Strength Of Single Crystals Research Articles

The Rehbinder effect in tests of superalloys in contact with molten salts

The results of the Rehbinder effect manifestation during testing of superalloys in contact with corrosive media containing Na2SO4 + NaCl are discussed. We present the experimental study of the effect of salts containing chlorine and sulfur on the mechanical properties and long-term strength of single crystal and powder nickel-based superalloys at high temperatures. The practical value of the work is associated with the possible operation of gas turbine parts in conditions of ingress of the particles containing chlorine, sulfur and sodium into the flow path. A simplified (compared to a previously used) procedure of testing wrought alloys for long-term strength in molten salts is developed. A comparative study of the mechanical properties and long-term strength of a single crystal superalloy in the initial state (previously damaged by corrosion of different duration) and being in contact with salts during testing is carried out. Due to the large scatter of experimental data, the method of lower envelopes has been proposed and implemented to determine the guaranteed values of the long-term strength. It is shown that the guaranteed values of the long-term strength at different temperatures and test durations can decrease by 2.5 – 5.0 times in the presence of salts. Metallographic studies of the nature of damage and destruction of samples are carried out. The revealed decrease in the long-term strength of heat-resistant alloys in contact with salts is interpreted as the Rehbinder effect and not as a manifestation of the effect of stress corrosion cracking. A methodology for using the obtained test results with a duration of up to several thousand hours in highly aggressive environments is proposed to predict the long-term strength in relation to long-term operation (tens of thousands of hours) under conditions of relatively low salt loads.

A new exponential function to represent the effect of grain size on the strength of pure iron over multiple length scales

Abstract

When ‘smaller is stronger’ no longer holds

ABSTRACT‘Smaller is stronger’ has been a well-established tenet for the mechanical strength of small-volume single crystals. However, emerging evidence suggests that this trend does not always hold in the deep sub-micrometre regime (<∼102 nm) when surface-mediated displacive and diffusive processes dominate. Here we present a perspective to highlight typical scenarios in which the single-crystal strength is influenced by surface stresses, diffusion-mediated surface creep, or dislocation-starvation-engendered high-stresses in the crystal interior, leading to ‘smaller is weaker’ or ‘plateau limited by ideal strength’. Such an unusual size dependence/independence of strength has implications for the reliability and stability of nano-structured devices.

Bounding the plastic strength of polycrystalline voided solids by linear-comparison homogenization techniques

The elastoplastic response of polycrystalline voided solids is idealized here as rigid-perfectly plastic. Bounds on the macroscopic plastic strength for prescribed microstructural statistics and single-crystal strength are computed be means of a linear-comparison homogenization technique developed by Idiart &amp; Ponte Castañeda (2007 Proc. R. Soc. A 463 , 907–924. ( doi:10.1098/rspa.2006.1797 )). Hashin–Shtrikman (HS) and Self-Consistent (SC) results in the form of yield surfaces are reported for cubic and hexagonal polycrystals with isotropic texture and varying degrees of crystal anisotropy. In all cases, the surfaces are smooth, closed and convex. Improvements over earlier linear-comparison bounds of up to 40% are found at high-stress triaxialities. New HS results can even be sharper than earlier SC results for some material systems. In the case of deficient crystals, the SC results assert that voided aggregates of crystals with four independent systems can accommodate arbitrary deformations, those with three independent systems can dilate but not distort, and those with fewer than three independent systems cannot deform at all. We report the sharpest bounds available to date for all classes of material systems considered.

Understanding size effects on the strength of single crystals through high-temperature micropillar compression

Compression tests of 〈111〉-oriented LiF single-crystal micropillars 1–5μm in diameter were carried out from 25°C to 250°C. While the flow stress at ambient temperature was independent of the micropillar diameter, a strong size effect developed with elevated temperature. This behavior was explained by rigorously accounting for the different contributions to the flow stress of the micropillars as a function of temperature and pillar diameter: the lattice resistance, the forest hardening; and the size-dependent contribution as a result of the operation of single-arm dislocation sources. This was possible because the micropillars were obtained by chemically etching away the surrounding matrix in directionally solidified LiF–NaCl and LiF–KCl eutectics, avoiding any use of focused ion beam methods, yielding micropillars with a controlled dislocation density, independent of the sample preparation technique. In particular, the role of the lattice resistance on the size effect of micrometer-size single crystals was demonstrated unambiguously for the first time. This result rationalizes the different values of power-law exponent for the size effect found in the literature for face-centered cubic and body-centered cubic metals as well as for covalent and ionic solids.

Effect of lanthanum on the high-temperature strength of single crystals of highly refractory alloy VZhM4-VI containing rhenium and ruthenium

The effect of lanthanum on the high-temperature oxidation resistance of single crystals of alloy VZhM4-VI used for casting single-crystal blades of gas turbine engines is studied. It is shown that lanthanum affects positively the resistance of single-crystal specimens of alloy VZhM4-VI to high-temperature oxidation and raises its heat resistance by a factor of 1.5 – 2.0 at 1150°C. This is proved by the data of an x-ray diffraction phase analysis of an oxide film and of a metallographic analysis of the structure of the near-surface layer of specimens of the alloy after oxidation.

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

Comparative study on lattice parameters of HAP nanoparticles with those of HAP whiskers

Hydroxyapatite whiskers are used as reinforcement for biomaterials because of their needlelike morphology and strong strength of single crystal. HAP nanoparitcles are used in drug delivery system, protein separation and anticancer drug besides their implant applications. Many new properties arise from nanoeffects while nanoeffects come from specialty of nanostructure. The Ca(NO3)2·4H2O and (NH4)2HPO4 were used as starting reagents for preparation of HAP whiskers and nanoparticles. The whiskers are 100–200 μm long while nanoparticles are 30–80 nm in size. XRD is applied to characterize the lattice parameter difference of whisker and nanoparticle. Compared with those of HAP whiskers, the a-axis and b-axis of HAP nanocrystals are shortened while the c-axis is elongated. That makes nanoparticles capable of their non-needle-like morphology, higher reaction activity and special bioeffects

Asymmetry of the yield strength in single crystals of titanium nickelide

Based on the results of the experimental studies of the temperature dependence of the yield stress of single crystals of TiNi alloys and of the mechanism of the mechanical twinning of the B2 phase as a martensite-type local reversible phase transformation, the asymmetry of the mechanical behavior of these materials during tension and compression in the [001] direction has been investigated. It is shown that a specific feature of this mechanism is the fact that the critical mode of distortion which determines stresses during mechanical twinning is a homogeneous deformation of transformation of the Bain-deformation type. To analyze the asymmetry of these stresses during tension and compression, a concept of the factor of normal stresses reduced to the principal axes of the tensor of the homogeneous deformation of transformation is introduced. It is shown that this factor can be used for analyzing the asymmetry of the mechanical behavior of single crystals of TiNi alloys.

Anomalies in the temperature dependences of the bulk and shear strength of aluminum single crystals in the submicrosecond range

The results of measurements of the dynamic yield stress and the ultimate strength of aluminum single crystals in the temperature range from 15 to 650°C, which is only 10°C lower than the melting point, are presented. The measurements are made on samples under the action of plane shock waves with a pressure up to 5 GPa behind the front and of a duration of ∼2×10−7 s. It is found that the dynamic yield stress anomalously increases, attaining, in the vicinity of the melting point, a value four times as high as that measured at room temperature. The dynamic strength of the single crystals in this temperature range decreases approximately by 40%, a high strength being preserved in the state in which melting during extension is expected.

Spallation strength of single crystal and polycrystalline copper

The spallation strength of single crystal copper foils 100 micrometers thick of the and orientations and of polycrystalline copper foils has been determined. Laser driving was used to launch miniature plates. Also, the spallation strength of polycrystalline copper was measured using conventional gas gun techniques and plates for comparison. Experimental measurements include VISAR records of the free surface particle velocity and micrographs of recovered flyers. A 1D hydrocode was used to simulate the VISAR records to determine the dynamic pressure history of the spallation process. Judging by the calculated pressures, it was found that the foils were somewhat stronger than the foils, which were about twice as strong as the polycrystalline foils. These were, in turn, substantially stronger than the larger sample plates (1.5 mm thick) of the gas gun. In the polycrystalline foils, small voids are found in recovered samples impacted at flyer plate velocities almost producing spallation in the gas gun, indicating that this sparse damage in the foils is able to grow substantially more in the much larger gas gun plates and under much slower gas gun loading.

Precipitation of NiHfsi phase in NiAl single crystals containing Hf

Small additions of Hf to NiAl produce a significant increase in the high-temperature strength of single crystals. Hf has a very limited solubility in NiAl and in the presence of Si, results in a high density of G-phase (Ni16Hf6Si7) cuboidal precipitates and some G-platelets in a NiAl matrix. These precipitates have a F.C.C structure and nucleate on {100}NiAl planes with almost perfect coherency and a cube-on-cube orientation-relationship (O.R.). However, G-phase is metastable and after prolonged aging at high temperature dissolves at the expense of a more stable Heusler (β'-Ni2AlHf) phase. In addition to these two phases, a third phase was shown to be present in a NiAl-0.3at. % Hf alloy, but was not previously identified (Fig. 4 of ref. 2 ). In this work, we report the morphology, crystal-structure, O.R., and stability of this unknown phase, which were determined using conventional and analytical transmission electron microscopy (TEM).Single crystals of NiAl containing 0.5at. % Hf were grown by a Bridgman technique. Chemical analysis indicated that these crystals also contained Si, which was not an intentional alloying addition but was picked up from the shell mold during directional solidification.

The effect of grinding direction on flaw character and strength of single crystal and polycrystalline ceramics

The effect upon the room-temperature strengths and fractures of flexure bars caused by grinding them either parallel or perpendicular to their tensile axis was investigated for selected single- and polycrystalline ceramics. Particular attention was given to the character of the flaws from which failure initiated. It was shown that grinding introduces two basic sets of flaws: one set forms at an average angle of ∼ 0°, and the other at an average angle of ∼ 90°, to the grinding direction. The angles of these flaws varied somewhat due to their formation on preferred fracture planes in single-crystal or larger-grain polycrystalline bodies as well as due to statistical effects. However, overall, the difference in these two sets of flaws was a major factor in the effect of grinding direction on strength. Flaws that formed approximately parallel with the grinding direction were typically more severe, and hence lead to lower strengths for grinding perpendicular to the bar axis, i.e. when the stress was perpendicular, or nearly so, to them.

Behavior of microcracks in neutron-irradiated molybdenum

Molybdenum single crystals with the 1100} surface orientation, obtained by electron-beam zone melting, were used in the investigation. The specimens were cut by using the electric-spark method. The thermal stresses arising in the surface layer in electric-spark cutting exceed the ultimate strength of single crystals, which causes the development of cleavage microcracks along the cube plane which extend to a depth of 60-65 pro. The specimens were irradiated at 200-300~ up to a fluence of 5"1019 neutrons/cm 2. The original and the irradiated specimens, as well as specimens annealed at T -- 0.95 Tree before or after irradiation, were investigated layer-by-layer by using metallographic analysis and x-ray methods. The thickness of the damaged layer containing the microcracks 'was determined by weighing the specimens on microanalytical scales before and after the layer was removed by polishing or by direct measurement by means of an optimeter. Figure 1 shows the statistically obtained curves of the total length of microcracks, reduced to the unit surface area of a polished section, as a function of the distance to the surface for neutron-irradiated molybdenum single crystals. Hightemperature annealing results in virtually complete closing-up of microcracks in the volume; neutron irradiation, on the contrary, promotes the opening of existing microcracks. Figure 2 shows the microstructure of a molybdenum monocrystai annealed at 2500~ over a period of 3 h, which developed at a depth of *20 pro. Etching pits corresponding to dislocation walls are visible at the locations of welded original microcracks. This suggests that annealing triggers the mechanism of microcrack welding as a result of the decomposition of microcracks into polygonal walls of edge dislocations due to diffusion over the crack surface [3]. The approximate closing-up time for cracks was calculated by means of the well-known [3] expression t5N6RTb 2 Tal I = 4e~DsAvcoN A ' where N is the number of dislocations in the wall; N A, Avogadro number; R, gas constant, b, Burgers vector; p, shear modulus; Ds, coefficient of surface self-diffusion; Av, effective difference between the volumes of atoms and vacancies; and

Orientation and temperature dependence of strength in single crystals of Mn-Sn with DO 19 structure

Single crystals with specified orientations of nonstoichiometric Mn-Sn having the hexagonal DO 19 structure have been compressed at temperatures mostly above 600 K to examine the activity and the deformation mechanisms of three types of slip systems. Basal slip, whose τ f - T curve showed a broad peak between 700 K and the melting point, was thought to be affected by the glissile-sessile transition, due to thermal activation, of a unit dislocation of the super-dislocation. The CRSS-T curve for prismatic slip showed a steep negative slope below 700 K probably due to the Friedel sessile dissociation mechanism for a screw dislocation. Yielding by the pyramidal { 1 011}〈112̄3〉 plane occurred by coarse slips on several definite planes forming large steps and was accompanied by a pronounced yield drop; the yield stress increasing from 1100 K with decreasing temperature. This behavior of the pyramidal slip has been interpreted as due to the difficulty in the nucleation of dislocation sources and the high Peierls stress of this system. An order-disorder transition was indicated at about 1135 K from both a CRSS-T and electrical resistivity measurements.

Orientation and temperature dependence of strength in single crystals of Mn  Sn with DO 19 structure

Orientation and temperature dependence of strength in single crystals of Mn  Sn with DO 19 structure

Anisotropy of strength in single crystals under plane strain compression

The strength of a crystal undergoing deformation depends on the crystal orientation and the geometry of deformation. In this paper, the Bishop and Hill analysis ( Phil. Mag. 42, pp. 414–427 and pp. 1298–1307 (1957)) is utilized to calculate the stress requirements for f.c.c. crystals of several orientations undergoing plane strain deformation. The latter is frequently found in plastic working operations such as rolling, deep drawing, and wire flattening. Compression tests were conducted on Permalloy (4% Mo-17 % Fe-79 % Ni) single crystals and on polycrystalline material with the samples confined to a channel to prevent lateral spreading. Strength differences by a factor of two may be obtained this way. The present Findings Are in good agreement with analysis, although some results are complicated by deformation banding and lattice rotations.

On size effects in polycrystal plasticity

Two cumulative ‘size’ effects are responsible for the increased strength of a polycrystalline aggregate above the single crystal strength at low temperatures : a ‘specimen size’ effect and a ‘grain size’ effect. The ‘specimen size’ effect occurs when few grains are present in a specimen cross-section (say less than twenty), and this effect is due mainly to the orientation dependence of crystal plastic flow. The ‘grain size’ effect occurs when many grains are in a specimen cross-section (say more than twenty), and this effect results because, in addition to the orientation dependence of plastic flow within grains, internal concentrations of stress are necessary at grain boundaries to cause bulk yielding and subsequent plastic flow of the polycrystalline aggregate.

The strength of single crystal and polycrystalline corundum

The strength of single crystal and polycrystalline corundum