High Back Stress Research Articles

Origins of back stress strengthening in Fe-22Mn-0.6C (-3Al) TWIP steels

The origins of back stress strengthening were investigated in Fe-22Mn-0.6C and Fe-22Mn-0.6C-3Al TWIP steels. In contrast to reports found in literature, this study revealed that the high back stress developed in these steels is primarily caused by pile-ups of high density of geometrically necessary dislocations near the grain boundary regions rather than at deformation twins.

Architecting a Gradient Multilayered Structure to Concurrently Strengthen and Toughen Ti/Mo Nanoscale Multilayers

Most nanoscale metallic multilayers (NMMs) exhibit ultra-high hardness but limited toughness. To toughen Ti/Mo NMMs, a gradient multilayered (GM) structure was architected via manipulating individual layer thickness (h) gradient distribution along the film growth direction. The resulting GM structure significantly raises the toughness of Ti/Mo NMMs without reducing their peak hardness. High back stress developed from the plastic incompatibilities stemmed from aggravated microstructural heterogeneity is primarily responsible for the unprecedented synergy of hardness and toughness. Our findings provide a promising approach to concurrently strengthen and toughen NMMs.

Outstanding Tensile Properties and Their Origins in Twinning-Induced Plasticity (TWIP) Steels with Gradient Substructures.

The low yield strength (~300 MPa) of twinning-induced plasticity (TWIP) steels greatly limits their structural applications in the industrial field. Conventional strengthening mechanisms usually cause an enhancement of yield strength but also a severe loss of ductility. In this research, gradient substructures were introduced in the Fe-22Mn-0.6C TWIP steels by different pre-torsional deformation in order to overcome the above limitations. The substructure evolution, mechanical properties, and their origins in gradient-substructured (GS) TWIP steels were measured and compared by electron backscattered diffraction (EBSD), monotonous and loading-unloading-reloading (LUR) tensile tests. It was found that a simple torsional treatment could prepare gradient twins and dislocations in coarse-grained TWIP steel samples depending on torsional strain. The uniaxial tensile tests indicated that a superior combination of high yield strength, high ultimate strength, and considerable ductility was simultaneously obtained in the GS samples. The high yield strength and high ultimate tensile strength were attributed to synergetic strengthening mechanisms, viz., dislocation strengthening, due to the accumulation of high density of dislocations, and very high back stress strengthening due to gradient substructure distribution, which was accommodated through pile-ups of extra geometrically necessary dislocations (GNDs) across the sample-scale. Additionally, high ductility originated from gradient substructure-induced back stress hardening. The present study is also beneficial to the design efforts of high strength and high ductility of other heterogeneous-structured TWIP alloy systems.

Isotropic and Kinematic Hardening of a High Entropy Alloy

The contribution of kinematic hardening to the overall strain hardening of a high entropy alloy (HEA) was investigated for the first time ever. It was assessed for a single-phase equiatomic CoCrFeMnNi alloy based on a study of the Bauschinger effect. The observed occurrence of high back stresses signifies substantial kinematic hardening. We attribute it to the low probability of cross-slip in this HEA which, however, rises in the process of straining. A model that captures the mentioned effects was proposed and validated.

Revealing extra strengthening and strain hardening in heterogeneous two-phase nanostructures

Heterogeneous metals and alloys are a new class of materials that emerged recently with outstanding mechanical performances such as excellent strength-ductility synergy, significant friction and wear reduction and high fatigue limits. Experiments demonstrated that back stress plays an important role in enhancing the strength-ductility balance. However, how does the back stress play the role remains fully unsolved. Here for a two-phase heterogeneous nanostructured Cu we proposed that the strong strain partitioning between the phases with high mechanical contrast due to the large variation of their grain sizes produces significant strain gradient and geometrically necessary dislocations (GNDs). The piling up of the GNDs at the phase interface generates high back stress towards the dislocation source in order to emit new dislocations. The above physical picture is incorporated into a newly developed theoretical model, in which the mechanical responses of the constituent phases with various grain sizes are described by a dislocation density based model, while the overall response is obtained by a secant method for an inclusion-matrix composite. The results show that the soft phase is considerably strengthened. The strain hardening capability of the heterogeneous composite is also enhanced to a large extent, even much higher than that the coarse-grained Cu. The strong strain hardening capability makes the heterogeneous Cu overcome the strength-ductility tradeoff that is usually the case of the homogeneous counterparts. The predicted stress-strain response agrees well with the existing experimental data. A strength-ductility map is also given for designing strong and ductile heterogeneous Cu by selecting smaller grain size in the hard phase, larger grain size in the soft phase and less soft materials. The developed model can be easily extended to investigate the strengthening and strain hardening behavior of other heterogeneous nanostructures such as gradient, laminate, nanotube strengthened and metallic glass/metallic substrate structures.

Achieving ultra-high strengthening and toughening efficiency in carbon nanotubes/magnesium composites via constructing micro-nano layered structure

Magnesium (Mg) is the lightest metal structural material but exhibits low toughness. By imitating the structure of nacre, constructing micro-nano layered structure is an effective way to balance strength and toughness. The micro-nano layered structure is constructed by electrophoretic depositing carbon nanotubes (CNTs) on Mg foils. The CNTs layers embedded into 50 μm Mg foils exhibit a strengthening efficiency of 500 while increasing the toughness of the composites. Experiments and theoretical calculation results reveal that the high back stress induced by CNTs layers is an important strengthening mechanism. The significantly improved toughness derives from the increase in energy required for crack propagation and high Schmid factor for the basal slip system {0 0 0 1} 〈1 1 2¯ 0〉. This work provides a feasible and universal way for conductive matrix, such as aluminum, titanium, copper and even conductive polymers to construct micro-nano layered structure to break through the mismatch between strength and toughness.

Simultaneous enhancement of strength and ductility through coordination deformation and multi-stage transformation induced plasticity (TRIP) effect in heterogeneous metastable austenitic steel

Here we reported a heterogeneous structural austenitic steel with super combination of high strength and ductility, which consisted of nano/ultrafine grains and a proper amount of micro grains. The high strength originates from high back stress hardening which evolved during coordination deformation process between different domains. The huge variation in austenite grain size leads to obvious difference in austenite stability and consequently leads multi-stage transformation induced plasticity (TRIP) effect, which can enhance the strain hardening at different stages and sustain the plastic deformation to a large value.

An outstanding synergy of high strength and ductility in gradient structured low-carbon steel

Both high strength and good ductility are desirable for structural materials for their applications. Here, an outstanding synergy of high yield strength (σy∼550.6 MPa) and promising uniform ductility (εu∼9.2%) is achieved in a low-carbon steel subjected to pre-torsion deformation and thermal annealing, which is much better than that (σy∼319.8 MPa, εu∼13.1%) of its coarse-grained (CG) counterpart. The enhanced mechanical properties is attributed to the formation of a gradient structure of ∼5 mm thickness, that is, the grain size of the ferrite increases gradually with decreasing the radius along the radial direction from the surface to the core. High back stress (over 400 MPa) is invoked by the gradient structure and increases with tensile strain, which devotes to both the high strength and ductility.

Strain hardening of novel high Al low-density steel consisting of austenite matrix and B2-ordered intermetallic second phase in the perspective of non-cell forming face-centered-cubic alloy with high stacking fault energy

It has recently been suggested that the extraordinary strain hardening rate in Fe-16Mn-10Al-0.86C-5Ni high specific strength steel [1] is due to the high back stresses arising from the strain incompatibility between austenite matrix and B2-ordered intermetallic second phase [2]. I argue here that the alloy matrix chemistry, particularly short-range ordering of constituent atoms, could be the intrinsic factor that affects the deformation and strain hardening, rather than the stress-strain partitioning caused by the microstructural heterogeneity.

Superior strength and ductility of 316L stainless steel with heterogeneous lamella structure

Strength and ductility are two of the most important mechanical properties for a metal, but often trade off with each other. Here, we report a 316L stainless steel with superior combinations of strength and ductility that can be controlled by fine-tuning its heterogeneous lamella structure (HLS). The HLS was produced by 85% cold rolling, which produced lamellar coarse grains sandwiched between mixtures of nano-grains and nano-twins. The HLS was fine-tuned by annealing at 750 °C for 5–25 min, which resulted in varying volume fractions of nano-grains, nano-twins, lamellar coarse grains, and recrystallized grains. During tensile testing, large amount of geometrically necessary dislocations were generated near the heterostructure interfaces to coordinate the deformation between soft domains and hard domains, which results in high back stress to achieve superior combination of strength and ductility. An optimal high yield strength of ~ 1 GPa with an elongation-to-failure of ~ 20% was obtained for an optimized HLS sample. Furthermore, the processing technique employed here is conducive to large-scale industrial production at low cost.

Compatible strain evolution in two phases due to epsilon martensite transformation in duplex TRIP-assisted stainless steels with high hydrogen embrittlement resistance

Two duplex TRIP-assisted stainless steels have been investigated showing distinct γ (face centered cubic) → ε (hexagonal close-packed) → α′ (body centered cubic) transformation sequences and a single γ → α′ martensite transformation. The microscopic strain evolution in two phases was revealed by in-situ high energy synchrotron X-ray diffraction combined with electron backscattering diffraction. The direct γ → α′ transformation route experiences a shift of strain localization from austenite to ferrite during deformation, which induces high back stress with massive dislocation accumulation in ferrite. The multistage γ → ε → α′ transformation sequences contribute to a good combination of strength and ductility. It is demonstrated that the compatible strain evolution in austenite and ferrite is achieved due to γ → ε transformation, leading to a mild dislocation multiplication in each phase. The following α′-martensite transformation supplies an extra work hardening capacity related to the piling up of dislocations. The obtained compatible strain evolution due to ε-martensite can not only prevent the nucleation of hydrogen-induced micro-cracks but also alleviate the localized plastic deformation in ferrite, which ensures a higher total elongation (TEL) and resistance to hydrogen embrittlement (HE).

Numerical study of the size-dependent deformation morphology in micropillar compressions by a dislocation-based crystal plasticity model

In recent years, size-dependent deformation morphology has been observed in uniaxial micropillar compression experiments. After deformation, large pillar shows a barrel shape while small pillar shows a clear shear band. Meanwhile, one or more slip systems are activated at different sizes. In order to reveal the underlying mechanism of this size-dependent phenomenon, a dislocation-based crystal plasticity model is developed in this paper to simulate the uniaxial compression tests for different sizes of pillars with material imperfection. The simulation results show that: (1) The transition from barrel shape to severe shear with decreasing pillar size is well captured by the newly developed model. Two slip systems operate equally in large pillar and only one slip system is activated in small pillar. (2) The back stress induced by dislocation mutual interactions plays an important role in size-dependent deformation morphology. High back stress in small pillar impedes the second slip system to operate. (3) The critical size of transition from double slip (or multiple slip) to single slip is obtained and it is quantitatively comparable with experiments in a specific case. (4) Material softening is necessary to trigger slip band. It can be concluded that the competition between the short range back stress and the external resolved shear stress results in the transition from barrel shape to shear band during the micropillar compression tests.

In-situ X-ray diffraction studies of slip and twinning in the presence of precipitates in AZ91 alloy

The effect of basal plate precipitates on the hardening of basal slip and {101¯2} twinning modes was investigated for a non-aged and aged AZ91 alloy in the twin dominated strain paths. Exploiting in-situ synchrotron and laboratory based X-ray diffraction methodologies, we quantified the critical resolved shear stress (CRSS) for basal slip and twinning modes. The twin volume fraction changes were quantified from the intensity changes with applied load. We observed that the twin volume fraction changes with plastic strain is sensitive to the initial texture, while the relative hardening of different deformation modes are considered as a secondary effect. We also found that the twin interior stresses were significantly smaller and consistent with the high twin back stresses in the presence of precipitates. We propose, based on a simple analytical equation, that the leading edge of the propagating twin have a Burgers vector equivalent to 100 twinning dislocations and when the propagating twin is blocked by a precipitate, relatively high resolved stress is required for bowing the twin dislocation and hence the propagation of the twin occurs by the dissociation of the leading edge of the twinning dislocation.

Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

We report a detailed study of the strain hardening behavior of a Fe–16Mn–10Al–0.86C–5Ni (weight percent) high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) during uniaxial tensile deformation. The dual-phase (γ-austenite and B2 intermetallic compound) HSSS possesses high yield strength of 1.2–1.4 GPa and uniform elongation of 18–34%. The tensile deformation of the HSSS exhibits an initial yield-peak, followed by a transient characterized by an up-turn of the strain hardening rate. Using synchrotron based high-energy in situ X-ray diffraction, the evolution of lattice strains in both the γ and B2 phases was monitored, which has disclosed an explicit elasto-plastic transition through load transfer and strain partitioning between the two phases followed by co-deformation. The unloading-reloading tests revealed the Bauschinger effect: during unloading yield in γ occurs even when the applied load is still in tension. The extraordinary strain hardening rate can be attributed to the high back stresses that arise from the strain incompatibility caused by the microstructural heterogeneity in the HSSS.

Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility

Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.

Reduction of Stress of Seated Reading Postures for Desk with Built-in Bookholder and Desktop-mounted Sliding Drawer

Objective: This study aims to develop a new desk with a built-in bookholder and desktop-mounted sliding drawer for reducing stress of seated reading postures and to analyze its effect.<BR><BR> Background: It is general for readers to bend their back and neck when reading books on desks, which may result in high back and neck postural stress. In addition, postures using vertically layered drawers positioned at the side of users may be awkward.<BR><BR> Method: For reducing the postural stress, a new desk with a built-in bookholder and sliding drawer was developed based on a standard of KS G 4208 for student desk. The muscle activities of reading postures on both existing and new desks were measured using EMG, in which seven male and three female college students participated. The postural stress, and musculoskeletal discomfort and ease of usage for existing and new drawers were analyzed by RULA and questionnaire survey, respectively.<BR><BR> Results: Compared to existing desk, the muscle activity for the new desk with a bookholder was reduced by 47% when reading books. In addition, musculoskeletal discomforts in the shoulder, neck and low back were significantly lower when using the new desk with a bookholder. Although the stresses for postures using the desktopmounted sliding drawer and general drawer were not significantly different, the questionnaire survey revealed that the new desktop-mounted sliding drawer is easier to use.<BR><BR> Conclusion: A new desk developed in this study significantly reduced postural stress and enhanced subjective preference when reading.<BR><BR> Application: This would be useful when developing new desks with low postural stress.

Modeling of pseudoelasticity via reversible slip in Fe3Ga

The Fe3Ga alloy with the cubic D03 lattice possesses considerable recoverable strain due to the slip reversibility. Pseudoelasticity via reversible slip in Fe3Ga is studied with atomistic simulations. An extended Peierls–Nabarro model incorporating the Generalized Stacking Fault Energy (GSFE) is established to determine Peierls stress in D03 and L12 Fe3Ga. The back stress and frictional stress are predicted during loading and unloading process. These stress magnitudes govern the reversible slip in Fe3Ga. The results show that the reversible slip observed experimentally in D03 Fe3Ga is induced by its larger back stress compared to its frictional stress. In contrast, the reversible slip cannot appear in L12 since its back stress is not large enough to pull back superpartials, and thus the existence of L12 will suppress the pseudoelasticity of D03 Fe3Ga and results in decreasing the strain recovery. The present study has explored the theoretical foundations of this phenomenon arising from high back stresses responsible for cyclic reversible dislocation motion.

Cyclic hardening mechanisms in [001] copper single crystals

Friction stress and back stress of [001] Cu single crystals during cyclic deformation were derived from cyclic stress–strain hysteresis loops by using the Cottrell method and the results are compared with those for single slip crystals. The cyclic hardening features of the [001] crystals are described in terms of the friction stress and back stress. Specific, detailed study was carried out on the sample cycled with a plastic strain amplitude of 1.8×10 –3. It has been shown that both the ‘true’ friction stress, i.e. the cyclic yield stress, and the back stress in the [001] crystals increase much more rapidly than those in single slip crystals in the hardening stage. In the saturation stage, however, only the friction stress in the [001] crystal is higher than its correspondent in the single slip crystals, while the back stresses in these two types of crystals are close to each other. The higher friction stress in the [001] crystal is related to the higher density of jogs produced by the dislocation reactions between the primary and critical slip systems, and the existence of secondary dislocations in channels. The higher back stress in the hardening stage is attributed to the higher dislocation density in loop patches introduced by dislocation reactions in the first dozen of cycles. By modifying the Kulhlmann-Wilsdorf and Laird model, it is possible now to express quantitatively both the back stress and the shear stress in a cyclically deformed crystal with a particular orientation as functions of the applied strain amplitude as well as an orientation-dependent factor: the hardening coefficient for the first stage of unidirectional deformation. The predictions from these expressions for multiple slip crystals are in very good agreement with experimental data.

Reactive spray processing of advanced materials: A.Lawley et al. (Drexel University, USA.)

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 HEA. Three types of precipitates, σ-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present after cold-rolling and annealing at 600 °C. Such hierarchical precipitates could lead to sluggish recrystallization and grain growth upon annealing. The partially recrystallized microstructures and hierarchical precipitates could lead to a high yield strength even for prolonged annealing conditions. Deformation mechanisms change with annealing time. The materials annealed for short times (< 2 h) are deformed by dislocation glide, deformation twinning, and deformation-induced ε phase. A longer annealing time (> 10 h) triggers a multi-variant ε phase, reverse transformation from ε to γ, and the multi-step sequential transformation, γ → ε → reverse transformed γ from ε → ε transformed from the reverse transformed γ. Further, materials annealed for longer times shows a higher contribution of back stress strengthening, which could be attributed to the increase in γ/ε and γ/σ interfaces. The activation of various deformation mechanisms and high back stress strengthening could lead to a superior strain hardening capacity and strength-ductility combination (YS: 699 MPa, UTS: 1041 MPa, TE: 45%) of the material annealed for 10 h. The present work provides the novel microstructure design solution of the metastable high entropy alloys with exceptional mechanical properties, utilizing hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening.

Influence of grain size and texture on the cyclic stress-strain response of nickel

The cyclic stress-strain behavior of fine-grain (24 μm) nickel was compared with previously published results on coarse-grain (290 μm) nickel to evaluate the effects of grain size on cyclic hardening. The fine-grain nickel exhibited a moderate 〈111〉〈100〉 fiber texture, while the coarse-grain material was essentially texture free. As a result, texture also influenced the cyclic responses. Fatigue experiments were conducted at plastic strain amplitudes ranging from 2.5 × 10 −5 to 2.5 × 10 −3. At a plastic strain amplitude of 2.5 × 10 −5, the cyclic responses of the two types of materials were nearly identical. Above that plastic strain amplitude the fine-grain-textured specimens exhibited higher saturation stresses. The difference became greaster plastic strain amplitude increased. The higher saturation stress in the fine-grain-textured material was primarily caused by a higher back stress. Both grain sizes exhibited cyclic stress-strain curve plateaux extending from plastic strain amplitudes of 2 × 10 −4 to 8 × 10 −4. The results are discussed in terms of traditional monotonic Hall-Petch mechanisms and the cyclic stress-strain behavior of multiple-and single-slip-oriented single crystals.