Creep Of Alloys Research Articles

Influence of permanent magnetic fields on creep and microhardness of iron-containing aluminum alloy

The necessity and urgency of studying plastic strain of metals is determined by both scientific significances of the problem and requirements of practice. So the objective of this work is investigation of aluminum alloy creep under action of static magnetic fields. This subject matter is of practical importance for engineering since the parts of engineering constructions are subject to various loads which may lead to their damage or even creep rupture. Based on the experiments performed by us, it is found that creeping increases under stable magnetic field; the main features appear at the first creep stage. Investigation of these processes will help to predict time dependence of creep strain and its rate as well as durability and plasticity at destruction.

Creep of Cu-Bi Alloys with High Bi Content Near and Above Melting Temperature of Bi

Cu-Bi alloy with high Bi content can be used for thermal surge protection and energy storage. For these applications, creep at high temperatures, including temperatures above the melting temperature of Bi, Tm,Bi, becomes important. Accordingly, the creep behavior of Cu-Bi alloys, comprising 30 and 40 vol pct Bi, was studied under compression at temperatures above and below Tm,Bi. At 200 °C, which is below Tm,Bi, Cu-Bi showed a stress exponent of ~ 4 at high stresses and ~ 1 at low stresses. Finite element analysis revealed that the creep behavior of Cu-Bi at 200 °C was predominantly governed by Bi. On the other hand, at temperatures higher than Tm,Bi, Cu-Bi showed a short transient stage at high stresses, followed by sudden failure of the material. However, at low stresses, the sample first continued to expand and then started to accumulate compressive strain. A qualitative model based on interaction between liquid Bi and Cu is developed to explain the observed creep behavior at temperatures higher than Tm,Bi. The results obtained here shed light on the creep behavior of alloys with constituents having significantly different creep behavior and containing a non-reacting liquid phase.

Effect of the addition of Cu on irradiation induced defects and hardening in Zr-Nb alloys

The irradiation hardening, growth, and creep of Zr alloys are strongly influenced by the presence of alloying elements. Cu is added to some Zr alloys due to its ability to form second phase particles which can cause strengthening. However, how the addition of Cu affects the microstructure and mechanical properties under irradiation remains unclear. In the current study, two Zr alloys, Zr-2.5Nb and Zr-2.5Nb-0.5Cu were selected and irradiated to 0.5 dpa and 5 dpa at 573 K with 5 MeV self-ions. The evolution of microstructure and hardness were characterized by Transmission Electron Microscopy and nanoindentation. Results show that the existence of Cu in the Zr alloys significantly altered the irradiated microstructure and resultant hardness. The addition of Cu resulted in a slight reduction of the irradiation loop size and density in both α and β phases, and its presence in β-Zr notably delayed the precipitation of ω phase. The redistribution of Fe from β phase to α phase was also slower in Zr-2.5Nb-0.5Cu than in Zr-2.5Nb. Irradiation caused hardening in both alloys, however, the hardening in Zr-2.5Nb-0.5Cu was not as significant as in Zr-2.5Nb due to the delayed ω precipitation.

Strain Rate during Creep in High-Pressure Die-Cast AZ91 Magnesium Alloys at Intermediate Temperatures.

During creep, magnesium alloys undergo microstructural changes due to temperature and stress. These alterations are associated with the evolution of the present phases at a microstructural level, creating different strain rates during primary and tertiary creep, and with the stability of the inter-metallic phase Mg17Al12 formed at these temperatures. In this paper, the results of creep testing of high-pressure die-cast AZ91 magnesium alloys are reported. During creep, continuous and discontinuous precipitates grow, which influences creep resistance. The creep mechanism that acts at these intermediate temperatures up to 150 °C is termed dislocation climbing. Finally, the influence of the type of precipitates on the creep behavior of alloys is determined by promoting the formation of continuous precipitates by a short heat treatment prior to creep testing.

Creep of a Titanium Nickelide-Based Alloy during Torsional Deformation

The short-term creep processes in a Ti–55.7 wt % Ni alloy are investigated. Wire samples of this alloy after preliminary annealing at 450–700°C are deformed by torsion under constant loading in an air atmosphere in the temperatures range 450–550°C. The effect of structural parameters (grain size, presence of fine Ti3Ni4 particles) on the creep mechanism and rate is shown. Grain-boundary diffusion is found to be one of the main creep mechanisms.

Thermal Stability of Ni3Al-Based Composite with Honeycomb Structure

By powder metallurgy, materials in the form of a single-phase alloy based on Ni3Al and the corresponding composite Ni3Al + W with honeycomb structure are produced. The structural unit of the composite is a round granule (mean size 25 μm) of nickel alloy surrounded by a continuous tungsten coating (thickness ~0.4 μm) applied by chemical gas-phase deposition. Compressive tests at room temperature show that the yield point of the composite Ni3Al + W at 20–1000°C exceeds that of the single-phase Ni3Al-based alloy (by a factor of as much as 1.7). However, at higher temperatures, the yield points of the alloy and composite are comparable. The unit yield point (standardized at a density of 7.8 g/cm3 for the alloy and 9.5 g/cm3 for the composite) behaves analogously. At 1300°C, the single-phase Ni3Al-based alloy exhibits solid–liquid behavior in compression. Creep tests with compression in vacuum at 1000–1200°C are conducted. By pair and parametric analysis of the creep processes according to Hollomon, regression equations for the creep rate, stress, and test temperature are obtained. The creep limit is calculated from the tolerances on the steady creep rate and its inverse. At all the test temperatures, the composite is characterized by lower creep rate (by a factor of seven) and higher creep limit (by a factor of 2.5) than for the nickel alloy on which it is based. The activation energy of creep is determined for the alloy and composite on the basis of an exponential relationship between the experimental quantities. The activation energy of creep for the nickel alloy is close to the activation energy of nickel self-diffusion in Ni3Al and materials based on it (230–310 kJ/mol). For the composite, it is close to the activation energy of tungsten self-diffusion (503 kJ/mol).

Modern data analytics approach to predict creep of high-temperature alloys

A breakthrough in alloy design often requires comprehensive understanding in complex multi-component/multi-phase systems to generate novel material hypotheses. We introduce a modern data analytics workflow that leverages high-quality experimental data augmented with advanced features obtained from high-fidelity models. Herein, we use an example of a consistently-measured creep dataset of developmental high-temperature alloy combined with scientific alloy features populated from a high-throughput computational thermodynamic approach. Extensive correlation analyses provide ranking insights for most impactful alloy features for creep resistance, evaluated from a large set of candidate features suggested by domain experts. We also show that we can accurately train machine learning models by integrating high-ranking features obtained from correlation analyses. The demonstrated approach can be extended beyond incorporating thermodynamic features, with input from domain experts used to compile lists of features from other alloy physics, such as diffusion kinetics and microstructure evolution.

Development of a novel methodology to study fatigue properties using the small punch test

Small scale mechanical test methods are now widely recognised as an established and quantifiable means of obtaining useful mechanical property information from limited material quantities. Much research has been gathered employing such approaches, but to date these methods have largely been restricted to characterising the creep, tensile and fracture characteristics of numerous materials and alloys through the small punch (SP) test. Clearly, a key element that is missing from this list of fundamental mechanical properties is understanding the cyclic response of the material, a significant form of damage that accounts for a large proportion of in-service failures in critical structural components. Therefore, in order to profit from the numerous benefits that SP testing has to offer, including a small sample size and hence reduced cost, a small scale fatigue testing methodology is now required to provide a holistic mechanical property evaluation. Such an innovative approach would provide real potential benefit to the engineering mechanical characterisation community. This paper will discuss the development and implementation of this highly bespoke SP fatigue testing methodology that can accommodate alternative loading ratios and frequencies to mimic conventional fatigue data. A number of novel experiments have been performed on the titanium alloy Ti-6Al-4V with accompanying analysis and fractography detailed. Numerical correlations to uniaxial fatigue data is also presented through the use of Finite Element Analysis.

Constitutive Relations of the Mechanics of a Damaged Medium for Evaluating the Creep-Rupture Strength of Structural Alloys

The main regularities of the degradation of structural materials (metals and alloys) resulting in a decrease in creep-rupture strength were investigated. A mathematical model describing the viscoplastic deformation and damage accumulation in polycrystalline structural alloys in creep was developed using the equations of the mechanics of damaged media. The paper presents the results of experimental studies of the short-term creep of 12Kh18N9 steel and VZh-159 high-temperature alloy at constant temperature and different stresses set in the samples and the results of experimental studies of creep recovery in some structural steels. Deformation and damage accumulation were studied numerically and the results were compared with the data of full-scale experiments.

Superplastic-Like Elongation by Transition of Deformation Mechanism from Grain Boundary Sliding to Solute Drag Creep in Fine-Grained Al-Mg Solid Solution Alloy

It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Effect of α<sub>2</sub> Precipitation on Creep and Tensile Properties of Ga-Added Near-α Titanium Alloys

The effect of α2 precipitation on the creep and tensile properties was investigated for bimodal and lamellar microstructures in two Ga-added near-α Ti alloys with Al equivalences of 10.6 and 11.5. Fine α2 phase formed in the α phase of both alloys. The volume fraction of the α2 phase for the Al equivalences of 10.6 and 11.5 is equivalent to 57.6 % and 73.3 %, respectively, in the binary Ti-Al system at 600 °C. Creep tests were carried out under a constant stress of 310 MPa at 600 °C and tensile tests were performed at room temperature. Lamellar microstructure showed lower minimum creep strain rates than bimodal microstructure for both alloys. The increase in Al equivalence increased creep life by a factor of 1.6 and decreased the minimum creep strain rate from 6.51 × 10-8 s-1 to 3.99 × 10-8 s-1 in bimodal microstructure. In addition, the increase in Al equivalence decreased room temperature tensile elongation although both alloys contained a similar volume fraction of equiaxed α in a bimodal microstructure.

Influence of Ca and Sr Addition on Impression Creep Behavior of Mg-4Al-RE Alloy

The impression creep behavior of Mg-4Al-RE-0.8Ca-0.2Sr (AEXJ4110) alloy was investigated under the applied stresses of 55, 75, 95 MPa and the temperatures of 398, 423, 448 K on a special apparatus. The results showed that the microstructure of AEXJ4110 alloy consisted of acicular Al11La3, granular Al2La, bone shape Al2Ca and fish-bone shape Al4Sr. The impression creep resistance of AE41 alloy was improved by adding Ca and Sr. The primary impression creep depth of AEXJ4110 alloy was increased with increasing applied stress and temperature. The creep strain rates of AEXJ4110 alloy under different stresses and temperatures were a power function of time. The constitutive equation of steady-state impression creep for AEXJ4110 alloy can be written as $$\dot{\varepsilon }_{\text{s}} = 9.71 \times 10^{ - 3} (E/T)(\sigma /E)^{4.3} \exp (( - 100.449 + 3368.98(\sigma /E))/RT)$$ with stress exponent n 4.3 and creep activation energy Qc about 100.449 kJ/mol. The impression creep mechanism of AEXJ4110 alloy in the temperature range of 398-448 K and stress range of 55-95 MPa is controlled by dislocation climb dominated by self-diffusion of magnesium atoms impeded by the precipitated phases with high melting temperatures.

Modelling of Creep in Alloys Strengthened by Rod-Shaped Particles: Al-Cu-Mg Age-Hardenable Alloys

In recent years, a creep model that does not involve adjustable parameters has been successfully applied to coarse-grained aluminum. The main feature of this model is that it is fully predictable. On the other hand, in the case of age-hardenable alloys, any physically-based creep model should take into account the changes in the volume fraction, size and distribution of strengthening precipitates, and the effect of grain size. With this aim in view, in this paper, the original model previously applied to single phase-alloys has been modified to describe the effects of the grain size and of the consequences of the high-temperature exposure on the strengthening role of precipitates. To this end, phenomenological equations describing the coarsening phenomena and their dependence on the applied stress have been introduced. The modified model has given an excellent description of the experimental behavior of an AA2024-T3 alloy tested at 250 and 315 °C and has provided a sound explanation of the difference observed when comparing the minimum creep rate obtained using two different testing techniques.

Assessment of creep in reactor-irradiated CuCrZr alloy intended for the ITER first wall panels

CuCrZr alloy is candidate heat sink material for the ITER blanket, first wall, and divertor. During ITER operation it will be exposed to a combination of elevated temperatures, heat flux, and intense fast neutron radiation. This environment will challenge the performance of components and joints based on CuCrZr. To address this issue, mechanical tests were performed with irradiated and reference specimens of CuCrZr and its joints with 316 L(N)-IG (ITER Grade) stainless steel made by hot isostatic pressing. The reactor exposure up to ∼0.7 dpa was performed in the BR2 reactor at SCK•CEN, in water at a temperature of 257 °C A special design was used to allow irradiation of specimens axially pre-stressed at different strain levels. The post-irradiation examination included: (i) tensile test, (ii) measurements of plastic deformation of samples axially loaded during irradiation (in situ creep test); (iii) thermal creep tests on irradiated samples. The fracture surfaces were examined in a hot cell using a Scanning Electron Microscope (SEM). The results were compared with data obtained from mechanical tests and SEM/EDX fracture surface analysis on non-irradiated reference samples. The level of a possible creep under irradiation is below the experimental uncertainty.

Determining the Form of a Hydrodynamic Flow upon Creep of an Amorphous Cobalt-Based Metal Alloy in a Variable Temperature Field

Deformation of an amorphous cobalt alloy of strip configuration in creep tests in a variable temperature field is studied. An analytical form of the creep curve is found, and a prediction calculation of the resistivity by means of the proposed creep function is carried out. The physical meaning of the parameters of the creep equation is established, and their dimensionality is found. It is shown that, under the action of a variable temperature field, the creep of the alloy is not Newtonian and the material under strain exhibits pseudoplastic properties. It was found that, in the absence of heating, the viscosity of amorphous strip alloys can be considered within the Bingham plastic model.

Creep of Slurry Squeeze-Cast ZA-27 Zinc Alloy with Different Solid Fractions at 140 °C

This research work was aimed at studying creep of a slurry squeeze-cast ZA-27 zinc alloy. The Gas-Induced Semi-Solid (GISS) technique was used for preparing semi-solid metal slurry with two different solid fractions. The ZA-27 alloy slurry was squeeze cast to form plate-like specimens with dimension of 100×100×15 mm3. The tensile specimens were machined from the squeeze-cast plates and heat-treated before mechanical test. Tensile creep tests were performed at 140 °C, at stresses of 20, 40, 60 and 80 MPa, in an ambient air. Two groups of specimens, the low solid fraction ZA-27 alloy (GISS 5s) and the high solid fraction ZA-27 alloy (GISS 15s) were tested for comparison of creep property. The results show that the GISS 5s has longer creep life than the GISS 15s. The power-law creep stress exponents (n) of GISS 5s and GISS 15s are 1.45 and 1.04, respectively. Based on the creep stress exponents found from the present study, it could be concluded that at the temperature of 140 °C and stress between 20 and 80 MPa, creep of both alloys was governed by the diffusional creep mechanism.

Ultrahigh-temperature tensile creep of TiC-reinforced Mo-Si-B-based alloy

In this study, the ultrahigh-temperature tensile creep behaviour of a TiC-reinforced Mo-Si-B-based alloy was investigated in the temperature range of 1400–1600 °C at constant true stress. The tests were performed in a stress range of 100–300 MPa for 400 h under vacuum, and creep rupture data were rationalized with Larson-Miller and Monkman-Grant plots. Interestingly, the MoSiBTiC alloy displayed excellent creep strength with relatively reasonable creep parameters in the ultrahigh-temperature range: a rupture time of ~400 h at 1400 °C under 137 MPa with a stress exponent (n) of 3 and an apparent activation energy of creep (Qapp) of 550 kJ/mol. The increasing rupture strains with decreasing stresses (up to 70%) and moderate strain-rate oscillations in the creep curves suggest that two mechanisms contribute to the creep: phase boundary sliding between the hard T2 and (Ti,Mo)C phases and the Moss phase, and dynamic recovery and recrystallization in Moss, observed with orientation imaging scanning electron microscopy. The results presented here represent the first full analysis of creep for the MoSiBTiC alloy in an ultrahigh-temperature range. They indicate that the high-temperature mechanical properties of this material under vacuum are promising.

Creep, fatigue, and fracture behavior of high-entropy alloys

Abstract

In-situ observation of an irradiation creep deformation mechanism in zirconium alloys

We report an original experimental study on a zirconium alloy deformed in situ under ion irradiation and applied stress inside a Transmission Electron Microscope. We observe that dislocations initially pinned on irradiation defects can be unpinned and glide at a lower stress under the effect of the irradiation. It is proposed that unpinning occurs by a local effect of the displacement cascade created by the incoming ion in the vicinity of the pinning point. This novel mechanism of dislocation glide assisted by irradiation is thought to play a crucial role on in-reactor irradiation creep of zirconium alloys.

Mechanism-based modeling of solute strengthening: Application to thermal creep in Zr alloy

In this work, a crystallographic thermal creep model is proposed for Zr alloys that accounts for the hardening contribution of solutes via their time-dependent pinning effect on dislocations. The core-diffusion model proposed by Soare and Curtin (2008a) is coupled with a recently proposed constitutive modeling framework (Wang et al., 2017, 2016) accounting for the heterogeneous distribution of internal stresses within grains. The Coble creep mechanism is also included. This model is, in turn, embedded in the effective medium crystallographic VPSC framework and used to predict creep strain evolution of polycrystals under different temperature and stress conditions. The simulation results reproduce the experimental creep data for Zircaloy-4 and the transition between the low (n∼1), intermediate (n∼4) and high (n∼9) power law creep regimes. This is achieved through the dependence on local aging time of the solute-dislocation binding energy. The anomalies in strain rate sensitivity (SRS) are discussed in terms of core-diffusion effects on dislocation junction strength.The mechanism-based model captures the primary and secondary creep regimes results reported by Kombaiah and Murty (2015a, 2015b) for a comprehensive set of testing conditions covering the 500–600 °C interval, stresses spanning 14–156 MPa, and steady state creep rates varying between 1.5·10−9s−1 to 2·10−3s−1. There are two major advantages to this model with respect to more empirical ones used as constitutive laws for describing thermal creep of cladding: 1) specific dependences on the nature of solutes and their concentrations are explicitly accounted for; 2) accident conditions in reactors, such as RIA and LOCA, usually take place in short times, and deformation takes place in the primary, not the steady-state creep stage. As a consequence, a model that accounts for the evolution with time of microstructure is more reliable for this kind of simulation.