Deformation-induced Microstructure Research Articles

Effect of Fe content on structure and mechanical properties of a medium: Entropy Fex(CoNi)100-xCr9.5C0.5 (x = 60 and 65) alloys after cold rolling and annealing

In this study two medium-entropy Fe60Co15Ni15Cr9.5C0.5 (hereinafter referred to as Fe60) and Fe65Co12.5Ni12.5Cr9.5C0.5 (Fe65) alloys (both compositions in at%) were studied after thermomechanical processing involving cold rolling and subsequent annealing at 800 and 1000 ℃. The Fe60 alloy was expected to have a higher stability of a fcc phase and to be less prone to the martensite formation than those of the Fe65 alloy as suggested the ∆Gfcc→bcc values calculated using a Thermo-Calc software. Both the Fe60 and Fe65 alloys underwent deformation-induced microstructure refinement upon cold rolling and recrystallization in the fcc phase and precipitation of Cr-rich carbides after annealing. However, the Fe60 alloy had the fcc-based structure, while the Fe65 alloy contained a significant amount (35–60 %) of the bcc martensite. After cold working, the Fe65 alloy was stronger than the Fe60 alloy (yield strength of 1050 MPa and 1390 MPa, respectively). Annealing resulted in considerable softening of the alloys - yield strength was in a range of 205–360 MPa, and no significant difference in strength between Fe60 and Fe65 was found. The latter finding was associated with the plastic flow initiation with the softer fcc phase. Both alloys demonstrated the transformation-induced plasticity (TRIP) effect associated with the bcc martensite formation during tensile deformation. At room temperature, the TRIP effect was more developed in the Fe65 alloy due to which a comprehensive combination of properties after cold rolling and annealing at 800 °C can be attained; i.e. yield strength of 360/1100 MPa, ultimate tensile strength of 1060/1900 MPa, uniform elongation of 20/26 %, and impact toughness (KCV) of 1300/885 kJ/m2 at 293/77 K, respectively. Quantitative analysis of the structure-properties relationships in the program alloys was performed to reveal similarities and differences in the behavior of the alloys.

Thermal stability of differently rolled pure tungsten plates in the temperature range from 1125 °C to 1250 °C

Application of rolled tungsten as armor of plasma-facing components in future fusion reactors requires its deformation-induced microstructure remaining stable during operation. At high operation temperatures, restoration processes as recovery, recrystallization and grain growth will unavoidably embrittle ductile tungsten achieved by rolling. Three pure tungsten plates manufactured by unidirectional rolling or cross-rolling in two different manners were qualified as baseline material for DEMO. To investigate their thermal stability, hardness testing is performed after isothermal annealing in the temperature range between 1125 °C to 1250 °C. The recrystallization kinetics of the different plates (and different regions within them) is analyzed thoroughly to allow extrapolation outside the investigated temperature range. Significant differences between the recrystallization kinetics of the three tungsten plates originate from the microstructure established by rolling. Two of the plates showed pronounced local differences in their recrystallization kinetics with the core of the plates recrystallizing ahead of the edge regions closest to the rolls. The third plate exhibits best thermal stability without notable differences between the local recrystallization kinetics. Where the two former plates may survive operation for two full power years only at temperatures less than 1005 °C, the later plate could sustain 1035 °C for the same period.

Temperature and microstructure dependent tensile behavior of coarse grained superelastic NiTi

In this work, effects of temperature and microstructure on tensile behavior of superelastic NiTi wires were analyzed. We analyzed cold-drawn NiTi wires of 1.78 mm that received four different final heat treatments using short direct electrical current pulses. Final coarse-grained microstructures ranged from recovered up to fully recrystallized microstructure with grain size of 25 µm. Effects of temperature in the range 30–130 °C were analyzed using monotonous tensile tests where a temperature gradient was induced along the sample axis. Deformation processes related to martensitic transformation and plastic deformation were characterized by temperature resolved stress–strain responses, deformation work, released heat, volume fraction of martensite. Deformation-induced microstructure changes were analyzed post-mortem using Electron Backscatter Diffraction and x-ray diffraction. Except the least recovered microstructure, the samples did not follow linear temperature trends according to Clausius-Clapeyron. With increasing temperature, the plateau stress of localized martensitic transformation stabilized at a constant stress while volume fraction of martensite decreased down to zero. Localized strains induced by martensitic transformation and ductility evolved non-monotonously with temperature, reaching maxima at temperatures that decreased with increasing microstructure recovery or recrystallization. Large ductility was positively correlated with occurrence of austenite deformation twins in deformed microstructure.

Simultaneously enhancing strength-ductility synergy and strain hardenability via Si-alloying in medium-Al FeMnAlC lightweight steels

We studied the initial microstructures, mechanical responses and deformation-induced microstructures of the medium-Al Fe-21Mn-6Al-1C-xSi (x = 0, 1.5, 3 wt.%) lightweight steels both in their solution-treated and aged states. Our results reveal that nano-sized long-range ordered domains exist in the solution-treated steels, which feature L12 type ordering in the 0Si and 1.5Si steels and L′12 type ordering in the 3Si steel. Aging at 550 °C for 20 h has little effect on the microstructures of the 0Si steel, but leads to κ′-carbide precipitation in the 1.5Si steel. For the 3Si steel, aging at 550 °C for 1 h already results in the precipitation of uniformly-distributed nano-sized κ′-carbides (4.2 nm). The strength, ductility and strain hardenability of the solution-treated steels all enhance with increasing Si content. The yield strength (YS) of the aged steels with κ′-carbides enhances with increasing aging time, accompanied by the reduction of ductility. The short-time (1 h) aged 3Si steel exhibits excellent strength-ductility synergy, with YS > 900 MPa and total elongation > 50%. The increase in YS with Si addition originates from the grain boundary, solid-solution and order strengthening. For the 0Si steel, the strain hardening is governed by the evolution of high-density dislocation walls and microbands. The occurrence of both dynamic slip band refinement (DSBR) and twinning-induced plasticity effects in the 3Si steel accounts for the increase of strain hardenability with Si-alloying. The DSBR effect still exists and the progressively-formed slip bands are homogeneously distributed in the short-time aged 3Si steel, explaining its high ductility.

Effect of chemical additives on the consolidation behaviours of mini-PVD unit cells –from macro to micro

The mechanism of adding NaCl, CaCl2 and small amounts of cement/lime (1–4% by dry weight) into clay slurry for mitigating the effect of apparent clogging around PVDs during vacuum consolidation was investigated by mini-PVD unit cell consolidation tests as well as microstructure observations via scanning electron microscopy (SEM) imaging. The consolidation test results indicated that for the specimens with CaCl2, cement and lime additives the rate of consolidation increased considerably, while the effect of the NaCl additive was limited. The SEM images show that for the specimen without additive, there were obvious localized deformation-induced microstructure anisotropies. For specimens with CaCl2 and cement/lime additives, the microstructures of the soils tended to be isotropic. The additives tested reduced the thickness of the diffusive double layer around the clay particles and promoted the formation of a more stable flocculated microstructure, therefore mitigating the effect of apparent clogging.

Outstanding cryogenic strength-ductility properties of a cold-rolled medium-entropy TRIP Fe65(CoNi)25Cr9·5C0.5 alloy

A new medium-entropy Fe65(CoNi)25Cr9·5C0.5 (at.%) alloy was investigated both at room and liquid nitrogen temperatures in both the as-cast and deformed conditions. The deformed sample was obtained by cold rolling the alloy until a thickness reduction of 80% had been reached. During testing at −196 °C, the cold-rolled program alloy exhibited a combination of very high strength (σ0.2 = 1360 MPa, σUTS = 2070 MPa), good ductility (δ = 26%) and excellent fracture toughness (approximately 700 kJ/m2). Examination of the microstructure suggested a deformation-induced phase transition from the initial face-centered cubic (fcc) phase to the martensite body-centered cubic (bcc) phase. The enhanced mechanical properties of the cold-rolled alloy at cryogenic temperatures, in comparison with the as-cast alloy, could be associated with an increased thermal stability of the fcc phase caused by deformation-induced microstructure refinement and shifting of the main martensite transformation to later stages of strain. Substructure strengthening, interphase strengthening, and solid solution strengthening could also contribute to the high strength and good ductility of the alloy. The obtained results can expand the possibilities for the development of medium-entropy ferrous alloys for use at extremely low temperatures.

Influence of cooling rate on ω phase precipitation and deformation mechanism of a novel metastable β titanium alloy

This work investigated the effect of cooling rate (water quenching and air cooling) on the precipitation of ω phase after solution treatment in β-phase region, and its effect on the mechanical properties in a novel metastable β titanium alloy (Ti–5Mo–3Cr–Fe–3Zr). The initial microstructures, phase composition and deformation-induced microstructures have been investigated using SEM, EBSD, XRD, and TEM. The phase composition of water-quenched alloy and air-cooled alloy are β, α", and ω phase. The size and volume fraction of ω phase of air-cooled alloy are larger than that of water-quenched alloy, resulting in an increase in tensile strength and a decrease in ductility. Deformation mechanisms of Ti–5Mo–3Cr–Fe–3Zr alloy with different cooling rate change from stress-induced ω phase transformation and dislocation slip to only dislocation slip. The stress-induced ω lamellas parallel to [1-11]β direction along the [0001]ω1 direction, which is formed by {112}β<111>β slip. Dislocations can cut through the encountered ω phase to form ω-free deformation bands, which accounts for the ductility.

Dynamic and Post-Dynamic Recrystallization of Haynes 282 below the Secondary Carbide Solvus

Thermomechanical processes, such as forging, are important steps during manufacturing of superalloy components. The microstructural development during processing, which controls the final component properties, is complex and depends on e.g., applied strain, strain rate and temperature. In this study, we investigate the effect of process parameters on the dynamic and post-dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. Specifically, we address the effect of deformation below the grain boundary carbide solvus temperature. During deformation, discontinuous and continuous dynamic recrystallization was observed at the grain boundaries, and particle-stimulated nucleation occurred at primary carbides. Strain rate was determined to be the governing factor controlling the recrystallization fraction for strain rates up to 0.5 s−1 above which adiabatic heating became the dominating factor. Careful examination of the temperature development during deformation showed that the response of the closed-loop temperature control system to adiabatic heating can have important effects on the interpretation of the observed behavior. During a 90 s post-deformation hold, grain growth and an increasing fraction of twin boundaries significantly changed the deformation-induced microstructure and texture. The microstructure developed during post-dynamic recrystallization was mainly controlled by the temperature and only weakly coupled to the prior deformation step.

Massively parallel finite difference elasticity using block-structured adaptive mesh refinement with a geometric multigrid solver

Computationally solving the equations of elasticity is a key component in many materials science and mechanics simulations. Phenomena such as deformation-induced microstructure evolution, microfracture, and microvoid nucleation are examples of applications for which accurate stress and strain fields are required. A characteristic feature of these simulations is that the problem domain is simple (typically a rectilinear representative volume element (RVE)), but the evolution of internal topological features is extremely complex. Traditionally, the finite element method (FEM) is used for elasticity calculations; FEM is nearly ubiquitous due to (1) its ability to handle meshes of complex geometry using isoparametric elements, and (2) the weak formulation which eschews the need for computation of second derivatives. However, variable topology problems (e.g. microstructure evolution) require either remeshing, or adaptive mesh refinement (AMR) - both of which can cause extensive overhead and limited scaling. Block-structured AMR (BSAMR) is a method for adaptive mesh refinement that exhibits good scaling and is well-suited for many problems in materials science. Here, it is shown that the equations of elasticity can be efficiently solved using BSAMR using the finite difference method. The boundary operator method is used to treat different types of boundary conditions, and the “reflux-free” method is introduced to efficiently and easily treat the coarse-fine boundaries that arise in BSAMR. Examples are presented that demonstrate the use of this method in a variety of cases relevant to materials science: Eshelby inclusions, fracture, and microstructure evolution. Reasonable scaling is demonstrated up to ∼4000 processors with tens of millions of grid points, and good AMR efficiency is observed.

Plastic deformation in advanced tungsten-based alloys for fusion applications studied by mechanical testing and TEM

In this work, we have assessed mechanical properties of several tungsten grades considered as perspective materials for applications in plasma facing components in the nuclear fusion environment, where the neutron irradiation damage is expected to cause embrittlement. In particular, the work focuses on two aspects: bending tests to deduce the onset of ductile deformation and microstructural analysis of the reference and plastically deformed materials. The microstructure in the reference state and the one induced by plastic deformation at 600 °C is studied by means of transmission electron microscopy (TEM). Six different types of tungsten-based materials were assessed: two commercial grades produced according to ITER specifications in Europe and China and four lab-scale grades utilizing different reinforcement options. The comparative assessment of tensile and bending strength was performed at 600 °C accompanied with a detailed TEM analysis. The deformation-induced microstructure was characterized and compared for all studied grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near grain boundaries and/or strengthening particles. The obtained data will serve as reference information required to assess the impact of neutron irradiation.

Trapping and release of helium and deuterium in recrystallized and heavily deformed tungsten after plasma exposure

Interaction of tungsten surface with plasma beam has tremendous importance for development of materials for fusion applications. Trapping and release of Helium (He) and Deuterium (D) in tungsten exposed to high flux plasma with varying He and D content are studied here. Recrystallized and plastically deformed tungsten samples were used to clarify the impact of the material microstructure on trapping and release of plasma components. Thermal desorption spectroscopy measurements were performed to reveal the release stages and quantify the integral desorption of both types of molecules. Comparison of reference and plastically deformed samples was used to clarify the impact of plastic deformation (expressed in the high dislocation density and dislocation networks) on the trapping and release processes. It has been demonstrated that in the mixed beam exposures, the integral He release weakly depends on the He/D ratio, but suddenly grows as the fraction of He/D increases from 80/20 to 100/0. The integral release of D is remarkably enhanced by He seeding in both reference and plastically deformed samples. It is concluded that under the mixed beam exposure conditions, the D release is primary controlled by the He seeding and not by the deformation-induced microstructure.

Revisiting \u201cSteady-State\u201d Monotonic and Cyclic Deformation: Emphasizing the Quasi-Stationary State of Deformation

High-temperature creep, cyclic deformation in saturation, and a number of technologically important processes are typical examples of the so-called “steady-state” deformation. These cases are usually defined in terms of the constancy of the mechanical parameters. Moreover, it is usually assumed that the deformation-induced microstructure undergoes no further changes. However, clear evidence shows that non-negligible microstructural changes continue to occur in the so-defined steady-state high-temperature creep and in cyclic saturation. It can be shown that the so-called “steady-state” deformation is actually a quasi-stationary deformation which is characterized by the initial development of a “mechanical steady state”, which is followed with a delay by a “microstructural steady state.” Only the latter can then be considered as a true steady state. A deeper analysis reveals a persistent slight increase of the dislocation density, with geometrically necessary dislocations in the cell walls/subgrain boundaries, causing the latter to transform gradually into sharper boundaries with higher misorientations. These findings, based on a detailed analysis of a wide range of experimental studies, are found to be almost identical for both high-temperature creep and cyclic deformation in saturation and are hence considered as characteristic of quasi-stationary deformation. The analysis clarifies, as a by-product, specific effects which arise from the increasing heterogeneity of the dislocation pattern (patterning). Thus, a marked decrease of the arrangement factor “alpha” in the Taylor flow stress is noted, as patterning proceeds, in agreement with predictions of the so-called composite model. Since this effect is compensated partially by the increase of the dislocation density, the flow stress remains rather insensitive to subtle microstructural changes. Based on these facts, the need for revision of current flow-stress formulations in future dislocation modeling is emphasized.

Nanoscale processes of trace element mobility in metamorphosed zircon

Several examples of zircon grains from high- to ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT) metapelites exhibit a characteristic, yet atypical, core–rim interface domain < 5-μm wide observed in cathodoluminescence (CL) imaging. The interface domain is located immediately against the magmatic core and is comprised of an irregular, 0–2-μm wide, CL-dark domain that is rimmed by a complex, 0–5-μm wide, CL-bright domain with cuspate margins. The outer margin of the interface domain is rimmed by intermediate-CL zircon with low contrast zoning. To characterize the nature of the interface domain and to identify mechanisms of trace element mobility in metamorphosed zircon, we analyzed several specimens prepared from zircon from the Rhodope Metamorphic Complex (eastern Greece) and the Goshen Dome (western Massachusetts, USA) via atom probe tomography (APT). The data reveal three types of geochemical anomalies, each with a unique morphology. (1) Toroidal clusters with high concentrations of Pb (+ Y, Al) are found exclusively within the core of the Rhodope grain. These clusters are interpreted as decorated dislocation loops that formed during metamorphism and annealing of radiation damage to the lattice. Geochronological and geochemical data support this interpretation. (2) Complex, cross-cutting planar and linear features with anomalous concentrations of Y + P + Yb or U are spatially restricted to the core–rim interface domain; these features do not correlate with inherited geochemical variation (oscillatory zoning) or deformation-induced microstructures. Instead, the planar features likely formed in response to an interface-coupled dissolution–reprecipitation reaction that propagated into the crystal during metamorphism. The observed cross-cutting relationships are the product of either multiple events or complexity of the process that originally formed the domains. (3) Ellipsoidal features with high concentrations of Y + P + Yb (+ Al) are found exclusively within the high-Y + P + Yb planar features. These features are interpreted as the product of spinodal decomposition that occurred during exhumation as the zircon passed the solvus where local equilibria favored nm-scale exsolution to minimize the Gibbs free energy. The presence of multiple types of geochemical features in these examples indicates that trace element mobility in zircon is driven by multiple processes over the course of orogenesis. Given that these atypical domains are apparently restricted to zircon metamorphosed at UHT and (U)HP conditions, their presence may represent a marker of metamorphism at very high-grade conditions.

Improved fatigue life of the newly developed Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy

Fe-15Mn-10Cr-8Ni-4Si is a newly developed alloy for seismic damper applications. The low- and high-cycle fatigue (LCF and HCF) properties of this alloy was investigated in this study. The LCF resistance of this alloy was found to be superior to that of other materials. The inelastic component was established to be the main contributor to the superior fatigue life of Fe-15Mn-10Cr-8Ni-4Si, including at relatively low strain levels. This alloy demonstrated a gradual and mild cyclic hardening behavior at a wide range of strains. In terms of the HCF property, the fatigue limit was 262.5 MPa, which is 96% of 0.2% proof stress of 274 MPa. A surface observation of the post-fatigued specimen tested at a stress amplitude of 250 MPa for 107 cycles without failure showed that while surface relief developed, no cracks with a size of at least several grains were detected. Since Fe-high Mn alloys generally exhibit several unique deformation modes involving the deformation-induced phase transformation, the microstructure of Fe-15Mn-10Cr-8Ni-4Si after fatigue was investigated using electron-backscatter diffraction (EBSD) microscopy. As a result, ε-martensite formed via the deformation-induced γ→ε martensitic transformation was detected in all specimens tested under different conditions. Even at a stress level lower than the fatigue limit, ε-martensite formed while the amount of it was very limited. Except for the specimen tested at a low stress level of 275 MPa, a well-developed deformation-induced microstructure was found near the crack; α′-martensite formed by the two-stage phase transformation γ→ε→α′ was detected only in the region near the crack in a specimen tested at a total strain range of 2%.

Pyrite deformation and connections to gold mobility: Insight from micro-structural analysis and trace element mapping

The metamorphic transition of pyrite to pyrrhotite results in the liberation of lattice-bound and nano-particulate metals initially hosted within early sulphide minerals. This process forms the basis for the metamorphic-driven Au-upgrading model applied to many orogenic Au deposits, however the role of syn-metamorphic pyrite deformation in controlling the retention and release of Au and related pathfinder elements is poorly understood. The lower amphibolite facies metamorphic mineral assemblage (Act-Bt-Pl-Ep-Alm ± Cal ± Qz ± Ilm; 550 °C) of Canada's giant Detour Lake deposit falls within the range of pressure-temperature conditions (450 °C) for crystal plastic deformation of pyrite. We have applied a complementary approach of electron backscatter diffraction (EBSD) mapping and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) 2D element mapping on pyrite from the Detour Lake deposit. Chemical element maps document an early generation of Au-rich sieve textured pyrite domains and a later stage of syn-metamorphic oscillatory-zoned Au-poor pyrite. Both pyrite types are cut by Au-rich fractures as a consequence of remobilization of Au with trace element enrichment of first-row transition elements, post-transition metals, chalcogens and metalloids during a late brittle deformation stage. However, similar enrichment in trace elements and Au can be observed along low-angle grain boundaries within otherwise Au-poor pyrite, indicating that heterogeneous microstructural misorientation patterns and higher strain domains are also relatively Au-rich. We therefore propose that the close spatial relationship between pyrite and Au at the microscale, features typical of orogenic Au deposits, reflects the entrapment of Au within deformation-induced microstructures in pyrite rather than the release of Au during the metamorphic transition from pyrite to pyrrhotite. Moreover, mass balance calculations at the deposit scale suggest that only a small percentage of Au could have been sourced from pyrite and instead point to the role of substructures in pyrite as depositional traps for Au during syn-metamorphic deformation- and fluid-assisted diffusion Au-upgrading.

Temperature-dependent strain hardening, precipitation and deformation-induced microstructure evolution in AA 6061

The effect of β′′ precipitation on strain hardening and yield stress evolution in an A6061 aluminium alloy is studied experimentally and through thermokinetic computer simulation. Samples were deformed by compression at temperatures from 25 to 500°C to strains of about 0.4 and three different precipitation states. Simulations on the thermal stability of β′′ are conducted on an equivalent model Al-Mg-Si alloy resulting in an explanation for the experimentally observed softening at intermediate temperatures. EBSD micrographs confirm that different dislocation storage and annihilation mechanisms are operative at low and high temperatures. The low temperature microstructure correlates very well with the subgrain structures typically observed during stage IV strain hardening.

The effect of microstructural evolution during deformation on the post-yielding behavior of self-associated polyamide blends

The deformation-induced microstructure evolution of self-associated polymer blends, i.e., long chain polyamide alloys (LCPAs), was investigated and correlated with their mechanical performance. A PA1012 (soft phase)/PA612 (hard phase) blend was selected for this study. It is interesting that both Young's modulus and yield stress exhibit a nearly linear dependence with composition, which follows a simple additive mixing law. With strong intra-associated hydrogen bonds, the LCPAs studied here were found to be immiscible, but mechanically compatible because of the strong interfacial adhesion of the two constituents. Moreover, when the blend was deformed close to or above its glass transition temperature (Tg), the simultaneous occurrence of fracture reinforcement and toughness was achieved, which was defined as “soft phase-reinforcing-hard phase” (SRH). On the other hand, below Tg, a “hard phase-reinforcing-soft phase” (HRS) mechanical behavior was identified at the expense of elongation at break. Upon uniaxial deformation above Tg, the PA1012 component imparted to the blends a superior fracture stress due to strong strain-induced crystallization effect and the subsequent formation of a microfibrillar structure. However, the PA612 phase contributed little to the improvement of the mechanical properties of the blends. The large discrepancy in the contributions of the blend components to the fracture stress, primarily originates from the flexible nature of the PA1012 chains and the thin crystals nucleated by the already crystallized PA612 phase. However, below Tg, tilt, slippage and fragmentation of the lamellae occur in both phases, which are accompanied by apparent void formation and directly lead to catastrophic fracture. Considering the higher density of hydrogen bonds in PA612, the PA612-rich blends displayed higher fracture stresses as compared to the rest of the compositions. Overall, the distinctive microstructure evolution of the constituent phases and their roles in stress enhancement at large strains have been successfully established. The results presented here shed light on the deformation mechanism of self-associated polymers and offer a new pathway for the development of mechanically reinforcing materials.

Evolution of plastic deformation in heavily deformed and recrystallized tungsten of ITER specification studied by TEM

Microstructure induced by plastic deformation at 600°C in as-fabricated and recrystallized tungsten grades of ITER specification is studied by means of transmission electron microscopy (TEM). Both grades are produced by AG Plansee and being used by fusion community as research material for plasma facing and high heat flux components. Reference microstructure was investigated by a combination of TEM, scanning electron microscopy and nano-indentation techniques. TEM was applied to investigate the necking region in the tensile samples deformed up to ~30% of stain. The deformation-induced microstructure was characterized and compared for the two grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near high angle grain boundaries. The obtained evolution of the dislocation density was compared with the prediction of the previously developed thermo-mechanical model for the plastic deformation of polycrystalline tungsten and a good agreement was found without modification of the original parameter set.

Multiple mechanisms of lath martensite plasticity

The multi-scale complexity of lath martensitic microstructures requires scale-bridging analyses to better understand the deformation mechanisms activated therein. In this study, plasticity in lath martensite is investigated by multi-field mapping of deformation-induced microstructure, topography, and strain evolution at different spatial resolution vs. field-of-view combinations. These investigations reveal site-specific initiation of dislocation activity within laths, as well as significant plastic accommodation in the vicinity of high angle block and packet boundaries. The observation of interface plasticity raises several questions regarding the role of thin inter-lath austenite films. Thus, accompanying transmission electron microscopy and synchrotron x-ray diffraction experiments are carried out to investigate the stability of these films to mechanical loading, and to discuss alternative boundary sliding mechanisms to explain the observed interface strain localization.

Transient microstructure in long alkane segment polyamide: Deformation mechanism and its temperature dependence

Deformation-induced microstructure evolution of Polyamide 1012, with an emphasis on lamella development, polymorphism transition and molecular orientation, is investigated. When deformed above Tg (100 °C), a series of complex SAXS patterns are continuously identified at intermediate strain including four-point pattern, figure-8 pattern and X-shaped pattern, accompanied with two-bar pattern on the meridian, which corresponds to the transient microstructure. Such particular structure is resulted from the approach of tilted lamellae along the drawing direction, the insertion and the orientation of new lamellae. To investigate the temperature dependence, the microstructure developments at 30 °C (below Tg), 60 °C (close to Tg) and 100 °C (above Tg) are compared. Considering 38% enhancement of the fracture stress with elevated temperature (from 60 °C to 100 °C), the temperature dependence of particular SAXS patterns further suggests that the elongation above Tg favors higher level of stretching-induced crystallization and better aligned microfibrillar structure, which competes with thermal-induced increased chain mobility. Based on the comprehensive results, the correlation between microstructure and mechanical response has been successfully established, which is featured by the synchronous occurrence of transient structure with slight strain hardening.