Rate-controlling Deformation Mechanism Research Articles

Rate controlling deformation mechanisms in SS316L stainless steel manufactured using laser powder bed fusion technique

We conducted uniaxial tensile strain rate jump tests to account for the strain rate controlling mechanisms in the additively manufactured SS316L stainless steel. Special emphasis is placed on the role of high dislocation density and chemical segregation on dictating the rate sensitive behavior. Lowest strain rate sensitivity is found, despite the highest dislocation density in the as build state, which signified the role of solute segregation at cell walls in increasing the activation volume and forms the strong basis for rejecting dislocation forest as the strain rate controlling obstacles. Consistent increase in the strain rate sensitivity with annealing is found consistent with the chemical homogenisation of the solutes causing smaller inter-obstacle spacing causing decreasing activation volume. Experimental findings are supported by CALPHAD simulations and mechanical threshold stress (MTS) modelling framework. Significant difference between the instantaneous and steady state rate sensitivity is understood using the dynamic pileup dislocation density readjustment mechanism causing considerable flow transient while adjusting the mobile dislocation density on strain rate change in the presence of solute decorated dislocation cells. Present investigation helps in gaining newer insights into the deformation mechanisms of the additively manufactured alloys featuring cellular microstructures.

Rate-controlling deformation mechanisms in drawn tungsten wires

ABSTRACT Undeformed tungsten suffers from a brittleness that makes it unsuitable for applications at low temperatures. Cold-worked tungsten materials such as drawn wires or rolled plates can however show considerable ductility even at low temperatures. The reason for this behaviour is so far not understood. We investigated a series of potassium-doped tungsten wires that were subsequently drawn from one sintered ingot, making them chemically identical. Hence, the properties of the wires could be studied without the influence of different impurity levels. Using transient mechanical tests, namely repeated stress relaxation experiments and strain-rate jump tests, the effective activation volumes and strain-rate sensitivities m of the wires were determined at room-temperature. Based on the obtained results, it is deduced that the motion of screw dislocations by formation and dissociation of kink-pairs is controlling the rate of plastic deformation in all wires that show plasticity at room temperature. It is hence concluded that the ductility of drawn tungsten wires at low temperatures is not due to a change in the rate-controlling deformation mechanisms, but should be a consequence of the microstructural and textural changes during wire drawing.

Heterogeneity of deformation, shear band formation and work hardening behavior of as-printed AlSi10Mg via laser powder bed fusion

The deformation heterogeneities and micro-to macro-structural evolution of shear bands were thoroughly characterized and assessed in correlation with the work hardening behavior of laser powder bed fused as-built AlSi10Mg using compression testing at 78 and 293 K in a wide range of constant strain rates. Using the Kocks-Mecking model, work hardening behavior was correlated with the evolved microstructure and rate-controlling deformation mechanisms. It was found that initial work hardening (athermal hardening rate) is dependent on the strain rate at both 78 and 293 K, where it increases by a transition from shearing to looping of nano-precipitates in the 10−2-10−1 s−1 strain rate range. Stages II (hardening stage) and Stage III (dynamic recovery stage) of work hardening were also observed and their dependence on temperature and strain rate was assessed. The former includes the dislocation-obstacle interactions resulting in work hardening, whereas the latter consists of recovery processes where density of dislocations decreases. Shear banding was reported for higher strain rates manifesting in micro- and then evolving into macro-shear bands with deformation, which led to the increase in the dislocation recovery rate in Stage III of work hardening. Compressive work hardening mechanisms were discussed for both 78 and 293 K temperatures.

High Strain Rate Quasi-Superplasticity Behavior in an Ultralight Mg-9.55Li-2.92Al-0.027Y-0.026Mn Alloy Fabricated by Multidirectional Forging and Asymmetrical Rolling.

To explore new approaches to severe plastic deformation and the ductility of a multicomponent magnesium-lithium alloy, an ultralight microduplex Mg-9.55Li-2.92Al-0.027Y-0.026Mn alloy was made by novel multidirectional forging and asymmetrical rolling, and the superplasticity behavior was investigated by optical microscope, hot tensile test, and modeling. The average grain size is 1.9 μm in this alloy after multidirectional forging and asymmetrical rolling. Remarkable grain refinement caused by such a forming, which turns the as-cast grain size of 144.68 μm into the as-rolled grain size of 1.9 μm, is achieved. The elongation to failure of 228.05% is obtained at 523 K and 1 × 10-2 s-1, which demonstrates the high strain rate quasi-superplasticity. The maximum elongation to failure of 287.12% was achieved in this alloy at 573 K and 5 × 10-4 s-1. It was found that strain-induced grain coarsening at 523 K is much weaker than the strain-induced grain coarsening at 573 K. Thus, the ductility of 228.05% is suitable for application in high strain rate superplastic forming. The stress exponent of 3 and the average activation energy for deformation of 50.06 kJ/mol indicate that the rate-controlling deformation mechanism is dislocation-glide controlled by pipe diffusion.

Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals

Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. Here we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.

Class I type creep behavior of coarse-grained Al0.5CoCrFeMnNi high entropy alloy

The high-temperature deformation behavior of homogenized cast Al0.5CoCrFeMnNi with a coarse-grained face centered cubic (FCC) phase and a small amount of body centered cubic phase-dispersed within the FCC matrix grains was investigated using tensile creep and elongation tests at three temperatures of 1023, 1098 and 1173 K in a wide strain rate range between 10−8 and 10−1 s−1. As the strain rate increased, the strain rate regime associated with n = 3 (where n is the stress exponent) and n = 5 and power law breakdown (n > 7) appeared in sequence. As the temperature increased, the upper limit of the strain rate for n = 3 increased. The inverse primary creep behavior occurred and subgrain formation was not observed at the low strain rates and high temperatures associated with n = 3, strongly suggesting that solute drag creep is the rate-controlling deformation mechanism. Furthermore, the deformation mechanism of Al0.5CoCrFeMnNi was very similar to that of binary Al–Mg alloys known as typical class I solid solution alloys. The concept of breakaway stress from solute atmosphere could explain the transition of deformation mechanism from solute drag creep to dislocation climb creep at high strain rates.

Low temperature deformation mechanisms of polycrystalline CoZr and Co39Ni11Zr50 B2-type intermetallic compounds

The B2-type intermetallic compounds CoZr and Co39Ni11Zr50 were deformed in tension at low temperatures. While CoZr is ductile down to 4 K, Co39Ni11Zr50 becomes brittle below 125 K due to a martensitic phase transformation. Thermal activation analysis shows that CoZr follows the Cottrell-Stokes law indicating forest dislocation cutting as the dominant rate-controlling deformation mechanism, similar to face-centered cubic metals. The moderate ductility of both intermetallic compounds at low temperatures may qualitatively be related to a significant metallic character of bonding giving rise to a low Peierls stress estimated for primary {110}<100> slip and most likely also leads to an easier activation of secondary {110}<110> slip which was proven by transmission electron microscopy. Secondary slip is necessary for the fulfillment of the von Mises criterion for homogeneous plastic deformation of polycrystalline materials. The present results generalize the findings made on the ductile rare earth intermetallics YAg and YCu and, therefore, may help to search for other ductile systems in the broad class of intermetallic compounds.

A natural example of brittle-to-viscous strain localization under constant-stress conditions: a case study of the Kellyland fault zone, Maine, USA

AbstractThis article integrates field, powder X-ray diffraction and microstructural data to constrain deformation mechanisms in and the rheology of granite-derived fault rocks exposed along the SE side of the crustal-scale, strike-slip Kellyland fault zone. Deformation in this area of the Kellyland fault zone localized during cooling and is marked by (1) foliated granite, (2) a ∼50 m wide band of pulverized foliated granite, (3) a ∼2.8 m wide breccia zone hosting coeval shear zones, and (4) a &gt;100 m wide ultramylonite zone. The earliest fabric in the foliated granite is defined by elongated quartz grains, and quartz dislocation creep was the rate-controlling deformation mechanism. Seismogenic deformation initiated when recorded flow stresses reached 96–104 MPa at temperatures of 400–450 °C and is marked by coeval pulverization and formation of breccia. Interseismic viscous creep at similar flow stresses is recorded by mutual cross-cutting relationships between breccia-hosted shear zones, brittle fractures and pseudotachylyte. Field and microstructural observations indicate that breccia-hosted shear zones are low-strain equivalents of the &gt;100 m wide ultramylonite zone, and seismogenic deformation abated as the ultramylonite formed. The rheology of ultramylonites was governed by grain-size-sensitive creep at 112–124 MPa flow stresses. Hence, from the onset of seismogenesis, the Kellyland fault zone was likely a constant-stress system wherein the rate-controlling mechanism shifted from episodic seismogenic slip and interseismic viscous creep to steady state grain-size-sensitive creep in ultramylonites derived from brittle fault rocks. Flow stresses recorded by these rocks also imply that the whole zone was relatively weak if the brittle–viscous transition and uppermost viscous zone are the strongest part of the crust.

High temperature nanoindentation as a tool to investigate plasticity upon phase transformations demonstrated on Cobalt

A new field of application for high temperature nanoindentation as a complimentary method to understand the mechanics of plasticity upon bulk phase transformations in thermodynamic equilibrium is introduced. The feasibility is outlined on polycrystalline Cobalt involving a low-temperature hexagonal closed packed phase, and above 700K, a high-temperature face centered cubic phase, which was conventionally characterized by means of differential scanning calorimetry and high temperature X-ray diffraction. Strain rate sensitivity, activation volume and activation energy of plastic deformation were determined up to 873 K to identify the rate-controlling deformation mechanism. From RT to 473 K plasticity was found to be controlled by lattice friction on the basal plane, where dislocations have to overcome the Peierls barrier and deformation twinning was apparent in the hexagonal phase. The thermal activation of cross slip lead to reduced twinning actions when the phase transition temperature is approached. In the high temperature face centered cubic phase deformation was found to be controlled by cutting of forest dislocations and no deformation twinning was observed.

Temperature and strain-rate dependence of flow stress of nanocrystalline nickel fabricated by electrolytic deposition

ABSTRACT Nanocrystalline nickel, having a grain size of about 40 nm, was fabricated by electrolytic deposition and strain-rate jump tests were performed at temperaures between 77 and 473 K to obtain the strain-rate sensitivity and the activation volume . The values of changed from about 0.05 to about 0.005 with increasing stress, indicating that the deformation is governed by the motion of glide dislocations. increased as the temperature increased from 77 to 200 K, but decreased with further increase in temperature. It is concluded that the rate-controlling deformation mechanism in nanocrystalline nickel is temperature sensitive and changes from forest dislocation cutting to dislocation bowing-out and depinning from grain boundaries as the temperature increases.

Understanding creep mechanisms of a Cu-Cr-Nb alloy by testing under constant structure conditions

Stress reduction creep tests conducted on a Cu-Cr-Nb alloy (GRCop-84) at 923 K have confirmed local dislocation climb to be the rate-controlling deformation mechanism. The activation energy of creep was measured to be consistent with that of self-diffusion in Cu matrix. An internal back stress of approximately -9 MPa was identified to act on the rate-controlling dislocations, which is believed to be the sum of the back stress for dislocation-particle interaction and the forward stress for dislocation-dislocation interaction. This back stress, however, does not lead to a true threshold in the framework of thermally activated dislocation motion.

Effect of temperature on the hot deformation behavior of AZ80 magnesium alloy

The present research studies the hot forgeability of a cast AZ80 alloy by means of uniaxial compression tests using a Gleeble® 3500 thermal-mechanical simulator. Compression tests were conducted for temperatures ranging from 300 °C to 450 °C, and constant true strain rates ranging from 0.001 s−1 to 1 s−1, while the characterization of deformed samples for microstructure and texture was performed only for the samples deformed within 300 °C–400 °C, at a strain rate of 0.001 s−1. The samples were deformed to true strains of 0.05, 0.15, 0.4 and 1.0, to study the microstructure and texture evolution with deformation strain, while the characterization was performed using optical microscopy, SEM-EDX, and EBSD. Analysis of the flow stress data suggested a transition in the rate controlling deformation mechanism from dislocation cross-slip at 300 °C to dislocation climb at 400 °C, with a corresponding transition in the dominant DRX mechanism from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). The thermodynamic stability of the Mg17Al12 phase (which is the dominant secondary phase in the AZ80 alloy) varied over the test temperature range, and significantly impacted the microstructure and texture evolution at different test temperatures. At 300 °C, multiple morphologies of the Mg17Al12 precipitates were present in the microstructure during deformation, and affected the DRX behavior of the material differently. At 400 °C, the precipitates rapidly dissolved in the α-Mg matrix, while the DRX took place by the grain boundary bulging mechanism, with the newly formed DRXed grains preserving the deformation texture. The study shows that the effect of temperature on the hot deformation behavior of the AZ80 alloy is especially pronounced due to a changing amount of the Mg17Al12 phase in the microstructure.

Low temperature deformation mechanisms of single crystalline intermetallic compound YAg

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that the low yield stress and high work-hardening rate observed cause the moderate ductility at low temperatures.

Low Temperature Deformation Mechanisms of Single Crystalline Intermetallic Compound Yag

YAg belongs to the family of B2-type rare earth intermetallic compounds that in polycrystalline form exhibit moderate low temperature ductility. In this work, YAg single crystals have been deformed at low temperatures down to 4 K at which significant ductility was observed. The activation parameters and their dependence on stress and temperature, as well as the comparison with the deformation behavior of polycrystalline YAg, indicate that the cutting of forest dislocations is the rate-controlling deformation mechanism, similar to face-centered cubic metals. The results suggest that a low Peierls stress causes the moderate ductility at low temperatures.

Operation of solute-drag creep in an AlCoCrFeMnNi high-entropy alloy and enhanced hot workability

Strong evidence for creep controlled by solute-drag processes, such as the yield-drop phenomenon, a stress exponent of 3 and a sluggish tendency to form subgrains, was found during hot compressive deformation of a cast BCC AlCoCrFeMnNi high-entropy alloy with coarse grains; these characteristics are typically observed in conventional metals that exhibit solute-drag creep. These three features were not observed in the FCC CoCrFeMnNi alloy with coarse grains, indicating that the addition of aluminum causes solute-drag creep to arise as the rate-controlling deformation mechanism. Compared with the FCC Al0.5CoCrFeMnNi matrix (with a small amount of BCC phase), the AlCoCrFeMnNi exhibited more distinct characteristics of solute-drag creep. This is likely because the Al concentration in the matrix of AlCoCrFeMnNi is notably higher than that in the matrix of Al0.5CoCrFeMnNi (17.8 vs. 6.5 at.%). Also, the solute-drag creep extended to an increased strain rate and a decreased temperature in AlCoCrFeMnNi because the critical stress for dislocations to break away from the solute atmospheres increases as the Al solute concentration in the matrix increases. This outcome improves hot workability of AlCoCrFeMnNi according to the processing maps because the power dissipation efficiency increases and the flow instability regime shrinks at high strain rates and low temperatures.

Primary hot working characteristics of an as-cast and homogenized nickel base superalloy DMR-742 in the sub and super-solvus temperature regime

Primary deformation behaviour of a medium alloyed nickel base superalloy DMR-742 in the as-cast and homogenized condition (furnace cooled) was evaluated in the sub-solvus and super-solvus temperature regimes. The flow stress in the sub-solvus regime was found to be highly sensitive to temperature. The contribution of back stress to the flow stress sensitivity was estimated using the microstructure dependent analytical equations and compared with the back stress estimated using experimental steady state flow stress. It was established that Orowan looping or by-passing mechanism contributes to the back stress resulting in high flow stress sensitivity in the sub-solvus regime. In the super-solvus regime, the material was relatively temperature insensitive and exhibited discontinuous dynamic recrystallization (DDRX) with a characteristic power dissipation value of 38–44% in three different temperature-strain rate regimes. Disregarding the industrially non-viable low strain rate regime, the recrystallization kinetics in the moderate and high strain rate regime was estimated using grain size factor compensated grain orientation spread maps. A higher rate of recrystallization observed in the material deformed at high temperature-strain rate was attributed to relatively strong dislocation interactions and weak recovery. The true stress exponent, activation volume and energy estimated using back stress and elastic modulus compensated flow stress revealed that diffusion controlled climb and cross-slip as the rate controlling deformation mechanism. Side-pressing under high secondary tensile stress conditions was used to evaluate the workability and validate the experimental findings.

Mechanical Properties of Laser Shock-Peened Regions of SS316LN and SS304 Studied by Nanoindentation

Nanoindentation has been used to evaluate various mechanical properties of affected regions of laser shock-peened (LSP) stainless steels, SS316LN and SS304 plates. Depending on the applied laser parameters (laser energy, pulse width), the hardness was varied in the range 3.5–6.2 GPa in comparison with the untreated sample hardness of 2.93 GPa for SS316LN. Similarly in SS304, the hardness has been improved to 5.73 GPa in comparison with the untreated specimen hardness of 2.57 GPa. The improvement in the hardness for the laser-treated stainless steel targets is attributed to various factors viz., increment in internal frictional stress, dislocation–dislocation interaction and solute–dislocation interactions. Effect of laser energy is not appeared in the hardness values but a difference in hardness values with the pulse width has been observed. The deformation characteristics such as strain rate sensitivity and activation volume were evaluated from the indentation creep data. Notable change in the strain rate sensitivity is not observed for the laser-treated samples in comparison with the untreated samples and the range of values measured between 0.01 and 0.02. The activation volumes for both the treated and untreated samples were in the range, 100–200 b3. There was a slight decrease in activation volume in the laser-treated samples. Several rate-controlling deformation mechanisms appear to be operative in these treated samples, in addition to the dislocation-based mechanisms.

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.

Strain rate sensitivity and activation volume of AISI 321 stainless steel under dynamic impact loading: Grain size effect

The rapid change in strain rate sensitivity (SRS) of metals at a critical true strain rate (ε̇Tc) is commonly attributed to change in rate-controlling deformation mechanism from thermally-activated process to viscous drag. In this study, metastable AISI 321 stainless steel with grain sizes of 0.24 μm (ultrafine [G1]), 3 μm (fine [G2]) and 37 μm (coarse [G3]) was deformed at high strain-rate. A change in SRS with increasing ε̇T is observed only in G2 and G3 and it occurs at ε̇Tc of ~5900 and 6800s−1, respectively; suggesting that grain size have an effect on the switch of rate-controlling deformation mechanisms. While the SRS of G1 is 0.101 and unchanged, those of G2 and G3 change from 0.094 to 0.326 and 0.091 to 0.634, respectively, once ε̇Tc is reached. Similarly, the estimated activation volume in G1 specimen is ~1.57b3, while those of G2 and G3 specimens change from ~2.95b3 to ~0.65b3 and from ~4.10b3 to ~0.45b3, respectively. Analysis of the deformed specimens using XRD, EBSD and TEM techniques revealed that the complexities in the activated deformation mechanisms in G2 and G3 specimens led to a change in ε̇Tc. Constant SRS in G1 specimen indicated no change in the operational deformation mechanisms.

Understanding activation energy of dynamic recrystallization in Inconel 718

Hot compression tests were performed on Inconel 718 to study high temperature flow stress-strain response and activation mechanism in the hot deformation. To model the flow stress response, we employed two physically based models namely Schӧcks-Seeger-Wolf and Kocks model in addition to extensively used Sellers model. Flow curves from all three models were compared and it was found that predictions of Sellers model closely resemble the experimental results. However, activation energy calculated using Sellers model differs significantly from the activation energy of micro mechanism as reported in the literature. Schӧcks-Seeger-Wolf model predictions are in close agreement with experiments at low strain rate and the activation energy corresponds to diffusion activation energy. Though Kocks model shows much deviation from the experimental results, qualitative comments on the activation energy profile can be made using this model. We utilized hardening models to derive a constitutive model for DRX and simultaneously establish an understanding of rate controlling deformation mechanism in Inconel 718.