both alloys. The critical onset strain is found to be lower and the magnitude of serration is found to be higher for the modified alloy, which suggests that, in addition to dynamic strain aging, Si particle size and morphology also play a role in serrated flow.

Compressive flow behavior of Al–Si based alloy: Role of heat treatment

The flow characteristics of a near eutectic Al–Si based cast alloy have been examined in compression at strain rates varying from 3×10−4 to 102s−1 and at three different temperatures, i.e., room temperature (RT), 100°C and 200°C. The dependence of the flow behavior on heat treatment is studied by testing the alloy in non-heat treated (NHT) and heat treated (HT) conditions. The heat treatment has strong influence on strain rate sensitivity (SRS), strength and work hardening behavior of the alloy. It is observed that the strength of the alloy increases with increase in strain rate and it increases more rapidly above the strain rate of 10−1s−1 in HT condition at all the temperatures, and at 100°C and 200°C in NHT condition. The thermally dependent process taking place in the HT matrix is responsible for the observed greater SRS in HT condition. The alloy in HT condition exhibits a larger work hardening rate than in NHT condition during initial stages of straining. However, the hardening rate decreases more sharply at higher strains in HT condition due to precipitate shearing and higher rate of Si particle fracture. Thermal hardening is observed at 200°C in NHT condition due to precipitate formation, which results in increased SRS at higher temperatures. Thermal softening is observed in HT condition at 200°C due to precipitate coarsening, which leads to a decrease in SRS at higher temperatures. Stress simulations by a finite element method support the experimentally observed particle and matrix fracture behavior. A negative SRS and serrated flow are observed in the lower strain rate regime (3×10−4–10−2s−1) at RT and 100°C, in both NHT and HT conditions. The observations show that both dynamic strain aging (DSA) and precipitate shearing play a role in serrated flow.

The thermal stability of high-energy ball-milled nanostructured Cu

The thermal stability of nanostructured (NS) Cu prepared by high-energy ball milling was investigated. The as-prepared samples were isothermal annealed for 1h in the temperature range of 200–1000°C. Effects of annealing on NS Cu samples were studied by means of Vickers hardness test, differential scanning calorimetry (DSC) and stress relaxation test. The exceptional high microhardness of as-prepared Cu sample of 1.7GPa was not detected to decrease after annealing at 500°C for 1h with corresponding small value of activation volumes V* of 22.6b3 and high value of strain rate sensitivity m of 0.0176. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volume and decrease of strain rate sensitivity. The present investigation demonstrates that the thermal stability of NS Cu prepared by high-energy ball milling is determined by not only the grain size but also the microstructure of grain boundaries, and during annealing process, the strain release process occurred prior to the grain growth process, therefore, the NS Cu has a relatively high thermal stability.

Influence of Precipitate Phase on the Strain Rate Sensitivity of Mg-Gd-Y Alloy

Supersaturated Mg-Gd-Y alloy followed by aging at 225 °C with different times were subjected to quais-static and dynamic strain rates to determine the influence of precipitate phase β′ on the strain rate sensitivity of magnesium alloy. Strain rate sensitivity (SRS) decreases with the increase of the size of β′. SRS decreases from initial condition to peak-aged condition due to the β′ increases the athermal component of flow stress. On the other hand, the influence of precipitate interfaces on dislocation generation and storage mechanisms may be responsible for the decrease of SRS from peak-aged to over-aged condition.

Impact deformation behaviour of Ti–6Al–4V alloy in the low-temperature regime

The impact response of Ti–6Al–4V alloy is investigated using a compressive split-Hopkinson pressure bar at strain rates of 1.0 × 10 3 s −1, 3.0 × 10 3 s −1 and 4.3 × 10 3 s −1 and temperatures of −150 °C, 0 °C and 25 °C, respectively. It is shown that for a constant temperature, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, while the activation volume decreases. Meanwhile, for a constant strain rate, the activation volume increases with increasing temperature, while the flow stress, work hardening rate and strain rate sensitivity decrease. Scanning electron microscopy (SEM) observations reveal that the fracture surfaces are characterised by a transgranular dimpled structure, indicating that Ti–6Al–4V alloy has excellent ductility. The density of the dimples increases with an increasing strain rate or increasing temperature. Transmission electron microscopy (TEM) observations show that the dislocation density increases with an increasing strain rate, but decreases with an increasing temperature. The linear correlation between the square root of the dislocation density and the true stress confirms the existence of a Bailey–Hirsch type relationship. Finally, the strengthening effect observed at higher strain rates and lower temperatures is attributed to a greater dislocation density.

Effects of twin and surface facet on strain-rate sensitivity of gold nanowires at different temperatures

We use classical molecular-dynamics simulations to examine the strain-rate sensitivity of single-crystalline and twinned Au nanowires (NWs) with a diameter of 12.3 nm deformed in tension at temperatures between 10 K and 450 K. It is found that the strain-rate sensitivity above 100 K is significantly smaller in twinned Au NWs with perfectly circular cross-section than in similar NWs without twins, while the activation volume remains in the same range of $1{b}^{3}--15{b}^{3}$ with $b$ the magnitude of Burgers vector. This behavior is markedly different from that generally observed in bulk face-centered cubic metals where addition of nanoscale twins increases both strength and strain-rate sensitivity. Furthermore, our simulations show a threefold decrease in strain-rate sensitivity in twinned Au NWs with zigzag morphology constructed by assembly of {111} surface facets in comparison to the different types of circular Au NWs. The rate-controlling deformation mechanisms related to surface dislocation emission and twin-slip interaction, and their dependence on temperature and surface morphology are analyzed in detail. The combination of ultrahigh strength and decreased sensitivity to strain-rate predicted above 100 K in twinned Au NWs with faceted surface morphology holds great promise for creating metallic nanostructures with increased failure resistance to extreme loading conditions.

Effect of Annealing Treatments on Strain Rate Sensitivity and Anisotropy in a Magnesium Alloy Processed by Severe Rolling

The effect of annealing treatments on the evolution of the strain rate sensitivity with strain of AZ61 magnesium alloy processed by severe rolling was investigated and related to previous results on normal plastic anisotropy (r-value). The various annealing treatments produce two effects on the microstructure: grain coarsening and slight weakening of the texture. In addition, these treatments produce a noticeable decrease in strain rate sensitivity and an increase of work hardening rate that is related to the decrease of the anisotropy. It is concluded that these effects are related to an enhanced contribution of basal slip as a consequence of the microstructural changes induced by the annealing treatments.