The effect of Sn segregation at grain boundary on the steady-state creep of Cu-10 wt % Sn alloy

The effect of Sn solutes segregating into grain boundaries on the steady-state creep characteristics was studied for wires of a Cu-10 wt % Sn alloy having various grain diameters,d. The creep tests were performed for samples in the-phase in the temperature range: (0·5–0·58 T_m). The steady state creep rate, _s, was found to vary linearly withd. The variation in the stress sensitivity parameter,m, was found to be much less appreciable for large grains (>50m). Activation energy calculation showed that two competitive mechanisms are operating for creep deformation, namely, dislocation climb and viscous motion of matrix dislocations due to dragging of Sn atmosphere. A value of 6·74 kJ/mol was obtained for the binding energy between a Sn atom and the dislocation, while the binding energy between a vacancy and the Sn atom was found to be 33·71 kJ/mol.     

Microstructure changes and steady state creep characteristics in Pb-Sn alloys during transformation

The steady state creep of Pb-10 wt.% Sn and Pb-61·9 wt.% Sn alloys have been investigated under different constant stresses near the transformation temperature. The temperature dependence of steady creep rate has shown two different transition points; at 423 K for Pb-10 wt.% Sn alloy and at 403 K for Pb-61·9 wt.% Sn (the eutectic composition). The strain rate sensitivity parameter (m) has been found to increase by raising the working temperature and to reach 0·45 and 0·85 for the first and second alloy, respectively. The activation energies of steady state creep of Pb-10 wt. % Sn have been found to be 46·2 kJ/mole and 88·2 kJ/mole in the low and high temperature regions (below and above 423 K) referring to dislocation and self diffusion mechanisms. While activation energies of steady creep in Pb-61·9 wt.% Sn have been found to be 42 kJ/mole and 63 kJ/mole in the low and high temperature region (below and above 403 K), characterizing grain boundary diffusion in Sn and Pb respectively. X-ray analysis and microscopic investigations of the test alloys have confirmed the above mentioned mechanisms.     

Dependence of creep characteristics on temperature and applied stress in Zn-1.0 wt.% Cu during transformation

The change in the transient and steady state creep deformation of zinc-1.0 wt.% Cu alloy was studied under various constant stresses ranging from 45.80 MPa to 56.02 MPa in the temperature range from 473 K to 573 K. From the transient creep results, the peak values of transient creep parametersB andn found in this temperature zone can be ascribed to dissolution of-phase (Cu-rich phase). The transient creep parameterB is related to the steady state creep rate through the exponent. This exponent has been found to range from 0.8 to 0.3. At the dissolution temperature (513 K) of-phase, the steady state strain sensitivity parameter has been 0.30±0.01 at the steady state strain peaks which is characteristic of dislocation climb along-grain boundaries. The activation energies of the transient and steady state creep in the phase transformation region have been found to be 42 kJ/mole and 63 kJ/mole characterizing the cross slipping of dislocations and dislocation climb along grain boundaries, respectively.     

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.     

Microstructure dependence of steady state creep in Cd–Sn eutectic alloy

The steady state creep of Sn–33 wt.% Cd alloy was studied under various constant stresses ranging from 25.56 to 30.85 MPa in the temperature range from 353 to 433 K. The stress exponent n was found to change from 6.25 to 4.55 in the above temperature range. The energy activating the steady state creep amounted to 59.3kJ/mol in the temperature range from 353 K to 393 K and to 37 kJ/mol in the temperature range from 413 K to 433 K characterizing the grain boundary diffusion in Cd and in Sn, respectively. Microstructure analysis confirmed that the above mentioned mechanisms took place during steady state creep.     

Stress enhanced diffusion process in Al-10 wt.% Zn alloy

The change in the steady state creep of Al-10 wt. % Zn alloy was studied under various constant stresses ranging from 77 MPa to 88·3 MPa and at different constant temperatures ranging from 423 K to 483 K. The strain rate sensitivity parameter (m) varied between 0·15 to (0·4 ± 0·05) in the above temperature range. The energy activating the steady state creep amounted to 70·3 kJ/mole in the temperature range from 423 K to 443 K and to 124·3 kJ/mole in the temperature range from 453 K to 483 K characterizing the grain boundary diffusion of Zn in B-phase and Al in-phase, respectively. This was affected by increasing the applied stress. The decrease in the activation energy was attributed to the stress enhanced diffusion processes. Microstructural analysis confirmed that the above mentioned mechanisms took place during steady state creep.     

Theories of Creep in Ceramics

Mathematical models that have been proposed for creep in ceramics are described. Emphasis is on models involving grain boundary motion (sliding or flow). In Lifshitz models the crystalline grains elongate with strain; the elongation results from diffusion, slip, or solution and precipitation. In Rachinger models the grains do not elongate during creep. The sliding strain can be accommodated by viscous flow of a glassy phase at the grain boundaries, or if there is no boundary glass by diffusion or slip in superplastic models. Sliding of a glass-free boundary can result in cavitation, cracking, or formation of boundary dislocations or triple point folds.

Most models of ceramic creep at high temperatures predict a steady state (stage II) creep rate that depends on the applied stress, grain size, and temperature. A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models. The models give different exponents for the functional dependence of creep rate on grain size and strain and different temperature dependencies. These differences are compared in tables, and the main mechanistic features of the models are described in the text.

The purpose of this review is to describe creep models rather than to compare them with experimental results or to select the most applicable models. There are few critical experimental tests that allow selection of the most accurate models; such experiments are suggested as the next step in choosing between the models for specific experimental results.     

Grain boundary sliding during ambient-temperature creep in hexagonal close-packed metals

Even at ambient temperature or less, below their 0.2% proof stresses all hexagonal close-packed metals and alloys show creep behaviour because they have dislocation arrays lying on a single slip system with no tangled dislocation inside each grain. In this case, lattice dislocations move without obstacles and pile-up in front of a grain boundary. Then these dislocations must be accommodated at the grain boundary to continue creep deformation. Atomic force microscopy revealed the occurrence of grain boundary sliding (GBS) in the ambient-temperature creep region. Lattice rotation of 5° was observed near grain boundaries by electron backscatter diffraction pattern analyses. Because of an extra low apparent activation energy of 20 kJ/mol, conventional diffusion processes are not activated. To accommodate these piled-up dislocations without diffusion processes, lattice dislocations must be absorbed by grain boundaries through a slip-induced GBS mechanism.     

Study of the mechanical properties of alloy 36NKhTYu in relation to the mechanisms underlying the precipitation of the γ' and η phases

The nature of the dispersion (precipitation) hardening of the alloy 36NKhTYu is examined. It is shown that the increase observed in the resistance of the alloy to the motion of dislocations inside the grains during the continuous precipitation of the γ' phase is due to the presence of long-range order in the hardening particles. In addition to this, there is also a considerable increase in that part of the yield stress which is associated with the grain boundaries; this is because of the precipitation of carbides at grain boundaries and the corresponding sharp increase in slip localization. Overaging is characterized by a reduction in the resistance to the motion of the dislocations inside the grains, and a diminishing influence of the grain boundaries (ultimately to no influence at all). These results are in satisfactory agreement with calculations based on the Orowan theory, and also with the assumption that, as overaging progresses, the range of the dislocations is reduced from a distance equal to the grain size to a distance equal to that separating the particles ofγ′ phase, while localized slip is replaced by uniform slip. When the precipitation of theη phase reaches a well-developed stage, the yield stress is reduced. The result of the present investigation confirm our earlier conclusion as to the substantial hardening (strengthening) of the meterial which occurs as a result of the discontinuous precipitation of theγ′ phase.     

Transient and steady state creep of Al-4.5 wt.% Mg during phase transformation

Transient and steady state creep of Al-4.5 wt. % Mg alloy was studied under various constant stresses ranging from 91 MPa to 117 MPa in the temperature range from 473 K to 553 K. The results of creep characteristics have shown two main deformation temperature regions (below 493 K and above 513 K as well as a transient region between these temperatures). Peak values of transient creep parametersB andn were obtained at 493 K. The transient creep parameterB was related to the steady state creep rate _st through the exponent which was found to range from 0.85 to 0.5. The stress exponentm of the steady state creep has been found to be minimum at the steady state strain peaks, which is characteristic of dislocation climb along the grain boundaries. Microstructural analysis confirmed that the above mentioned mechanism took place in the dissolution region of-phase.     

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.     

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

Abstract

Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions.     

Interdiffusion in two-layer Pd/Ag films III. TEM investigation of diffusion-induced grain boundary migration

Diffusion-induced grain boundary migration (DIGM) is studied by the transmission electron microscopy method in polycrystalline two-layer Pd/Ag thin films with a grain size (100–2000 nm). In addition to the typical features of DIGM known for coarse-grained bulk objects and foils, new features are found which are caused by a quite dense network of triple junctions and by misfit dislocations: fast increase of grain boundary curvature and inclination; back motion of grain boundaries owing to recrystallization forces and termination of DIGM. Homogenization resulted from diffusion-induced migration of misfit dislocations is observed in addition to DIGM.     

Effects of grain boundary diffusion of copper on nickel creep

We have investigated the effects of surface copper diffusion on the creep of large-grain nickel (average grain size about 20 m) and submicron crystalline nickel (grain size about 0.3 m). For both structural states of nickel we find an acceleration of creep over its value in vacuum, and an increase in plasticity during creep when copper has diffused deeply into the nickel from the surface. The temperature range over which these effects are observed in the submicron nickel is nearly 300° lower than that for the large-grain samples. This result is probably due to a significant increase in the grain-boundary and bulk diffusion coefficients of copper in submicron nickel when compared with large-grain nickel.Siberian Branch. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 83–86, December, 1994.     

Structural changes during annealing of submicro- and nanocrystalline aluminum alloys

The annealing-induced evolution of the structure and microhardness of submicro-and nanocrystalline Al—3% Mg and Al 1570 alloys produced by torsional severe plastic deformation are studied. Annealing of the Al-3% Mg alloy at 373–423 K and annealing of the Al 1570 alloy at 373–473 K are shown to result in the relaxation of internal stresses and subsequent normal grain growth. As the annealing temperature increases, the microhardness decreases. At higher temperatures (473 K for the Al—3% Mg alloy and 573 K for the Al 1570 alloy), anomalous grain growth takes place. This growth is accompanied by the appearance of numerous grains with a high dislocation density, a high concentration of impurity atoms in grain boundaries, and an increase in the microhardness. These effects are explained.     

Creep of lead alloys. I

The effect of traces of Sb, Te and Cu on the creep of lead under various stresses () atroom temperature is considered. The greatest sensitivity to the value of is exhibited by Pb itself, and the least by the Pb-Sb-Te-Cu alloy, in which the creep rate is low over a wide stress range. Tests on alloys treated in such a way as to produce the same grain size show that the addition of Sb, Te, and Cu reduces the creep rate of lead. The greatest influence is exerted by the addition of copper and the least by the addition of antimony. For all the materials studied the creep rate falls with increasing grain size. This influence is the greater, the lower the value of . For alloys annealed at the same temperature and possessing a grain size smaller than that of lead, in the case of small values of the beneficial effect of alloying is partly masked by the grain-size effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp. 77–81, September, 1973.     

The kinetics of barium precipitation at dislocations in NaCl monocrystals

The kinetics of barium precipitation at dislocations in NaCl monocrystals has been studied in thermally and mechanically treated NaCl + 4 ppm BaCl₂ samples by investigating the isothermic variation of ionic conductivity as a function of time. The course of precipitation which takes place at dislocations located at grain boundaries can be divided into three time regions characterized by diffusion of impurities to dislocation cores at grain boundaries, nucleation, formation of new grain boundaries, etc. At higher number of dislocations an interruption of the precipitation appears due to a local free energy of nucleation minimum at radiusr ₀=6·92×10^–8 cm.     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

Variation of apparent creep activation energy in plastic deformation of molybdenum in diffusion contact with nickel

The variation of the apparent creep activation energy as a function of the state of grain boundaries is investigated in the deformation of molybdenum in the presence of diffusion fluxes of nickel at the grain boundaries. It is shown that this energy varies in the same way as in the plastic deformation of classical superplastic materials.Physics Institute of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 110–113, May, 1993.     

A study on anomalous yield drop behaviour in modified beta titanium alloys

ABSTRACT

The yield drop phenomenon observed in the Ti–15V-3Al–3Sn-3Cr (Ti–15–3) beta-titanium alloy and its anomalous behaviour in the boron and carbon added Ti–15–3 alloys have been studied. While the base and the carbon containing alloys exhibit yield drop, the boron containing alloy with smaller grain size than base alloy does not appear to show this phenomenon. Tensile tests were interrupted at different stress levels followed by analyses of slip lines and sub-structural characteristics using scanning and transmission electron microscopes to understand this anomalous yield point phenomenon. Infrared thermal imaging technique was used to map the strain localisation and the spatiotemporal evolution of deformation along the gauge length of the specimens during the tensile tests. Deformation in these alloys initiates only in a few grains. Pile-up of dislocations in these grains subsequently triggers the formation of dislocations in other grains and their rapid multiplications. The spreading of deformation by the generation of dislocations from pile up dislocations in one grain to neighbouring un-deformed grains and their rapid multiplication to new regions influence the yield drop phenomenon and its characteristics. It is shown in this study that microscopic instability in the grain level is a necessary, but not the sufficient condition for the manifestation of macroscopic instability during tensile deformation in polycrystalline materials. The presence of boride particles at grain boundaries restricts the slip transfer across the grains as well as the spreading of deformation to new regions, which causes the suppression of yield drop in the boron containing alloy.