Change In Deformation Mechanism Research Articles

Orientation-selected micro-pillar compression of additively manufactured 316L stainless steels: Comparison of as-manufactured, annealed, and proton-irradiated variants

Irradiation response and deformation mechanisms of additively manufactured (AM) 316 L stainless steel were studied by atomic scale characterization and micro-pillar compression. The AM 316 L stainless steels were fabricated by direct energy deposition, a laser-based additive manufacturing process. Irradiation with 2 MeV protons at 360 °C was performed to create ∼1.8 displacements-per-atom (dpa) damage in AM 316 L. Deformation behaviors of the as-manufactured, annealed, and proton-irradiated variants were studied, focusing on the effects of manufacturing-induced pores, residual stress, and irradiation-introduced defects (dislocation loops and voids). Micro-pillars were prepared from grains of pre-selected orientation, avoiding contributions of grain boundaries and allowing determination of resolved shear stress on {111} glide planes. Transmission electron microscopy was used to characterize the pre- and post-deformation microstructure. It was found that in the as-manufactured alloy variant, moving dislocations were the major deformation carrier, with noticeable blocking by fabrication-induced pores, In the annealed variant, hardness was reduced, and deformation was also accomplished by dislocation gliding. In the proton-irradiated variant, significant twinning was observed. Comparing measured resolved shear stress and predicted critical stress for dislocation dissociation, we conclude that irradiation hardening became high enough to activate twinning. Therefore, the deformation mechanism changes from dislocation gliding to twinning. The study is important for both processing optimization and performance evaluation of AM alloys for reactor applications.

Implication of Dynamic Recrystallization Mechanism for the Exhumation of Lower Crustal Rocks: A Case Study in the Shear Zones of the Ambaji Granulite, NW India

Abstract Shear zones are important channels for the exhumation of lower crustal rocks. The Ambaji granulite of the Aravalli-Delhi mobile belt (ADMB) has been exhumed along several shear zones, and earlier studies have shown a two-stage exhumation process during a continuous compressional tectonic event, consisting of an initial phase of vertical flow that brought the granulites to the brittle-ductile transition zone and a successive phase during which the granulite underwent a lateral flow. In this contribution, we studied the microtectonics of granulites by analyzing the dynamic recrystallization behaviour of quartz, while the granulite was passing through the vertical flow regime to the horizontal flow regime. We show that the dynamic recrystallization process assists the flow pattern at different levels of exhumation. The vertical flow is dominated by grain boundary migration (GBM), registering high temperatures for recrystallization between 490 and 600°C and low flow stresses of 12-15 MPa. The horizontal flow at the brittle-ductile transition (BDT) is characterized by bulging (BLG) and subgrain rotation (SGR), which occurred at low temperatures of 390-490°C and high flow stresses of 18-26 MPa. Strain rates are between 1.20×10−12 and 7.26×10−14/s. For the ductile exhumation of the granulite, we suggest that at depths of ~22 km, the granulite exhumed in a vertical direction facilitated by GBM. Once the granulite reached the BDT, at ~16 km depth, the material flowed laterally assisted by BLG and SGR. Once an exhuming body reaches the BDT, the deformation mechanism changes to BLG-SGR, and the only direction in which the material can move further is in the horizontal plane.

Load redistribution in eutectic high entropy alloy AlCoCrFeNi[formula omitted] during high temperature deformation

The load redistribution between and within phases in eutectic high entropy alloy AlCoCrFeNi2.1 was measured using in-situ neutron diffraction during tensile deformation at 973 K. The load partitioning between phases is reversed compared to lower temperatures, with L12 becoming the stronger phase. The evolution of the orientation-specific stresses and strains in the L12 phase suggests that cube slip dominates the response. The low strength, internal load transfer and ideally plastic response of the B2 phase indicate a change in deformation mechanism compared to lower temperatures.

Creep-Fatigue Response, failure mode and deformation mechanism of HAYNES 282 Ni based superalloy: Effect of dwell position and time

Creep-fatigue (C-F) response of HAYNES-282 as a function of dwell position and time, along with corresponding deformation micro-mechanisms and failure modes are presented here. An inertia effect in inelastic strain beyond the total-strain reversal point is reported for the first time. Damage (DC-F) in the material is found to decrease in the order of both-dwell, compressive-dwell and tensile-dwell. DC-F, an effective stress relaxation per cycle and strain energy density are found to correlate well with life. A change in deformation mechanism from shearing to Orowan-looping explains the difference in damage as a function of dwell position. HAYNES 282 shows an inherent anisotropy in strain hardening behavior, being higher under compressive loading. We postulate that compressive dwell would be more damaging in such material, with dwell-time-softening nullifying the beneficial effect of strain hardening of the same segment. Failure occurs through a mixed-mode, with dominance of either transgranular or intergranular fracture, depending on dwell position and time.

Postponing the fracture strain of ultrafine-grained 316LN steel by sequential activations of deformation modes at 77 K

The yield strength of 316LN stainless steel is always insufficient due to the single-phase austenitic structure. We processed bulky specimens with recrystallized ultrafine-grained microstructure by cold rolling and annealing treatments. The specimens are strong and ductile at 293 K as a result of dislocation-dominated plastic deformation process. In contrast, the yield strength was doubled, and the ductility was further enhanced due to the sequentially activated deformation modes at 77 K. The yielding behavior and plastic flow were discussed based on the dramatic change of deformation mechanisms at 293 K and 77 K.

Temperature-dependent yield strength and deformation mechanism of a casting Ni-based superalloy containing low volume-fraction γ′ phase

This study investigates the temperature-dependent yield strength and deformation mechanism of a new casting Ni-based superalloy K4750. This alloy exhibits yield strength anomaly (YSA) at 650–750 °C though it contains only low volume-fraction γ′ phase (22%). Using the interrupted samples after ~1.0% plastic strain, the dominant deformation mechanisms at the initial yielding stage are identified as: (ⅰ) (anti-phase boundary)APB-coupled dislocation pairs shearing γ′ from room temperature to 650 °C, (ⅱ) dislocations bypassing γ′ and creating loops above 650 °C, and (ⅲ) dislocations overcome γ′ by the thermally activated cross-slip and local climb at 850 °C. Additionally, with the proceeding of deformation, superlattice stacking faults shearing is active above 750 °C, implying that the deformation mechanism changes as the deformation progresses. According to the theoretical calculation, the critical resolved shear stress of Orowan looping decreases faster than APB shearing with the temperature increase, which is in agreement with the observed transition of deformation mechanism. Moreover, the remaining loops hinder the movement of subsequent dislocations, resulting in severe dislocation entanglement and high dislocation density in the heterogeneous slip bands, contributing to YSA. By contrast, dislocations can easily surmount γ′ phase by cross-slip and climb at 850 °C, leading to a rapid degradation of yield strength.

Quantification of the temperature-dependent evolution of defect structures in a CoNi-base superalloy

Quantifying the individual contributions of specific deformation mechanisms to the total plastic strain is of primary importance to understand the underlying deformation behavior and assess the significance of the different mechanisms. However, this quantification is nearly impossible in Ni-base superalloys due to the inherent difficulties in determining the propagation distances of the respective shearing dislocations. In this study, the occurrence of stacking faults (SFs) at temperatures between 750 °C and 1000 °C in the single crystalline CoNi-base superalloy ERBOCo-4 provides an opportunity to systematically quantify the evolution of the defect structures as a function of temperature and strain after constant strain-rate compressive tests. The quantification approach revealed that deformation is still localized in the matrix at 750 °C. With increasing temperature, the main deformation mechanism changes to SF shearing, which contributes 74% of the total deformation during the initial yielding stage (∼ 0.3% plastic strain) at 900 °C. Deformation spreads significantly more homogeneously across the γ/γ’ microstructure with increasing plastic strain. Extremely high stacking fault densities and frequent fault interactions cause a pronounced work-hardening anomaly at 850 °C. Structural characterization on the atomic scale confirms the strengthening effect of SFs as barriers to partial dislocation propagation. A strong correlation between the possible diffusion length of segregating solutes and the operant shearing mechanisms reveals the significance of segregation on the occurring deformation mechanisms.

Enhanced cold deformability of a marine Cu-Ni-Al–Fe-Mn alloy produced by HCCM vertical continuous casting: effect of deformation mechanism and second phases

ABSTRACT Cu-Ni-Al–Fe-Mn alloy billets were prepared by heating–cooling combined mould (HCCM) continuous casting at different casting speeds. Then, the cold deformation behaviour of billets was studied by multi-pass cold rolling, and the effects of HCCM continuous casting on cold deformability were discussed by comparing with the mould casting (MC) alloy. The results show that HCCM continuous casting significantly improves the cold deformability of Cu-Ni-Al–Fe-Mn alloy billets. When the rolling reduction is 94%, HCCM continuous casting samples have good edge quality without obvious crack, while the MC sample has severe edge cracks. With the rolling reduction increasing from 40% to 94%, the elongation of the MC samples decreases from 6% to 2%, while that of HCCM continuous casting samples maintains in 6%∼9%, showing good continuous deformability. The HCCM continuous casting samples with lower casting speeds have [001] oriented columnar grains along the casting direction, which make the deformation mechanism change from nano-crystallization to parallel stacking faults. In the meanwhile, HCCM continuous casting improves deformation coordination of as-cast grains and restrains intergranular cracking, which is reflected in the influence on misorientation between as-cast grains and distribution of coarse NiAl phases. In addition, nano-twins found in the rolled MC sample promote the initiation and propagation of cracks.

Macromolecular Insights into the Altered Mechanical Deformation Mechanisms of Non-Polyolefin Contaminated Polyolefins

Current recycling technologies rarely achieve 100% pure plastic fractions from a single polymer type. Often, sorted bales marked as containing a single polymer type in fact contain small amounts of other polymers as contaminants. Inevitably, this will affect the properties of the recycled plastic. This work focuses on understanding the changes in tensile deformation mechanism and the related mechanical properties of the four dominant types of polyolefin (PO) (linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP)), contaminated with three different non-polyolefin (NPO) polymers (polyamide-6 (PA-6), polyethylene terephthalate (PET), and polystyrene (PS)). Under the locally elevated stress state induced by the NPO phase, the weak interfacial adhesion typically provokes decohesion. The resulting microvoids, in turn, initiate shear yielding of the PO matrix. LLDPE, due to the linear structure and intercrystalline links, is well able to maintain high ductility when contaminated. LDPE shows deformation similar to the pure material, but with decreasing ductility as the amount of NPO increases. Addition of 20 wt% PA-6, PET, and PS causes a drop in strain at break of 79%, 63%, and 84%, respectively. The typical ductile necking of the high-crystalline HDPE and PP is strongly disturbed by the NPO phase, with a transition even to full brittle failure at high NPO concentration.

Nanoindentation creep testing: Advantages and limitations of the constant contact pressure method

Different loading protocols have been developed in the past to investigate the creep properties of materials using instrumented indentation testing technique. Recently, a new indentation creep method was presented, in which the contact pressure is kept constant during the creep test segment, similar to the constant stress applied in a uniaxial creep experiment. In this study, the results of constant contact pressure creep tests are compared to uniaxial and constant load hold indentation creep experiments on ultrafine grained Cu and CuAl5. The constant contact pressure method yields similar stress exponents as the uniaxial tests, down to indentation strain rates of 10–6 s−1, whereas the constant load hold method results mainly in a relaxation of the material at decreasing applied pressures. Furthermore, a pronounced change in the power law exponent at large stress reductions is found for both uniaxial and constant contact pressure tests, indicating a change in deformation mechanism of ultrafine grained metals.Graphical abstract

Correlating temperature-dependent stacking fault energy and in-situ bulk deformation behavior for a metastable austenitic stainless steel

In-situ high-energy synchrotron X-ray diffraction experiments during uniaxial tensile loading are performed to investigate the effect of temperature (25, 45 and 70 °C) on the deformation behavior of a 301 metastable austenitic stainless steel. The micromechanical behavior of the steel at the three deformation temperatures is correlated with the stacking fault energy (γSF) experimentally determined through the same in-situ X-ray experiments. The applied measurements provide a unique possibility to directly interrogate the temperature-dependent γSF in relation to the active bulk deformation mechanism in a metastable austenitic stainless steel. The determined γSF is 9.4 ± 1.7 mJ m−2 at 25 °C, 13.4 ± 1.9 mJ m−2 at 45 °C and 25.0 ± 1.1 mJ m−2 at 70 °C. This relatively minor change of γSF and temperature causes a significant change of the dominant deformation mechanism in the alloy. At room temperature (25 °C) significant amounts of stacking faults form at 0.05 true strain, with subsequent formation of large fractions of deformation-induced α′- and ε-martensite, 0.4 and 0.05, at 0.4 true strain, respectively. With increasing temperature (45 °C) fewer stacking faults form at low strain and thereupon also smaller α′- and ε-martensite fractions form, 0.2 and 0.025, at 0.4 true strain, respectively. At the highest temperature (70 °C) plastic deformation primarily occurs by the generation and glide of perfect dislocations at low strain, while at higher strain these dislocations dissociate to form stacking faults. The α′-martensite fraction formed is significantly less at 70 °C reaching 0.1 at 0.4 strain, whilst ε-martensite is not found to form at any strain at this temperature. The temperature-dependent mechanical behavior of the alloy is consistent with the observed dominant deformation mechanisms; the strong work hardening from the TRIP effect at low temperature, and low γSF, decreases significantly with increasing temperature, and γSF.

Orientation dependence of microstructure deformation mechanism and tensile mechanical properties of Nickel-based single crystal superalloys: A molecular dynamics simulation

In this paper, molecular dynamics (MD) simulations are performed to investigate the tensile deformation mechanisms and mechanical properties of superalloys with different orientations. The research results show that the superalloys exhibit anisotropy of mechanical properties and are consistent with previous experimental research results. The anisotropy of mechanical properties is related to the differences of stability at the interfacial dislocation network and the 1/6〈112〉{111} slip system startup of superalloys with different orientations. When the yield point is reached, dislocations and stacking faults on both sides of the slip plane will contact each other and merge into a stacking fault band to penetrate into the γ' precipitates. Meanwhile, the dislocation density and the proportion of face-centered cubic (FCC) structure with different orientation models have a sudden change at the yield point. As the temperature increases, the yield strength and elastic modulus decrease in the (110) and (111) orientation models, whereas the (100) orientation model has an anomalous yield behavior. This is mainly related to the destruction of dislocation network, the change of deformation mechanism and the thermally activated cross-slip mechanism when the temperature increases.

Room-temperature deformation of single crystals of transition-metal disilicides (TMSi2) with the C11b (TM = Mo) and C40 (TM = V, Cr, Nb and Ta) structures investigated by micropillar compression

The room-temperature deformation behavior of single crystals of transition-metal (TM) disilicides with the tetragonal C11b (TM=Mo) and hexagonal C40 (TM = V, Cr, Nb and Ta) structures has been investigated by micropillar compression as a function of specimen size, paying special attention to the deformation behavior of the equivalent slip ({110}<1¯11> and (0001)<21¯1¯0>, respectively for the two structures). In contrast to bulk single crystals, in which high temperature at least exceeding 400 °C is usually needed for the operation of the equivalent slip, plastic flow is observed by the operation of the equivalent slip at room temperature for all these TM disilicides in the micropillar form. The critical resolved shear stress (CRSS) value exhibits the ‘smaller is stronger’ behavior following an inverse power-law relationship for all these TM disilicides. The bulk CRSS values at room temperature estimated from the specimen size dependence are 620 ± 40, 240 ± 20, 1,440 ± 10, 640 ± 20 and 1,300 ± 30 MPa for MoSi2, VSi2, CrSi2, NbSi2 and TaSi2, respectively. Transmission electron microscopy reveals that the equivalent slip at room temperature occurs by a conventional shear mechanism for all TM disilicides, indicating the change in deformation mechanism from synchroshear in bulk to conventional shear in micropillars occurs in CrSi2 with decreasing temperature.

Molecular dynamics simulations of dynamics mechanical behavior and interfacial microstructure evolution of Ni-based single crystal superalloys under shock loading

In this paper, molecular dynamics simulations are performed to study the dynamic mechanical response and microstructural evolution of Ni-based single crystal superalloys under different shock velocities. The results show that when the shock velocity (Up ≤ 0.75 km/s), the interfacial dislocation network composed of the Stair-rod dislocation will prevent the Shockley dislocation from shearing the γ′ phase, the deformation of the microstructure is dominated by the slipping and dragging of dislocations. When the shock velocity (Up ≥ 1 km/s), the interfacial dislocation network is destroyed under the shock loading, the dislocations basically disappear, the atomic structure becomes disordered and undergoes structural phase transformation. In addition, the increase of the shock velocity leads to the regular increases of the shock pressure, internal energy and normal stress of the superalloys. However, the shear stress increases sharply once the shock velocity is greater than 1 km/s due to the change of the microstructural deformation mechanism. This work has an important guiding significance for in-depth understanding of the failure mechanism of Ni-based single crystal superalloys under shock loading.

Microhardness Indentation Studies of 2‐4‐6 Trinitrotoluene

AbstractThe microhardness of the {001} faces of 2‐4‐6 trinitrotoluene crystals has been investigated using both Vickers and Knoop indentation methods. The Vickers hardness number was found to be 22.5 kg mm−2 independent of crystal orientation and perfection. At ambient temperatures (∼20 °C) the Knoop hardness number varied between 20.5 kg mm−2 and 24.0 kg mm−2 with crystal orientation. At higher temperature (50 °C) the Knoop hardness anisotropy curve retained its shape, although the overall hardness decreased by 10 %. We interpret this change as reflecting a simple temperature dependant loosening of the crystal lattice rather than any change in deformation mechanism. No variation of Knoop hardness was evident with changing load. The hard direction was [010] and the soft [100]. The dominant operative slip system was defined to be {001}[010].

Cyclic plastic deformation mechanism and cyclic hardening model of Sanicro 25 steel welded joint

The low cycle fatigue (LCF) behaviors of Sanicro 25 steel welded joint were investigated under different total strain amplitudes (Δεt/2) at 700 °C. The results revealed that the stress amplitude increased with an increase in Δεt/2, while the fatigue life decreased. In addition, the cyclic hardening ratio first increased and then decreased with increasing Δεt/2. This abnormal trend was attributed to the variation in the cyclic yielding stress, indicating the evolution of the dislocation morphology. The microstructure analysis revealed a significant change in the cyclic plastic deformation mechanism from the dislocation tangle (0.2%–0.4%) to the entanglement of dislocations on the precipitate (0.5%) with an increase in Δεt/2. Furthermore, the cyclic hardening models considering the entanglement of dislocations on the precipitate and the dislocation tangle were unified according to the modified Taylor stress. The validated results indicated that the unified cyclic hardening model accurately calculated the maximum cyclic stress during LCF for a 316H welded joint.

Effect of grain size on the microstructure and deformation mechanism of Mg-2Y-0.6Nd-0.6Zr alloy at a high strain rate

The dynamic mechanical behaviour of Mg-2Y-0.6Nd-0.6Zr alloys with different grain sizes at a high strain rate (1100 s-1) was investigated in this study, and the effect of grain size on the microstructure and deformation mechanism was analysed. In the process of dynamic compression, fine grains (3 μm) can inhibit the formation of twinning, the majority of twins in coarse grains are {101(—)2} tensile twins, and the number of {101(—)2} tensile twins increases with increasing grain size. The accumulation of dislocations is more obvious in fine grains. Under dynamic conditions, <c+a> dislocations more easily move in the samples with smaller grain sizes. With increasing grain size, the main deformation mechanism change from <a> dislocation and <c+a> dislocation slip to {101(—)2} tensile twin and <a> dislocation slip.

Superplastic deformation mechanical behavior and constitutive modelling of a near-α titanium alloy TNW700 sheet

TNW700 is a newly developed near-α high temperature titanium alloy and has great application potential in the aerospace industry owing to excellent creep strength and short-term service temperature up to 700 °C. The superplastic tensile deformation behavior and constitutive model of TNW700 alloy in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1 were investigated. Results indicated that TNW700 alloy exhibits significant work hardening behavior, which may be related to dynamic growth of β grains, dislocation evolution, and silicide precipitates. In addition, work hardening coefficient (n) as well as the critical strain between flow hardening and softening increases with decreasing strain rate, and first increases and then decreases with increasing deformation temperature. TNW700 alloy exhibits excellent superplasticity at 900 °C–950 °C corresponding to strain rate sensitivity exponent (m) greater than 0.3, and expresses poor superplasticity at 975 °C with lower m value. The m value decreases monotonously with strain owing to decrease of grain boundary sliding contribution caused by dynamic grain growth of β grains. The deformation activation energy increases with decreasing strain rate due to the change of deformation mechanism. A phenomenological constitutive model considering strain hardening, strain rate hardening and temperature softening was proposed and material constants were calibrated by true stress-strain data. The constitutive model was implemented into Abaqus code by UHARD subroutine to simulate the superplastic forming of cone parts, and thickness distribution and bulging heights under different forming time were used to verify the validity of constitutive model.

Deformation microstructures as well as strengthening and toughening mechanisms of low-density high Mn steels for cryogenic applications

Abstract The three Al-bearing high Mn steels were prepared to investigate the effect of Al on microstructures as well as tensile and cryogenic impact properties. The Al addition leads to three strengthening modes of solid-solution, grain refinement and δ-ferrite. The former two strengthening modes can increase yield strength from 406 to 467 MPa with increasing Al content from ~3.0 to ~5.0 wt%. Once the Al content reaches ~8.0 wt%, the yield strength reaches 588 MPa owing to the formation of δ-ferrite. The changes in major deformation mechanisms are sufficient twinning plus deformation bands → a little twinning plus deformation bands → nearly no twinning plus highly dense dislocation walls as the Al content increases from ~3.0 to ~8.0 wt%. A loss in total elongation is mainly due to the suppression of twinning in the 5Al and 8Al steels. However, under an impact loading at −196 °C, the major deformation mechanisms are cross twinning and highly dense dislocation walls in the 3Al and 5Al steels, leading to ductile dimpled fracture as well as high impact absorbed energies of ~122 and ~138 J for the 3Al and 5Al steels, respectively. The toughening modes contain high angle grain boundaries induced deflections of a crack, cross planar defects induced tortuousness of a crack in grain interior and a large plastic deformation dissipating energy. In addition, it is due to the formation of δ-ferrite that the 8Al steel becomes a brittle material at −196 °C. The δ-ferrite induced cryogenic brittlement (δ-ICB) effect was sufficiently discussed.

Mechanical properties and deformation mechanisms of a Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy at elevated temperatures

A Ni2Co1Fe1V0.5Mo0.2 medium-entropy alloy (MEA) with a single-phase face-centered cubic (fcc) crystal structure are casted. The as-cast MEA possesses remarkable work hardening ability, yield strength (σy), ultimate tensile strength (σUTS) and uniform elongation (σu) at high temperatures up to 800 °C. Up-turn of strain hardening rate after elasto-plastic transition (secondary strain hardening) is observed for the MEA during tensile deformation in the temperature range of 25-800 °C. A high density of dislocation forests with solute atmospheres in concentrated solid solution is considered sufficient for back stress strengthening, thus causing the up-turn of strain hardening rate. Post-mortem microstructural analysis reveals that the temperature effects on mechanical properties of the MEA are closely related to dislocation structures, dislocation densities and dynamic-strain aging (DSA). With increasing deformation temperature from 25 °C to 800 °C, the dislocation density undulates and the fraction of screw dislocations increases. Strong solute pinning effect in conjunction with dominant forest strengthening mechanism (indicated by negative strain rate sensitivity and large activation volume) boost the strain hardening rate at the temperatures from 400 ℃ to 700 ℃. At 800 °C, the dominant deformation mechanism changes from the forest dislocation cutting mechanism to dislocation cross-slip with increasing strain, resulting in the sharp up-turn of strain hardening rate. On the other hand, DSA poses plastic instability to deteriorate the strain hardening of the MEA. These phenomena are considered to have material impacts on mechanical properties and fracture mechanisms of MEAs and high-entropy alloys at high temperatures.