Prevailing Deformation Mechanism Research Articles

Influence of phase stability on postshock mechanical properties and microstructure evolution in Ti-17 alloy

There has been a growing surge of interest in examining the shock response of titanium alloys, owing to their considerable potential for military applications. The present study aims to reveal the influence of phase stability on the shock-induced mechanical response and substructure evolution of a metastable β titanium alloy, namely, Ti-17. This investigation included extensive work, such as plate impact tests, quasi-static reloading compression tests, and electron microscope analyses. The microstructural evaluations following the shock-wave loading unveil planar slip as the prevailing deformation mechanism in Ti-17 with a bimodal microstructure with stable α and β phases. However, when the shock stress exceeds 10 GPa, the activation of {101¯1}α nano-sized twins was observed, leading to improved reloading ductility. This implies a novel strategy to achieve excellent strength-plasticity compatibility in titanium alloys through appropriate shock-wave loading. Conversely, in Ti-17 with an equiaxed β microstructure, the metastability of the β phase leads to the activation of shock-induced α″ martensite, shock-induced ω, and planar slip. Two distinct forms of interaction involving the α″ laths, i.e., shear and truncation, were also observed. Phase stability greatly influences substructure evolution, which ultimately controls the reloading mechanical properties of the postshock Ti-17 alloy.

In-situ synchrotron diffraction analysis of deformation mechanisms in an AA5083 sheet metal processed by modified equal-channel angular pressing

Severe plastic deformation (SPD) is a process to significantly improve the mechanical properties of materials by grain refinement. For forming at room temperature (RT), higher strengths can thus be achieved; for increased temperatures (HT), greater elongations can be achieved. This great potential has been chiefly applied only on laboratory scale due to a lack of industrial process design. Equal-channel angular pressing (ECAP) offers the possibility to integrate these advantages in industrial processes. This paper investigates a modified ECAP process for aluminum AA5083 sheet metal. Two passes of ECAP for sheet metal and an additional heat treatment were used. To study the material deformation on a micro-scale, in-situ synchrotron measurements at DESY (Hamburg) were performed to analyze the evolution of lattice strains and dislocation densities during tensile testing. The mechanical properties of the ECAP-processed sheet material reveal an increase in yield strength and a decrease in elongation at RT. Higher strains can be measured for the processed material at elevated test temperatures. In-situ diffraction results show a changed behavior of the ECAP sheet material compared to the reference material for both temperature regimes. These changes are based on the increased defect density in the material, which leads to increased strength at RT. At HT, dynamic recrystallization occurs during tensile testing, and the prevailing deformation mechanisms reconfigure. Electron backscatter diffraction (EBSD) and micrographs are used to interpret and supplement the observations. The findings provide the basis for the in-depth understanding of the deformation behavior of the material and help to apply the ECAP process on an industrial scale.

Role of precipitation and solute segregation on micro-scale deformation of additively manufactured Inconel 718

Inconel 718, a Ni-based superalloy, was prepared by powder laser bed fusion (P-LBF) additive manufacturing (AM) method. AM of the Inconel 718 is challenging, as this particular alloy possesses a higher melting point, as well as resistance to temperature, which induces ‘hot-cracking’. Thus, the selection of proper input parameters are essential, which was successfully achieved in this present study. The microstructure exhibits a hierarchical structure, that starts with equiaxed and sub-cellular (∼100–600 nm) grains, and ends up on the formation of melt-pool and grain boundaries. This structure is exceptional and completely dissimilar to that of wrought alloy, which is infested with twins and precipitates of different secondary phases. However, the effectiveness of such microstructure of L-PBF alloy towards material's strength was undermined, as it lacks the presence of abundant precipitates, which is the major contributor to the strength of such alloys. The strength of the L-PBF processed alloy was in the range of 413–491 MPa and 562–665 MPa respectively, for yield and ultimate compressive strength. This was marginally higher than that of wrought alloy (408 MPa of Yield and 656 MPa of compressive strength). This was attributed to the absence of abundant gamma/gamma double prime (γ'/γ'') phases, and the presence of laves and delta (δ) phases, that are inferior towards the overall strengthen of the Inconel 718. Cross-sectional SEM and TEM investigation of the deformed micro-pillars confirmed the prevailing deformation mechanism that can be attributed to the slip/share plane formation in the presence of dislocation loops.

Hot deformation constitutive analysis and processing maps of the as-cast and wrought Mg–2.5Gd–0.5Zr alloy

Hot deformation behavior and constitutive analysis of the as-cast and extruded Mg–2.5Gd–0.5Zr alloy were compared, using the shear punch testing (SPT) method in the temperature range of 623–723 K and shear strain rate range of 2.8 × 10–3–2.2 × 10–2 s–1. The respective hyperbolic-sine exponents of 2.67 and 2.59 as well as activation energies of 183 and 275 kJ mol–1 obtained for the as-cast and extruded alloy suggested grain boundary sliding as the prevailing deformation mechanism. Surprisingly, regardless of the instability domains, the efficiency of power dissipation exceeded 30% in most areas in the processing maps of both cast and wrought conditions. The safe and instability domains were rather similar in both processing maps. The relatively high potential of the as-cast alloy for hot deformation, despite its initial coarse grain size of about 150 µm, can be ascribed to the severe dynamic recrystallization (DRX) during hot deformation caused by Zr. This could be economically advantageous, as there would be no need for homogenization of the as-cast material before deformation processing. The most noticeable optimum window occurred in the temperature range of 673–723 K for both as-cast and extruded conditions with a vaster strain-rate range for the extruded one (5.6 × 10–3–1.4 × 10–2 s–1), owing to its initial finer grain size. The instability domains, however, took place for both conditions in the temperature range of 648–673 K and strain rates of about 5.6 × 10–3 s–1, manifested by cracking in the deformed zone, and also at temperatures higher than 673 K and medium to high strain rates.

High-temperature deformation processes in migmatites from the Atuba Complex, southern Ribeira Belt: Constrains from microstructures and EBSD analyses

The Atuba Complex occurs in the southern region of the Ribeira Belt, in the eastern Paraná State. It comprises part of the Curitiba Terrane and is mainly composed of stromatic metatexites and migmatites subject to varying degrees of mylonitisation. The present work aims to perform microstructural analyses on ductile tectonites, as well as assess the prevailing deformation mechanisms by characterizing quartz crystallographic textures and microstructures, in order to understand in a more accurate way the structural and tectonic evolution of the Atuba Complex and of the Curitiba Terrane. The rocks have two phases of migmatization (granodiorite-tonalitic and granitic leucosomes) and constitute two deformation phases (Dn-1 and Dn), represented by the Sn-1 and Sn foliations, generated by ductile shear. Sn-1 foliation may be associated with Paleoproterozoic overthrusting tectonics and Sn foliation may be the result of Neoproterozoic sinistral transpressive tectonics. The electron backscatter diffraction (EBSD) results related to foliation Sn show that the deformation was accommodated by dislocation creep achieving dynamic recrystallization in quartz. Microscopically, the quartz may exhibit lobate grain boundaries, suggesting grain boundary migration (GBM), and may also show microstructures indicative of recrystallization by subgrain rotation (SGR) and, as very fine grains developed along grain boundaries, suggesting dynamic recrystallization by bulging (BLG), pointing to deformation superposition under greenschist facies conditions. The feldspar crystals show undulose extinction, subgrains and new grains, formed mainly by bulging and subgrain rotation. The domain comprised by the Sn foliation show activation of the basal slip with rhombohedral component or rhombohedral slip system for quartz, which indicate the deformation occurred at low temperature conditions (<500 °C) and may be associated with retrometamorphism that occurred in the Atuba Complex.

Temperature-dependent deformation in silver-particle-covered copper nanowires by molecular dynamics simulation

Cu nanowires covered by Ag particles is studied for potential applications in the next-generation microelectronics. To date, the deformation mechanism in the CuAg core-particle is not clear. Here, molecular dynamics simulation is used to describe the CuAg core-particle system. The results show that the equilibrium structure of Ag particles is reconstructed, when the particle ≤1.0 nm. At low temperature (1 K) indicate that three different deformation processes take part in the core-particle structure, depending on the size of Ag particles. When the particle diameter ≤2.0 nm, the prevailing deformation mechanism is the emission of dislocations from the Cu surface. For the particle diameters ranging from 3.0 to 6.0 nm, the emission of misfit dislocations from the AgCu interface is the dominant deformation mechanism. If the Ag particle ≥6.0 nm, the deformation mechanism can be characterized by the slip band, consisting of the dislocations and amorphous atoms. For elevated temperatures (2–400 K), the mechanical properties of the AgCu core-shell system are nearly independent of temperature, whereas the structure with particles larger than 2.0 nm showed a strong dependence of its mechanical properties on temperature. Based on the results, the diameter-temperature plastic deformation map is proposed.

Deformation Behavior of Nanocrystalline Body-Centered Cubic Iron with Segregated, Foreign Interstitial: A Molecular Dynamics Study.

In the present work, modified embedded atom potential and large-scale molecular dynamics’ simulations were used to explore the effect of grain boundary (GB) segregated foreign interstitials on the deformation behavior of nanocrystalline (nc) iron. As a case study, carbon and nitrogen (about 2.5 at.%) were added to (nc) iron. The tensile test results showed that, at the onset of plasticity, grain boundary sliding mediated was dominated, whereas both dislocations and twinning were prevailing deformation mechanisms at high strain. Adding C/N into GBs reduces the free excess volume and consequently increases resistance to GB sliding. In agreement with experiments, the flow stress increased due to the presence of carbon or nitrogen and carbon had the stronger impact. Additionally, the simulation results revealed that GB reduction and suppressing GBs’ dislocation were the primary cause for GB strengthening. Moreover, we also found that the stress required for both intragranular dislocation and twinning nucleation were strongly dependent on the solute type.

On microboudin paleopiezometers and their applications to constrain stress variations in tectonites

Accounting for interfacial shear-induced near-end fracturing, we propose a new microboudin piezometer, τe = τ/C = 0.25/(L/W), where τ is the shear flow stress of the ductile matrix (e.g., polycrystalline quartz), C is the tensile fracture strength of a columnar mineral (e.g., tourmaline), which ranges generally between E/1000 to E/250 (E is Young's modulus), τe is a dimensionless parameter named ‘equivalent shear strength’, and L/W is the mean length/width ratio of the microboudins formed by near-end fracturing. This provides a straightforward method of estimating the magnitude of shear flow stresses responsible for ductile deformation of natural rocks. The piezometer is easy to apply because it does not require the use of TEM, SEM-EBSD or the identification of prevailing deformation mechanism (e.g., dislocation creep, diffusion creep, hardening or weakening). The samples from the Gaoligong metamorphic belt (Yunnan, China) yield the equivalent shear flow stresses of 0.145–0.424 (36–106 MPa if taking C = E/800 or 250 MPa) for felsic gneisses, mylonites and phyllonites. The earlier formed microboudins had consistently a larger mean length/width ratio than the later formed ones, suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity.

Superplasticity in a multi-directionally forged Mg–Li–Zn alloy

Superplastic behavior of the Mg–8Li–1Zn alloy, processed by extrusion at 573 K followed by multi-directional forging (MDF) at 423 K, was studied using shear punch testing (SPT) of miniature specimens at various temperatures and strain rates. Microstructural analysis indicated moderate grain refinement, reducing the gran size from 8 μm in the initial as-extruded condition to 4 μm after 8 passes of MDF. Processing by MDF not only refined the grain structure but also provided a more homogeneous microstructure and a larger fraction of high-angle grain boundaries. SPT was performed at shear strain rates in the range of 3.3 × 10−3–1.3 × 10−1 s−1 and temperatures in the range of 498–573 K. The strain rate sensitivity (SRS) index (m-value), obtained from the slope of the central region of the sigmoidal shear stress–shear strain rate curves, was as high as 0.51 for the MDF processed material at 548 K. This is in contrast to the as-extruded material that exhibited a linear behavior with a maximum m-value of only 0.29 at the same temperature. For the MDF processed material, the activation energy of 61 kJ mol−1, which is close to 67 kJ mol−1 for the grain boundary diffusion in β-Li, and the high m-value of 0.51 suggest that the prevailing deformation mechanism is grain boundary sliding (GBS) controlled by grain boundary diffusion.

The interaction and migration of deformation twin in an eutectic high-entropy alloy AlCoCrFeNi2.1

An eutectic high-entropy alloy consisting Al, Co, Cr, Fe and Ni elements was prepared by vacuum directional solidification technology. The alloy exhibits excellent comprehensive mechanical performance during tension at temperature range of 600–700 °C. The microstructure reveals the intersection of twin-twin is the prevailing deformation mechanism and the twins play a dual role in strengthening and toughening the alloy in the thermomechanical process. The deformation twin variants I and Π were formed by the edge dislocation 112¯ and the mixed dislocation 211¯ on the {111} crystal planes, respectively. Besides, the dislocation jogs and kinks caused by twin intersection on the slip planes can strengthen the alloy, which may contribute to the high strength (the tensile strengths at the 600° and 700° tensile tests are respectively780 MPa and 630 MPa.). Moreover, the coherent twin boundary migration has the function of coordinating deformation and contributes to the high ductility of the alloy.

Effect of processing temperature on the microstructural characteristics of Cu-Ag nanocomposites: From supersaturation to complete phase decomposition

Initially micrometer-sized powder blends of different compositions (Cu-6at%Ag, Cu-37 at%Ag and Cu-84 at%Ag corresponding to Cu-9vol%Ag, Cu-46 vol%Ag and Cu-89 vol%Ag) were deformed by high-pressure torsion for varying applied strain and temperature in order to study their effect on the degree of supersaturation and the prevailing deformation mechanisms. The resulting microstructures were comprehensively characterized with synchrotron X-ray diffraction and transmission electron microscopy. A gradual transition from complete supersaturation to complete phase separation was observed with increasing processing temperature. The critical temperature for full supersaturation strongly depended on the composition. In composites with low Ag and Cu contents single-phase alloys could be obtained up to processing temperatures of 100 °C. In the medium composition range strain localization in shear bands prevented full supersaturation for room temperature deformation. Only in regions deformed by shear bands a complete single-phase supersaturated solid solution was obtained, while the lamellar matrix retained a dual-phase structure. By lowering the processing temperature, using liquid nitrogen as coolant, a homogenous single-phase alloy could be also attained at medium compositions. The present results unravel a clear correlation between dominating deformation mechanisms and the degree of deformation-induced supersaturation.

Influence of air injection on the liquefaction-induced deformation mechanisms beneath shallow foundations

Earthquake-induced liquefaction of soils frequently causes serious damage to structures with shallow foundations. Reducing the degree of saturation of liquefiable soils by air injection is offered as a cost-effective and reliable method of mitigating liquefaction hazards. Nevertheless, very little experimental research is available on the performance of this method. Particularly, the way that air injection influences the deformation mechanisms beneath shallow foundations is not well defined. Gaining a deeper insight into soil displacements during and after air injection can pave the way for developing effective guidelines for the use of this particular technique. For this purpose, a series of dynamic centrifuge tests are presented in this paper. The prevailing deformation mechanisms are identified in a novel way using displacement vector fields. The results indicate that air injection alters the deformation mechanisms that develop underneath and in the ground surrounding a shallow foundation, substantially reducing the average settlements.

About mechanisms of radiation-induced effect of nanostructurization of near-surface volumes of metals

Mechanisms of the radiation-induced development of nanostructures in subsurface metal regions have been analyzed based on field-ion microscopy data. It is concluded that the modification of near-surface metal regions on a nanometer scale as a result of the interaction with Ar+ ion beams proceeds by several mechanisms. In particular, for a fluence of F = 1016 ion/cm2 (at an ion energy of E = 30 keV), the main contribution is due to the ion channeling. A tenfold increase in the ion fluence leads to prevailing deformation mechanism in nanostructure formation in the subsurface metal regions.

Segregation assisted microtwinning during creep of a polycrystalline L12-hardened Co-base superalloy

A polycrystalline L12-hardened Co-base superalloy was creep deformed at 750 °C. The investigation of the deformed microstructure in the transmission electron microscope revealed microtwinning to be the prevailing deformation mechanism. The detected twins spanned the entire grain and cut through both, γ and γ′. Detailed high-resolution transmission electron microscopy investigations indicated that twin growth takes place by the slip of single a/6 〈112〉 partial dislocations along the twin boundary. Further analysis of the twin boundaries in the γ′ phase revealed segregation of elements known to decrease the stacking fault energy and a local depletion of γ′ forming elements. We propose that this segregation behavior enables subsequent a/6〈112〉 dislocations to easily slip along the twin boundary and further thicken the twins in the process.

Prediction of the local deformation mechanisms in metastable austenitic steels from the local concentration of the main alloying elements

The activation of different deformation mechanisms in high-alloy austenitic steels depends strongly on the concentration of the alloying elements, as they change the driving force for the martensitic phase transformation and the stacking fault energy in the austenite. Consequently, local chemical gradients alter the local deformation behaviour tremendously. In this contribution, the correlations between the local chemical composition, the corresponding difference between the Gibbs energies of austenite and α′-martensite, martensite start temperature, stacking fault energy, observed stacking fault density and the prevailing deformation mechanism are illustrated.

Analysis of the phenomenon of radiation-induced nanostructure development in subsurface metal regions

Mechanisms of the radiation-induced development of nanostructures in subsurface metal regions have been analyzed based on field-ion microscopy data. It is concluded that the modification of near-surface metal regions on a nanometer scale as a result of the interaction with Ar+ ion beams proceeds by several mechanisms. In particular, for a fluence of F = 1016 ion/cm2 (at an ion energy of E = 30 keV), the main contribution is due to the ion channeling. A tenfold increase in the ion fluence leads to prevailing deformation mechanism in nanostructure formation in the subsurface metal regions.

Scratch Resistance and Adhesion Properties of PEEK Coating Filled with h-BN Nanoparticles

Effects of h-BN particles on mechanical properties and adhesion of semi-crystalline poly (ether-ether-ketone) (PEEK) coatings were studied. PEEK powder was mixed with various contents of h-BN nanoparticle in ethanol under ultrasonic mixing. As-mixed powders were sprayed onto low carbon steel substrate with thermal spray technique. The hardness, scratch hardness, specific scratch wear rate, prevailing deformation mechanisms and adhesion were obtained from scratch tests by varying the applied load. The damage geometry on scratched polymer surfaces and scratch wear volume loss were examined using scanning electron microscope (SEM) and surface profilometer, respectively. The results indicated that the addition of h-BN content improved the hardness of the composite. In addition, the specific scratch wear rate and the adhesion between PEEK coatings and substrate decreased with increasing h-BN content. The poor adhesion resulted from the reductions of flow-ability and viscosity between PEEK and substrate caused by heat dissipation from the h-BN particles.

On the Compression Behavior of an Austenitic Fe–18Mn–0.6C–1.5Al Twinning‐Induced Plasticity Steel

High manganese austenitic TWIP steels are of great potential in the field of transportation‐related industries owing to their exceptional combination of strength and ductility. A series of compression experiments were conducted on a Fe–18Mn–0.6C–1.5Al alloy at various strain rates (from 1.0 × 10−2 to 6.4 × 103 s−1) and total strains (≈15 and ≈20%) with a Gleeble 3500 thermo‐mechanical simulator and a Split Hopkinson Pressure Bar system. Under compressive deformation, results showed this alloy possessed excellent strain‐hardening behavior, attributed to the occurrence of mechanical twinning during deformation. The prevailing deformation mechanism was observed to be twinning, which was substantiated by microstructural analyses, as well as phase identification and evolution of crystallographic texture.

Scratch deformations of poly(etheretherketone)

Abstract This paper presents the experimental results obtained from the scratching of semicrystalline poly(etheretherketone) (PEEK). Various surface mechanical properties of the PEEK such as the hardness, the friction coefficient, the scratch depth and the prevailing deformation mechanisms are presented in the form of scratching maps, generated as a function of the contact conditions. These scratch maps provide a first order evaluation of the relative scratch resistance of materials for comparison purposes. The penetration depth dependent scratches (in the range 20–200 μm) were produced on the PEEK surfaces using rigid cones of different cone included angles and under different normal loads. The prevailing deformation mechanism and geometry of damage on the scratched polymeric surfaces were assessed using scanning electron microscope (SEM) and an optical profilometer. The deformation modes were seen to be influenced by the imposed experimental parameters; such as the contact geometry, the load, the nominal sliding velocity, the contact temperature and the contact lubrication. In addition to the common deformation mechanisms observed in amorphous polymers; ranging from elastic, ironing, viscoelastic plastic ploughing and brittle deformations, a fibrillation of the semicrystalline PEEK was seen when the polymeric surface was scratched under the severe contact conditions (the higher contact strains and the normal loads).

Development of Microstructure and Texture of Hematite Ores Deformed to Large Strain in Torsion: Can Texture Identify the Prevailing Strength and Creep Mechanisms During Deformation?

AbstractSamples of fine‐grained (approximately 9 µm) and coarse‐grained (approximately 45 µm) hematite ores with almost random crystallographic preferred orientation (CPO) were deformed in high pressure, high temperature (400 MPa, 850–1 000 °C) torsion experiments up to shear strains of 4.7. Samples with large initial grain size, preferably deformed by dislocation creep attended by dynamic recrystallization, showed grain size reduction and a weak CPO (J ∼ 1.4 at 950 °C). In contrast, fine‐grained ores, deformed mainly by grain boundary sliding accompanied by dislocation activity with slip on the dominant basal glide system, showed grain growth and a strong CPO (J ∼ 2.9). At high strain both ores attained similar strength and grain size, but different CPOs. The experiments demonstrate that the texture intensity of highly deformed rocks strongly depends on initial microstructure and may not reflect unambiguously the prevailing deformation mechanism and strength as often assumed in field studies.