Dominant Deformation Mechanism Research Articles

Deformation kinking and highly localized nanocrystallization in metastable β-Ti alloys using cold forging

• Microstructural refinement involving deformation kinking was investigated. • Formation of kink band involves dislocation gliding, pre-kinking and its ripening. • Nanograins ∼15 nm in diameter are produced but highly localized in kink bands. • Localized nanocrystallization results from the intrinsic soft nature of kink bands. • The softening of kink bands originates from the inner degraded dislocation density. Deformation kinking as an uncommon plastic deformation mechanism has been reported in several materials while the relevant microstructure evolution and grain refinement behavior at a large strain remain unclear so far. In this study, the issue was systematically investigated by utilizing cold forging to impose severe plastic deformation (SPD) on Ti-11 V metastable β -Ti alloys. It is found that the formation of kink bands experiences dislocation gliding, pre-kinking and the ripening of pre-kinks in sequences. The kink bands are subsequently thickened through the coalescence of multiple kink bands in a manner of high accommodation. Ordinary dislocation slip is developed as a dominant deformation mechanism when deformation kinking is exhausted. The resulting grain refinement involves transverse breakdown and longitudinal splitting of dislocation walls and cells, which fragment kink bands into small β -blocks. Further refinement of the β -blocks is still governed by dislocation activities, and finally nanograins with a diameter of ∼15 nm are produced at a large strain of 1.2. Alternatively, it is revealed that nanocrystallization is highly localized inside kink bands while the outer microstructure maintains original coarse structures. Such localized refinement characterization is ascribed to the intrinsic soft nature of kink bands, shown as low hardness in nanoindentation testing. The intrinsic softening of kink bands is uncovered to originate from the inner degraded dislocation density evidenced by both experimental measurement and theoretical calculation. These findings enrich fundamental understanding of deformation kinking, and shed some light on exploring the deformation accommodation mechanisms for metal materials at large strains.

Rheology of Naturally Deformed Antigorite Serpentinite: Strain and Strain-Rate Dependence at Mantle-Wedge Conditions.

Antigorite serpentinite is expected to occur in parts of subduction plate boundaries, and may suppress earthquake slip, but the dominant deformation mechanisms and resultant rheology of antigorite are unclear. An exhumed plate boundary shear zone exposed near Nagasaki, Japan, contains antigorite deformed at 474°C ± 30°C. Observations indicate that a foliation defined by (001) crystal facets developed during plate‐boundary shear. Microstructures indicating grain‐scale dissolution at high‐stress interfaces and precipitation in low‐stress regions suggest that dissolution‐precipitation creep contributed to foliation development. Analysis of crystal orientations indicate a small contribution from dislocation activity. We suggest a frictional‐viscous rheology for antigorite, where dissolution‐precipitation produces a foliation defined by (001) crystal facets and acts to resolve strain incompatibilities, allowing for efficient face‐to‐face sliding between facets. This rheology can not only explain aseismic behavior at ambient plate boundary conditions, but also some of the contrasting behaviors shown by previous field and laboratory studies.

Distinguishing contributions of metal binder and ceramic phase to mechanical performance of cermets at different temperatures

It is crucial to clarify the effects of the ceramic phase, the metal binder, and temperature on the mechanical properties of WC-Co cermets, as a typical representative of cermet materials, due to highly demanded excellent high-temperature properties under harsh service conditions. Based on uniaxial compression tests and detailed microstructural characterization, this study quantified the deformation characteristics of the ceramic phase and the metal binder at room temperature and high temperatures applying multiple analysis methods. It distinguished different contributions of the ceramic phase and metal binder to the plasticity of the cermets during deformation, and proposed the mechanisms for the plastic deformation of cermets. The results indicated that the dislocation plasticity is the dominant deformation mechanism of the cermets, and the ceramic matrix is mainly responsible for the fracture strength and contributes significantly to the plasticity of the cermets both at room temperature and high temperatures. At high temperatures, grain boundary slip and phase boundary sliding also contribute to the plastic deformation of cermets.

Hot deformation behaviour of Ti alloys: A review on physical simulation and deformation mechanisms

Titanium and its alloys, owing to their properties like high strength, toughness, corrosion resistance and thermal stability, are employable in various engineering and medical applications. The properties of titanium alloys depend upon the processing routes and their final microstructure. Therefore, the present review paper compiles the deformation behaviour of various alloys during hot deformation using a physical simulation. Subsequently, the flow stresses are analysed and utilized to develop the processing maps and the constitutive equations that are advantageous for predicting deformation mechanisms during the hot deformation. Specific features reported in the flow stress curves include work hardening, flow softening and steady-state behaviour. In certain cases, the yield point drop and oscillatory behaviour or serrations are also observed. Softening and oscillatory behaviour is an indicator of either dynamic recrystallization or flow-instability, whereas yield discontinuity signifies locking and unlocking of dislocations. In the processing maps, high power dissipation efficiency, η, reveals safe processing conditions, and the η value higher than 40% demonstrates dynamic recrystallization or globularization in the deformed microstructure, whereas the instability domain expresses shear band, flow localization, void formation and wedge cracks in the deformed microstructure. Amongst all the constitutive equations, the Arrhenius-type hyperbolic sine equation is the most suitable for Ti alloys for calculating flow stress and predicting dominant deformation mechanism using activation energy Q and stress exponent n.

The Effect of Strain Rate on the Deformation Behavior of Fe-30Mn-8Al-1.0C Austenitic Low-Density Steel

Automotive steels suffer different strain rates during their processing and service. In this study, the effect of strain rates on the tensile properties of fully austenitic Fe-30Mn-8Al-1.0C (wt.%) steel was investigated, and the dominant deformation mechanism was clarified. Conventional and interrupted tension tests and various microscopic characterization methods were carried out in this study. The results indicate that the yield strength increases with the increasing strain rate in the range of 10−4–10−1 s−1, and a good strength–ductility combination was achieved in the sample deformed at 10−3 s−1. In the process of straining at 10−3 s−1, microbands and deformation twins were observed. Thus, the combination of microband induced plasticity (MBIP) together with twinning induced plasticity (TWIP) leads to a continuous strain hardening behavior, and consequently to superior mechanical properties. However, adiabatic heating that leads to the increase in stacking fault energy (SFE) and inhibits the TWIP effect, as well as thermal softening jointly induces an anomalous decrease in tensile strength at the high strain rate of 10−1 s−1.

Temperature effects on tensile behaviors and relevant deformation mechanisms of a low-cost nickel-based single crystal superalloy containing 1.5% Re

In this work, tensile properties and relevant deformation mechanisms were investigated particularly in a novel low-cost second-generation nickel-based single crystal superalloy containing 1.5 wt% Re. The results showed that the experimental alloy exhibited an anomalous yielding phenomenon. The yield strength of alloy reached a peak value of 1120 MPa but the corresponding elongation was only 8.2% at 760 °C. Furthermore, the double yield phenomenon occurred at 900 °C and above whereas the strain softening behavior appeared at high temperatures. With increasing temperature, the main fracture mode gradually evolved from pure shear to microvoid accumulation fracture. The dislocations sheared into γ′ particles and led to the formation of stacking faults, which was the dominant deformation mechanism below 760 °C. Particularly, the formation of both K-W locks and L-C locks is closely related to the peak yield strength of the alloy at 760 °C. At 900 °C, the tensile deformation was controlled by the combination of APBs shearing and Orowan bypassing γ′ particles. As the temperature further increased to 1000 °C and 1100 °C, the dislocation networks formed on γ/γ′ interface could hinder the dislocations shearing into γ′ particles. Meanwhile, Orowan bypassing and dislocations climbing became the dominant deformation mechanisms. The results of this work could provide vital support for development and application of low-cost SX superalloys.

Quantitative assessment of the microstructural factors controlling the fatigue crack initiation mechanisms in AZ31 Mg alloy

The deformation and fatigue crack nucleation mechanisms were studied by means of slip trace analysis and secondary electron microscopy in a textured AZ31B-O Mg alloy subjected to fully-reversed cyclic deformation at two different cyclic strain semi-amplitudes. Samples were deformed in two orientations leading to symmetric and non-symmetric cyclic stress-strain curves due to the activation of different deformation mechanisms. They were ascertained in longitudinal sections of the specimens, which included a large number of grains (from 1500 to 4500 for each specimen), to obtain statistically significant results. If the dominant deformation mechanisms were basal slip and tensile twinning/detwinning, the most damaging fatigue cracks were nucleated along twins in large grains, together with cracks parallel to basal slip bands associated with the localization of deformation in clusters of small grains suitably oriented for basal slip. If the main deformation mechanisms were tensile twinning/detwinning and pyramidal slip, the longest fatigue cracks were nucleated along pyramidal slip bands in large grains. Grain boundary cracks around small grains were found in all cases, but they were not critical from the viewpoint of fatigue failure. This information is relevant to assess the effect of the microstructural features on the fatigue life of Mg alloys and as input to simulate the fatigue behavior of Mg alloys using fatigue indicator parameters.

Low-cycle fatigue behavior and surface treatment of a twinning-induced plasticity high-entropy alloy

The low-cycle fatigue life and cyclic deformation behavior of a metastable high-entropy alloy were investigated in this study. Additionally, the effects of the ultrasonic nanocrystal surface modification (UNSM) process on tensile properties and fatigue life were evaluated. Heat treatment following the cold rolling of Fe40Mn40Co10Cr10 alloy plates resulted in the formation of a coarse grain size of 46.7 ± 19.6 μm. Additional mechanical twins were activated during tensile testing compared to the equiatomic CoCrFeMnNi alloy, leading to increased elongation. However, mechanical twins in cyclic loads appear only at high strain amplitudes. Therefore, similar to the CoCrFeMnNi alloy, the slip of dislocations was the dominant cyclic deformation mechanism and the fatigue life of the alloys were comparable. The UNSM process formed a deformed gradient layer with a thickness of approximately 200 μm on the alloy surface. In addition to mechanical twins and low-angle grain boundaries, ultrafine grains with diameters as small as 200 ± 120 nm are detectable in this layer. Although this surface treatment successfully increased the yield strength by more than 70%, fatigue crack initiation was accelerated and fatigue crack growth resistance was degraded.

Temperature dependence of tensile behavior, deformation mechanisms and fracture in nitrogen-alloyed FeMnCrNiCo(N) Cantor alloys

Among a family of the multi-principle-element alloys, a high-entropy Cantor alloy attracts much attention due to the striking temperature dependence of strength properties, strong grain boundary hardening and the excellent low-temperature ductility. Cantor alloy is a substitutional concentrated solid solution, thus, its strength properties and strain hardening could be additionally enhanced by interstitial hardening. Here we explore the effect of nitrogen alloying with atomic concentrations of 0.8%, 1.4% and 1.6% on a temperature dependence (temperature range from 77 to 473 K) of the microstructure, mechanical properties, and fracture micromechanisms of FeMnCrNiCo alloy with the focus on low-temperature deformation regime. Nitrogen-alloying promotes the expansion of a crystal lattice with a linear increase of a lattice parameter of the alloy with nitrogen content, ∆a/∆CN = 0.625 pm/at%. Nitrogen-bearing FeMnCrNiCo(N) alloys possess stronger temperature dependence of a yield strength relative to the Cantor alloy, and nitrogen forces both athermal and thermal components of the stress. The solid-solution strengthening of Cantor alloy is proportional to the nitrogen concentration Δτ ~ CN, and the value (ΔYS/ΔCN) for the coarse-grained FeMnCrNiCo(N) alloys is a temperature-dependent parameter (ΔYS/ΔCN = 97 MPa/at% at RT and ΔYS/ΔCN = 146 MPa/at% at 77 K). Dislocation slip is a dominating deformation mechanism of the nitrogen-alloyed alloys along the whole range of the deformation temperatures. A decrease in test temperature and nitrogen-alloying both force the tendency to the planar slip and facilitate strain hardening. Despite a weak effect of nitrogen on a stacking fault energy of the Cantor alloy and high deforming stresses in FeMnCrNiCo(N) alloys, nitrogen-containing alloys show low activity of the mechanical twinning. Opposite effects of the nitrogen-alloying on the elongation of the Cantor alloy were found for different deformation regimes: positive at T > 250 K (elongation increases with nitrogen-alloying) and negative at T < 250 K (it decreases in nitrogen-bearing alloys). For FeMnCrNiCo(N) alloys, the appearance of the ductile-to-brittle transition in low-temperature deformation regime is associated with the brittle intergranular fracture of the specimens alloyed with nitrogen CN ≥ 1.4 at%.

Tuning deformation mechanisms of face-centered-cubic high-entropy alloys via boron doping

Face-centered-cubic (FCC) high-entropy alloys (HEAs) can be strengthened through boron doping. However, the existing form of boron and its effect on the deformation mechanism of FCC HEAs have been rarely reported. The present work demonstrates for the first time the effect of the interstitial boron and boron-rich precipitate on the deformation mechanisms of the FCC (CoCrFeNi) HEAs. The dominant deformation mechanism of CoCrFeNi HEAs is dislocation slip, whereas the interstitial boron atoms dissolved in the matrix, thereby reducing the stacking fault energy (SFE) and transforming the dominant deformation mechanism to deformation twins. The loss of Cr from the matrix after boride formation increased the SFE, which led to transforming the dominant deformation mechanism from deformation twins to dislocation slip. Based on the results, we firstly propose to tune the deformation mechanism of FCC HEAs via boron doping, which provide a new strategy for designing and optimizing the HEAs. • Adding boron to CoCrFeNi HEA can significantly improve the yield strength. • The different existing form of boron will change the SFE of CoCrFeNi HEA, thereby affecting its deformation mechanism. • We firstly propose to tune the deformation mechanism of FCC HEAs via boron doping.

Effects of carbon and molybdenum on the nanostructural evolution and strength/ductility trade-off in Fe40Mn40Co10Cr10 high-entropy alloys

The effects of carbon and molybdenum on the strain-induced nanostructural evolution and strength-ductility trade-off in Fe 40 Mn 40 Co 10 Cr 10 , Fe 39.5 Mn 40 Co 10 Cr 10 C 0.5 , and Fe 38.3 Mn 40 Co 10 Cr 10 Mo 1.7 high-entropy alloys (HEAs) were investigated at room temperature (RT) and cryogenic temperature (CT). Deformation twinning was the dominant deformation mechanism for all the three HEAs at RT. The addition of Mo enhanced the metastability and thus increase the volume fraction of ε-martensite with strain in Fe 38.3 Mn 40 Co 10 Cr 10 Mo 1.7 at 77 K. The fractions of annealing twins played important roles in the nanostructural evolution and grain refinement of the Fe 38.3 Mn 40 Co 10 Cr 10 Mo 1.7 at CT. The nanostructural evolution was found to be controlled by modifying SFE, the friction stress for cross slip and Gibbs free energy for phase transformation (ΔGfcc−hcp) via alloying with Mo or C. The nucleation of mechanical twinning at RT and the preferential occurrence of strain-induced martensitic transformation at CT was found to induce the grain partitioning through the hierarchical structure formation and enhanced work hardening rate. The simultaneous occurrence and synergistic effect of twinning-induced plasticity/transformation-induced plasticity in Fe 39.5 Mn 40 Co 10 Cr 10 C 0.5 exhibited an excellent strength/ductility combination (1022 MPa/~ 110%) at 77 K. • Effects of C and Mo on the nanostructural evolution and deformation were studied. • Deformation is dependent on changes of SFE, friction stress & ΔG fcc−hcp by alloying. • TRIP effect is stimulated by addition of Mo in Fe 40 Mn 40 Co 10 Cr 10 high entropy system. • Combined TWIP/TRIP in Fe 39.5 Mn 40 Co 10 Cr 10 C 0.5 enhanced both strength/ductility at CT. • Superior mechanical properties achieved via metastability engineering at CT.

On the plasticity of a hot-rolled Zircolay-4 via a combination of in-situ acoustic emission and ex-situ EBSD techniques

The plastic anisotropy of a hot-rolled Zircaloy-4 (Zr-1.56Sn-0.22Fe-0.11Cr) occurring during compressive loading in three orthogonal directions was analysed using the acoustic emission (AE) technique and scanning electron microscopy (SEM). In particular, AE was used for in-situ monitoring of the activity of individual deformation mechanisms. The microstructure was characterized by the electron backscatter diffraction (EBSD) technique in the initial state and after deformation of 2% and 12%, respectively. The grain reference orientation deviation (GROD) maps estimated from the EBSD data reveal the evolution of slip system activities in individual grains. During compressive loading along the rolling and transverse directions (RD and TD, respectively), dislocation glide and twinning are dominant deformation mechanisms. Moreover, the AE technique in combination with EBSD analysis indicates that more intensive twinning in RD, reflected by a high twin volume fraction, is rather given by twin thickening than massive twin nucleation. With respect to the initial texture, the twinning is almost suppressed during loading along the normal direction (ND) and plastic deformation is realized by several slip systems, including multiple slip.

Study on the high temperature creep deformation and fracture behaviors of Inconel 625 deposited metal

The creep behaviors of as-welded Inconel 625 deposited metal were studied over broad ranges of temperature (600 °C–900 °C) and stress (50 MPa–500 MPa), and the corresponding deformation mechanisms and fracture behaviors after rupture were investigated by various techniques. The results showed that the average slope of the creep life and the stress fitting curve was −0.159 (600 °C–800 °C), which was almost the same as that of the cast 625 alloy (−0.16). However, the slope decreased at 900 °C. According to transmission electron microscopy (TEM) observations, the dominant creep deformation mechanism transformed from shearing and climbing of dislocations at 600 °C, 700 °C and 800 °C/200 MPa to cross-slip of dislocations at 800 °C/80 MPa and 900 °C. The fracture behaviors were characterized by optical microscopy (OM) and scanning electron microscopy (SEM). With increasing temperature, the fracture behavior changed from mixed to intergranular. Similarly, the fracture behavior changed with increasing applied stress at 800 °C. At 600 °C, 700 °C and 800 °C/200 MPa, the carbides and intermetallic compounds produced microcracks, which eventually led to fractures. However, at 900 °C and 800 °C/80 MPa, the aggregation of micropores was the main cause of grain boundary failure.

Calcite Deformation Twins: From Crystal Plasticity to Applications in Geosciences

E-twinning is the dominant mechanism of plastic deformation of calcite at low temperature (&lt;300 °C), and in most limestones, e-twins are, at the crystal scale, the dominant microstructures [...]

Analysis of anisotropy mechanism in relation with slip activity in near α titanium alloy pipe after Pilger cold rolling

Understanding of anisotropy mechanism for Pilger rolling Ti80 (Ti-6Al-3 Nb-2Zr-1Mo) alloy pipe is of great significance to improve its service performance. In this work, the strength, plasticity and fracture mode of Ti80 alloy pipe with rolling texture in three different directions (RD, TD and 45°) were comparatively investigated. Firstly, the tensile strength is in the descending order of TD, RD and 45°, and plasticity shows the opposite trend. According to the statistics of a large number of deformed grains, the underlying mechanism of anisotropy was revealed by analyzing the slip activation, slip transfer, damage and fracture for three different loading directions. The results show that prismatic<a> slip is the dominant deformation mechanism in RD, and the basal<a> slip is mostly activated in TD. In 45°, the number of activated prismatic<a> slip and basal<a> slip is almost the same. Meanwhile, the pyramidal<a> slip plays an important role in accommodating the plastic deformation. A method for predicting the yield strength was established based on the critical resolved shear stress (CRSS) of basal<a> slip, prismatic<a> slip and pyramidal<a> slip, accompanying by different SFs distributions of activated slip systems. Subsequently, slip transfer, which can relieve the incompatible deformation, occurred between adjacent α grains when the slip transfer factor (m) and Schmidt factor (SF) are at a high value. Slip transfer was prone to occurring between the same slip systems, such as between prismatic<a> slip or basal<a> slip. However, it is discovered that the slip transfer between prismatic<a> slip and basal<a> slip is also likely to occur, when the adjacent α grains maintain 90° variant relationship. The slip transfer between 90° variants in 45° have higher probability than that in RD and TD due to high SFs of activated basal<a> slip and prism<a> slip, resulting better plasticity. Lastly, the micro-defects are uniformly distributed and the fracture mode is microvoids coalescence in RD. More basal<a> slip induces the cleavage cracks and cause the shear fracture in 45°. In TD, a mixed fracture mode of microvoids coalescence and cleavage is presented due to the heterogeneous deformation.

Grain boundary evolution in nanograined metals: from relaxation to Schwarzation

High density of grain boundaries (GBs) in nanograined metals may become unstable and evolve in various ways such as migration, sliding, or relaxation under thermal or mechanical stimuli. In this report, we will present our recent studies on structural evolution of GBs in nanograined Cu during intensive plastic deformation. As the grain size decreased below 200 nm, GB migration become obvious assisted by dislocation activities. Below 70 nm, accompanied by a transition in dominant deformation mechanism from full dislocations to partials, GB migration decays gradually, while the GB structures relax into lower energy states through their interactions with partials. The population of low-energy GBs increases with a decreasing grain size. Below 10 nm, further refinement of grains results in a violent structural evolution of the GB network into a 3-dimensional periodic minimal surface structure, Schwarz-D, through a phase-transformation-like process, which is termed as Schwarzation.

Shear direction induced transition mechanism from grain boundary migration to sliding in a cylindrical copper bicrystal

Control over grain boundary (GB) motion ways provides an effective mean for tailoring the mechanical and physical properties of nanocrystalline metals. However, shear direction induced transition from GB migration to sliding in nanocrystalline metals has remained elusive. In this work, we used molecular dynamics (MD) simulations to track the dynamic process of a representative Σ11(113) high-angle GB in a cylindrical copper bicrystal under controllable shear directions parallel to the GB plane. When the shear direction changed from the [332¯] direction to the [11¯0] direction (the shear angle θ increased from 0° to 90°), the dominant GB deformation mechanism switched from GB migration to sliding. The critical orientation (critical shear angle) of the transition was [85¯1¯] (75°). Atomistic observations indicated that the disconnection and the surface step nucleation provoked the GB migration and sliding, respectively. This shear direction induced transition was also found in other 〈110〉 symmetrical and asymmetric tilt GBs, as well as nano polycrystalline metals. Analytical models revealed that the transition mechanism originated from the competition between the nucleation energies of the disconnection and surface step. These insights may contribute to the ongoing search for favoring manageable design of nanocrystalline metals by GB-mediated plastic deformation.

Prolonged grain boundary sliding in naturally deformed calcite marble at the middle crustal level

It is generally accepted that grain boundary sliding (GBS) is the dominant deformation mechanism in fine-grained strain localization zones. However, the lifetime of GBS during deformation in monomineralic rocks is hotly debated, as grain growth may inhibit the operation of GBS. In this contribution, recrystallized matrix grains in calcite marbles from the Jinzhou detachment fault zone at the middle crustal level, Liaodong Peninsula, Northeast China, show clear evidence of GBS, e.g., 1) fine grains with sizes of 5–30 μm, nearly equant grain shapes and slightly curved grain boundaries, 2) rare evidence for intracrystalline deformation, 3) existence of four-grain junctions and aligned straight grain boundaries, and 4) weak crystallographic preferred orientations. The existence of high misorientation zones along the margins of the matrix grains, especially at their triple junctions, and differences in cathodoluminescence (CL) characteristics between rims and cores of the grains indicate simultaneous accommodation of GBS by dislocation motion and grain boundary diffusion. The different CL characteristics between rims and cores of calcite matrix grains and the presence of calcite-filled cracks imply fluid activity during deformation. Fluids could dramatically enhance GBS by promoting dislocation motion and grain boundary diffusion. Additionally, the nucleation of fine grains by dynamic recrystallization and cavitation-sealing processes during GBS guaranteed the maintenance of GBS. At the same time, the occurrence of micropores and the existence of second phases also inhibit grain growth. These features make GBS the dominant deformation mechanism in naturally deformed marbles in the middle-upper crust.

Grain-size-evolution controls on lithospheric weakening during continental rifting

Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from nature and rock deformation experiments suggest a transition to grain-size-dependent mechanisms due to a reduction in grain size can cause lithospheric weakening. Here, we employ a two-dimensional thermo-mechanical numerical model of the upper mantle to investigate the nature of deformation and grain-size evolution in a continental rift setting, on the basis of a recent growth law for polycrystalline olivine. We find that the average olivine grain size is greater in the asthenospheric mantle (centimetre-scale grains) than at the crust–mantle boundary (millimetre-scale grains). This grain-size distribution could result in dislocation creep being the dominant deformation mechanism in the upper mantle. However, we suggest that along lithospheric-scale shear zones, a reduction in grain sizes due to localized deformation causes a transition to diffusion creep as the dominant deformation mechanism, causing weakening of the lithosphere and facilitating the initiation of continental rifting.

Nanoindentation investigation of mechanical and creep properties of continental Triassic Yanchang Formation shale, Ordos Basin

The mechanical and creep properties of shale have a great impact on wellbore stability and artificial fracturing performance during shale gas exploration. We have applied nanoindentation to study the preceding two aspects of the lacustrine Triassic Yanchang Formation shale in Ordos Basin on the microscale, which is the representative exploration target for shale gas occurring in continental shales in China. A massive nanoindentation campaign finds that the Yanchang shale is highly mechanically heterogeneous in local areas but statistically homogeneous in large areas. The sequentially increasing indentation loads (5–400 mN) result in a two-stage change of values of Young’s modulus and hardness, i.e., descending first and then remaining steady with a turning point at approximately 300 mN, which suggests that the large indentation load of not less than 300 mN can detect the mechanical response of bulk shale on the microscale. In addition, the indentation tests reveal a strong anisotropy of mechanical properties of Yanchang shale, as the measured Young’s modulus and hardness in the bedding plane parallel are much larger than those in the bedding plane normal direction. Furthermore, the Yanchang shale presents strong creep behavior and weak fracture toughness, which are different from those of current gas-producing marine shales worldwide. We attribute this to the higher content of clay minerals and relatively more loose texture of the Yanchang shale. In particular, the creep strain-rate sensitivity ( m) is calculated to be 0.102–0.134, suggesting that the dominant deformation mechanism of the lacustrine Yanchang shale may be dislocation creep.