Stress For Slip Research Articles

Physical mechanisms of nanocrystallization of a novel Ni-based alloy under uniaxial compression at cryogenic temperature

A novel Ni-based alloy, which possesses a nominal chemical composition of Ni52Cu20Cr15Mo5Ti2Al1Nb5 (wt.%), is designed and fabricated in order to be used for cryogenic environments. The Ni-based alloy exhibits high plasticity and high strength in the case of uniaxial compression at −150°C. Nanocrystalline phases, in particular, are observed in the matrix of Ni-based alloy subjected to plastic deformation at −150°C. Statistically stored dislocation (SSD) density and geometrically necessary dislocation (GND) density are found to play a significant role in nanocrystallization of Ni-based alloy. SSD density and GND density increase with the increase in the plastic strain. There appears a critical dislocation density above which the critical resolved shear stress (CRSS) for dislocation slip was greater than the CRSS for deformation twinning. Consequently, deformation twinning occurs instead of dislocation slip with increasing plastic strain. Under the simultaneous action of dislocation slip and deformation twinning, dislocation wall, dislocation cell, deformation twin, low angle grain boundary and high angle grain boundary occurs in such an alternative way that nanocrystalline grains are formed in the end.

Effects of thermal treatment on microstructure and mechanical properties of a Mg-Gd-based alloy plate

The Mg-8Gd-4Y-Nd-Zr alloy plate was prepared by casting, homogenization and extrusion. Microstructure of the as-cast alloy was comprised of α-Mg and eutectics Mg5.05RE. These eutectics were able to dissolve in matrix during subsequent homogenization, but rare earth (RE) solutes precipitated from the matrix again during extrusion in the form of irregular Mg5.05RE particles. In order to dissolve particles and improve mechanical property, companion samples were solution treated at temperatures ranging from 450°C to 520°C. With the solution temperature increasing, the volume fraction of Mg5.05RE particles decreased, and finally, almost all of the particles dissolved in the matrix when solution temperature increased to 520°C. The strength of the solution and ageing treated samples increased first and then decreased as the solution temperature increasing, and the best property was achieved by 475°C/0.5h treatment followed by ageing. The ultimate tensile strength, tensile yield strength and elongation of the sample were 419MPa, 321MPa and 2.6%, respectively. Fracture analysis indicated that the residual Mg5.05RE particles were responsible for the sample′s failure. It was also the reason for the failure of other samples solution treated at lower temperatures. However, in 520°C/0.5h-T6 treated sample, the fracture was induced by promoted twinning behavior, which was caused by the formation of β′ precipitates during ageing. The critical resolved shear stress (CRSS) for basal slip was improved much higher than that for twinning, which suppressed slip but promoted twinning and finally resulted in the deterioration in elongation.

Slip Resistance of Ti-Based High-Temperature Shape Memory Alloys

Titanium with Nb, Zr, and Ta alloying substitutions possesses high plastic slip resistance and high transformation strains upon bcc (β) to orthorhombic $$(\alpha^{\prime\prime})$$ transformation. In the current study, we determine the critical resolved shear stress (CRSS) for slip in Ti alloyed for a wide composition range of Nb, Ta, and Zr. The CRSS is obtained with a proposed Peierls–Nabarro formalism incorporating the generalized stacking fault energy barrier profile for slip obtained from the first-principles Density Functional Theory (DFT) calculations. The CRSS for slip of the orthorhombic martensite increases from 80 to 280 MPa linearly with increasing unstable fault energy. The addition of tantalum is most effective in raising the energy barriers. We also demonstrate the composition dependence of the lattice parameters of both β and $$\alpha^{\prime\prime}$$ crystal structures as a function of Nb, Ta, and Zr additions showing agreement with experiments. Using the lattice constants, the transformation strain is determined as high as 11 % in the [011] pole and its magnitude increases mainly with Zr addition.

A study of stress relaxation in AZ31 using high-energy X-ray diffraction

The micro-plasticity of the Mg alloy AZ31 is explored through high-energy X-ray diffraction (HEXD). Through cyclic loading of the sample, a softening response is found to follow the resolved shear stress for basal slip. Stress relaxation is studied by applying an incremental elongation increase while continuously collecting images from a detector array. The rate exponent associated with a particular reflection is developed by evaluating the average lattice strain in the loading direction from the composite detector image collected at a particular point in time, and then following the time evolution. A distinct rate exponent is identified for grain orientations that have a propensity for slip on second-order pyramidal planes, highlighting the capability to capture the simultaneous action of different deformation mechanisms through HEXD.

Room-temperature yield and fracture strength of single-crystalline 6H silicon carbide

Silicon carbide (SiC) has excellent physical and electrical properties with potential for nuclear applications and power semiconductors. The properties of SiC, however, have not been fully determined. One property in question is the critical resolved shear stress (CRSS) for slip at room temperature. Here we evaluated the CRSS of 6H-SiC using micro-compression tests. Single-crystalline micro-pillars were fabricated on the surface of a 6H-SiC(0001) wafer. Brittle fracture occurred in all the fabricated micro-pillars. The compressive fracture strength of the material was determined to be near 24 GPa. Micro-pillars were also fabricated on a tilted specimen, to facilitate slip on the basal plane of hexagonal close-packed structure. Plastic deformation was observed in micro-pillars below 0.49 μm in diameter. Cross-sectional TEM observation of the compressed micro-pillars showed clear slip traces and dislocations on the basal {0001} planes. The CRSS of 6H-SiC was determined to be 9.8 ± 0.69 GPa from the measured stress–strain curves and the sample geometry. The CRSS evaluated here was compared with that determined from first-principle calculations reported in the literature.

Atomistic study on the cross-slip process of a screw dislocation in magnesium

The cross-slip process of a screw <a> dislocation from the basal to the prismatic plane in magnesium was studied using the density functional theory and the molecular dynamics calculations. An atomistic method for calculating the total Peierls energy map has been devised to track the transition path of a dissociated and/or constricted screw <a> dislocation in the cross-slip process. The barrier of a screw <a> dislocation from the basal to the prismatic plane is estimated by the density functional theory for the first time to be meV per Burgers vector length. The activation enthalpy for the cross slip is calculated using a line tension model based on the density functional theory to be 1.4–1.7 eV, which is in reasonable agreement with experiments. On the basis of the results, the effect of temperature on the cross-slip process of the dissociated screw <a> dislocation on the basal plane is studied in detail using the molecular dynamics method with the embedded-atom-method (EAM) interatomic potential, in which the critical resolved shear stress for the cross slip is evaluated. It is confirmed that the bowed-out dislocation line on the prismatic plane consists of slightly dissociated rectilinear segments with connecting jogs at low temperatures and, as the temperature rises, the curved dislocation line becomes smooth with many segments. The motion of an <a> dislocation on the prismatic plane is jerky in the low temperature region, while it is retarded by the formation of the largely dissociated plateau segment above the room temperature. A large reduction of the critical shear stress for the cross slip is obtained when the <a> screw dislocation interacts with a hard-sphere particle placed on the basal plane in the low temperature region.

Effects of Mn Content on the Deformation Behavior of Fe–Mn–Al–C TWIP Steels—A Computational Study

This paper presents a double-slip/double-twin polycrystal plasticity model using finite element solution to investigate the kinetics of deformation twinning of medium manganese (Mn) twinning-induced plasticity (TWIP) steels. Empirical equations are employed to estimate the stacking fault energy (SFE) of TWIP steels and the critical resolved shear stress (CRSS) for dislocation slip and deformation twinning, respectively. The results suggest that the evolution of twinning in Fe–xMn–1.4Al–0.6 C (x = 11.5, 13.5, 15.5, 17.5, and 19.5 mass%) TWIP steels, and its relation to the Mn content, can explain the effect of Mn on mechanical properties. By comparing the double-slip/double-twin model to a double-slip model, the predicted results essentially reveal that the interaction behavior between dislocation slip and deformation twinning can lead to an additional work hardening. Also, numerical simulations are carried out to study the influence of boundary conditions on deformation behavior and twin formation. The nucleation and growth of twinning are found to depend on internal properties (e.g., mismatch orientation of grains and stress redistribution) as well as on external constraints (e.g., the applied boundary conditions) of the material.

Preparation of sound ribbons with submicrometer-grained microstructure on a Mg–Zn alloy

In this paper, a viable way to fabricate Mg alloy sound ribbons with ultra-fine-grained microstructure was presented. The hot-rolled and annealed Mg–0.4Zn (at%) alloy exhibited excellent rollability to form sound ribbons with submicrometer grains when subjected to one-pass cold rolling process. The more balanced multi-mode dislocation slips originated from the significant decrease of critical resolved shear stress for non-basal slip with the addition of solute Zn and the favorable crystallographic orientation were suggested to be responsible for the excellent cold rollability. The formation of ultra-fine-grained microstructure was attributed to low-temperature dynamic recrystallization occurring during the cold rolling process with large strain.

Influence of temperature and strain rate on flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys

The effects of temperature and strain rate on the flow stress behavior of twin-roll cast, rolled and heat-treated AZ31 magnesium alloys were investigated under uniaxial tension. At high temperatures, dynamic recovery, continuous dynamic recrystallization, grain boundary sliding and the activation of additional slip systems lead to an improvement of the ductility of the alloys. The elongation to failure is nearly independent of the strain rate between 473 and 523 K at 10−2 s−1 and 10−1 s−1, which is related to the strain rate dependence of the critical resolved shear stress (CRSS) for nonbasal slip. Despite the high temperature, twins are even observed at 573 K and 10−3 s−1 because they have a low CRSS.

Improvement in the Mechanical Properties of High Temperature Shape Memory Alloy (Ti50Ni25Pd25) by Copper Addition

High temperature shape memory alloys Ti50Ni25Pd25and Ti50Ni20Pd25Cu5were developed, characterized, and tensile tested in both martensite ( Mf− 50°C) and austenite ( Af+ 50°C) phases. The transformation temperatures of ternary Ti50Ni25Pd25alloy were increased by 11 to 12.5°C by substitution of Ni with 5 at% Cu. At the same time, transformation heat absorbed and released during forward and reverse martensitic transformation was also increased. In the martensite phase, the mechanical properties, that is, the stress for reorientation of martensite variants and fracture stress, were increased by 33 and 60 MPa, respectively, whereas the fracture strain was decreased by 1.5%. In the austenite phase, the critical stress for slip and fracture stress were increased by 62 and 40.9 MPa, respectively, whereas the fracture strain was decreased by 1.2%. The increase in both stresses was attributed to the solid solution strengthening by substitution of Ni atoms with relatively greater atomic radius of copper (Cu) atoms. The overall results suggest that the addition of 5 at% Cu in place of Ni in Ti50Ni25Pd25alloy is very beneficial to improving the mechanical and shape memory properties and increasing the transformation temperatures.

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti55.5Ni45.5−xCux (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti55.4Ni32.8Cu11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti2Ni, Ti2Cu and TiNi, Ti2Cu grains grew faster than Ti2Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.

Superelastic properties of biomedical (Ti-Zr)-Mo-Sn alloys.

A new class of Ti-50Zr base biomedical superelastic alloys was developed in this study. The (Ti-Zr)-Mo-Sn alloys exhibited a shape memory effect and superelastic property by adjusting Mo and Sn contents. The (Ti-Zr)-1.5Mo-3Sn alloy revealed the most stable superelasticity among (Ti-Zr)-(1-2)Mo-(2-4)Sn alloys. The superelastic recovery strain showed a strong dependence on heat treatment temperature after cold working in the (Ti-Zr)-1.5Mo-3Sn alloy. The superelastic recovery strain increased as the heat treatment temperature increased although the critical stress for slip decreased. The (Ti-Zr)-1.5Mo-3Sn alloy heat treated at 1073K exhibited excellent superelastic properties with a large recovery strain as large as 7% which is due to the strong {001}β<110>β recrystallization texture.

Microstructure and shape memory behavior of annealed Ti44.5Ni55.5−XCuX (X = 15.3–32.8) thin films with low Ti content

(Ni,Cu)-rich Ti–Ni–Cu amorphous films with a low Ti content of 44.5at% and a Cu content of 15.3–32.8at%, formed by sputtering, were annealed for 1h at 773, 873, and 973K, and their microstructure and shape memory behavior were investigated. Ti(NiCu)2 precipitates were observed in all the annealed films. In addition to these precipitates, a TiCu phase was detected in the Ti45.6Ni21.6Cu32.8 films. These precipitates were densely distributed within the B2 grains in the films annealed at 773K. As the annealing temperature increased, the precipitates grew in size within the B2 grains. Unlike Ti–Ni–Cu films with 48.5at% Ti or greater, a few precipitates exclusively grew along the {100} planes and divided the original grains for the films annealed at 873K. The grain size increased with increasing Cu content from 15.3 to 27.3at%, but decreased for the Ti45.6Ni21.6Cu32.8 film. The films annealed for 1h at 773K were found to be brittle because of serrated grain boundaries with intergranular precipitates. The critical stress for slip decreased with increasing annealing temperature, whereas the martensitic transformation start (Ms) temperatures and maximum recoverable strains of the films annealed at 873 and 973K were almost the same. As the Cu content increased, the critical stress and the Ms temperature increased, but the maximum recoverable strain decreased. The relationship between the microstructure and shape memory behavior of the films was discussed.

Nanostructured extension twins in rapidly compressed Mg–3.0Al–1.0Zn alloy

Magnesium alloy components are potentially subjected to dynamic strain rate loading. High strain rate usually results in a mechanical behavior quite different from that under quasi-static loading. In the present study, twin characteristics were investigated in Mg–3.0Al–1.0Zn (AZ31B) alloy compressed at dynamic strain rate of 8.0×10−1s−1. In addition to large extension twins, nanostructured lamella extension twins were found in a large number in the matrix. The reason that such nanostructured twins co-exist with large twins was explained in terms of comparison between dislocation and twinning dynamics, i.e., due to the high strain rate, the dynamic critical resolved shear stress for slip could be so high as to result in a viscous drag and the abundant occurrence of the rapid shear of the extension twinning should be needed in widely oriented grains to accommodate strain.

Evaluating the effect of yttrium as a solute strengthener in magnesium using in situ neutron diffraction

In situ neutron diffraction of two binary Mg alloys, Mg–0.5wt.% Y and Mg–2.2wt.% Y have been carried out in compression. The experimental data has been modelled using the elastoplastic self-consistent methodology in order to determine the critical resolved shear stress for basal slip, second-order 〈c+a〉 pyramidal slip and {101¯2} twinning. It was found that the addition of Y strengthens all three of the deformation modes examined. However, increasing the Y concentration from 0.5% to 2.2% showed no additional hardening in the basal slip and {101¯2} twinning modes, indicating that solute strengthening of these deformation modes is already exhausted by a concentration of 0.5% Y. Second-order pyramidal slip showed additional solute hardening at the higher concentration.

Particle evolution in Mg–Zn–Zr alloy processed by integrated extrusion and equal channel angular pressing: Evaluation by electron microscopy and synchrotron small-angle X-ray scattering

The evolution of intermetallic precipitate particles in Mg–Zn–Zr alloy ZK60 during thermomechanical processing by integrated extrusion and equal channel angular pressing was investigated in detail. Electron microscopy was employed to analyse individual particles and their orientation within the Mg matrix, while small-angle X-ray scattering (SAXS) was used for an assessment of global particle behaviour and statistical significance of their volume in the microstructure. A significant redistribution of prismatic rod-type and basal platelet-type precipitates, as well as their resolutioning followed by the formation of prismatic platelets, was found. The platelet-type precipitates lying on prismatic {21¯1¯0}α planes were hitherto unknown for the Mg–Zn–Zr system. These precipitates were present in a statistically significant amount detectable by SAXS. Such precipitates should favour an increase of critical resolved shear stress for basal slip in the Mg matrix, thus contributing to an improved performance of the Mg–Zn–Zr alloy. The possibility of formation of prismatic platelet-type precipitates in Mg alloys proven in this paper opens up a new avenue for the design of relatively inexpensive high-performance magnesium alloys.

A new dual-plate slipometer for measuring slip between molten polymers and extrusion die materials.

In this work, we study the slip behaviors common to plastics die extrusion metals or platings using a new instrument called a dual-plate slipometer. By dual-plate, we mean that whereas the stationary plate incorporates a local shear stress transducer, the moving plate does not. The stationary plate and transducer are made of one stainless steel, but the moving plate is made from, or plated with, different extrusion die materials under study. This new instrument allows slip velocity to be measured without having to build a new shear stress transducer from each extrusion metal or plating under study. We explore the effect of extrusion die composition and die metal surface morphology on the slip properties of polyolefins using a sliding plate rheometer. In this work, we studied the slip behaviors of polyolefins on four common plastics die extrusion metals or platings, without having to build a new shear stress transducer from each. Specifically, our new method replaces the moving plate; with each of the four die metals or platings under study without changing the stainless steel material of the shear stress transducer and its stationary plate. Our experiments include high-density polyethylene, low-density polyethylene, and polypropylene (PP) on four different die metals or platings. We use steady simple shear to obtain shear stress versus nominal shear rate for different gaps, from which we can then deduce the slip velocity using the Mooney analysis. We then fit four slip models to our experimental measurements, and we find the Hatzikiriakos hyperbolic sine model to be accurate, even for the measured inflections in the slip velocity as a function of shear stress curves. Our analysis includes detailed characterization of the die metal plating surfaces, including measurements of the composition of the sliding plates by energy dispersive spectroscopy, surface energy by contact angle goniometry, and surface roughness by both white light interference and stylus profilometries. We use our slip measurements to evaluate the Allal-Vergnes equation for the critical shear stress for slip [A. Allal and B. Vergnes, "Effect of die surface on the onset of stick-slip transition in the flow of molten linear polymers," J. Non-Newtonian Fluid Mech. 167-168, 46-49 (2012)]. We conclude our analysis by dedimensionalizing slip, and we then use these dimensionless groups to analyze slip. This paper provides a set of reference data for extrusion die designers for polyolefins.

Effects of Ti content on microstructure and shape memory behavior of TixNi(84.5−x)Cu15.5 (x = 44.6–55.4) thin films

Ti–Ni–Cu films with Cu and Ti contents of 15.5 and 44.6–55.4at.%, respectively, were prepared by sputtering and their structure and shape memory behavior after annealing for 1h at 773, 873 and 973K were investigated. The Cu content of the B2 phase was constant for the Ti-rich films, but decreased with decreasing Ti content for the (Ni,Cu)-rich films. The (Ni,Cu)-rich films annealed at 773K were supersaturated with Ni. The grain size of the Ti-rich films was smaller than that of the (Ni,Cu)-rich films. These differences corresponded to the difference in the shape memory behavior. The martensitic transformation start temperature was almost constant for the Ti-rich films annealed at 873 and 973K, but decreased with decreasing Ti content for the (Ni,Cu)-rich films. The critical stress for slip was larger for the Ti-rich films than for the (Ni,Cu)-rich films. The maximum recoverable strain was smaller for the Ti-rich films than for the (Ni,Cu)-rich films. The relationship between the structure and shape memory behavior is discussed.

Effect of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi shape memory alloy

As-received nickel–titanium (NiTi) shape memory alloy with a nominal composition of Ni50.9Ti49.1 (mole fraction, %) was subjected to solution treatment at 1123 K for 2 h and subsequent aging for 2 h at 573 K, 723 K and 873 K, respectively. The influence of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi alloy was systematically investigated by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and compression test. Solution treatment contributes to eliminating the Ti2Ni phase in the as-received NiTi sample, in which the TiC phase is unable to be removed. Solution treatment leads to ordered domain of atomic arrangement in NiTi alloy. In all the aged NiTi samples, the Ni4Ti3 precipitates, the R phase and the B2 austenite coexist in the NiTi matrix at room temperature, while the martensitic twins can be observed in the NiTi samples aged at 873 K. In the NiTi samples aged at 573 and 723 K, the fine and dense Ni4Ti3 precipitates distribute uniformly in the NiTi matrix, and thus they are coherent with the B2 matrix. However, in the NiTi sample aged at 873 K, the Ni4Ti3 precipitates exhibit the very inhomogeneous size, and they are coherent, semi-coherent and incoherent with the B2 matrix. In the case of aging at 723 K, the NiTi sample exhibits the maximum yield strength, where the fine and homogeneous Ni4Ti3 precipitates act as the effective obstacles against the dislocation motion, which results in the maximum critical resolved shear stress for dislocation slip.

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ∼70nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [100] stock was subjected to shock compression generated by high-energy laser (∼350–850J), creating pressure pulses with initial duration of ∼3ns and amplitudes of up to ∼145GPa. The laser beam, with a spot radius of ∼1mm, created a crater of significant depth (∼135μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24GPa for the monocrystalline to ∼150GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684J.