High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.     

Creep Constitutive Model and Component Lifetime Estimation: The Case of Niobium-Modified 9Cr-1Mo Steel Weldments

The θ-projection parametric method was used to analyze the creep strain versus time data, obtained in uniaxial tension, from weldments fabricated using a niobium-modified 9Cr-1Mo steel as the weld metal (Ellis, Private communication, 1991, provided the data). We used these data to illustrate a methodology whereby the θ-projection method may be used to obtain estimates of component design creep lifetimes, for specified sets of design stress, temperature, and strains. Furthermore, it is suggested that the creep strain results may be consistent with dislocation climb being the creep deformation mechanism in the alloy.     

Creep behaviour of Ti-6Al-2Sn-4Zr-2Mo: II. Mechanisms of deformation

Constant load creep tests are performed in Ti-6242(Si) alloy with a lath microstructure, at temperatures of 538 and 565 °C. A change in the stress exponent values from ˜1 at low stresses to between 5 and 7 at high stresses, is indicative of a change in creep mechanism. TEM analysis indicates that the deformation is dominated by a-type dislocations in the phase, with little evidence of dislocation activity in the β laths. At higher stress (310 MPa), the a-type dislocations are pinned frequently along their screw direction by tall jogs. A creep model is proposed based on the premise that movement of these jogged screw dislocations may control the creep rate. In contrast, at low stress (172 MPa), the a-type dislocations have long straight screw segments with no apparent pinning points. The near-edge segments are in climb configurations. The creep rates here are close to those predicted, based on Harper–Dorn creep, although the dislocation density is larger than that normally associated with this regime.     

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti₃Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (10₃<σ/E<3x10^-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion     

Physically-based model for creep in nickel-base superalloy C263 both above and below the gamma solvus

The polycrystalline nickel-base superalloy (C263) is used for stationary components in aero-engines such as combustion chambers, casings, liners, exhaust ducting and bearing housings. It is a fine-precipitate strengthened alloy at 800°C, with a precipitate solvus temperature of 925°C. Below the solvus, the precipitate coarsens at elevated temperature. A critical precipitate size exists below which particle cutting is the rate-controlling creep mechanism. Above the critical size, it is dislocation pinning and climb. Once the solvus temperature has been exceeded, however, the rate-controlling mechanism becomes pinning and climb within a dislocation network.This paper presents physically-based constitutive equations for creep deformation in C263. In the presence of the γ′ precipitate, populations of climbing and gliding dislocations are assumed to exist in a similar way to that proposed by Dyson and Osgerby [3]. Climbing dislocations are assumed to be pinned at precipitates. The dependence of steady state glide dislocation flux can then be obtained as a function of precipitate volume fraction. Above the γ′ solvus, the pinning process is different and results from the establishment of a dislocation network, the size of which determines the pinning distance. The physical constants arising in the equations have been determined by the conventional minimisation of errors between experimental and calculated creep curves. They have also been determined quite independently using fundamental data and experiments. Remarkable agreement between the two sets of physical constants is achieved. The constitutive equations have been shown to capture the material’s creep deformation characteristics over a broad range of temperature, both below and above the γ′ solvus. In addition, the effect of precipitate coarsening on creep rate is correctly captured, together with the effect of prior ageing on subsequent creep rate.The damage processes of cavitation and multiplication of mobile dislocation density have been coupled with the constitutive equations and used to predict failure in constant and variable stress creep.     

Impressions and Conclusions of the Conference on Mechanical Properties and adherence of scale layers and their influence on the oxidation of metals

Review of the papers presented to the Conference and of discussions. The topics dealt with included the various ways in which growing oxide scales can deform to accommodate their growth stresses. According to that plastic deformation occurs by the following mechanisms: dislocation glide, dislocation climb, Herring-Nabarro stress-assisted diffusion creep, grain-boundary sliding, mechanical twinning and viscous flow of amorphous oxides. The deformation depends on the scale configuration and the stress system; many oxide systems are rather plastic at high temperatures, but pores, defect doping elements and second phase inclusions play important parts, too. It is generally accepted that the evidence for some plastic deformation in growing oxide scales is very strong, the most favoured mechanism at high temperatures being diffusion controlled creep associated with grain boundary sliding. Factors influencing adhesion of scales include electrostatic forces at the metalloxide interface, interface irregularities, stress/strain interactions between oxide and metal, stress relaxation in metal and oxide, and the presence of stress raisers such as voids, second phase particles and pores at the metal/oxide interface. The necessity of developing new measuring methods is pointed out, too.     

Mg-5.5Zn-2Gd-0.6Zr铸造镁合金的蠕变机制

在ZM-1(Mg-5Zn-0.6Zr)合金的基础上,适量增加Zn的含量并加入重稀土元素Gd,设计了Mg-5.5Zn-2Gd-0.6Zr实验合金。采用砂型铸造工艺制备实验合金试样,在不同温度和应力条件下对该实验合金和ZM-1合金的蠕变曲线进行了测试。结果表明:在相同条件下,Mg-5.5Zn-2Gd-0.6Zr实验合金的稳态蠕变速率较ZM-1合金的降低了一个数量级;当施加应力为40 MPa时,实验合金的蠕变激活能Q200-250℃=142.0 kJ/mol,接近镁的自扩散激活能,蠕变受位错攀移控制,而ZM-1合金在相同应力下蠕变激活能Q200-250℃=88.5 kJ/mol,接近镁的晶界扩散激活能,蠕变受晶界滑移控制。合金在200℃条件下的应力指数n=4.21,而ZM-1合金的应力指数n=2.21。因此,认为加入重稀土元素Gd后实验合金的蠕变机制发生改变,200℃时的蠕变机制为位错攀移机制。     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Creep mechanisms in Ti–3Al–2.5V alloy tubing deformed under closed-end internal gas pressurization

Creep tests were carried out on Ti–3Al–2.5V alloy tubing in the temperature range of 723–873 K under closed-end internal pressurization. The data thus obtained were analyzed to obtain the mechanistic creep parameters (stress exponent and activation energy). Transitions in creep mechanisms were noted as the stress exponent varied from a lower value of 1 through 2 to a higher value of 5 with increasing stress where the activation energy assumed values of 232 and 325 kJ mol⁻¹, respectively. The creep mechanisms were elucidated in the light of standard creep models supported by the substructures studied by transmission electron microscopy. Newtonian viscous creep (n = 1) at lower stresses was identified to be in accordance with a slip band model named after Spingarn and Nix. Grain boundary sliding with n = 2 was noted in an intermediate stress region while climb of edge dislocations was observed to control creep at higher stresses. Microstructural observations along with parametric variations of creep rates were useful in identifying the underlying deformation mechanisms.     

Experimental evidence for diffusion creep in the superplastic 3 mol% yttria-stabilized tetragonal zirconia

Although there have been numerous studies on the high temperature deformation characteristics of the superplastic 3 mol% yttria stabilized tetragonal zirconia (3YTZ), the rate controlling deformation mechanism has not been identified unambiguously. In the present study, experiments were conducted on 3YTZ at high stresses and at coarser grain sizes than used conventionally for superplasticity. The experimental results reveal, for the first time, an intragranular dislocation motion controlled high stress regime that is independent of the grain size. With a decrease in stress, there is a transition to a Newtonian viscous deformation regime consistent with Coble grain boundary diffusion creep. At sufficiently low stresses, or in materials with finer grain sizes, there is a further transition to a grain size dependent interface controlled deformation regime. Analysis of the experimental data suggests strongly that superplastic flow in 3YTZ occurs by an interface controlled deformation mechanism.     

TC6钛合金的高温拉伸蠕变行为研究

通过高温拉伸蠕变实验,获得了TC6合金的蠕变应变-时间曲线,并计算了其不同应力与不同温度下的稳态蠕变速率、应力指数及在350～450℃范围内的蠕变激活能,借助OM、TEM等手段对合金蠕变前后的显微组织进行了观察和分析,并在此基础上研究了其蠕变变形机制.结果表明:TC6合金的稳态蠕变速率随温度或恒应力的增加而增大,该合金在此温度范围内的蠕变受位错和扩散双重机制的控制,晶界滑动对蠕变也有一定的作用.     

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

Constitutive relations for determining the critical conditions for dynamic recrystallization behavior

A series mathematical model has been developed for the prediction of flow stress and microstructure evolution during the hot deformation of metals such as copper or austenitic steels with low stacking fault energies, involving features of both diffusional flow and dislocation motion. As the strain rate increases, multiple peaks on the stress-strain curve decrease. At a high strain rate, the stress rises to a single peak, while dynamic recrystallization causes an oscillatory behavior. At a low strain rate (when there is sufficient time for the recrystallizing grains to grow before they become saturated with high dislocation density with an increase in strain rate), the difference in stored stress between recrystallizing and old grains diminishes, resulting in reduced driving force for grain growth and rendering smaller grains in the alloy. The final average grain size at the steady stage (large strain) increases with a decrease in the strain rate. During large strain deformation, grain size reduction accompanying dislocation creep might be balanced by the grain growth at the border delimiting the ranges of realization (field boundary) of the dislocation-creep and diffusion-creep mechanisms.     

High temperature creep property of silicon particles reinforced high Al Zn-based alloys

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Multi-stage spark plasma sintering to study the densification mechanisms of boron carbide

The densification kinetics of boron carbide (B₄C) during multi-stage spark plasma sintering was studied. The densification mechanisms were analyzed according to the stress exponent n and the apparent activation energy Q_d using a creep deformation model. The results showed that the densification mechanisms were controlled by viscous flow and grain boundary diffusion at the low effective stress with initial temperature range of 1600–2000 °C, while the dominant mechanism is the dislocation climb at the effective stress regime with final temperature of 2100 °C and the multi-stage sintering can reduce the apparent activation energy. Meanwhile, the scheme of multi-stage sintering can obtain nearly theoretical dense B₄C and avoid grain growth. Therefore, the basic mechanical properties suggesting a good combination of high hardness (37.63GPa) and bending strength (539.86 MPa) was obtained by the multi-stage sintering.     

An investigation of the deformation mechanisms in sn5% sb alloy using tensile, creep and abi tests from ambient to 473k

Tensile, creep, and automated ball indentation (ABI) tests have been conducted to study deformation mechanisms in Sn5%Sb alloy between ambient and 473 K. A power law relationship was obtained between minimum creep rate and applied stress, with stress exponent,n=5 and activation energy,Q=12.6±1.1 kCal/ mole. At 473 K, a transition fromn=5 ton=3 was observed at low stresses. ABI tests showed a power law relationship between strain rate and ultimate tensile stress with values ofn=5 andQ=13.0±1.8 kCal/mole. Tensile results were in broad agreement with the creep and ABI data. A new deformation mechanism is proposed for then=5 region involving viscous glide of dislocations assisted by dislocation core diffusion.     

Deformation of fine-grained alumina by grain boundary sliding accommodated by slip

Creep data from over 40 different polycrystalline alumina materials are reviewed. Most of these studies have attempted to describe the creep data using models based on diffusional creep. In the present paper, however, it is concluded that the dominant deformation mechanism in creep of fine-grained alumina is grain boundary sliding (GBS) accommodated by slip. The slip accommodation process is related to the sequential steps of dislocation glide and climb. When the accommodation process for GBS is that of dislocation climb, the stress exponent is always 2. In this case, the activation energy for creep is either that for oxygen ion diffusion in the lattice or that for oxygen ion diffusion in the grain boundary. When the accommodation process for GBS is that of solute-drag dislocation glide, the stress exponent is 1. For this case, the activation energy is that for solute diffusion at the dislocation site during glide.     

Hot shear deformation constitutive analysis of fine-grained ZK60 Mg alloy sheet fabricated via dual equal channel lateral extrusion and sheet extrusion

Dual equal channel lateral extrusion (DECLE) process with various passes followed by sheet extrusion process was performed to produce fine-grained ZK60 alloy sheets. The coarse grain structure of the annealed sample after applying sheet extrusion (size: 68 µm) changed to fine grains of 6.0 and 5.2 µm after 3 and 5 passes of DECLE and following extrusion. The hot shear deformation behavior of samples was studied by developing constitutive equations based on shear punch test (SPT) results. SPT was carried out in the temperature range of 200?300 °C and strain rate range of 0.003?0.33 s^–1. The activation energy of 125?139 kJ/mol and the stress exponent of 3.5?4.2 were calculated for all conditions, which indicated that dislocation creep, controlled by dislocation climb and solute drag mechanism, acted as the main hot deformation mechanism. It was concluded that material constants of n and Q are dependent on the microstructural factors such as grain size and second phase particle fraction, and the relationship of which was anticipated using a 3D surface curve. Moreover, the similar strong basal texture of extruded sheets gave rise to the same deformation mechanisms during SPT and similar n and Q values for ZK60 alloy.     

Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.