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.     

Low temperature superplasticity of AZ91 magnesium alloy with non-equilibrium grain boundaries

The superplastic behavior of a fine-grained AZ91 alloy, processed by equal channel angular extrusion, has been investigated in a low temperature range of 423–523 K. The experimental results showed a stress exponent of 2 and the activation energy for superplastic flow was in agreement with that for grain boundary diffusion of magnesium. The alloy with non-equilibrium grain boundary structures exhibited lower superplastic elongation than the alloy with equilibrium grain boundaries. Furthermore, the strain rate for superplastic flow of the former was lower than that of the latter. These differences probably arise because the accommodation process for grain boundary sliding is hampered by the long-range stresses associated with the non-equilibrium grain boundaries.     

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.     

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.     

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     

Microstructure and high-temperature mechanical behavior of the NiAl–27 at.% Cr intermetallic composite

A NiAl–27 at.% Cr composite material was prepared by a powder metallurgical route, involving argon atomization and consolidation by hot isostatic pressing at 1350°C for 4 h at 400 MPa. The consolidated material exhibited a fine-grained microstructure consisting of a fine dispersion of Cr particles of about 1.7 μm in a NiAl matrix. The mechanical behavior at temperatures ranging from 650 to 1100°C was investigated by tensile-strain-rate-change tests. Analysis of the strain–stress data with both power law creep and Garofalo’s hyperbolic sine relation shows the transition to a low stress exponent creep regime with decreasing stress and/or increasing testing temperature. The measured activation energy for deformation of 300 kJ/mol is consistent with the activation energy for Ni self-diffusion in Ni–50Al. Experiments with coarse grain sizes established that the creep rate is independent of grain size which suggests that the deformation mechanisms must be associated with the motion of lattice dislocations.     

Nanoquasicrystalline Al–Fe–Cr-based alloys. Part II. Mechanical properties

Nanoquasicrystalline Al-based alloys show considerable promise for elevated temperature applications compared with commercial Al-based alloys. In particular, a group of Al–Fe–Cr-based alloys-containing Ti, V, Nb or Ta have outstanding thermal stability. In the present work, the elevated temperature mechanical properties of these nanoquasicrystalline alloys were studied by tensile tests at a constant strain rate. Tests were designed in order to compare the mechanical behaviour at different test temperatures. Fractographic analysis was also carried out. The apparent activation energy for plastic deformation was found to be close to that for lattice self-diffusion for pure Al in the Al–Fe–Cr ternary alloy and in the Ti-containing alloy, and for grain boundaries diffusion for pure Al in the V-containing alloy, whereas the activation energy of the alloy with Ta additions was three times higher. All of the alloys showed similar sensitivity of plastic deformation to the strain rate in the range of 10^?3–5 × 10^?6 s^?1 at 350 °C. The apparent true stress exponent was n_app ～ 7, which can be associated with a deformation process controlled by dislocation mechanisms.     

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.     

Creep deformation mechanisms in fine-grained niobium

Creep rates in fine-grained Nb were measured at 600 °C using free-standing Cu/Nb polycrystalline multilayered foils. For specimens with layer thicknesses ranging from 0.5 to 5 μm and Nb grain sizes ranging from 0.43 ± 0.05 to 1.87 ± 0.13 μm, two distinct regimes were observed. At high stresses, the stress dependence, grain size dependence and activation energy for creep are consistent with power-law creep, with an average stress exponent of 3.5. At low stresses, creep rates exhibited a linear dependence on stress and an inverse linear dependence on grain size. A model is presented for a vacancy generation-controlled creep mechanism, whereby deformation rates are controlled by the rate of vacancy generation at or near grain boundaries, not by their diffusion. The proposed model is consistent with experimental observations of stress and grain size dependence, as well as the measured activation energy for creep.     

Hot deformation behavior of TiAl alloys prepared by blended elemental powders

A nearly full dense Ti-45Al-7Nb-0.4W (at.%) alloy billet with dimension of 120 mm in diameter and 50 mm in height was fabricated by reactive sintering of blended elemental powders. The high temperature deformation behavior was investigated by isothermal compressive tests, performed at temperature in 1000–1200 °C with strain rates from 1 × 10^?3 s^?1 to 1 × 10^?1 s^?1. Results indicate that the dependence of flow stress on temperature and strain rate is well fit for a hyperbolic-sine relationship using the Zener–Hollomon parameter. The measured apparent activation energy Q and stress exponent are determined as 420 kJ mol^?1 and 3.7, respectively. High oxygen content, high Nb content and fine grain size are main reasons for the high activation energy and high strength of PM TiAl alloy. An appropriate set of deformation processing parameters of 1200 °C and 1 × 10^?3 s^?1 are recommended for the present TiAl alloy.     

The high temperature tensile and compressive deformation characteristics of magnesia doped alumina

The mechanical characteristics of alumina have not yet been characterized completely in tension due in part to strain hardening accompanying grain growth and premature cavitation failure. Tensile tests were conducted on fine grained magnesia doped alumina over a range of strain rates, grain sizes and temperatures to evaluate the stress exponent, inverse grain size exponent and activation energy. Constant stress compression creep tests were also carried out under a similar range of experimental conditions. Extensive microstructural characterization after deformation indicated that there was considerable grain growth during deformation; however, the grains retained their initially equiaxed structure after significant deformation. Although a standard plot of strain rate versus stress indicated a stress exponent of ∼2, a complete analysis including the compensation of data for concurrent grain growth revealed that true stress exponent was ∼1, consistent with diffusion creep. It is argued that grain rearrangement processes accompanying grain growth will tend to mask the development of an elongated grain structure predicted by diffusion creep processes. In contrast to several ceramics with a significant amount of glassy phase, there is no significant difference between the elevated temperature tensile and compressive behavior of alumina.     

Ordering and grain growth in nanocrystalline Fe75Si25 alloy

The grain growth and ordering behaviors in nanocrystalline Fe₇₅Si₂₅ powders produced by mechanical alloying was studied. In the temperature range 573–703 K, the disordered α-Fe(Si) showed only limited growth with a small activation energy of 32.3 kJ mol⁻¹. However, in the range 773–853 K ordering to DO₃ structure occurred accompanied by a sharp increase in average grain size to a maximum value that depended on the temperature. A grain growth model available in the literature for nanocrystalline materials yielded a good fit to the experimental data. Activation energy of 278.9 kJ mol⁻¹ determined using this model for grain growth is similar to that for lattice diffusion of Si in DO₃ Fe₃Si. The reason for the concurrence of ordering and grain growth is attributed to the Si desegregation at grain boundaries upon ordering transformation. The levels of segregation are estimated from lattice parameter measurements.     

Spark plasma sintering of W-10Ti high-purity sputtering target: Densification mechanism and microstructure evolution

The densification mechanism and microstructure evolution of W-10Ti sputtering target prepared by spark plasma sintering (SPS) method at a temperature ranges from 900 to 1600 °C, with dwelling time of 6 min and fixed pressure of 30 MPa were investigated. Densification occurs mainly at low temperatures (900 to 1300 °C), while grain growth occurs at high temperatures (1400 to 1600 °C). The creep model has been used to reveal the densification process. The effective stress exponent n is calculated systematically, which indicates that the densification process is mainly due to the particle rearrangement (n < 1), grain boundary diffusion (n = 1–2), and dislocation climbing (n = 3.77 or 4.14). In addition, the apparent activation energy Q_d is calculated to be 119.30 and 271.79 kJ/mol when the effective stress exponent n is equal to 1 and 2, respectively. It is also found that the microstructure of W-10Ti alloys is greatly affected by the sintering temperatures. The solution between W and Ti significantly improves with the increase of the sintering temperature. The solubility of W in βTi(W) exceeded the eutectoid point (28.97 wt% W) and the eutectoid structure (βW(Ti) + αTi) forms in cooling process when the temperature is up to 1300 °C. With the temperature increasing to 1500 °C, the composition of the βTi(W) phase is located in the miscibility gap of the (βTi(W), βW(Ti)) system, which tends to decompose in to βTi(W) and βW(Ti) phases.     

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.     

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.     

Aluminum Impurity Diffusion in Magnesium

The diffusion of Al in polycrystalline Mg (99.9%) was studied via depth profiling with secondary ion mass spectrometry in the temperature range of 573-673 K, utilizing the thin film method and thin film solution to the diffusion equation. Multiple samples with multiple depth profiles on each sample were obtained to determine statistically confident coefficients with a maximum standard deviation between measurements of 16%. The activation energy and pre-exponential factor of Al impurity diffusion in Mg were determined as 155 kJ/mole and 3.9 × 10⁻³ m²/s, respectively. The Mg substrates have a small grain size (~10 μm) and therefore some contributions from grain boundary diffusion are expected in the measurements. Sputter roughening during depth profiling, which is inherent to the SIMS process, also contributes to the measured diffusion coefficient, especially in samples with smaller grain sizes.     

Superplasticity in electrodeposited nanocrystalline nickel

Electrodeposited nanocrystalline Ni films were processed with different levels of S, to evaluate the role of S on superplasticity. All the materials exhibited high strain rate superplasticity at a relatively low temperature of 777 K. Microstructural characterization revealed that the S was converted to a Ni₃S₂ phase which melts at 908 K; no S could be detected at grain boundaries. There was no consistent variation in ductility with S content. Superplasticity was associated with a strain rate sensitivity of ～0.8 and an inverse grain size exponent of ～1, both of which are unusual observations in superplastic flow of metals. Based on the detailed experiments and analysis, it is concluded that superplasticity in nano-Ni is related to an interface controlled diffusion creep process, and it is not related to the presence of S at grain boundaries or a liquid phase at grain boundaries.     

Creep of cast Fe–36Al–2Ti alloy

Creep of an alloy based on the intermetallic compound FeAl with Ti addition was studied by compressive tests at constant stress in the temperature range from 873 to 973 K. The stress exponent n and the activation energy of creep Q were determined for the minimum creep rate. The stress exponent is slightly decreasing with increasing temperature. Its value is in agreement with stress exponents reported for Fe–Al alloys with similar additions of titanium. The values of n together with the observed shapes of the creep curves are consistent with the expected behaviour for solid solution hardened alloys where the dislocation motion is controlled by viscous glide of dislocations. Annealing of the alloy for 2 h at 1423 K with subsequent oil-quenching had no influence on the minimum creep rate in comparison with the as-cast material. On the other hand, this annealing accelerated “the inverse primary behaviour”.     

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.     

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。