Operative slip systems and anomalous strengthening in Ni3Nb single crystals with the D0a structure

The plastic deformation behavior of Ni₃Nb single crystals was examined in tension and compression to determine the operative slip and twinning systems, and to explore the anomalous strengthening behavior. A strong temperature dependence of the CRSS for both the slip and twinning systems was observed, which was dependent on the sample orientation. Anomalous flow behavior was also observed in Ni₃Nb crystals deformed by (010)[100] and (001)[100] slip. The anomalous strengthening mechanism is discussed on the basis of both the anisotropy of APB energy and the formation of dragging atmosphere around moving dislocations.     

Inter-martensitic transitions in Ni–Fe–Ga single crystals

The strain–temperature response of Ni–Fe–Ga single crystals underscores the role of the inter-martensitic transformation in creating intersecting heating and cooling segments; the separation of these segments occurs due to irreversibilities at high stresses and at high temperatures. An ultra-narrow tensile (1 °C) and compressive (<10 °C) thermal hysteresis are observed for the A ⇌ 10M ⇌ 14M case, accompanied by a small stress hysteresis (<30 MPa) in compressive and tensile stress–strain responses. The hysteresis levels increase and the intersecting segments disappear at high stresses and at high temperatures. This paper reports the use of a thermo-mechanical formulation to rationalize the role of inter-martensitic transformations. Plotting the transformation stress as a function of temperature indicates that inter-martensitic transformations enable a very wide pseudoelastic temperature range, as high as 425 °C. The measured Clausius–Clapeyron curve slope in compression (2.75 MPa °C⁻¹) is eight times the tensile slope (0.36 MPa °C⁻¹); the higher slope is attributed to the predominance of A ⇌ L1₀ at high temperatures.     

Certain features of the structure and anisotropy in Ni3Al single crystals

The structure of single crystals of an intermetallide, the composition of which is close to stoichiometric, was investigated. It was shown that it is sensitive to microadditions of rare-earth metals within limits of 0.05–0.3% and crystallization rate of 10–210 mm/min.All-Union Scientific-Research Institute for Aviation Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 25–27, February, 1994.     

Texture memory effect in heavily cold-rolled Ni3Al single crystals

In Ni₃Al the cold-rolled Goss texture changed to a complicated one after primary recrystallization and returned to the original Goss during the subsequent grain growth, which can be referred to as the texture memory effect. In this study, we examined the evolution of grain orientations during the grain growth using the electron backscatter diffraction (EBSD) method. It was found that just after the primary recrystallization most of the grains had a 40°〈1 1 1〉 rotation relationship to the Goss texture, the remaining grains being Goss and other textures. The formation of the 40°〈1 1 1〉 rotated grains can be explained by a multiple twinning mechanism. In the grain growth, the Goss grains, which were surrounded by the 40°〈1 1 1〉 rotated grains, grew preferentially due to the high mobility of the 40°〈1 1 1〉 grain boundaries, leading to the texture memory effect.     

High pressure effects on the photoluminescence intensity of sexithiophene single crystals

We report on the influence of interchain interactions on the light emission properties of a sexithiophene single crystal. The strength of the intermolecular interactions is controlled by applying hydrostatic pressure. The combined use of both steady-state and time-resolved photoluminescence techniques permits to show that the pressure-induced quenching of the photoluminescence is caused by a reduction of the radiative recombination rate.     

Effect of long-range order on the cyclic deformation behaviour of Ni3Fe single crystals

Effect of ordering on cyclic deformation in disordered and ordered Ni₃Fe single crystals was investigated focusing on stress–strain response and deformation substructure. The cyclic hardening depended strongly on the long range order. The maximum stress in the disordered crystals increased gradually with increasing number of cycles and then reached a saturation, while ordered ones exhibited cyclic softening after an initial strong cyclic hardening. The cyclic hardening at an early stage of fatigue in ordered crystals may be due to APB tubes and debris which were produced by the intersection between primary and secondary slips. Coarse slip bands were observed in fatigued ordered Ni₃Fe single crystals. In the bands, three-dimensional dislocation structure was formed accompanied by a decrease in the degree of order, which was responsible for the cyclic softening.     

Stress corrosion cracking in single crystals of Fe-25Cr-20Ni

The passivation kinetics and the s.c.c of Fe-25Cr-20Ni single crystals in boiling MgCl₂ has been examined under single glide conditions. Stress corrosion cracks form at the open circuit potential and at small anodic polarization. Formation of slip steps and thus exposure of new surface area is a necessary condition for s.c.c. to occur. Large slip steps enhance s.c.c. The cracks generally start at pits. The crack planes lie near 100 planes. At small cathodic polarization crystallographically oriented pits form with side faces composed of only 111 planes.     

Fatigue mechanism in Ni3Fe single crystals with L12 superlattice structure

Ni₃Fe single crystals with the L1₂ structure were cyclically deformed with various loading axes at different plastic strain amplitudes. The crystals demonstrated initial strong cyclic hardening followed by rapid cyclic softening at any orientation tested. There existed a typical plateau region in the cyclic stress–strain curves of the crystals oriented for single slip, while the saturation shear stress (τ_s) monotonically increased with increasing plastic shear strain amplitude (γ_pl) for double-slip oriented crystals. Persistent slip bands (PSBs) with cell-like dislocation structure developed in fatigued Ni₃Fe single crystals. The relationship between the volume fraction of PSBs and γ_pl suggests that Winter’s two-phase model can be applied to explain the cyclic stress–strain response in Ni₃Fe single crystals. Activation of the secondary slip is closely related to the cyclic hardening/softening behaviour and the formation of PSB in Ni₃Fe single crystals.     

Effect of orientation on the peak temperature of the yield-stress anomaly in single crystals of the Ni3Ge alloy

Temperature dependence of the yield stress τ_c(T) and shear stresses, as well as the orientation dependence of the peak temperature T _p of the yield-stress anomaly have been studied on single crystals of the Ni₃Ge alloy. The peak temperature T _p corresponds to the maximum in the τ_c(T) dependence. Profiles of the T _p isolines have been determined based on the equality of the values of the yield stress in the octahedral and cube planes of cross slip. It is shown that in the case of the above equality it is possible to describe the orientation dependence of the peak temperature T _p of the temperature anomaly of the yield stress in single crystals of Ni₃Ge.     

An enhanced continuum model for size-dependent strengthening and failure of particle-reinforced composites

We present an enhanced continuum model for the size-dependent strengthening and failure of particle-reinforced composites. The model accounts explicitly for the enhanced strength in a discretely defined “punched zone” around the particle in a metal matrix composite as a result of geometrically necessary dislocations developed through a mismatch in the coefficients of thermal expansion. We incorporate the punched zone explicitly through a unit-cell model within this work, but the approach can be used more generally to account for discrete particle distributions and particle shapes. Smaller particles lead to greater strengthening, and this size effect is larger for larger volume fractions. An equation for the coupling of the size-dependent increase of yield strength of metal matrix composites with the particle volume fraction is obtained. The results indicate that the punched zone effect may amplify the occurrence of a variety of failure modes such as matrix localization, particle fracture and/or particle–matrix interface failure; smaller particles perceive higher stresses. We account for interface failure through a cohesive approach, and show that the interface damage mechanism is also particle-size-dependent. Some implications are presented for microstructural design of metal matrix composites.     

Acoustical properties of Ni3Al single crystals alloyed with cobalt and niobium

High-temperature nickel-base alloys (superalloys) are widely adopted in industry as materials for structural elements of devices operating upon high temperatures (aerospace turbine blades, etc.). Successful application of these alloys is based on their high mechanical properties. Today, the range of service conditions expands toward progressively higher temperatures. Thereupon, the investigation of mechanical and thermal properties of nickel high-temperature alloys appears to be a topical question.     

Superelastic behavior and stabilization of stress-induced martensite in Cu-13.4Al-4.0Ni single crystals

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Orientation dependence of the shape memory properties in aged Ni45.3Ti29.7Hf20Pd5 single crystals

The shape memory properties of Ni_45.3Ti_29.7Hf₂₀Pd₅ single crystals aged at 550 °C for 3 h and at 600 °C for 48 h were investigated along the [111], [011] and [−117] orientations in compression. The material was stronger along the [−117] orientation compared to the [111] and [011] orientations based on load-biased thermal cycling experiments. The shape memory properties such as reversible strain, temperature hysteresis, critical stress for stress-induced martensite transformation and Clausius–Clapeyron relations were also strong functions of orientation and aging condition (precipitate characteristics). Shape memory effect with no or negligible irrecoverable strain was observed under stress levels as high as 1000 MPa. After aging at 550 °C for 3 h, the maximum reversible strains were 2.2%, 2.7% and 0.7% along the [111], [011] and [−117] orientations, respectively. Aging at 600 °C for 48 h resulted in maximum reversible strains of 2.3%, 3.2% and 0.9% along the [111], [011] and [−117] orientations, respectively. In both cases, similar levels of transformation strain, as a function of orientation, were observed during superelastic testing. The maximum work output reached 27 J/cm³ in the [011] orientation after aging at 550 °C for 3 h.     

Searching for feedback between the process parameters for directional solidification and the structure of Ni3Al single crystals

We demonstrate the possibility of changing the strength characteristics in single-crystalline samples of the intermetallic compound Ni₃Al, whose production is closely connected with three-dimensional growth of the primary -phase (unhardened solid solution of aluminum in nickel).All-Union Scientific Research Institute of Aeronautical Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 26–30, November, 1994.     

Evolution of microstructure and thermomechanical properties during superelastic compression cycling in Cu–Al–Ni single crystals

The aim of this work is to relate the macroscopic evolution of the compression superelastic effect in Cu–Al–Ni shape memory alloy single crystals with the evolution of the microstructure during cycling. The analysis has been carried out as a function of the number of cycles, the maximum reached deformation and the kind of induced martensite. Moreover, the new microstructure after mechanical cycling and the evolution of the thermal transformation have been also studied. The presence of two new families of dislocations created by different mechanisms has been observed and the influence of each one on the stress-induced and thermal transformations has been analyzed. In the samples where both kinds of dislocations are present at the same time, the observed behaviour is a combination of their effects in proportion to their density.