Basal Plane Shear Research Articles

First-principles investigation on the chemical bonding, elastic properties and ideal strengths of MoAlB and WAlB nanolaminated MAB phases

The present work deals with first-principles calculations on chemical bonding characteristics, elastic properties, and ideal strengths under both shear and tensile deformations of MoAlB and WAlB nanolaminated MAB phases. Diverse chemical bonds were found in MoAlB and WAlB, including strong BB covalent bonds, AlB covalent-ionic bonds, MoB bonds and metallic MoAl and AlAl bonds. Elastic properties of MoAlB and WAlB are not highly anisotropic with shear modulus of (0 1 0)[1 0 0], (0 1 0)[0 0 1], (0 0 1)[1 0 0] similar to each other. Nevertheless, ideal strengths display significant anisotropy. The results show that ideal shear strength τm of basal plane shears are lower than those of non-basal plane shears, while ideal tensile strength σm of the basal plane is lower than those of non-basal planes. In addition, ideal tensile strength of the basal plane is also much lower than ideal shear strengths of basal plane shears. The basal plane cleavage and shear failure both take place along the weakly bonded AlAl layers. Moreover, the basal plane shear failure is found accompanied by remarkable expansion of the AlAl layers, which reduces the resistance against basal plane shears. It suggests that MoAlB and WAlB tend to cleavage along the AlAl layer or dislocations tend to multiply and slip in the AlAl layer, which is similar to MAX phases.

Easily tiltable B[sbnd]Al[sbnd]B linear chain: The origin of unusual mechanical properties of nanolaminated MAB phases (CrB2)nCrAl

The present work deals with first-principles simulations on the shear responses of nanonaminated (CrB2)nCrAl (n = 1–3) MAB phases. The simulated results show that the ideal shear strengths τm of basal plane shears are much lower than those of non-basal plane shears, indicating that dislocations tend to nucleate in basal planes. The lower resistance against basal plane shear is associated with the easily tiltable nature of BAlB linear chains, which may result from the local open structure around Al layers. In addition, the easily tiltable nature of BAlB chains also well explains the reduction tendency of τm with the increase of boride nano-layer thickness under basal plane shears. It suggests that the easily tiltable BAlB chains facilitate the release of shear strains in Al layers, which may give rise to MAB phases with the unusual mechanical properties in analogous to MAX phases, including readily machinability, thermal shock resistance, and damage tolerance.

Precursors to stress-induced martensitic transformations and associated superelasticity: Molecular dynamics simulations and an analytical theory

Precursor phenomena are critical issues for martensitic transformation, as they provide important clues for understanding the origin of the transformation and the structure of the transformation products. Prior to temperature-induced martensitic transformation, it has been recognized for a long time that the basal plane shear modulus ${C}^{\ensuremath{'}}$ ({110} $⟨1\overline{1}0⟩$ shear mode) of the parent phase decreases on approaching the transformation temperature. On the other hand, martensitic transformation can also be induced by stress; but little has been known about whether similar precursor phenomenon also exists prior to such stress-induced martensitic transformation. In the present study, we successfully simulated the stress-induced martensitic transformation and associated superelasticity in a generic martensitic system by means of molecular dynamics method. Through calculating ${C}^{\ensuremath{'}}$ as a function of applied stress, we found a significant softening of ${C}^{\ensuremath{'}}$ prior to the stress-induced martensitic transformation. This is a clear evidence for the existence of lattice softening prior to stress-induced martensitic transformation. Our results suggest that lattice softening is a common feature for both temperature- and stress-induced martensitic transformation. An interesting result is that stress does not soften all the crystallographically equivalent {110} $⟨1\overline{1}0⟩$ shear moduli (all are ${C}^{\ensuremath{'}}$ by definition); it softens some of them while it hardens the rest. This is contrasting with the temperature-induced ${C}^{\ensuremath{'}}$ softening, in which all equivalent {110} $⟨1\overline{1}0⟩$ shear moduli simultaneously soften to the same extent. It corresponds to the well-observed fact for given stress direction only certain martensite variant(s) are induced while other variants are prohibited. We also formulated an analytical theory to investigate the variation of ${C}^{\ensuremath{'}}$ under stress, and obtained similar result as that of our molecular dynamics simulations.

Study on elastic constant softening in stress-induced martensitic transformation by molecular dynamics simulation

Precursor phenomena are critical issues for martensitic transformation, as they provide important clues for understanding the origin of the transformation and the structure of transformation products. Prior to temperature-induced martensitic transformation, it has been widely recognized for a long time that the basal plane shear modulus C′ (=( C 11 − C 12)/2) of the parent phase decreases with approaching transformation temperature. On the other hand, martensitic transformation can also be induced by stress. But little is known about whether similar precursor phenomenon also exists prior to such a stress-induced martensitic transformation (SIM). In the present study, we used molecular dynamics method to simulate the precursory stage of a stress-induced martensitic transformation. A new method for calculating the elastic constants C′ prior to SIM was proposed. The relationship between C′ and applied stress at different temperatures was calculated. Our results showed that the softening of elastic constant C′ is a common feature for both temperature and stress-induced martensitic transformation. For stress-induced martensitic transformation, the critical stress to induce the transformation increases with increasing temperature and obey Clausius–Clapyeyron relationship; at a given temperature, C′ decreases with the applied stress approaching the critical stress.

Shear at twin domain boundaries in YBa2Cu3O7-x.

The microstructure and strain state of twin domains in YBa2Cu3O7-x are discussed based upon synchrotron white-beam x-ray microdiffraction measurements. Intensity variations of the fourfold twin splitting of Laue diffraction peaks are used to determine the twin domain structure. Strain analysis shows that interfaces between neighboring twin domains are strained in shear, whereas the interior of these domains are regions of low strain. These measurements are consistent with the orientation relationships of twin boundaries within and across domains and show that basal plane shear stresses can exceed 100 MPa where twin domains meet. Our results support stress field pinning of magnetic flux vortices by twin domain boundaries.

Understanding the martensitic transformations in TiNi-based alloys by elastic constants measurement

Abstract The origin of the unique monoclinic martensite B19′ in TiNi and TiNi-based alloys has remained obscure for many years. The formation of B19′ martensite also challenged the well-known basal-plane shear/shuffle theory of martensitic transformation in β phase alloys. Recently we proposed that the B2–B19′ transformation stems from a strong coupling between a non-basal-plane shear (c44 shear) with the basal-plane shear (c′ shear), being manifested by a low-lying and decreasing anisotropy factor (c44/c′) towards Ms. This model has gained experimental support from elastic constants measurement on Ti50Ni30Cu20 alloy which exhibits B2–B19 transformation (i.e. absence of the monoclinic shear). In the present study, we attempt to further verify this model by measuring the elastic constants of Ti50Ni40Cu10 alloy which undergoes a two-stage transformation B2–B19–B19′. The results clearly demonstrated once again that whenever the parent phase B2 does not transform directly into B19′, the anisotropy factor exhibits an increase towards Ms, indicating that the c44 shear is not included into the transformation. Therefore, the present study gives additional support to the coupling model of TiNi-based alloys. Furthermore, we tried to understand the multi-stage transformation in terms of coupling strength between the c′ shear and c44 shear.

The Role of Softening in Elastic Constant c44 in Martensitic Transformation

In the present paper, the authors show that the puzzling monoclinic distortion in TiNi martensite B19{prime} has a simple origin. It is due to the softening of {l_brace}001{r_brace} shear modulus c{sub 44} (which is also the {l_brace}001{r_brace} shear modulus by definition), which leads to a correlation of this shear mode with that of basal shear mode {l_brace}110{r_brace} . They give further support to this view by demonstrating how martensite structure of TiNi is altered by the addition of alloying element Cu through changing the relevant elastic moduli c{prime} and c{sub 44}. The also proposed a Landau type model to incorporate the influence of softening c{sub 44} on martensitic transformation. The present study suggest strongly that MT as a whole cannot be understood only in terms of basal plane shear and shuffle, and non-basal plane strain also plays an important role.

Creep and recovery in graphites at ambient temperature: An acoustic emission study

Nuclear graphites subject to compressive or tensile stress cycling show a Felicity effect, that is, acoustic emission (AE) is detected at stresses less than the previous peak stress. This is attributed to recovery processes that occur upon unloading and at zero stress. The extent of recovery increases with time (up to 10 5 mins) at zero stress between cycles. For two graphites (IM1-24 and PGA) held under constant compressive or tensile strain, AE over ∼16 h is attributed to creep. For the same graphites at zero stress (after application of a compressive or tensile prestress), AE over ∼16 h is attributed to creep recovery. Both types of AE time curve follow logarithmic rate laws similar to those derived earlier for high-temperature primary creep and creep recovery. The micromechanical processes that give rise to creep and AE on loading graphites are basal plane shear and microcracking; creep recovery is attributed to the reverse of these processes.

Extended basal plane shear in pyrolytic graphite at 3035 K

Extended basal plane shear in pyrolytic graphite at 3035 K

The deformation and twinning of graphite crystals under a point load

Abstract The deformation produced by point loading on the cleaved basal surfaces of graphite crystals has been studied in the scanning electron microscope. The experiments were of two types, viz. sliding and static contact. In the first a small tungsten stylus (tip radius ∼ 1 μm) was slid on the graphite surface inside the scanning electron microscope and the resulting damage observed as it occurred. The marked anisotropy and the limited number of slip systems of the graphite structure result in a characteristic deformation mode which involves the cleavage of thin flakes followed by folding of the partially separated layers. In the second type of experiment the deformation produced by a diamond indenter was studied. These experiments showed that the hardness value of c. 4.8 × 107 N/m2 for naturally-occurring graphite and 22 × 107 N/m2 for neutron-irradiated graphite are determined by a cleavage and folding process and are not related to the basal plane shear strength. It is suggested that the mechanism of folding in both types of experiment is one of mechanical twinning. An important feature of the deformation process is that the dislocation walls forming the twin boundaries are mobile under the applied stress and are free to run in the newly formed chips and flakes in a direction parallel to the basal plane. The greatly increased hardness of neutron-irradiated graphite is explained by the reduction in mobility of the twin boundaries due to the high point defect concentration.

Elastic constants and grüneisen parameters of pyrolytic graphite

Abstract The room-temperature elastic constants of a pyrolytic graphite were measured by an ultrasonic transmission technique and compared with values calculated on the basis of the constant strain and constant stress models from the elastic constants of compression-annealed pyrolytic graphite (CAPG). It was found that the constant stress model gives better agreement; and that the measured C 11 and C 44 can be bracketed by calculations with the basal plane shear constants of neutron-irradiated and unirradiated CAPG respectively as inputs, as would be expected because of the high mobility of dislocations in the basal plane. Grüneisen parameters, for sample temperatures between 80 and 300°k, were determined from measurements of inertial thermoelastic stress. Calculations from single-crystal thermal expansion and elastic constant values showed reasonable agreement with the measured parameters and indicated that the elastic constants of a given pyrolytic graphite, being extremely structure sensitive, can be used to estimate Grüneisen parameters from single-crystal data.

68. High temperature basal plane shear in pyrolytic graphite

68. High temperature basal plane shear in pyrolytic graphite