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.     

Critical assessment of high-temperature deformation and deformed microstructure in high-purity tetragonal zirconia containing 3 mol.% yttria

Tensile creep has been investigated in a fine-grained yttria-stabilized tetragonal zirconia (3Y-TZP). The creep data corrected for grain growth reveal a high stress region with a stress exponent of n∼2.7 and a grain size exponent of p∼2.0–3.0, an intermediate stress region with n∼5.0, and a low stress region with n∼1.3 and p∼1.8. In the whole region, the apparent activation energy takes a value for the lattice diffusion of cations (580 kJ/mol). Microstructural observation reveals that intragranular dislocation motion is noticeably activated in the high stress region, limited in the intermediate stress region and absent in the low stress region. The results suggest that the rate of deformation is controlled by the recovery of the intragranular dislocations in the high stress region and by Nabarro–Herring creep in the low stress region. The intermediate stress region is related to a threshold stress for the dislocation motion.     

Superplastic behaviour of Ti-48at.%Al two-phase titanium aluminide compacts made from mechanically alloyed powder

An HIP compact of MA-processed powder having a nominal composition of Ti-48at.% Al was produced. The compact consisted of a large amount of TiAl(λ) and a small amount of Ti₃Al (₂), in a completely ultra-fine equiaxed grain structure. This two-phase compact showed typical superplastic deformation behaviour. A maximum elongation of 550% was obtained. A strain exponent, n = 2, and grain size exponent, p = 2, were determined from the results of a strain-rate-change test and a creep test at constant initial stress using samples having various grain sizes, respectively. The activation energy for creep, Q_c at constant stress was calculated to be 350 kJ/mole. It is concluded that the superplastic deformation mechanism of the material under study is grain boundary sliding controlled by lattice diffusion in the TiAl phase.     

Separate contributions of texture and grain size on the creep mechanisms in a fine-grained magnesium alloy

The influence of texture and grain size on the creep behavior of a fine-grained magnesium alloy, over the temperature range 423–723 K was investigated. Equal channel angular pressing and rolling were used to produce samples with different textures. Two deformation regimes could be distinguished by their stress exponents. A stress exponent close to 2 and activation energy of 91 kJ mol⁻¹, close to that for grain boundary diffusion, were found at the lower strain rates. In this range, there is no detectable effect of texture. In the high stress exponent regime, within the range 3 < n < 12, a noticeable effect of texture and grain size has been found. The texture effect is related to the orientation of the basal planes. The influence of grain size distribution on flow stress is satisfactorily explained by modeling the deformation as a combination of grain boundary sliding and slip creep.     

Superplasticity in alumina enhanced by co-dispersion of 10% zirconia and 10% spinel particles

Superplastic deformation behavior is examined for Al₂O₃-based ceramics dispersed with 10 vol% ZrO₂ and 10 vol% spinel (MgO·1.3 Al₂O₃) particles. The multiple-phase dispersion considerably decreases the rate of grain growth during deformation, leading to enhanced superplasticity (larger tensile elongation and higher strain rate). Maximum tensile elongation reaches 850% at a strain rate of 5.0×10⁻⁴ s⁻¹ and at 1500°C. Grain growth during deformation is found to follow a theoretical model based on a grain boundary diffusion mechanism. The creep parameters corrected for concurrent grain growth are 2.2 as the stress exponent, 3.2 as the grain size exponent and 751 kJ/mol as the activation energy. Spherical ZrO₂ particles embedded in elongated Al₂O₃ grains in deformed specimens suggest that the deformation mechanism of the present material is strongly related to grain boundary diffusion. Being different from other superplastic aluminas, cavities in the present material tended to grow in the direction parallel to the stress axis.     

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     

Stress-enhanced grain growth in a nanocrystalline material by molecular-dynamics simulation

Molecular-dynamics simulations are used to elucidate the coupling between grain growth and grain-boundary diffusion creep in a polycrystal consisting of 25 grains with an average grain size of about 15 nm and a columnar grain shape. Consistent with our earlier simulations of grain-boundary diffusion creep, albeit in the absence of grain growth, we find that initially, i.e. prior to the onset of significant grain growth, the deformation proceeds via the mechanism of Coble creep. Also, consistent with our earlier grain-growth simulations in the absence of stress, two growth mechanisms are observed during the deformation: growth due to curvature-driven GB migration and growth resulting from grain rotation-induced grain coalescence. The comparison of the grain growth observed in the presence of the applied stress with that solely in response to temperature as the driving force enables us to identify the mechanisms by which external stress affects grain growth. In particular, we find that both GB migration and grain rotation are accelerated by the deformation.     

Creep of polycrystalline SrCo0.8Fe0.2O3−δ

In this study, a series of compressive creep tests on rectangular specimens of SrCo_0.8Fe_0.2O_3−δ of grain sizes in the range of 2.4–6.8 μm was performed in air, covering a temperature range of 850–975°C and a stress range of 2–80 MPa. The stress exponent has been found to be close to unity in the 10–20 MPa stress range. The apparent activation energy assumes two different values, 471 kJ/mole below 925°C and 275 kJ/mole above 925°C. The microstructural observations of crept samples revealed equiaxed grains and negligible grain growth indicating a diffusion-accommodated grain switching mechanism. The logarithmic plot of strain rate vs inverse grain size has been found to be non-linear. A possible explanation for this behavior is discussed through the incorporation of a threshold stress. The possibility of estimating the lattice and grain-boundary diffusion coefficients from the data is discussed.     

High temperature deformation of a submicron grained Al-12 wt% Ti alloy

Compressive deformation behavior of a MA Al-12wt%Ti alloy has been studied at 623–773 K with strain rates from 4 × 10⁻⁵ to 1 × 10⁻¹s⁻¹. A high stress exponent was observed in the stress dependence of strain rate. By assuming that the presence of a threshold stress is the cause of the high stress exponent, the experimental data were analyzed and compared with existing models. The creep behavior of this alloy is found similar to the dispersion strengthened Al-Al₂O₃ alloys. It is suggested that the high temperature deformation of this alloy is mainly governed by the fine carbide and oxide strengthened aluminum matrix, which can be described by lattice-diffusion controlled creep with a “constant structure” [11]. In addition, a strong temperature dependence of the threshold stress was observed. It might be related to certain grain boundary processes and/or thermally activated deformation of strengthening particles.     

新型高强铝合金的热变形行为及组织演变

采用高温压缩实验研究了新型Al-Zn-Mg-Cu高强铝合金在温度300~450℃、应变速率0.001~10 s-1和压缩变形程度30%~80%范围内的热变形行为和组织演变。分析了该合金在实验参数范围内变形的应力-应变曲线特征。动力学分析获得该合金热变形的应力指数和激活能分别为4.97和150.07 kJ/mol,表明合金的热变形主要受扩散所控制。金相组织观察发现,随着变形温度的升高或应变速率的降低,变形组织晶内析出相逐渐溶入基体组织,晶内组织逐渐趋于均匀;同时粗大的晶粒沿变形方向拉长,晶界难溶相的碎化和弥散化程度增大。TEM和EBSD(electron back-scattered diffraction)组织分析表明,该合金在高温压缩变形过程中组织演变主要是亚晶的形成和完善的过程,热变形组织演变机理为动态回复和大应变几何动态再结晶。     

Steady-State Creep Behavior of Super304H Austenitic Steel at Elevated Temperatures

Creep behavior of Super304 H austenitic steel has been investigated at elevated temperatures of 923-973 K and at applied stress of 190-210 MPa.The results show that the apparent stress exponent and activation energy in the creep deformation range from 16.2 to 27.4 and from 602.1 to 769.3 kJ/mol at different temperatures,respectively.These high values imply the presence of a threshold stress due to an interaction between the dislocations and Cu-rich precipitates during creep deformation.The creep mechanism is associated with the dislocation climbing governed by the matrix lattice diffusion.The origin of the threshold stress is mainly attributed to the coherency strain induced in the matrix by Cu-rich precipitates.The theoretically estimated threshold stresses from Cu-rich precipitates agree reasonably with the experimental results.     

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.     

The effects of film formation and mechanical factors on the initiation of stress corrosion cracking of unalloyed steels in carbonate solutions

Investigations into the intergranular stress corrosion cracking (SCC) of unalloyed steel St 36 in carbonate-bicarbonate solutions have shown that SCC susceptibility is limited to the potential range of magnetite formation for both constant load and constant strain rate tests. Experiments to determine the creep behaviour of the steel in the corrosive solution show that creep rates corresponding to the critical strain rates in the constant strain rate test occur during the crack initiation state after loading of the tensile specimen with a constant crack initiating load. These experimental findings establish, therefore, that critical crack initiating strain rates of the unalloyed steel are always present during the initial stage for SCC regardless of the test method used. The cause of crack initiation at grain boundaries can be attributed to the fact that the magnetite layers feature faults and grooves in the oxide along the grain boundaries in the entire critical potential range. These oxide grooves, which are also observed on unstressed steel, cause a notch-like localized corrosion attack. The depth and crack tip radius of the resultant grain boundary notches appear to be sufficient to initiate propagating cracks under the single action of predominately pure stresses without macroscopic deformation of the steel. This does not necessarily mean, however, that a purely stress induced corrosion process is involved, for it is not out of question that the concentration of stress in the base of grain boundary notches is sufficient to cause localized deformation processes required for crack initiation.     

Effect of fully lamellar morphology on creep of a near γ-TiAl intermetallic

Creep properties of a polycrystalline binary near γ-TiAl intermetallic in two fully lamellar microstructural conditions are presented. Creep tests (760°C/240 MPa) indicate that a lamellar structure with fine interface spacing and planar grain boundaries improves creep resistance. A lamellar structure with wide lamellar interface spacing and interlocked grain boundaries has less than half the creep life, five times higher minimum creep strain rate and a greater tertiary creep strain. The deformation substructures are presented in terms of the lamellar orientation to the stress axis and indicate that creep strain is accommodated by dislocation motion in soft oriented grains, but the creep strain rate is controlled by hard oriented grains. The extent of tertiary creep is controlled by the grain boundary morphology, with planar grain boundaries susceptible to intergranular cracking. The results suggest that to maximize the creep resistance of near γ-TiAl intermetallics with lamellar microstructures requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

Superplastic behavior of zirconia-reinforced alumina nanocomposites from powder alcoxide mixtures

Here we report the synthesis, microstructure and superplastic creep behavior of alumina-based composites reinforced with 5–20 wt% ZrO₂, prepared from high-purity alumina powders and zirconium alcoxide precursor mixtures. The composites reached 100% true strain at 1350 °C, deformed by stationary creep in the range of strain rates ~10⁻⁷ to 10⁻⁴ s⁻¹. The microstructure remained stable during creep; no evidence of grain growth, grain boundary opening or cracking were observed in deformed samples. We conclude from the analysis of creep parameters that creep takes place mainly by interface-reaction-accommodated grain boundary sliding, and that the mobility of alumina grain boundaries controls the creep rate. The observed behavior is explained by the role of alumina–zirconia interphases, stronger and with a faster relaxation kinetics than alumina interfaces, to achieve an effective grain boundary pinning. Zirconia particles (~0.2–0.4 μm) are homogeneously placed at alumina grain boundaries (~1–2 μm).     

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.     

晶粒尺寸与保载载荷对Cu膜纳米压入蠕变性能的影响

利用纳米压入仪对Si片上的多晶Cu膜进行压入蠕变研究．实验结果显示晶粒尺寸大于200nm时，应力指数对晶粒尺寸不敏感．当晶粒尺寸小于200nm时，因压头底部更多的晶粒参与变形，应力指数随晶粒尺寸的降低而增大．认为薄膜材料存在一个对应力指数不敏感的最小If缶界晶粒尺寸ιc，Cu膜的应力指数随保载载荷增大而增大，其主要原因在于高载荷下位错强化机制使蠕变率降低。     

Degradation of RSP/PM Al-8Fe-4Ce during creep

The creep behavior of Al-8Fe-4Ce powder metallurgy alloy produced by rapid solidification processing (RSP/PM alloy) was studied within the 623 to 773 K temperature range and at initial stresses ranging from 10 to 52 MPa. The activation energy, Q, for creep in RSP/PM Al-Fe-Ce alloy is 2.3QL, where Ql is the activation energy for lattice diffusion in pure aluminum and the stress exponent is 8.6. The high-temperature creep deformation is associated with deformation of matrix and Al₁₃Fe₄ incoherent particles. In addition, particle coarsening is an important factor in alloy degradation. The formation and growth of cavities during creep at all stress levels at 698 K is also a contributing factor.     

Effect of thermomechanical parameters on dynamically recrystallized grain size of AZ91 magnesium alloy

Dynamic recrystallization of the AZ91 alloy was studied by conducting hot compression tests at temperature range of 325-400 °C and strain rate of 0.001-1 s⁻¹. The influence of the hot deformation variables on flow stress as well as recrystallized grain size was investigated. The results showed that by decreasing temperature and increasing strain rate, flow stress increases while dynamically recrystallized grain size decreases. A power-law relation developed between the characteristic peak strain and Zener-Hollomon parameter and the exponent was determined as 0.17. Besides, the linear regression between the Zener-Hollomon parameter and dynamically recrystallized grain size developed another power law equation, with a stress exponent equivalent to −0.13.