Apparent Activation Energy For Creep Research Articles

Creep and Corrosion Properties of the Extruded Magnesium Alloy Containing Rare Earth

Effects of microstructures on the creep and corrosion properties were investigated in the Mg-8A1 alloys with addition of the rare earth elements (La-rich Mischmetal, RE). The addition of RE to Mg-8A1 alloy may form a stable intermetallic phase, Al 11 RE 3 at elevated temperature, and suppress the βhase (Mg 17 Alj 2 ) with poor stability at high temperature. The corrosion rate of the alloy slightly decreases with increasing the added RE contents. The constant-load creep behavior was examined at 423, 448 and 473 K under stresses between 40 and 100MPa. The RE-containing alloy showed a prolonged period of stead-state creep compared to the Mg-8Al base alloy and slightly reduced minimum creep rates. A stress exponent of 2 estimated suggests that the creep behavior can be controlled by the grain boundary sliding. The apparent activation energy for creep of Mg-8Al and Mg-8Al-2RE alloys are 114 and 104 kJ mol -1 , respectively.

Effect of trace impurities in helium on the creep behavior of Alloy 617 for very high temperature reactor applications

The effect of trace impurities, methane and oxygen, in helium on the creep behavior of Alloy 617, has been investigated. The creep rupture life at relatively low applied stresses was shortest in a helium environment containing 500 vppm oxygen (He + O 2), while it was the longest in helium containing 675 vppm methane (He + CH 4). However, the rupture strain was significantly lower in the He + CH 4 environment compared to that in pure helium (He) and He + O 2. The low rupture strain in the He + CH 4 is caused by cleavage fracture. In the He + CH 4 environment, the fracture mode was cleavage at lower applied stresses and ductile at higher applied stresses while in the He and He + O 2, a ductile fracture was observed at all stress levels. The apparent activation energy for creep was determined in all three environments, and it appears to be independent of stress in the He, dependent in the He + CH 4, while in the He + O 2 environment the stress dependence could not be conclusively established.

Interpretation of the creep behavior of nanocrystalline Ni in terms of dislocation accommodated boundary sliding

Creep behavior and deformation-induced grain growth in electrodeposited (ED) nanocrystalline (nc) Ni with a grain size of about 20 nm were studied over more than five orders of magnitude of strain rate (10−9 s−1 to 2×10−4 s−1) at 393 K (0.23 Tm, where Tmis the melting point). In addition, the activation energy for the creep in ED nc-Ni was determined by using the temperature change procedure. The results show that the creep behavior of the material is characterized by (a) a stress exponent that increases continuously from about 4.5 to about 30 with increasing applied stress; (b) an apparent activation energy for creep in the range of 126 to 141 kJ/mol; (c) an activation volume of about 20 b3 where b is the Burgers vector; and (d) a grain size that upon loading, grows, attaining a constant value once steady-state creep is approached. The mechanical characteristics cannot be accounted for by current deformation processes. Analysis of the creep data along with consideration of available information leads to the suggestion that the creep behavior of nc-Ni arises from a deformation process that is based on the concept of dislocation-accommodated boundary sliding. By quantitatively developing this concept, a rate-controlling deformation process is formulated. It is shown that the predictions of this process, are in good agreement with experimental results and trends.

Assessment of creep behaviour of the die-cast cylinder-head alloy AlSi6Cu4-T6

Abstract Mechanical properties of a T6-AlSi6Cu4 cylinder-head alloy are investigated at 4 stress levels up to 545 K through tensile and compressive test to assess creep damage. The effect of over-ageing for 200 h at comparable temperatures was studied through RT hardness tests. Creep behaviour was characterised by a minimum creep rate and an extended steady state was not observed. No threshold stresses were apparent from minimum creep rate vs. stress plots at any of the temperatures studied here. Apparent stress exponents were both temperature and stress dependent and the apparent creep activation energy was 2–3 times the lattice selfdiffusion activation energy for aluminium. The Norton plot was best described by an exponential relation. The total strain to failure exhibited a maximum for a particular applied stress and decreases for both higher and lower applied stress. Creep life is well described by the Monkman– Grant relation at the highest temperature, but exhibits deviations from the idealised behaviour at the lower test temperatures.

Internal stress and activation energy of creep

The stress σ acting on a moving dislocation can be divided into two components: the effective stress σ * that opposes the velocity dependent lattice friction and the internal stress σ i that overcomes long-range stress fields generated by neighbouring dislocations or other obstacles. The activation energy of plastic deformation has to be determined for the same state of structure, i.e. at an identical level of the internal stress, and at the same applied stress Q i * = − k ∂ ln ⁡ ε ˙ ∂ ( 1 / T ) σ i , σ , where ε ˙ is the creep rate, T the absolute temperature and k is the Boltzmann's constant. A new method for determining this quantity is proposed. It is shown that at zero applied stress this energy is identical to the apparent activation energy of creep determined at constant applied stress.

Hot hardness and indentation creep study on Al–5% Mg alloy matrix–B 4C particle reinforced composites

Al–5% Mg matrix composites reinforced with 10 and 20% B 4C were prepared by stir casting method. Microhardness of unreinforced alloy and composites were measured using a Nikon hot hardness tester. It was observed that hardness values decrease gradually with increase in temperature. Transition temperature increases marginally from 262 to 274 °C due to addition of 20% B 4C. Stress exponent for all the materials varies from 4.3 to 4.5 at temperatures in the range 310 and 370 °C. Apparent activation energy for creep increases with increase in B 4C content. At temperatures below transition temperature, deformation is attributed to the increased interatomic spacing due to increase in temperature and the unpinning of attractive junction between glide and forest dislocations. At temperatures above the transition temperature, deformation is diffusion assisted phenomenon such as dislocation glide and dislocation climb.

Effect of growth rate on microstructure and mechanical properties of directionally solidified multiphase intermetallic Ni–Al–Cr–Ta–Mo–Zr alloy

The effect of growth rate on microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic alloy with the chemical composition Ni–21.9Al–8.1Cr–4.2Ta–0.9Mo–0.3Zr (at.%) was studied. The DS ingots were prepared at constant growth rates V ranging from 5.56 × 10 −6 to 1.18 × 10 −4 ms −1 and at a constant temperature gradient at the solid–liquid interface of G L = 12 × 10 3 K m −1. Increasing growth rate increases volume fraction of dendrites and decreases primary dendritic arm spacing, mean diameter of α-Cr (Cr-based solid solution) and γ′(Ni 3Al) precipitates within the dendrites. Room-temperature compressive yield strength, ultimate compressive strength, hardness and microhardness of dendrites increase with increasing growth rate. All room-temperature tensile specimens show brittle fracture without yielding. The brittle-to-ductile transition temperature for tensile specimens is determined to be about 1148 K. Minimum creep rate is found to depend strongly on the applied stress and temperature according to the power law with a stress exponent of n = 7 and apparent activation energy for creep of Q a = 401 kJ/mol.

Heat treatment effects on creep behavior of polycrystalline SiC fibers

Polycrystalline SiC fibers are being considered as potential reinforcement for ceramic matrix composites (CMCs). For these fibers with fine grain size, basic issues arise concerning thermo-mechanical properties and microstructural instability during fabrication and application of CMCs. To examine these issues, three commercially available SiC fibers were heat treated at elevated temperatures for 1 h in an Ar atmosphere. The creep resistance of SiC fibers was evaluated by the bend stress relaxation (BSR) method, and it was found that the creep resistance could be improved by heat treatment. Combining the results of the BSR tests with the results of X-ray diffraction examinations indicated that the creep resistance of SiC fibers is mainly related to the β-SiC grain size and the composition at or adjacent to the grain boundaries. Also, the apparent activation energy of creep for both Hi-Nicalon™ and Hi-Nicalon™ type S fiber increased with increasing heat treatment temperature. In the case of Tyranno™-SA fiber, the apparent activation energy of creep did not show an obvious dependence on the heat treatment temperature.

Creep behavior of extruded Al–6Mg–1Sc–1Zr–10 vol.% SiC p composite

Creep tests were performed on a fine-grained Al–6Mg–1Sc–1Zr–10 vol.% SiC particulate composite in the temperature range of 423–533 K. The composite exhibited a high value of apparent stress exponent and apparent activation energy for creep at low temperatures. The flow behavior of this composite was characterized by the presence of a threshold stress. Incorporation of threshold stress into analysis reduces the high values of apparent stress exponent and activation energy to those anticipated from the creep of solid solution alloys. The result showed increased creep resistance in the composite compared to the unreinforced alloy.

On high creep activation energy for cast NiAl–9Mo eutectic alloy

The stress change-strain transient creep experiment was conducted and the side surface of creep-rupture samples was examined in order to study the apparent creep activation energy for NiAl–9Mo alloy. Most of the recoverable strain is established and the strain rate decreases sharply during the initial deformation after unloading the major stress. The magnitude of the recoverable deformation increases only slightly and the strain rate changes slowly during the following course. The relationship between recoverable strain rate and relax time during the relative steady-state stage is fitted in the equation ɛ˙ r = At m ( A=1.294 E−4, m=−0.450). The backflow process is temperature dependent and is controlled by the flow behavior of the matrix material. The apparent creep activation energy under constant stress is a combination of the activation energy for creep flow of the matrix and that for the damage process at the interface.

Threshold Stress for Super Plastic Flow in Spatially Extended Crystalline Systems

In general, the mechanical behavior of superplastic spatially extended crystalline systems (SP-SECS) is characterized by a sigmoidal relationship between the applied stress and the steady state strain rate. The sigmoidal curve is defined by three regions: low stress - region I, intermediate stress or superplastic - region II and high stress - region III. The region I is known as the region where the threshold stress exists, but there are controversies on their existence. In this way, some experimental results reported in the past are analyzed. Earlier investigations have reported the apparent activation energy for creep in SP-SECS as a function of the applied stress, where it is exhibited a marked dependence of impurities or precipitates concentration. In addition, recent experimental evidences have revealed that the unified interpretation of creep, plasticity and superplasticity is the deductive rule [1]. In order to describe the interaction between precipitates at grain boundaries and dislocations during deformation processes a phenomenological expression for the threshold stress at the Grain Boundary in SP-SECS is described in this work.

Analysis of the creep response of an Al–17Si–4Cu–0.55Mg alloy

The creep response of a hypereutectic Al–Si alloy containing 4% Cu and 0.55% Mg was investigated between 553 and 653 K. Minimum creep rate as a function of applied stress by using the power-law equation suggested the existence of two different regimes: a low-stress regime characterised by a stress exponent close to 4–5, and a high-stress regime with a higher stress exponent. Although the magnitude of the stress exponent in the low-stress regime was equivalent to that observed in pure Al, the apparent activation energy for creep was higher ( Q = 210 kJ/mol) than the activation energy for self-diffusion in Al ( Q d = 143 kJ/mol). The microstructural analysis suggested a similarity between this alloy and Al-matrix discontinuously reinforced composites. This approach, based on threshold-stress concept, permitted to rationalise both the magnitude of the stress exponent and the apparent activation energy for creep. In addition, the substantial similarity between hypereutectic Al–Si alloys and Al-based composites with similar composition was confirmed.

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.

Analysis of the creep response of an Al–17Si–4Cu–0.55Mg alloy

Abstract The creep response of a hypereutectic Al–Si alloy containing 4% Cu and 0.55% Mg was investigated between 553 and 653 K. Minimum creep rate as a function of applied stress by using the power-law equation suggested the existence of two different regimes: a low-stress regime characterised by a stress exponent close to 4–5, and a high-stress regime with a higher stress exponent. Although the magnitude of the stress exponent in the low-stress regime was equivalent to that observed in pure Al, the apparent activation energy for creep was higher (Q = 210 kJ/mol) than the activation energy for self-diffusion in Al (Qd = 143 kJ/mol). The microstructural analysis suggested a similarity between this alloy and Al-matrix discontinuously reinforced composites. This approach, based on threshold-stress concept, permitted to rationalise both the magnitude of the stress exponent and the apparent activation energy for creep. In addition, the substantial similarity between hypereutectic Al–Si alloys and Al-based composites with similar composition was confirmed.

Study on creep behavior of Ti–V–Cr burn resistant alloys

The creep behavior of two Ti–V–Cr burn resistant alloys is studied. During creep, some phases such as Ti 5Si 3, α and β′ phases precipitate from β matrix. The Ti 5Si 3 phase distributes discontinuously along grain boundary and in the grain in Ti40 alloy, which has pinning effects on dislocation and obstructs the dislocations from moving. Rod-like α phase are precipitated in AlloyC alloy, which reduces the creep resistance. Ti40 alloy shows better creep property than AlloyC alloy. The apparent creep activation energy of Ti40 alloy is calculated to be 94.0 kJ/mol. The creep behavior of Ti–25V–15Cr–0.2Si alloy in the range of 460–540 °C is controlled by double mechanisms of diffusion and dislocation.

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

Abstract The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The α 2 (Ti 3 Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/α 2 interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.

Creep in ODS copper reinforced with tungsten short fibres – an ODS copper-matrix/tungsten-short-fibre composite

Abstract Creep in copper dispersion-strengthened with fine alumina particles and reinforced with tungsten short fibres is investigated at three temperatures ranging from 923 to 1023 K. It is shown that creep is associated with a true threshold stress decreasing with increasing temperature more strongly than the shear modulus of copper. The true threshold stress in the composite is higher than that in the composite matrix by a factor λ, by which the flow stress in the matrix is reduced due to the load transfer. The minimum creep strain rate is found to be lattice diffusion controlled and to depend on the fifth power of applied stress. The values of the apparent activation energy of creep are higher than that of the activation enthalpy of lattice self-diffusion in the composite matrix, which is fully explained in terms of the temperature dependence of the true threshold stress. The values of the apparent stress exponents are higher than the value of the true stress exponent since the true threshold stress i...

Effect of Zn-addition and structural transformation on the creep behaviour of Pb–10 wt.% Sn alloy

Pb–10 wt.% Sn alloy is one of the materials considered for the spheres used in Ball Grid Array (BGA) of the most popular packaging technologies. The tensile creep behaviour of Pb–10 wt.% Sn and Pb–10 wt.% Sn–1.5 wt.% Zn alloys was investigated at different working temperatures ranging from 373 to 443 K and under different constant applied stresses. Stress exponent n and activation energy Q c were determined to clarify the deformation mechanism. X-ray analysis and differential thermal analysis (DTA) were studied to evaluate the structural changes during transformations. Attention has been paid to the role of zinc on the creep behaviour of Pb–10 wt.% Sn, the influence of Zn on the melting temperature ( T m) and apparent activation energy of creep. Microstructure examination of the deformed samples below and above the transition temperatures was performed.

Effect of Zn-addition and structural transformation on the creep behaviour of Pb–10wt.% Sn alloy

Pb–10 wt.% Sn alloy is one of the materials considered for the spheres used in Ball Grid Array (BGA) of the most popular packaging technologies. The tensile creep behaviour of Pb–10 wt.% Sn and Pb–10 wt.% Sn–1.5 wt.% Zn alloys was investigated at different working temperatures ranging from 373 to 443 K and under different constant applied stresses. Stress exponent n and activation energy Q c were determined to clarify the deformation mechanism. X-ray analysis and differential thermal analysis (DTA) were studied to evaluate the structural changes during transformations. Attention has been paid to the role of zinc on the creep behaviour of Pb–10 wt.% Sn, the influence of Zn on the melting temperature ( T m ) and apparent activation energy of creep. Microstructure examination of the deformed samples below and above the transition temperatures was performed.

Creep Characteristic of NiAl-9Mo Eutectic Alloy

The creep behavior of NiAl-9Mo eutectic alloy has been studied in an effort to understand the characteristic that the alloy exhibits of the low stress exponent of 4.75 and the high apparent activation energy of 410 kJ/molduring the steady-state creep of under the test conditions. The material exhibits threshold behavior with a threshold stress of 17.9 MPa. TEN observation reveals that the creep deformation is mainly governed by dislocation climb in NiAl matrix phase. Large recoverable strain is observed after unloading under the transient creep and increases with the increase of stress or temperature. It exhibits an activation energy 232 kJ/mol, suggesting the processes is controlled by the flow behavior of the matrix phase. The apparent creep activation energy under constant stress is a combination of the creep flow activation energy of the matrix and the activation energy for the damage process at the phase interface.