Strain Rate Cycling Tests Research Articles

Study on X-Ray-Induced Defects as Barriers for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillation

Study on X-Ray-Induced Defects as Barriers for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillation

Study on Thermal Activation Barrier for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillations

Study on Thermal Activation Barrier for Dislocation Motion by Strain-Rate Cycling Tests Combined with Ultrasonic Oscillations

High-temperature plastic deformation of langle 110rangle-oriented BaTiO3 single crystals

BaTiO3 single crystals were deformed in compression along the langle 110rangle crystal axis to study the plastic deformability and dislocation structures at high temperatures under different loading conditions. The yield strength is determined from stress–strain curves under strain rate control, load control, strain rate cycling tests, and under step-wise loading conditions to elucidate the impact of measurement approach in yield strength behavior. A comparison between the chosen methods based on stress-dependent strain rate plots indicates that load control measurements are a suitable alternative to the commonly used strain rate-control experiments in metals. This allows avoiding overloading and providing an estimate of the overall achievable strain rates in a ceramic. Activation energies and activation volumes in the temperature range of 1100–1170 °C indicate a similar mechanical deformation behavior to SrTiO3.Graphical abstract

Study on Dislocation-Dopant Ions Interaction in Ionic Crystals by the Strain-Rate Cycling Test during the Blaha Effect

The interaction between a dislocation and impurities has been investigated by measurements of yield stress and proof stress, micro-hardness tests, direct observations of dislocation, internal friction measurements, or stress relaxation tests so far. A large number of investigations has been carried out by the separation of the flow stress into effective and internal stresses on the basis of the temperature dependence of yield stress, the strain rate dependence of flow stress, and stress relaxation. Nevertheless, it is difficult to investigate the interaction between a dislocation and obstacles during plastic deformation by the mentioned methods. As for the original method which combines strain-rate cycling tests with the Blaha effect measurement, the original method is different from above-mentioned ones and would be possible to clear it up. The strain-rate cycling test during the Blaha effect measurement has successively provided the information on the dislocation motion breaking away from the strain fields around dopant ions with the help of thermal activation, and seems to separate the contributions arising from the interaction between dislocation and the point defects and those from dislocations themselves during plastic deformation of ionic crystals. Such information on dislocation motion in bulk material cannot be obtained by the widely known methods so far. Furthermore, the various deformation characteristics derived from the original method are sensitive to a change in the state of dopant ions in a specimen by heat treatment, e.g., the Gibbs free energy (G0) for overcoming of the strain field around the dopant by a dislocation at absolute zero becomes small for the annealed KCl:Sr2+ single crystal (G0 = 0.26 eV) in comparison with that for the quenched one (G0 = 0.39 eV).

X-ray-induced defects as obstacles to dislocation motion in alkali halide crystals

NaBr and KBr single crystals were exposed to X-ray (W-target, 30 kV, 20 mA) at room temperature and strain rate cycling tests associated with ultrasonic oscillation were carried out for the crystals at 77–293 K. The stress decrement (Δτ) due to oscillation and the stress change due to strain rate cycling have been measured during plastic deformation. The relative curve of ∆τ and strain rate sensitivity (λ) of flow stress has a stair-like shape for the two kinds of crystals. That is to say, the curve has two bending points and is divided into three regions: two plateau regions and the region between the two bending points, where λ decreases gradually with increasing ∆τ. The first region is a plateau within the small ∆τ. This implies that X-ray-induced defects have weak interaction with dislocation and act as obstacles to dislocation motion. Furthermore, the dependence of stress decrement (τ p) at the first bending point on the activation volume (V) obtained from the difference between λ in the first and second plateau regions reflects the interaction between dislocation and defects induced by the X-irradiation. The activation energy for the break-away of a dislocation from the defect could be obtained on the basis of τ p–V curve fitting the Barnett model to the experimental results. Then, the values of activation energy are 0.76 and 0.81 eV for NaBr and for KBr, respectively.

Activation energy for dislocation breakaway from various monovalent ions in NaCl single crystals

Combination method of strain-rate cycling tests and application of ultrasonic oscillations was carried out during plastic deformation of NaCl single crystals doped with Li+, K+, Rb+, Cs+ or F− at 77 K to room temperature. Relative curves between the stress decrement (Δτ) due to ultrasonic oscillatory stress and strain-rate sensitivity () of flow stress under superposition of the oscillation are obtained by the original method (strain-rate cycling test during the Blaha effect measurement) and have stair-like shapes for the specimens at low temperature. Assuming that the Cottrell–Bilby relation is suitable for the force distance between dislocation and the dopant ion, the values of G0, Tc, etc. are calculated from the data analysed in terms of Δτ vs. . G0 is the Gibbs free energy for overcoming of the dopant by dislocation at absolute zero and Tc the critical temperature at which the dopant does not act as obstacle to dislocation motion. The obtained energies G0 are found to be arranged linearly with the isotropic defect (Δε) around it in the each specimen.

I<sup>–</sup> Ions as Obstacles to Dislocation Motion in NaCl:I<sup>–</sup> Single Crystals

Strain-rate cycling tests associated with the ultrasonic oscillation were conducted for the purpose of investigation on the interaction between dislocation and I– ions during plastic deformation of NaCl:I– (0.5 mol% in the melt) at 77 K to room temperature. The relative curves of stress decrement (Δt) due to the oscillation and strain-rate sensitivity ( ) have stair-like shape for NaCl single crystals doped with I– at low temperatures. There are two bending points and two plateau regions. λ decreases with Δt between the two bending points. τp at Δt of first bending point and λp between λ at first plateau place and at second one depend on the dopant ions as weak obstacles to dislocation motion. Not only temperature dependence of τp and λp but also τp versus V (activation volume) reflects the interaction between dislocation and I– ions. On the basis of the data (i.e. τp and λp) analyzed in terms of the relative curves of Δt and λ, the activation energy, G0, for the overcoming of dislocation from the dopant ion is found to be 0.47 and 0.53 eV for NaCl:Br– and NaCl:I–, respectively. This result that G0 for NaCl:I– is somewhat larger than for NaCl:Br– leads to the phenomenon that I– ions are slightly stronger than Br– ones as weak obstacles to dislocation motion because of the difference between isotropic strains around I– ion and around Br– in NaCl single crystal. Furthermore, the values of τp0 and Tc are also obtained for the two kinds of specimens. τp0 and Tc are the value of τp at absolute zero and critical temperature at which τp becomes zero.

Interaction between dislocation and defects induced by X-irradiation in alkali halide crystals

Abstract Interaction between dislocation and defect induced by X-irradiation has been investigated in NaCl and KCl single crystals by strain rate cycling tests under superimposition of ultrasonic oscillation during plastic deformation. The interaction energy between dislocation and radiation-induced defect has been obtained by fitting Barnett model to experimental results, assuming that the defect is tetragonal. The interaction energies between dislocation and defect were determined to be 0.39 and 0.87 eV for NaCl and KCl, respectively. The larger interaction energy for KCl presents its larger tetragonality of defect induced by X-irradiation than NaCl.

Bending angle of dislocation pinned by an obstacle and the Friedel relation

The Friedel relation between the effective stress and the average length of dislocation segments agrees with the Fleischer relation () in the L 0/L versus φ 0 curve within 150° ≤ φ 0 ≤ 180°, where L 0 is the average spacing of impurities on a slip plane, L is the impurities along a dislocation and φ 0 is the angle at which the dislocation embraces the impurity at a temperature of 0 K. Therefore, it was considered that the Friedel relation is suitable for impurity obstacles that impede the dislocation above φ 0 of about 150° during steady and plastic deformation. In addition, it was confirmed from the value of φ 0 that the Friedel relation is appropriate for the interaction between a dislocation and the impurity in alkali halides doped with monovalent impurities. This is based on the data obtained by strain-rate cycling tests associated with ultrasonic oscillation at 80 to 300 K.

Interaction between dislocation and divalent impurity in KBr single crystals

Interaction between dislocation and divalent impurity has been investigated in KBr single crystals doped with various impurities by strain rate cycling tests under ultrasonic oscillation. The interaction energy between dislocation and impurity-vacancy dipole (I-V dipole) has been obtained by fitting Barnett model to experimental results. The interaction energies between dislocation and I-V dipole were determined to be 0.51, 0.30, 0.23 and 0.36eV for Ba2+, Sr2+, Ca2+ and Mg2+, respectively. The energy increases with increasing ionic radius of divalent impurity except Mg2+. This may be because the I-V dipole is made of a divalent impurity and a nearest vacancy for Ba2+, Sr2+ and Ca2+ but of a divalent one and a next nearest vacancy for Mg2+.

Study on the interaction between a dislocation and impurities in KCl:Sr2+ single crystals by the Blaha effect—Part IV influence of heat treatment on dislocation density

Strain-rate cycling tests associated with ultrasonic oscillation were carried out at 80–239 K for two kinds of KCl:Sr2+ (0.05 mol.% in the melt) single crystals: one is a quenched specimen and the other an annealed one. In this study, it was found that the density of moving dislocation is not influenced by the heat treatment. Furthermore, the increase in forest dislocation density for the annealed specimen seemed to be remarkable under the compression test, compared with that for the quenched specimen. As a result, the strain-hardening rate increased and the extent of plastic deformation region became short at a given temperature by annealing the quenched specimens. The investigation concerning forest dislocation density was conducted on the basis of the \( \Updelta (\Updelta \tau^{\prime}/\Updelta \ln \dot{\varepsilon })/\Updelta \varepsilon , \) which will represent the variation of the strain-rate sensitivity due to dislocation cuttings with shear strain.

STRAIN RATE SENSITIVITY OF ULTRAFINE-GRAINED Cu WITH NANOSIZED TWINS

An ultrafine-grained Cu sample with a high density of growth twins was synthesized by means of pulsed electrodeposition technique. The strain rate sensitivity of the Cu sample was measured by strain rate cycling tests under tension. The effects of grain size as well as twin density on the strength and strain rate sensitivity were discussed.

Strain rate sensitivity of flow stress under superimposition of ultrasonic oscillatory stress during plastic deformation of RbCl doped with Br − or I −

Strain rate cycling tests were carried out under superimposition of ultrasonic oscillation during plastic deformation of RbCl single crystals doped with Br − or I −. The relation between the strain rate sensitivity λ ( = d τ / d ln ⁡ ε ˙ ) of flow stress and stress decrement Δ τ due to application of oscillation has a stair-like shape and is divided into three regions, two plateaus at low and high Δ τ and the decreasing λ region between them. The stress decrement τ p at the first bending point is considered to be the effective stress due to an impurity and the value of λ p, the difference in λ between the first and second plateau regions is also a part of λ due to an impurity. Values of τ p and λ p were measured from 77 K to room temperature. Assuming the Cottrell–Bilby relation for RbCl crystals doped with Br − or I −. The interaction energies between dislocation and Br − or I − were determined to be 0.50 or 0.58 eV in RbCl, respectively.

Plastic deformation of Fe-Al polycrystals strengthened with Zr-containing Laves phases

Abstract Fe-10 at.% Al-2.5 at.% Zr and Fe-20 at.% Al-2.5 at.% Zr alloys were deformed between room temperature and 700 °C. The materials show a flow stress plateau at about 300 MPa up to 600 °C for the material with 10 at.% Al and above 600 MPa up to 400 °C for the alloy with 20% Al. The high flow stresses compared to Fe-Al reference materials are partly due to the addition of Zr. The strain rate sensitivity of the flow stress was measured by stress relaxation and strain rate cycling tests. It is low up to 400 °C and high between 450 and 600 °C, i.e. in the range of the flow stress decrease. The microstructures of the undeformed materials are described in Part I of this paper. Micrographs of the deformed specimens taken in a high-voltage electron microscope reveal that the deformation occurs mainly within the soft Fe-Al grains and in the Fe-Al component of the grain boundary eutectic. The deformation data are interpreted in terms of solution hardening from the Al solute, dynamic strain ageing due to the Cottrell effect of the same defects, the athermal stress component of elastic dislocation interactions, the Hall–Petch contribution from the grain size, and the strengthening effect of the grain boundary layers.

Strain rate sensitivity and apparent activation volume measurements on equal channel angular extruded Cu processed by one to twelve passes

The strain rate sensitivity m and the apparent activation volume V ap of equal channel angular extruded Cu up to 12 passes was measured by strain rate cycling tests in compression. With increasing number of passes of ECAE an increase in m and a decrease in V ap were measured.

Study on the interaction between a dislocation and impurities in KCl:Sr2+single crystals by the Blaha effect Part III Influence of heat treatment on various characteristics

Strain-rate cycling tests associated with ultrasonic oscillation were conducted at 80–239 K for two kinds of KCl:Sr2+ (0.05 mol% in the melt) single crystals: quenched and annealed specimens. Examining the relationship of temperature and dislocation velocity-effective stress exponent, m*, estimated from the data obtained in this series, we could find the suitable force-distance relation between a dislocation and the impurity. The force-distance relation for the quenched specimen approached to the Fleischer's model taking account of the Friedel relation rather than the Fleischer's model. As for the annealed specimen, the SQ was the most appropriate of the three models: the SQ, the PA, and the TR indicate a square, a parabolic, and a triangular force-distance profile respectively. The three force-distance relations are taken account of the Friedel relation. By annealing the quenched specimen, m* became low at a given temperature. This may have been caused by the following two phenomena. First, the concentration of weak obstacles to dislocation motion decreased after the heat treatment. Secondly, the resistance to movement of a dislocation in the quenched specimen was weakened by annealing it, e.g., F0 was reduced to about one-third and ϕ0 increased from 154 to 172 degrees. F0 and ϕ0 are the force acted on the dislocation and the bending angle of dislocation by the weak obstacle such as the impurity. Therefore, it may be deduced that the dislocation velocity in the quenched specimen is more sensitive to the effective stress due to the impurities than that in the annealed specimen at the temperature.

Internal friction of γ-TiAl-based alloys with different microstructures

A Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) intermetallic alloy with different microstructures (fine-grained primary annealed (PA) and coarse-grained fully lamellar (FL)) was examined by internal friction experiments. The influence of microstructure on the internal friction properties was studied by high-temperature (300–1270 K) mechanical loss experiments using a low frequency subresonance apparatus (0.01–10 Hz). The mechanical loss spectra show two phenomena: (i) a loss peak of Debye type at about 1000 K (1 Hz) which occurs only in samples with fully lamellar microstructure. The activation enthalpy, determined from the frequency shift, is 3.0 eV. The peak is assigned to thermally activated reversible local movement of dislocations that are part of the mismatch structure of semicoherent lamellar interfaces. (ii) A high-temperature damping background above 1000 K which is controlled by an activation enthalpy of 3.8–3.9 eV. The activation enthalpy agrees well with that of creep and strain rate cycling tests (3.5–3.7 eV) and is in the range of values reported for self-diffusion indicating that both properties (high-temperature background (HTB) and creep) are controlled by volume diffusion assisted climb of dislocations.

Investigation of interaction between a dislocation and a Cl − or I − ion in NaBr single crystals

Strain rate cycling tests were carried out under superimposition of oscillatory stress for NaBr single crystals doped with Cl − or I −. The relationship between strain rate sensitivity (SRS) of flow stress and stress decrement Δ τ due to superimposition of oscillatory stress is stair-like and consists of three regions, i.e. two plateau regions at low and high Δ τ and a middle region where the SRS decreases with increasing Δ τ. The stress τ p at the first bending point between the first plateau region and the middle one, which is considered to be the effective stress due to Cl − or I −, decreases with increasing temperature. The difference (SRS p) between the SRS at the first and second plateau regions decreases with temperature. The SRS p is attributed to Cl − or I −. The interaction energies between a dislocation and a Cl − or I − are 0.40 or 0.47 eV in NaBr assuming that the force–distance profile is expressed by the Cottrell–Bilby relation. The force–distance profile is different from that for KCl:Br −.

Investigation of Interaction between a Dislocation and a Br Ion in NaCl:Br Single Crystals

Strain rate cycling tests were carried out under simultaneous Blaha effect measurement on NaCl single crystals doped with Br—. Curves of strain rate sensitivity (SRS) of flow stress versus stress decrement due to superimposition of oscillatory stress were obtained. The curves are stair-like and show two plateau regions at low and higher stress decrement, and a middle region where the SRS decreases with increasing stress decrement, as reported in a previous paper concerning KCl:Br—. The stress τp at the first bending point of the curve, which is considered to be the effective stress due to Br—, decreases with increasing temperature and becomes zero at a critical temperature Tc = 270 K. The difference SRSp between the SRS at the first and second plateau regions decreases with temperature in contrast to the case of KCl:Br—. This means that the force–distance profile of the interaction between a dislocation and a Br— ion for NaCl:Br— is different from that for KCl:Br— and is pointed near the top. It is found that the interaction energy between a dislocation and a Br— impurity for NaCl:Br— crystal is 0.37 eV.

Investigation of Interaction between a Dislocation and a Br? Ion in NaCl:Br? Single Crystals

Strain rate cycling tests were carried out under simultaneous Blaha effect measurement on NaCl single crystals doped with Br—. Curves of strain rate sensitivity (SRS) of flow stress versus stress decrement due to superimposition of oscillatory stress were obtained. The curves are stair-like and show two plateau regions at low and higher stress decrement, and a middle region where the SRS decreases with increasing stress decrement, as reported in a previous paper concerning KCl:Br—. The stress τp at the first bending point of the curve, which is considered to be the effective stress due to Br—, decreases with increasing temperature and becomes zero at a critical temperature Tc = 270 K. The difference SRSp between the SRS at the first and second plateau regions decreases with temperature in contrast to the case of KCl:Br—. This means that the force–distance profile of the interaction between a dislocation and a Br— ion for NaCl:Br— is different from that for KCl:Br— and is pointed near the top. It is found that the interaction energy between a dislocation and a Br— impurity for NaCl:Br— crystal is 0.37 eV.