Climb-controlled Dislocation Creep Research Articles

Stress relaxation during isothermal annealing in electroplated Cu films

The deformation mechanisms of electroplated Cu thin films on TaN/SiO 2/Si were investigated by performing isothermal annealing between 180 °C and 400 °C. The stress relaxation behavior during isothermal annealing was analyzed in terms of the grain growth, power-law creep and diffusional creep. The approximated relaxed stress caused by grain growth was too small to consider as the main deformation mechanism in the Cu films. In the case of the diffusional model, the relaxation curve should be able to be fit by two exponent curves only under compressive stress. Furthermore, an extremely low activation energy was obtained. The stress relaxation curves obtained by power-law creep were the best fit. The stress exponent, n, for power law creep, was around 6 during heating and this suggested that the dislocation core diffusion was activated for the power-law creep. On the other hand, the large n value of over 20 during cooling was attributed to the breakdown of the power-law due to the high tensile stress in the cooling stage. Therefore, among the possible mechanisms for stress relaxation in electroplated Cu films, climb-controlled dislocation creep was found to be the most plausible one.

High Strain Rate Deformation Behavior of Mg–Al–Zn Alloys at Elevated Temperatures

High temperature deformation behavior of AZ31 and AZ91 magnesium alloys was examined by compression tests over a wide strain rate range from 10–3 to 103 s–1 with emphasis on the behavior at high strain rates. The dominant deformation mechanism in the low strain rate range below 10–1 s–1 was suggested to be climb-controlled dislocation creep. On the other hand, experimental results indicated that the deformation at a high strain rate of ~103 s–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures. The solid-solution strengthening was operative for high temperature deformation at ~103 s–1.

Dislocation creep behavior in Mg–Al–Zn alloys

The climb-controlled dislocation creep behavior was investigated in Mg–Al–Zn alloys of AZ31, AZ61 and AZ91 with different aluminum contents. The flow stress increased with the increase of aluminum content under the same deformation conditions. At high temperatures, the stress exponent was 5 and the activation energy was close to that for lattice diffusion of magnesium, whereas at low temperatures the stress exponent was 7 and activation energy was close to that for pipe diffusion for all alloys. By incorporating the effective diffusion coefficient and stacking fault energy into a constitutive equation, the dislocation creep behavior of magnesium alloys can be described by a single relation.

High-temperature behaviour of as die-cast and heat treated Mg–Al–Si AS21X magnesium alloy

Abstract The mechanical behaviour of the magnesium alloy AS21X (Mg–2% Al–1% Si alloy AS21 modified by addition of rare-earth elements) was experimentally investigated in the as die-cast and in the solution treated and artificially aged conditions in the temperature range from 70 to 210 °C. The as die-cast microstructure was characterized by coring effects and by the presence of Mg2Si and Mg17Al12 particles. Holding at temperatures below 180 °C caused massive formation of fine-scale precipitates at the grain boundaries. The homogeneous distribution of Al in the Mg-rich α-phase of heat treated material caused at these temperatures the formation of further amounts of Mg17Al12 particles, also within the grains. At 210 °C, Mg17Al12 particles dissolved in both material conditions. At low temperatures, longer times to creep rupture and better creep resistance were displayed by the as cast (AC) material. The properties in the heat treated (HT) condition approached those of the as cast material at the highest test temperature. The suggested deformation mechanism of the alloy up to 180 °C is climb-controlled dislocation creep, for which core diffusion could play a significant role at 70 °C in the as cast material.

Deformation behavior of an ultrahigh carbon steel (UHCS-3.0Si) at elevated temperature

Abstract Isothermal compression tests were carried out on a spray-formed ultrahigh carbon steel (UHCS-3.0Si) by Gleeble 1500 system. The true stress increases with the decrease of temperature and increase of strain rate. The deformation activation energies, Q , of UHCS-3.0Si are not only temperature dependent but also strain rate dependent. The value of Q increases as the deformation temperature decreases and it is in proportion to the weight percent of undissolved carbides. At high deformation strain rates (5–30 s −1 ), dislocation slip and cross slip are the main deformation mechanism and at low deformation strain rates (0.1–1 s −1 ), climb-controlled dislocation creep is the main deformation mechanism. Microstructural evolution was analyzed during deformation at 800 °C, cementite lamellas have fair formability and some are broken up which leads to great softening.

The Effect of Temperature and Flow Stress for Climb-Controlled Dislocation Creep in Magnesium Alloy

The Effect of Temperature and Flow Stress for Climb-Controlled Dislocation Creep in Magnesium Alloy

Creep behavior of Fe–C alloys at high temperatures and high strain rates

The creep behavior of Fe–C alloys (1–1.8%C) has been studied at high temperatures (0.7–0.9 T m) and high strain rates (1–100 s −1). The dominant deformation resistance has been found to be climb-controlled dislocation creep and thus the creep rates are a function of elastic modulus, lattice diffusivity and stacking fault energy. The self-diffusion coefficient of iron in austenite was found to be solely a function of T m/ T and to vary as D=6.8×10 −6 exp(−17 T m/ T) m 2 s −1. The Fe–C alloys were observed to have a high stacking fault energy which was unaffected by carbon and manganese. The stacking fault energy was observed to decrease with increasing concentrations of silicon, aluminum and chromium. At high stresses, deviation from power law behavior was accounted for by considering the contributions to diffusivity by dislocation pipe diffusion. The results have been used to develop a rate equation for these steels of varying composition that depends on only three material characteristics – alloy melting temperature, elastic modulus and stacking fault energy.

The rheology of synthetic rocksalt in uniaxial compression

Synthetic polycrystalline halite (rocksalt) has been deformed in uniaxial compression in the temperature range 250–780°C (0.5−0.98 T m ) at strain rates between 10 −3 and 10 −7 s −1 . Constant strain rate and strain-rate decrement, increment and cycling tests have been carried out. At temperatures from 250 to 450°C, the steady-state flow law is given by ⋗e= 5.75 × exp (−129/RT)σ 5.7 , which is thought to describe climb-controlled dislocation creep, in which climb is controlled by diffusion through dislocation cores (pipe diffusion). The flow law in the temperature range 500–780°C is given by ⋗e = 6.57 × 10 7 exp (−227/RT)σ 4.4) , which is inferred to describe lattice diffusion-controlled dislocation creep. In the temperature range 250–350°C, polygonization and rotation recrystallization are the dominant microstructural processes. However, with increasing temperature, grain boundary migration becomes important. The transition to migration recrystallization-dominated microstructures, at c. 450°C, coincides with the change from pipe diffusion- to lattice diffusion-controlled creep. This is explained as due to relative changes of grain-boundary mobility, nucleation rate and driving force, which allow a balance to be established between the migration rate of nucleated grains and the nucleation rate of new grains. Below 450°C, the strain-rate history has a profound effect upon the observed microstructural development and mechanical behaviour. The subgrains do not increase and the stresses do not fall to the expected steady-state values upon a lowering in strain rate. This process of subgrain refinement results in relative hardening effects and in an increase of the stress sensitivity upon strain rate up to 10. Above 450°C, however, the strain-rate history does not affect the microstructural development and steady-state behaviour is always attained because migration recrystallization resets the microstructure which adjusts to the newly imposed conditions. The crystallographic preferred orientation under all conditions investigated is characterised by rotation of the {110} poles towards the compression direction. Recrystallization processes appear to have no influence on the texture development.

Deformation of polycrystalline salt in compression and in shear at 250–350°C

Abstract Dry synthetic polycrystalline salt (NaCl) has been deformed in uniaxial compression and in shear in order to gain insight into the influence of deformation mode on the development of crystallographic preferred orientation, microstructural evolution and mechanical behaviour (flow strength). The experiments were carried out between 250 and 350°C, at strain rates between 10 −5 and 10 −7 s −1 . Under these conditions halite deforms by climb-controlled dislocation creep. In our samples, the presence of deformed grains containing cellular networks of subgrains, and the development of texture are consistent with such mechanisms. The weaker {110} <110> slip plane appears to align parallel to the shear plane and perpendicular to the uniaxial compression direction. In the sheared samples, a <111> maximum is observed parallel to the shear direction. The textures obtained agree favourably with texture simulations. When the mechanical behaviour seen in uniaxial compression and shear is compared in terms of equivalent stress and strain, as defined in the von Mises theory of isotropic plasticity, the uniaxially deformed samples appear to be stronger. From a detailed consideration of the data, it is inferred that the difference in flow strength is largely due to anisotropy resulting from texture development. The results suggest that the assumptions underlying the von Mises theory of isotropic plasticity are not applicable in the present case, and that the associated flow rule does not offer an accurate method of generalizing creep laws for salt under the conditions investigated.

High-temperature deformation of forsterite single crystals doped with vanadium

Creep experiments have been performed on samples from a single crystal of vanadium-doped forsterite under controlled\(p_{{\text{O}}_2 } \) conditions to investigate the effects of the addition of substitutional defects in the tetrahedral lattice sites. The addition of vanadium causes marked changes in the flow behavior of the forsterite, with a net increase in the creep rate at high\(p_{{\text{O}}_2 } \) and a new\(p_{{\text{O}}_2 } \) -dependent flow regime at low\(p_{{\text{O}}_2 } \) conditions. These observations can be interpreted as resulting from changes in the majority defect species that maintain the charge neutrality within the crystal. A climb-controlled dislocation creep model for the high-temperature deformation of vanadium-doped forsterite is proposed in which either (i) movement of uncharged jogs is rate-limited by the diffusion of silicon via a vacancy mechanism or (ii) movement of positively charged jogs is rate-limited by diffusion of oxygen via a vacancy mechanism.

Mechanical properties of silver at low temperatures

In this study the authors obtained detailed information about the stress, strain, strain rate, and temperature-dependence of plastic flow in cold-worked high-purity (99.99 wt%) silver. They performed tensile and creep tests at temperatures from 77 K to 473 K and at strain rates of 10/sup -8/ to 10/sup -3/ per second. It is generally agreed that silver, like most facecentered-cubic metals, deforms at low temperatures by the dislocation intersection mechanism. The rate-controlling process in the intersection mechanism is the overcoming of obstacles (forest dislocations) by glide dislocations. The process involves the formation of jogs and constrictions, with an activation energy appropriate to those processes. The amount of local deformation per successful event is expressed by an activation volume, or activation area times the Burgers vector. As the density of the forest increases--at higher strains and lower temperatures--the activation area is expected to decrease. At temperatures near half the absolute melting point (617 K) the metal begins to deform predominantly by climb-controlled dislocation creep. The data generated thus far show some interesting phenomena concerning the transition between these two mechanisms.