Dominant Deformation Mechanism Research Articles

Thermal processing behaviour and microstructural evolution of high W and high γ’ content extruded nickel-based powder metallurgy superalloy FGH4109

ABSTRACT Nickel-based powder metallurgy superalloys with high W and high γ’ content exhibit excellent high-temperature properties, but their high deformation resistance poses significant challenges for hot processing. In this study, the hot compression behaviour of an extruded nickel-based powder superalloy FGH4109 with high W (6.1%) and high γ’ phase contents (60%) was systematically investigated at temperatures ranging from 1060 ℃ ~ 1140 ℃, strain rates from 0.001 s−1 ~1 s−1, and maximum true strains of 0.7. Combined with the hot working map and the microstructure evolution in different hot working zones, the optimal hot working window for the alloy was determined to be in the temperature range of 1060 ℃ ~ 1140 ℃ and strain rates of 0.001 s−1 ~0.012 s−1, where the power dissipation efficiency η was greater than 0.5, and the dominant deformation mechanism was discontinuous dynamic recrystallisation.

The microstructure evolution and mechanical properties of Ti–1300 alloy during ECAP deformation and subsequent aging treatment

The effect of Equal channel angular pressing (ECAP) deformation on the microstructure, mechanical properties, and aging behavior of solution treated Ti–1300 alloy was investigated. The results show that the grains are not broken during ECAP deformation due to coordination of grain rotation. The dominant deformation mechanism involved dislocation slip and twinning. The ECAP deformation has a significant effect on the subsequent aging behavior. The microstructure of aged samples comprised αs phase in β matrix. The precipitation of αs phase in the pre-deformed sample is higher than that in the solution treated sample, which is reflected in the improvement of microhardness of the pre-deformed + aging sample. The microhardness of the alloy after ECAP deformation increases with decreasing aging temperature and increasing aging time.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Exceptional Strength-Ductility Combinations of a CoCrNi-Based Medium-Entropy Alloy via Short/Medium-Time Annealing after Hot-Rolling.

Strong yet ductile alloys have long been desired for industrial applications to enhance structural reliability. This work produced two (CoCrNi)93.5Al3Ti3C0.5 medium-entropy alloys with exceptional strength-ductility combinations, via short/medium (3 min/30 min) annealing times after hot-rolling. Three types of intergranular precipitates including MC, M23C6 carbides, and L12 phase were detected in both samples. Noticeably, the high-density of intragranular L12 precipitates were only found in the medium-time annealed sample. Upon inspection of the deformed substructure, it was revealed that the plane slip is the dominant deformation mechanism of both alloys. This is related to the lower stacking fault energy, higher lattice friction induced by the C solute, and slip-plane softening caused by intragranular dense L12 precipitates. Additionally, we noted that the stacking fault and twinning act as the mediated mechanisms in deformation of the short-time annealed alloy, while only the former mechanism was apparent in the medium-time annealed alloy. The inhibited twinning tendency can be attributed to the higher energy stacking faults and the increased critical twinning stress caused by intragranular dense L12 precipitates. Our present findings provide not only guidance for optimizing the mechanical properties of high/medium-entropy alloys, but also a fundamental understanding of deformation mechanisms.

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Hot deformation behavior and processing map of the nanocrystalline pure Mg

The thermal compression test of the nanocrystalline, as-extrusion and as-cast pure Mg was carried out by the AGX-XD universal testing machine. The results demonstrated that flow stress of pure Mg with different grain sizes decreased with increasing temperature and decreasing strain rate. Accurate prediction of the thermal deformation behavior was achieved through establishing the constitutive equations. In comparison, the nanocrystalline pure Mg represents stronger strain hardening rate and weaker sensitivity to strain rate. Based on processing map, the optimal processing area for the nanocrystalline pure Mg is at 300–400 °C, 0.01– 0.1 s−1. The grain distribution of the nanocrystalline pure Mg remains stable and uniform after hot compression, and the grain boundary migration is the dominant deformation mechanism.

TPMS-based strut-shell interpenetrating lattice metamaterial with wide-range customizable mechanical properties and superior energy absorption

Mechanical metamaterials (MMs) are artificially designed structures with superior properties that originate from unit cells. Compared with the widely studied single-morphology MMs, multi-morphology interpenetrating MMs offer, by the virtue of their fused lattice characteristics, the potential for customizable mechanical properties and new application fields. Inspired by the fact that the isosurface of triply periodic minimal surfaces (TPMS) can divide a lattice into two independent regions, this paper proposes a novel TPMS-based strut-shell interpenetrating (TSSI) lattice metamaterial. This study demonstrates that the TSSI lattice has a larger specific surface area than traditional TPMS lattices. Compared with the volume fraction, the fusion proportion parameter is not only more conducive to effectively adjusting the mechanical properties in a wide range and controlling the anisotropy of the lattice metamaterials to achieve elastic isotropy but significantly changes the dominant mechanism of deformation under uniaxial and shear loads. Moreover, the TSSI lattice metamaterial can avoid sharp stress drops due to the mutual support and wrapping of the internal interpenetrating lattices after failure, resulting in a more stable energy absorption efficiency and a maximum increase in energy absorption of 74%. This work provides new opportunities to design lightweight components with a wide range of customizable mechanical properties and superior energy absorption.

Unveiling mechanical response and deformation mechanism of extruded WE43 magnesium alloy under high-speed impact

To clarify the mechanical behavior and deformation mechanism of rare earth magnesium (Mg) alloy WE43 under extreme service loads, high-speed impact tests under various deformation temperatures and loading paths were conducted using a split Hopkinson pressure bar. The flow stress along extrusion direction (ED) and extrusion radial direction (ERD) decreases apparently with deformation temperature. Compared with conventional Mg alloys, it exhibits a slight anisotropy and an unusual C-shaped characteristic. Cellular dislocation, mechanical twin and fine grain that occur after high-speed impact deformation are insensitive to the loading direction, but strongly dependent on the deformation temperature, especially superimposed with adiabatic temperature rise. As a result, dynamic recrystallization (DRX) occurs even at an ambient temperature of 25 °C. Double twinning and prismatic slip or pyramidal slip are the dominant deformation mechanisms at 25 °C. These twins induce mechanical cutting refinement to form some fine-grained structures, accompanied by a small number of fine grains by twinning induced DRX. In contrast, the deformation at 250 °C is mainly controlled by prismatic slip and pyramidal slip, accompanied by various types of twinning in early deformation stage. Compared with 25 °C, more fine-grained microstructures are formed at 150 and 250 °C through a synergy mechanism of twinning induced mechanical cutting and twinning induced DRX.

Tailoring size and fraction of coherent L12 nanoprecipitates to achieve strong hardening in medium entropy alloys with heterogeneous grain structures

Heterogeneous grain structures with coherent L12 nanoprecipitates were constructed in a medium entropy alloy with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (at. %). The volume fraction and size of L12 nanoprecipitates are observed to become higher and larger, and the interspacing is found to become smaller with increasing aging time, resulting in a severer heterogeneity. The domain of tensile properties was expanded by varying the size and volume fraction of coherent L12 nanoprecipitates after aging treatment. The samples after longer-time aging treatment show stronger strain hardening as compared to those after shorter-time aging treatment. The hetero-deformation-induced (HDI) hardening plays a more vital role in the longer-time aged samples than the shorter-time aged samples, especially at the elasto-plastic transition stage. The dominant precipitation hardening mechanism is observed to transit from dislocation-cutting to Orowan dislocation-looping with increasing precipitation size, as predicted by a theoretical model and validated by experimental results. Multiple deformation defects, such as deformation twins, stacking faults and Lomer-Cottrell locks, are the dominant deformation mechanisms for all samples, while interactions between these defects and L12 nanoprecipitates were observed to be more intensive in the longer-time aged samples than in the shorter-time aged samples, resulting in stronger precipitation and HDI hardening.

Anomalous size dependent softening behaviors and related deformation mechanisms of Zr/Mo multilayers

As a representative of hcp/bcc metallic multilayers, the correlation between microstructures and mechanical properties of nanostructured Zr/Mo multilayers with were studied. Zr/Mo multilayers with different layer thicknesses were prepared by magnetron sputtering method. XRD and TEM analyses revealed that Zr/Mo multilayers have Zr (0002) and Mo (110) out-of-plane crystalline texture. The nanoindentation hardness of Zr/Mo multilayers is in the range of ∼9.9–10.8 GPa at h ≤ 10 nm, while decreases with reducing layer thickness at larger h, which is on the contrary of the common rule - smaller is stronger. The aforementioned strengthening mechanism applicable in multilayers, i.e., dislocation bowing-out in nanolayers is not suitable to explain this size dependent softening behavior, while the IBS model fits well with the measured hardness at h = 10 nm. This anomalous softening behavior at large layer thickness scales may be caused by its special structures and deformation mechanisms. Dislocations interact with the grain boundaries, instead of the interfaces, and grains rotation/grain boundaries sliding are the dominant deformation mechanism at large layer thickness scales. The size-dependent strain rate sensitivity and activation volume were also explored, to revealing the rate dependent deformation mechanisms.

Activation of Dissolution‐Precipitation Creep Causes Weakening and Viscous Behavior in Experimentally Deformed Antigorite

AbstractAntigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low‐strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring‐shear experiments on antigorite and antigorite‐quartz mixtures at temperatures between 20 and 500°C at 150 MPa effective normal stress. Pure antigorite is strain hardening, with frictional coefficient (μ) > 0.5, and developed cataclastic microstructures. In contrast, antigorite‐quartz mixtures (10% quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200°C to 0.22 at 500°C. Antigorite‐quartz mixtures developed foliation similar to natural serpentinite shear zones. Although antigorite‐quartz reactions may form mechanically weak talc, we only find small, localized amounts of talc in our deformed samples, and room temperature friction is higher than expected for talc. Instead, we propose that the observed weakening at temperatures ≥200°C primarily results from silica dissolution leading to a lowered pore‐fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution‐precipitation creep. We suggest that under our experimental conditions, efficient dissolution‐precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional‐viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution‐precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example, in faults juxtaposing serpentinite with quartz‐bearing rocks.

Temperature Effect on Microstructure Evolution and Mechanical Properties of Fe–28Mn–8Al–1C Lightweight Steel via Supersonic Fine Particle Bombardment

The influence of room temperature (RT) and cryogenic temperature (CR) supersonic fine particle bombardment (SFPB) on the surface, microstructure, and mechanical properties of Fe–28Mn–8Al–1C lightweight steel is investigated in this work. The results indicate that both RT‐SFPB and CR‐SFPB successfully induce gradient nanostructures on the surface of steel, refining the grains to the nanoscale. Furthermore, CR‐SFPB results in a finer grain size and higher dislocation density compared to RT‐SFPB. Additionally, the dominant deformation mechanism shifts from dislocation slip for RT‐SFPB to a combination of dislocation slip and twinning for CR‐SFPB. CR‐SFPB is seen to be superior to RT‐SFPB in terms of surface integrity and strength due to low‐temperature lubrication effect, suppression of dynamic recovery and reduced stacking fault energy of the material. Interestingly, while CR‐SFPB enhances strength, elongation remains comparable to that of untreated material. However, excessive impact times during SFPB treatment promote microcrack formation on the surface, compromising plasticity.

Strengthening mechanisms for microstructures containing unimodal and bimodal γ′ precipitates in ATI 718Plus

The influence of γ′ precipitate size distribution on the deformation mechanisms under tensile loading in ATI 718Plus was studied. A set of aging treatments within the temperature range of 720 °C–900 °C was performed on solution-treated samples to obtain various γ′ precipitate size distributions. Unimodal and bimodal γ′ precipitate size distributions were achieved through single-step and two-step aging sequences, respectively, and such microstructures were tensile tested to failure to assess their yield strength, ultimate tensile strength, and elongation-to-failure. Some of the tensile samples were interrupted after achieving 3–4 % plastic strain, and the deformed microstructures were examined using transmission electron microscopy to investigate the γ′ precipitate-dislocation interactions. For the unimodal γ′ precipitate size distribution samples with the smaller γ′ precipitates (radius ∼ 7 nm), dislocations sheared through the precipitates. Both dislocation loops and paired dislocations were observed for the microstructures containing larger γ′ precipitates (radius ∼ 24 nm). The microstructure containing a bimodal γ′ precipitate size distribution, which included average γ′ precipitate radii of ∼6 nm and ∼28 nm, exhibited shearing as the dominant deformation mechanism, and this microstructure exhibited the highest strength values. The experimental observations were rationalized based on the theoretically-calculated critical resolved shear stress values for shearing and looping and a modified model for predicting the yield strength for bimodal microstructures was introduced.

Numerical investigation on Taylor bubble mass transfer in microchannel based on CO2 gas with the consideration of gas compressibility

This paper focused on the mass transfer of Taylor bubbles composed of pure carbon dioxide in organic solvent in microchannels. Combined with the Henry’s law, the carbon dioxide concentration at the two-phase interface was dynamically updated by using a compressible fluid model and considering the variation of gas density in the bubble. The critical bubble shape factor could help to judge the dominant mechanism of bubble deformation. The entire mass transfer process was divided into three stages: the rapid dissolution stage, the two-end dissolution stage, and the diffusion-like dissolution stage. The gas–liquid mass transfer capacity and influencing factors of each stage were analyzed, and the dominant mass transfer mechanism of each stage was summarized. It was found that the non-uniform degree of gas density in the bubble was related to the bubble Reynolds number and bubble volume by analyzing the distribution variation of solute density.

Enhanced yield stress and reduced elastic modulus of metastable β Ti–Nb alloys deformed by equal channel angular pressing attributed to the Synergistic effect of deformation mechanisms

This work aims to obtain Ti–Nb alloys with a high yield stress, good plasticity, and low elastic modulus. To this end, metastable β Ti–Nb alloys were subjected to multipass equal channel angular pressing (ECAP) deformation at room temperature, and a thorough investigation of their microstructure evolution, deformation mechanisms, and strengthening/toughening mechanisms was conducted through X-ray diffraction, transmission electron microscopy, electron backscatter diffraction, and tensile tests. In the initial deformation stages, multiple deformation mechanisms are observed, including dislocation slip, {332} <113> twinning, stress-induced martensite (SIM) α″-phase, {112} <111> twinning, and stress-induced ω-phase. However, as the strain accumulates, the ω-phase disappears, the volume fraction of the SIM α″-phase gradually decreases, and the dominant deformation mechanism changes to dislocation slip and {332} <113> twinning. The ECAP deformation results in a complex-network microstructure and the precipitation of the nanoscale SIM α″-phase, which leads to the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects. As a consequence, the Ti–Nb alloy simultaneously exhibits a high yield stress and a good plasticity. Furthermore, the high-density dislocations and interfaces generated by the ECAP deformation reduce the stability of the β-phase, and the introduced nanopores cause a decrease in density, resulting in a decrease in the elastic modulus of the alloy.

Hot deformation characteristics and processing map analysis of Al-Zn/stainless steel particles-based composite

The hot deformation behavior of Al-Zn/martensitic stainless steel particles-based composite (Al-Zn/6 %SSp), was examined in this study. The composite was tested using isothermal compression at 200–350 °C/0.01–10 s−1 and a global strain of 0.5. From the results, it was noticed that the composite’s flow stress increased with strain rate increase and drop in temperature. The constitutive equation from the hot-worked composites resulted in an estimated activation energy of 226.27 kJ/mol, which was 58 % more than that for the self-diffusion of aluminum alloy (142 kJ/mol). These findings suggest dynamic recrystallization (DRX) as the dominant deformation mechanism, as confirmed from the microstructures of the hot worked samples mostly at high temperatures and strain rates. Work hardening was predicted to dominate the deformation process by the stress exponent (n) value of 10.36 (which exceeded 5), but this was inconsistent with the microstructural observations. Comparing the linear fitting of calculated flow stress data with the estimated flow stress yielded a correlation coefficient (R2) of approximately 0.97. This observation demonstrates an effective relationship involving the calculated stress with the computed stress value for the composite material that was fabricated. Based on the processing map analysis, the instability regime occurs at 200270 °C/0.01–10 s−1. The stable domain established was at 280–340◦C/0.01–10 s−1 which is most suitable for achieving the best microstructural conditions for enhanced service performance.

Quantitative study on the influence of strain amplitude on deformation mechanisms and cracking modes during cyclic torsion of AZ31 magnesium alloy

AbstractThe influence of strain amplitude on deformation mechanisms and cracking modes during cyclic torsion of the AZ31 magnesium (Mg) alloy was quantitatively investigated via EBSD‐SEM characterization. At 0.42% strain amplitude, basal slip was the dominated deformation mechanism throughout the entire fatigue life. At 1.732% strain amplitude, the activation of deformation mechanisms was featured by the periodic transition: tension twinning + basal slip (during counterclockwise torsion) then detwinning + tension twinning + basal slip (during reverse torsion). Moreover, cracking along slip traces (ST) and tension twins (TTW) were the primary cracking modes at 0.42% and 1.732% strain amplitude, respectively. ST cracking and TTW cracking preferentially appeared in soft‐oriented grains for basal slip and tension twinning, respectively. The underlying mechanisms governing the activation of various deformation mechanisms, cracking modes, and cyclic stress–strain responses were systematically discussed.

Microstructural evolution and cryogenic and ambient temperature deformation behavior of the near-α titanium alloy TA15 fabricated by laser powder bed fusion

Titanium alloys produced by additive manufacturing show excellent mechanical properties at room temperature. However, their deformation behavior at low temperature is still not fully understood. In this study, the microstructural evolution and tensile behavior of the near-α titanium alloy Ti-6.5Al-2Zr-Mo-V (TA15), fabricated via laser powder bed fusion (LPBF), were determined at both 25 °C and −196 °C. The LPBFed TA15 alloy shows the microstructure of α laths and acicular α′ martensite due to the rapid cooling rate during LPBF processing. Pyramidal <c+a> slip and numerous twins, including nano-twins and triple twins, were activated at cryogenic temperature, leading to the high ultimate tensile strength (UTS) up to 1750±8 MPa and yield strength (YS) up to 1548±25 MPa, which is higher than the strength at room temperature (YS∼1103 MPa, UTS∼1281 MPa). The dominant deformation mechanism changes from dislocation slip to twinning with decreasing temperature, leading to uniform deformation with an elongation of 5.2±0.1 %. The results can guide the control of the microstructure of LPBFed near-α titanium alloys at cryogenic temperature.

Anisotropic mechanism of cold-rolled pure titanium plate during the two-step tensile process of the variable path

The effects of pre-deformation on the macro-mesoscopic deformation characteristics of the titanium plate during variable path tensile were studied by a two-step tensile test and electron backscatter diffraction (EBSD). The Schmid law, Kernel Average Misorientation (KAM), Taylor axis, and In-Grain Misorientation Axis (IGMA) were used to forecast and identify the types and change laws of the activated deformation modes under different loading conditions. The results demonstrated that dislocation slip is the primary cause of the evolution of the basal split texture components. The nucleation and de-twinning of {10–12}ET resulted in the sudden appearance and disappearance of the <0001>//RD texture. The pre-tensile causes a texture redirection that reduces the activability of both prismatic <a> slip and pyramidal <a> slip in the following variable path tensile. Among them, prismatic <a> slip is the dominant deformation mechanism that causes yield and hardening behavior during RD tensile and TD-RD tensile (rolling direction-RD, transeverse direction-TD). A portion of the prismatic <a> slip is preferentially activated in TD tensile and RD-TD tensile deformation, and the deformation is subsequently jointly controlled by the prismatic <a> slip and the pyramidal <a> slip. The pre-tensile along RD promotes nucleation of {10–12} extension twin (ET) during subsequent RD-TD variable path tensile, while tensile along TD-RD coordinates plastic deformation by consuming a significant quantity of {10–12}ET generated during pre-tensile along TD. The formation of slip-assisted twins in neighboring grains is facilitated by the dislocation pile-up at grain boundaries, and the de-twinning phenomenon also cause the local dislocation pile-up to weaken.

A modified viscous flow law for natural glacier ice: Scaling from laboratories to ice sheets

Glacier flow modulates sea level and is governed largely by the viscous deformation of ice. Multiple molecular-scale mechanisms facilitate viscous deformation, but it remains unclear how each contributes to glacier-scale deformation. Here, we present a model of ice deformation that bridges laboratory and glacier scales, unifies existing estimates of the viscous parameters, and provides a framework for estimating the parameters from observations and incorporating flow laws derived from laboratory observations into glacier-flow models. Our results yield a map of the dominant deformation mechanisms in the Antarctic Ice Sheet, showing that, contrary to long-standing assumptions, dislocation creep, characterized by a value of the stress exponent [Formula: see text], likely dominates in all fast-flowing areas. This increase from the canonical value of [Formula: see text] dramatically alters the climate conditions under which marine ice sheets may become unstable and drive rapid rates of sea-level rise.