Activation Energy For Creep Research Articles

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.

Modeling the creep behavior of 2024-T3 Al alloy

Exposure of 2024-T3 Al alloy to an elastic loading, either for “creep age forming” and other manufacturing processes at relatively high temperature, may lead to the lasting creep deformation. The high-temperature creep behavior of 2024-T3 Al alloy was studied by the constant-stress uniaxial tensile creep experiments under the temperatures of 423, 448 and 473K. Constitutive models, based on the θ projection method, are established to describe the high-temperature creep behavior of 2024-T3 Al alloy. The material parameters of the established constitutive models are associated with the applied creep stress and temperature. The creep strains predicted by the proposed models well agree with experimental results, which confirm that the established creep constitutive models can give an accurate and precise estimate of the high-temperature creep behavior for 2024-T3 Al alloy. The evaluated power–law stress exponent n=3.405 and the activation energy for secondary creep Q=85.390kJmol−1 indicate that the creep processes of 2024-T3 Al alloy are controlled by the dislocation viscous glide mechanism.

Effect of zinc addition on microstructure and mechanical properties of Mg–7Y–3Sm–0.5Zr alloy

The effect of zinc addition on the microstructure and mechanical properties of Mg–7Y–3Sm–0.5Zr casting alloy was investigated. Creep test was carried out at 200–300 °C under 50-120 MPa. Within the limits of the creep test conditions used in this study, the creep activation energy of the investigated alloys was in the range of 156-221 kJ/mol. The microstructure evolution during creep was characterized by optical metallography, SEM and TEM. The results show that the creep life of the alloy is increased from 52.2 to 152.8 h at 300 °C under 50 MPa by only 1% addition of Zn, though both the alloys have similar creep behaviors below 250 °C, which suggests that the thermally stable compound and lamellar structure can improve the high temperature creep resistance of the alloy with the addition of Zn.

DC and AC susceptibility study of sol–gel synthesized Bi2Sr2CaCu2O8+δ superconductor

We study DC susceptibility along with amplitude and frequency dependence of AC susceptibility of sol–gel synthesized polycrystalline samples of Bi2Sr2CaCu2O8+δ (Bi-2212) sintered at different temperatures. In particular, it is demonstrated that susceptibility techniques are an effective tool to characterize granular characteristics of high temperature superconductors. The results are discussed in the framework of Bean's critical state model whence the field and temperature dependence of critical current density is determined. Flux pinning force density is calculated and possibility of the pinning mechanisms prevalent in type II superconductors are investigated. Flux creep activation energy is determined in the light of vortex dynamics exhibited by frequency dependence of AC susceptibility. Since polycrystalline samples are granular in nature, we calculated grain volume fraction and separated the contribution of grain and matrix susceptibility from total measured AC susceptibility. We establish that increase in the sintering temperature not only changes the grain morphology but affects the superconducting properties significantly, validating the impact of grain boundaries on superconducting performance of studied Bi-2212 superconductor.

Microstructure and creep characteristics of dissimilar T91/TP316H martensitic/austenitic welded joint with Ni-based weld metal

This paper deals with characterization of microstructure and creep behavior of dissimilar weldment between the tempered martensitic steel T91 and the non-stabilized austenitic steel TP316H with Ni-based weld metal (Ni WM). Microstructure analyses were performed using light microscopy, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy. The martensitic part of the welded joint exhibited a wide heat-affected zone (HAZ) with typical microstructural gradient from its coarse-grained to the fine-grained/intercritical region. In contrast, the HAZ of austenitic steel was limited to only a narrow region with coarsened polygonal grains. The microstructure of Ni WM was found to be very heterogeneous with respect to the size, morphology and distribution of grain boundaries and MC-type precipitates as a result of strong weld metal dilution effects and fast non-equilibrium solidification. Cross-weld creep tests were carried out in a temperature range from 600 to 650 °C at applied stresses from 60 to 140 MPa. The obtained values of apparent stress exponents and creep activation energies indicate thermally activated dislocation glide to be the governing creep deformation mechanism within the range of used testing conditions. The creep samples ruptured in the T91 intercritical HAZ region by the “type IV cracking” failure mode and the creep fracture mechanism was identified to be the intergranular dimple tearing by microvoid coalescence at grain boundaries. The TEM observations revealed pronounced microstructural differences between the critical HAZ region and the T91 base material before as well as after the creep exposure. ► Phase transformations affect the microstructures of T91 and TP316H HAZ regions. ► High weld metal dilution results in heterogeneous microstructure with MC carbides. ► Creep behavior of the studied weldment is controlled by its martensitic T91 part. ► The lowest hardness of T91 ICHAZ region corresponds with its lowest creep strength.

Transient creep in titanium alloys: Effect of stress, temperature and trace element concentration

The influence of stress, temperature and trace element concentration viz. Fe, Ni on the transient creep behavior of Ti-834 alloy, i.e. Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.5Mo–0.35Si–0.06C (in wt%) was studied. Two Ti-834 alloys, DMR Ti834 and TIMETAL 834 with higher and lower contents of (Fe, Ni) respectively were chosen for this study. Both alloys were given controlled heat treatments to develop microstructures containing 15vol% primary α in a matrix of transformed β. The creep behavior of both alloys was studied in the temperature range 873–1023K and stress range 80–350MPa. Subsequent analysis showed that the limiting transient creep strain reduced with increasing stress and temperature. Also it was observed that DMR Ti834 yields a smaller transient creep strain value in comparison to TIMETAL 834. The creep activation energy (Q) in the transient creep region was also determined and again found to be smaller in DMR Ti834 (Q=284kJ/mol) in comparison to TIMETAL 834 (Q=399kJ/mol). The difference in transient creep strain and Q values was rationalized vis-a-vis the (Fe, Ni) content. The stress and temperature dependence of the transient creep strain and transient time were studied to understand the rate controlling mechanism of transient creep.

Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

Abstract Constrained sintering kinetics of 8 mol% Y 2 O 3 /92 mol% ZrO 2 (8YSZ) films approximately 10–15 μm thick screen-printed on dense YSZ substrates, and the resulting stress induced in the films, were measured in the temperature range 1100–1350 °C. The results are compared with those reported earlier for 3YSZ films. Both materials behave similarly, although there are differences in detail. The constrained densification rate was greatly retarded compared with the unconstrained densification rate due to the effect of the constraint on the developing anisotropic microstructure (3YSZ) and, in the case of 8YSZ, considerable grain growth. The stress generated during constrained sintering was typically a few MPa. The apparent activation energies for free sintering, constrained sintering, creep and grain growth are found to cover a wide range (135–670 kJ mol −1 ) despite all probably being mainly controlled by grain boundary cation diffusion.

Analysis of interactions between damage and basic creep of HPC and HPFRC heated between 20 and 80 °C

The present study concerns the uniaxial compressive creep of High Performance Concrete (HPC) at moderate temperatures, 20–80 °C. The study was conducted on four formulations of HPC including two fibrous concretes envisioned for future storage structures of Intermediate Level Long-Life Nuclear Wastes. These wastes are exothermic and lead to maximal temperatures in the field ranging from 50 to 70 °C (Andra, Referentiel des materiaux de stockage de dechets a haute activite et a vie longue, 2005). Here, we investigate the basic creep under uniaxial compression at 50 and 80 °C and compare it to that obtained on the same HPC at 20 °C. The objective of this research is to contribute to a better understanding of the phenomenon of interaction between damage and basic creep of HPC at moderate temperature, especially with a view to its integration in Thermo-Hydro-Mechanical models dealing with the design of special structures (massive structures, specific serviceability conditions in nuclear or hydroelectric power plants, etc.). This test campaign allowed us to assess the effect of temperature on the magnitude of basic creep of HPC, and also the impact of various temperature and mechanical loading conditions on the Young’s modulus of HPC. Heating to 80 °C damages HPC (instantaneous Young’s modulus decrease) and thereby increases the creep capacity, showing a relation between damage and creep amplitude. Moreover, this study gives global activation energy of basic creep of HPC that should be useful for practitioners dealing with concrete structures sensitive to delayed strains and subjected to moderate temperature.

Creep of AZ31 Mg alloy: A comparison of impression and tensile behavior

Creep behavior of AZ31 magnesium alloy was investigated in temperature range of 423–498K by the localized impression creep and the conventional tensile creep testing methods, in order to evaluate the correspondence of the creep results obtained by these two testing techniques. Impression creep tests were conducted under punch stresses in the range of 125–600MPa, while tensile creep tests were performed under tensile stresses in the range 35–140MPa. According to the obtained results, the creep behavior could be divided into two stress regimes with different stress exponents and activation energy values. Based on the steady-state power-law creep relationship, the average stress exponents of about 3 and 6 were, respectively, obtained at the low- and high-stress regimes in both impression and tensile creep tests. The respective average impression creep activation energies of 96.9 and 139.2kJ/mol, at the low- and high-stress regimes, were close to 93.8 and 126.6kJ/mol determined in the tensile creep tests. Based on the obtained stress exponents and activation energies, it is suggested that the dominant creep mechanism is pipe-diffusion-controlled dislocation viscous glide in the low-stress regime, and dislocation climb in the high-stress regime. The achievement of similar creep characteristics in the employed tests implies that the localized impression creep test is capable of acquiring reliable information on the creep behavior of the present wrought Mg alloy.

Creep and fracture behavior of as-cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%)

The tensile-creep and creep–fracture behavior of as-cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%) (WGZ1152) was investigated at temperatures between 523 and 598 K (0.58–0.66Tm) and stresses between 30 and 140 MPa. The creep stress exponent was close to five, suggesting that dislocation creep was the dominant creep mechanism. The activation energy for creep (233 ± 18 kJ/mol) was higher than that for self-diffusion in magnesium, and was believed to be associated with cross-slip, which was the dominant thermally-aided creep mechanism. This was consistent with the surface observations, which suggested non-basal slip and cross-slip were active at 573 K. The minimum creep rate and fracture time values fit the original and modified Monkman–Grant models. In situ creep experiments highlighted the intergranular cracking evolution. The creep properties and behavior were compared with those for other high-temperature creep-resistant Mg alloys such as WE54-T6 and HZ32-T5.

Mechanical Behavior of Epoxy-Graphene Platelets Nanocomposites

Various aspects of the mechanical behavior of epoxy-based nanocomposites with graphene platelets (GPL) as additives are discussed in this article. The monotonic loading response indicates that at elevated temperatures, the elastic modulus and the yield stress are significantly improved in the composite as compared to neat epoxy. The activation energy for creep is smaller in neat epoxy, which indicates that the composite creeps less, especially at elevated temperatures and higher stresses. The composites also exhibit larger fracture toughness. When subjected to cyclic loading, fatigue crack growth rate is smaller in the composite relative to neat epoxy. This reduction is important by at least an order of magnitude at all stress intensity factor amplitudes. Optimal property improvements in the monotonic, cyclic, and fracture behaviors are obtained for very low filling fraction of approximately 0.1 wt. %. Similar differences in the mechanical behavior are observed when the composite is probed on the local scale by nanoindentation.

Creep and precipitation behaviors of AL6XN austenitic steel at elevated temperatures

Creep behaviors of the solution-treated AL6XN austenitic stainless steel have been investigated at 873–1023K and 120–260MPa. The results showed that the creep stress exponent and activation energy of the AL6XN steel are 5 and 395.4kJ/mol, respectively in the power–law breakdown regime. TEM observations revealed that dislocations distributed homogenously in grains. The creep deformation mechanism is mainly attributed to viscous dislocation glide. Precipitates in the steel after creep deformation were additionally analyzed by TEM, and the results showed that there are four different types of precipitates, such as M23C6, M6C, σ phase and Laves phase. The M23C6 carbides were observed at grain boundaries in the steel after creep at 873K. The M6C, σ phase and Laves phase precipitates were found when the creep temperature increases to 923–1023K. Although the AL6XN steel exhibited low steady state creep rates, a high volume fraction of brittle precipitates of σ and Laves phases reduced the creep lifetime of the steel at elevated temperatures.

Creep properties of different γ′-strengthened Co-base superalloys

Abstract The influence of various alloying elements on the creep properties of polycrystalline Co-base superalloys hardened by a ternary L1 2 compound, Co 3 (Al,W) (γ′-phase), was investigated. A Ti containing quaternary alloy shows creep strength similar to Ni-base superalloys IN100 and IN713C at 850 °C and strongly superior to conventional Co-base superalloys as Haynes188. The activation energy for creep between 850 and 950 °C is similar to the polycrystalline Ni-base superalloy IN100 in the same temperature range. Strengthening of the grain boundaries by third phase precipitates was found to be crucial for the mechanical properties. This can be achieved either by precipitation of borides or by additional intermetallic phases which precipitate due to oversaturation. During compressive creep at 850 °C only a slight tendency for directional coarsening occurs, while at 950 °C distinct γ/γ′-rafts perpendicular to the external compressive stress axis are formed which indicate a positive lattice misfit even at 950 °C.

Creep and Fracture Behavior of Peak-Aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct)

The tensile-creep and creep-fracture behavior of peak-aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct) (WGZ1152) was investigated at temperatures between 523 K (250 °C) to 598 K (325 °C) (0.58 to 0.66Tm) and stresses between 30 MPa to 140 MPa. The minimum creep rate of the alloy was almost two orders of magnitude lower than that for WE54-T6 and was similar to that for HZ32-T5. The creep behavior exhibited an extended tertiary creep stage, which was believed to be associated with precipitate coarsening. The creep stress exponent value was 4.5, suggesting that dislocation creep was the rate-controlling mechanism during secondary creep. At T = 573 K (300 °C), basal slip was the dominant deformation mode. The activation energy for creep (Qavg = 221 ± 20 kJ/mol) was higher than that for self-diffusion in magnesium and was believed to be associated with the presence of second-phase particles as well as the activation of nonbasal slip and cross slip. This finding was consistent with the slip-trace analysis and surface deformation observations, which revealed that the nonbasal slip was active. The minimum creep rate and time-to-fracture followed the original and modified Monkman-Grant relationships. The microcracks and cavities nucleated preferentially at grain boundaries and at the interface between the matrix phase and the second phase. In-situ creep experiments highlighted the intergranular cracking evolution.

Effect of solute segregation on thermal creep in dilute nanocyrstalline Cu alloys

The effect of solute segregation on thermal creep in dilute nanocrystalline alloys (Cu–Nb, Cu–Fe, Cu–Zr) was studied at elevated temperatures using molecular dynamics simulations. A combined Monte-Carlo and molecular dynamics simulation technique was first used to equilibrate the distribution of segregating solutes. Then the creep rates of the diluted Cu samples were measured as functions of temperature, composition, load and accumulated strain. In Cu–Nb samples, the creep rates were observed to increase initially with strain, but then saturate at a value close to that obtained for alloys prepared by randomly locating the solute in the grain boundaries. This behavior is attributed to an increase in grain boundary volume and energy with added chemical disorder. At high temperatures, the apparent activation energy for creep was anomalously high, 3 eV, but only 0.3 eV at lower temperatures. This temperature dependence is found to correlate with atomic mobilities in bulk Cu–Nb glasses. Calculations of creep in nanocrystalline Cu alloys containing other solutes, Fe and Zr, show that the suppression of creep rate scales with their atomic volumes when dissolved in Cu.

Microstructure and creep characteristics of Zn–3Cu–xAl ultra high-temperature lead-free solders

This study investigates the microstructure and impression creep behavior of the ultra high-temperature Zn–3Cu–4Al (ZCA34), Zn–3Cu–5Al (ZCA35), and Zn–3Cu–6Al (ZCA36) solder alloys under constant punch stress in the range of 70–800MPa and at temperatures in the range of 345–495K. The results showed that for all loads and temperatures, increasing Al content of the alloys results in higher creep rates, and thus lower creep resistances. This is attributed to the partial spheroidisation of the lamellar eutectic structure, and the four phase transformation α+ε→T′+η. The former may ease grain boundary sliding at high temperatures and the latter results in a lesser amount of the more creep resistant ε-phase. The stress exponents of 5.0–7.9 and activation energy values of 52.5–100.3kJmol−1, are indicative of a dislocation climb controlled creep mechanism. The observed increasing trend of creep activation energy with temperature means that two parallel mechanisms of lattice-diffusion-controlled and pipe-diffusion-controlled dislocation climb are competing. Dislocation climb controlled by pipe diffusion is the rate controlling mechanism at low temperatures, whereas dislocation climb controlled by lattice diffusion is the dominant creep mechanism at high temperatures.

On the effect of grain boundary segregation on creep and creep rupture

The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu–0.008wt.% Bi and Cu–0.92wt.% Sb at stresses ranging from 10 to 20MPa and temperatures between 773 and 873K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu–Bi) and high apparent activation energies for creep (Cu–Bi and Cu–Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures.

Interface Controlled Diffusional Creep of Cu + 2.8 at.% Co Solid Solution

High-temperature creep experiments were performed on a Cu-2.8 ат.% Co solid solution. Cylindrical foils of 18 micrometers thickness were used for this purpose. Creep tests were performed in a hydrogen atmosphere in the temperature range of about from 1233 K to 1343 K and at stresses lower than 0.25 MPa. For comparison, a foil of pure copper and Cu-20 at.% Ni solid solution were investigated on high temperature creep. Measurements on the Cu foil showed classical diffusional creep behavior. The activation energy of creep was defined and turned out to be equal 203 kJ/mol, which is close to the activation energy of bulk self-diffusion of copper. There was a significant increase in activation energy for the Cu-20 at.% Ni solid solution. Its activation energy was about 273 kJ/mol. The creep behavior of Cu-Co solid solution was more complicated. There were two stages of diffusional creep at different temperatures. The extremely large activation energy (about 480 kJ/mol) was determined at relatively low temperature and a small activation energy (about 105 kJ/mol) was found at high temperatures. The creep rate of Cu-Co solid solution was lower than that of pure copper at all temperatures. In addition, the free surface tension of Cu-2.8 ат.% Co was measured at different temperatures from 1242 K to 1352 K. The surface tension increases in this temperature range from 1.6 N/m to 1.75 N/m. There were no features on the temperature dependence of the surface tension.

Kinetics of nonisothermal hot pressing of boron carbide powder and its mixtures with alumina and additions of metallic aluminum

The paper proposes a method to determine the kinetic parameters of temperature-controlled hot pressing from the current temperature and height of specimens. This method substantially reduces the scope of experimental work in comparison with the conventional method for studying isothermal hot pressing. The kinetics of the densification of boron carbide powder and its composites with 20, 50, and 80 wt.% Al2O3 and 2 wt.% Al is studied during nonisothermal hot pressing. It is experimentally established that the kinetics of densification is controlled by the creep mechanisms in the matrix that forms a porous body. The creep activation energy for the matrix is estimated to be 8.1 eV for boron carbide, 7.6 eV for B4C + 20% Al2O3 + 2% Al, and 6.6 eV for B4C + 80% Al2O3 + 2% Al. The densification of the composite with 50 wt.% Al2O3 is controlled by the weekly temperature-dependent viscous flow of the matrix, for which the estimated activation energy is 1.7 eV. At higher temperatures, the viscous flow of the matrix is controlled by nonlinear creep the activation energy of 4.6 eV, which may correspond to superplasticity. The structure, phase constitution, and some mechanical properties of the composites are studied.

The microstructure and impression creep behavior of cast AZ80 magnesium alloy with yttrium additions

The effects of 0.5, 1.0 and 2.0wt.% Y additions on the microstructure and creep behavior of the as-cast AZ80 alloy were investigated by impression tests. The tests were performed at temperatures in the range 423–523K, under punching stress in the range 150–650MPa. At low temperatures up to 473K, the AZ80+0.5Y alloy had the highest creep resistance among all materials tested, whereas with increasing temperature from 473K to 523K, the AZ80+1.0Y alloy had a better performance. This can be attributed to the fact that at low temperatures the presences of β-Mg17Al12 and Al2Y phases together with solid solution hardening effects of Al in the Mg matrix strengthen the AZ80+0.5Y alloy. At higher temperatures, AZ80+1.0Y with a higher volume fraction of the more thermally stable Al2Y and lower amounts of the less stable β-Mg17Al12 exhibits better creep behavior. The stress exponents and activation energies were almost the same for all alloy systems studied, 6.0–8.8 and 90–119kJ/mol, respectively. The observed decreasing trend of creep-activation energy with stress suggests that two parallel mechanisms of lattice and pipe-diffusion-controlled dislocation climb are competing. Climb of dislocations with an additional particle strengthening effect controlled by dislocation pipe diffusion is dominant at high stresses, whereas climb of dislocations is the controlling mechanism at low stresses.