Climb-controlled Dislocation Creep Research Articles

The dynamic and static mechanical characteristics of Sn-7Zn-based solder alloy modified with microalloying of In, Fe and Co elements

Tensile creep experiments and pulse-echo overlap (PEO) method were utilized to analyze the impact of small additions of 2.5%wt In, Fe and Co (0.1%wt for each) on the creep resistance and elastic properties of a cast Sn-7Zn alloy. The In-modified alloy displays noticeably enhanced creep resistance and increased the fracture time (~ 2.7 times) due to precipitation strengthening and formation of γ-InSn4 particles. The In-modified alloy exhibits an as-solidified grain structure, which is finer than the Sn-7Zn alloy, and predicted to enhance deformation resistance by lattice self-diffusion creep. Nonetheless, the creep resistance of Sn-7Zn alloy deteriorates after Fe and Co addition, highlighting the excellent coarsening of the new γ-Zn21Co5, γ-Co2Sn2Zn, and α-Fe0.92Sn0.08 precipitates. The obtained results implied that the creep strain rate follows the Garofalo hyperbolic sine equation, and the computed creep stress exponent is consistent with a climb-controlled dislocation creep. Consequently, for all PEO tests, the values of resulting Young's modulus (E) (66.7 GPa) and shear modulus (G) (20.5 GPa) of In-modified Sn-7Zn alloy were superior to those of the conventional plain Sn-7Zn alloy and Fe and Co modified alloy in its cast condition.

Investigation on multi-temperature short-term stress and creep relaxation of Sn-Ag-Cu-In solder alloys under the effect of rotating magnetic field

In this study, using one single specimen and viscous strain εv of ≤0.0001, the short-term stress and creep relaxation tests are developed and performed on SAC(307)-2In alloy before and after applying rotating magnetic field (RMF), to predict the creep mechanisms at different temperatures and strain rates. The global strain rate εp is shown to follow the same power-law with stress exponents of 4.8–8.1, while the activation energies Q were 55.7–51.2 kJ/mol, indicating the climb-controlled dislocation creep enhanced by pipe diffusion mechanism. Because of the grain refinement after applying RMF, significant enhancement has been detected in viscous strain εv, elastic strain εe, delayed strain εd, young modulus E, stress exponents n and activation energy Q, whereas the thermal properties are rather sensitive to it. Moreover, significant drop in stress-time relationship rather than saturation stress was observed at different strain rates after applying RMF, indicating the viscoelastic response of creep, which has good predictions for next-generation electronic devices and engineering alloy design due to the contribution of εv/εe ratio.

Effect of Cu on the Creep Behavior of Cast Al-15Si-0.5Mg Alloy

In the present article, the effect of Cu on the microstructure and creep properties of Al-15Si-0.5Mg cast alloy was studied by using optical microscopy, scanning electron microscopy, energy dispersion spectrometry and the impression creep test. The microstructure is changed considerably in the presence of copper. The number of the Cu-rich phases increased noticeably with rising Cu content. Formation of a dendritic structure of α(Al) and modification of the eutectic Si were the other effects of increasing the Cu amount. The results showed that creep properties of the alloy increase considerably with the Cu addition. Calculating the stress exponent (n) and creep activation energy (Q) revealed that pipe diffusion climb-controlled creep is the dominant creep mechanism of the main alloy. Although Cu had no effect on the dominant creep mechanism at the lower stress, it was changed to lattice diffusion climb-controlled dislocation creep with increasing stress.

Tensile Properties at Room and Elevated Temperatures and High-Cycle Fatigue Properties of Extruded AZ80 and TAZ711 Alloys

The microstructure, tensile properties at room and elevated temperatures, and high-cycle fatigue behavior of an extruded Mg-7Sn-1Al-1Zn (TAZ711) alloy were investigated in comparison with those of an Mg-8Al-0.5Zn (AZ80) alloy extruded under the same conditions. The extruded AZ80 alloy exhibited a fully recrystallized structure, whereas the extruded TAZ711 alloy had a bimodal structure consisting of coarse nonrecrystallized grains and fine recrystallized grains. The TAZ711 alloy had smaller recrystallized grains and a larger amount of fine dynamic precipitates than the AZ80 alloy. Both alloys showed the typical texture of extruded Mg alloys in which the basal planes of the grains are arranged parallel to the extrusion direction, but the texture intensity of the TAZ711 alloy was greater than that of the AZ80 alloy. The yield strength of the TAZ711 alloy was higher than that of the AZ80 alloy at room temperature because of the finer grain size, stronger texture intensity, and greater amount of fine precipitates in the TAZ711 alloy. However, at an elevated temperature of 150 °C, the yield strength of the TAZ711 alloy was lower than that of the AZ80 alloy because the climb-controlled dislocation creep, grain boundary sliding, and diffusional creep behaviors were more easily generated in the TAZ711 alloy owing to its finer grain structure. As the applied tensile strain rate was reduced, the decrease in the strength of the TAZ711 alloy was greater than that of the AZ80 alloy owing to the higher strain rate sensitivity of the former alloy. Although the yield strength of the TAZ711 alloy at room temperature was higher than that of the AZ80 alloy, the TAZ711 alloy exhibited lower high-cycle fatigue resistance, which can be attributed to the occurrence of fatigue cracks at the twins formed in the coarse non-recrystallized grains remaining in the TAZ711 alloy. Key words: magnesium alloy, extrusion, microstructure, tensile properties, fatigue resistance

Theoretical derivation of flow laws for quartz dislocation creep: Comparisons with experimental creep data and extrapolation to natural conditions using water fugacity corrections

AbstractWe theoretically derived flow laws for quartz dislocation creep using climb‐controlled dislocation creep models and compared them with available laboratory data for quartz plastic deformation. We assumed volume diffusion of oxygen‐bearing species along different crystallographic axes (//c, ⊥R, and ⊥c) of α‐quartz and β‐quartz, and pipe diffusion of H2O, to be the elementary processes of dislocation climb. The relationships between differential stress (σ) and strain rate ( ) are written as and for cases controlled by volume and pipe diffusion, respectively, where Dv and Dp are coefficients of diffusion for volume and pipe diffusion. In previous experimental work, there were up to ~1.5 orders of magnitude difference in the water fugacity values in experiments that used either gas‐pressure‐medium or solid‐pressure‐medium deformation apparatus. Therefore, in both the theories and flow laws, we included water fugacity effects as modified preexponential factors and water fugacity terms. Previous experimental data were obtained mainly in the β‐quartz field and are highly consistent with the volume‐diffusion‐controlled dislocation creep models of β‐quartz involving the water fugacity term. The theory also predicts significant effects for the transition of α‐β quartz under crustal conditions. Under experimental pressure and temperature conditions, the flow stress of pipe‐diffusion‐controlled dislocation creep is higher than that for volume‐diffusion‐controlled creep. Extrapolation of the flow laws to natural conditions indicates that the contributions of pipe diffusion may dominate over volume diffusion under low‐temperature conditions of the middle crust around the brittle‐plastic transition zone.

Tensile flow behavior of AZ31 magnesium alloy processed by severe plastic deformation and post-annealing at moderately high temperatures

The uniaxial tensile flow behavior of an AZ31 magnesium alloy was examined after processing by severe plastic deformation (SPD) and short time post-annealing. The SPD process of constrained groove pressing (CGP) was performed to three cycles at temperatures of 503–453K. After CGP, the alloy was further annealed at 473K for a short duration of 3min. By using this procedure, a bimodal fine-grained microstructure with an average grain size of 3.5μm was produced. In the room temperature tensile tests, the yield strength and elongation to failure were improved by 34% and 11%, respectively as compared with the initial material before SPD. The tensile deformation behavior and deformation mechanism of the alloy were further examined at moderately high temperatures from 373 to 523K under initial strain rates from 1×10−3 to 1.0s−1. Examination of the microstructures near the fractured tips indicate that dynamic recrystallization could occur in the fine-grained alloy at a relatively low temperature of 423K and the maximum strain rate sensitivity of 0.14 was measured at 523K. The apparent activation energy tested at 423–473K with initial strain rates of 1×10−3-1.0s−1 was estimated to be 123.6kJmol−1, which is close to the value for lattice self-diffusion of magnesium. The results suggest that climb-controlled dislocation creep is the dominant deformation process associated with lattice diffusion. Despite the relatively fine-grained microstructure, superplastic-like ductility was not observed because of the low thermal stability of the microstructure at elevated temperatures, and is attributed to the high stored strain energy in the microstructure imparted by the CGP. Nevertheless, enhanced ductility was measured at 523K and 1×10−3s−1 where a maximum elongation to failure was 100±2.5%, and dynamic recovery was observed as the main restoration process.

Identification of the threshold stress and true activation energy for characterizing the deformation mechanisms during hot working

For characterizing the origins of threshold stress and the evolution of deformation mechanisms in hot working region, the hyperbolic-sine constitutive analyses were conducted to determine the threshold stress σ0, the true stress exponent nt and the true activation energy Qt in AA7050 Al alloys. The origins of σ0 could be related to the four bypassing mechanisms of dislocations overcoming an obstacle. The origins of σ0 are the local climb at 603–633K, and it may change from the local climb to the detach stress with increasing strains at 663–693K. The nt and Qt after Young's modulus and threshold stress correction can be associated with the specific deformation mechanism. The deformation mechanisms at peak strains may change from the climb-controlled dislocation creep (lattice diffusion) at 603K to the Zn diffusion controlled solute drag creep which is controlled by (sub)grain boundary diffusion at 693K. The nt and Qt decrease with increasing strains may be attributed to the grain boundary self-diffusion accelerated by the fluxes of vacancies for lower temperatures (603–663K) and the atom diffusion controlled solute drag creep accelerated by the vacancies, (sub)grain boundaries, and the motion of high angle grain boundaries for higher temperatures (663–693K), respectively.

Microstructure, mechanical and creep properties of high Ca/Al ratio Mg-Al-Ca alloy

In the present study, the microstructure, mechanical and creep properties of Mg-3.7Al-3.8Ca (AC4a), Mg-4.4Al-4.5Ca (AC4b) and Mg-4.9Al-5Ca (AC5) alloys were investigated. Direct water-cooling semi-continuous ingots were subject to hot extrusion at 673K with the extrusion ratio of 16:1. After being extruding, these Mg-Al-Ca alloys turned into situ composites consisting of ɑ-Mg (solid solution)+Al2Ca. Such alloys showed high strength at high temperatures. The ultimate tensile strength of as-extruded AC4b at 448K and 523K are 200MPa and 112MPa, respectively. The strain rate sensitivity exponent, m, estimated from the slope of the stress-strain curve, exhibits from 0.11 to 0.14, implying that climb-controlled dislocation creep could be a dominant deformation process of AC4a, AC4b and AC5. During the creep test, the creep properties of as-extruded AC5 is better than that of as-extruded AC4a and AC4b, the total elongation of as-extruded AC5 is 0.97% under 60MPa at 448K after 100h and the total elongation increased to 1.3% under 70MPa at 448K after 100h. The stress exponent (n) value of as-extruded AC5 is 6.67, which implies that the dislocation climb is the dominant creep mechanism in the creep test at 448K.

The microstructure and impression creep behavior of cast Mg–4Sn–4Ca alloy

Because of low creep properties of magnesium–aluminum alloys, magnesium–tin alloys have received much attention in applications where high mechanical properties in high temperatures required. In this study creep properties of Mg–4Sn–4Ca alloy were investigated by the aim of impression creep test, scanning electron microscopy, energy dispersion spectrometry and X-ray diffraction analysis. The impression creep tests were carried out under different shear modulus normalized stress at high temperatures. According to the measured stress exponent values and activation energies the climb-controlled dislocation creep was determined as the dominant mechanism. The creep resistance of this alloy was related to the presence of Ca–Mg–Sn and Mg2Ca phases which are distributed uniformly in the matrix and exhibit high thermal stability.

Hot deformation behavior and workability of pre-extruded ZK60A magnesium alloy

The hot deformation behavior and workability of pre-extruded ZK60A magnesium alloy were investigated by compression tests in the temperature range of 250–450 °C and the strain rate range of 0.001–10 s−1. The constitutive equation for the pre-extruded ZK60A alloy can be described by hyperbolic sine function. Processing maps were constructed from true strains of −0.2 to −0.8. The alloy experienced complete dynamic recrystallization (DRX) and showed good workability in the temperature range of 300–400 °C and the strain rate range of 0.01–0.001 s−1, where hot working in pre-extruded ZK60A, such as forging, can be carried out. For large deformation to true strain of over −0.5, strain rates above 0.1 s−1 are not recommended at all temperatures, where flow instability such as local strain concentration, twinning deformation, abnormal grain growth, micro-cracks, and shear fracture were observed. Climb-controlled dislocation creep dominates both the plastic deformation and nucleation of DRX of the pre-extruded ZK60A magnesium alloy.

Effect of tin on the microstructure and impression creep behavior of MRI153 magnesium alloy

The effect of tin on the microstructure and creep behavior of as cast magnesium MRI153 alloy was investigated in the present study. Creep behavior of the alloys were studied using impression creep test under shear modulus normalized stress ranging from 0.025 to 0.04 at temperatures of 152–217°C. Results showed that adding tin to MRI153 alloy reduces the amount of Al2Ca via forming CaMgSn phase, whereas the continuity and value of Mg17Al12 phase are increased. Solubility of calcium in Mg17Al12 phase is also reduced in the presence of tin. It was shown that adding tin to MRI153 alloy decreases the alloy creep strength, which can be attributed to the reduced value of Al2Ca phase and formation of CaMgSn phase instead as well as reduced thermal stability of Mg17Al12 phase and its increased value and continuity. By calculating creep activation energy and stress exponent, it was shown that although climb-controlled dislocation creep is the dominant mechanism in the creep of these alloys, the creep activation energy is decreased with increasing the amount of tin.

Strain-hardening and warm deformation behaviors of extruded Mg–Sn–Yb alloy sheet

Abstract Strain-hardening and warm deformation behaviors of extruded Mg–2Sn–0.5Yb alloy (at.%) sheet were investigated in uniaxial tensile test at temperatures of 25–250 °C and strain rates of 1 × 10−3 s−1–0.1 s−1. The data fit with the Kocks–Mecking type plots were used to show different stages of strain hardening. Besides III-stage and IV-stage, the absence of the II-stage strain hardening at room temperature should be related to the sufficient dynamic recrystallization during extrusion. The decrease of strain hardening ability of the alloy after yielding was attributed to the reduction of dislocation density with increasing testing temperature. Strain rate sensitivity (SRS) was significantly enhanced with increasing temperature, and the corresponding m-value was calculated as 0.07–0.12, which indicated that the deformation mechanism was dominated by the climb-controlled dislocation creep at 200 °C. Furthermore, the grain boundary sliding (GBS) was activated at 250 °C, which contributed to the higher SRS. The activation energy was calculated as 213.67 kJ mol−1, which was higher than that of lattice diffusion or grain boundary self-diffusion. In addition, the alloy exhibited a quasi superplasticity at 250 °C with a strain rate of 1 × 10−3 s−1, which was mainly related to the fine microstructure and the presence of the Mg2Sn and Mg2(Sn,Yb) particles.

Dislocation Creep in Al-22.2, 53.6 and 101 at.ppm Fe Solid Solution Alloys

Creep tests of ultra-high-purity (99.999%) Al and Al-22.2, 53.6, 101 at.ppm Fe solid solution alloys were conducted at 773 K in the stress range of 2-6 MPa in order to investigate effect of solute Fe on high temperature deformation of Al. Creep resistance was enhanced by addition of Fe in solid solution. The stress exponents of the samples exhibited values of about 5, which indicate that climb-controlled dislocation creep was dominant deformation mechanism. It could be suggested that Fe atoms segregating in dislocations due to the strong interaction between solute Fe atoms and the dislocation enhanced the creep resistance.

Compressive deformation behavior of Mg–Zn–Ca alloy at elevated temperature

The compressive deformation behavior at elevated temperature of the as-extruded and as-ECAPed Mg–5.3Zn–0.6Ca (wt%) alloys was investigated in the current study. The flow stress in the as-ECAPed alloy was lower than that of the as-extruded alloy when the compressive temperature was above 200°C, due to the grain boundary softening effect. The strain rate sensitivity (SRS) was remarkably enhanced with the decrease of grain size or the increase of test temperature, and the corresponding m-value was calculated as 0.06–0.13, which indicated that the deformation was mainly dominated by the climb-controlled dislocation creep. Furthermore, the grain boundary sliding (GBS) mechanism might also be activated, which contributed to the higher SRS in the as-ECAPed alloy. The true deformation activation energy (Q') modified by threshold stress was calculated as 92–98 and 70–76kJ/mol in the as-extruded and as-ECAPed alloy, respectively, which was close to the activation energy for grain boundary diffusion, and the lower Q'-value in the as-ECAPed alloy might be derived from the non-equilibrium grain boundaries.

High temperature deformation and microstructural instability in AZ31 magnesium alloy

The high temperature deformation behaviour of AZ31 magnesium alloy is analysed by comparing numerous investigations, including work by these authors and by other researchers. Three main deformation mechanisms are observed, i.e grain boundary sliding, solute drag creep and climb-controlled dislocation creep. A combined set of constitutive equations, which takes into account the concurring effect of these different deformation mechanisms, is proposed. Grain boundary sliding is observed to cause a superplastic behaviour in fine-grained materials, but grain growth due to excessively prolonged high temperature exposure invariably results in a transition to either viscous glide or dislocation climb as a rate-controlling mechanism. On the basis of these considerations, the differences observed by testing the same material under constant strain rate or by strain rate change experiments are rationalised by quantifying the effect of static and dynamic grain growth and dynamic recrystallisation. This procedure provides a unitary description of the high temperature deformation of AZ31 in a wide range of strain rates and temperatures.

Optimization of the Mg–Al–Zn–Ca–Sr alloy composition based on the parameter A′ in the constitutive equation for the climb-controlled dislocation creep including the stacking fault energy

Abstract In this study, calcium, Ca-, and strontium, Sr-added AZ91 magnesium alloys, which were composed of magnesium, Mg–9 wt.% aluminum, Al–0.8 wt.% zinc, Zn–x wt.% Ca–y wt.% Sr, were formulated and designated as AZXJ91xy alloys. The optimum composition of die-cast AZXJ alloys has been studied by micro-structural analysis and through the application of the constitutive equation of deformation behavior at elevated temperatures that includes the stacking fault energy. The normalized plots of the tensile test results of AZXJ alloys at 448 K by the constitutive equation of deformation behavior indicated that the deformation mechanism of the alloys was climb-controlled dislocation creep. The value of the constant A′ in the constitutive equation differs in the different AZXJ alloys, even though the effect of the solid solution of added elements, which is considered in the stacking fault energy term, was eliminated from A′. Using A′ as an indicator parameter of the creep resistance with the tensile strength at ambient temperature, the optimum composition of the creep-resistant Mg–Al–Zn–Ca–Sr alloy has been determined, and its creep resistance was comparable to those of known heat-resistant magnesium alloys.

The influence of anisotropic diffusion on the high-temperature creep of a polycrystalline aggregate

The influence of anisotropic diffusion coefficients on diffusion-controlled high-temperature creep is examined. Anisotropic diffusion affects anisotropy of diffusion-controlled deformation of a single crystal. The shape change of a single crystal by diffusional mass flux is controlled directly by the anisotropy in diffusion coefficients. The rate of shape change of a single crystal by diffusion-controlled dislocation glide is controlled by the anisotropy of diffusion coefficients on the plane normal to the dislocation line. Consequently, if a polycrystalline aggregate is deformed uniformly by diffusion creep, then the rate of deformation is controlled by that of diffusion (of the slowest diffusion species) along the slowest direction. In contrast, when a polycrystalline aggregate is deformed by climb-controlled dislocation creep, the rate of deformation is controlled by the diffusion (of the slowest diffusion species) along the direction where the diffusion coefficient has the intermediate value. The results are applied to olivine and post-perovskite. For olivine, the observed large plastic anisotropy and small anisotropy in diffusion suggests that high-temperature power-law creep is controlled not only by diffusion but also by some other factors such as jog density. For post-perovskite, the results of numerical calculations on the mobility of vacancies would suggest that the viscosity of post-perovskite aggregates is higher than or comparable to that of the perovskite aggregates if the defect concentrations were the same among these minerals. However, currently nothing is known about the defect concentrations in post-perovskite and other coexisting phases, and therefore it is impossible to compare the creep strength among coexisting phases from these results.

Investigation on Dynamic Friction Properties of Extruded AZ31 Magnesium Alloy Using by Ring Upsetting Method

The dynamic friction properties of the extruded AZ31 magnesium alloy of the grain size of 20 mm were investigated by ring upsetting method test at 523, 548 and 573 K at strain rate of 1:0 � 10 � 2 s � 1 , where all the initial testing conditions were the climb-controlled dislocation creep. The MoS2 lubricant maintained lower dynamically friction coefficient (m value) than the oil lubricant. The difference in m values between machined surface and polished surface was unclear. The m values for WC-Co and diamond like carbon (DLC) tools were similar in MoS2 lubricant. The m values for DLC tool were lower than those for the WC-Co tool in the oil lubricant. The extruded direction influenced to the friction properties. The aspect ratio of the inner diameter on 90 � to extruded direction after testing was almost isotropic; on the other hand, the anisotropy occurred on 0� and 45� . The extent of anisotropy at 548 K was the highest, although the lower temperature, the higher the critical shear stress of non-basal plane. The condition at 523 K, where the fine grain sizes less than 3 mm could be obtained by dynamic recrystallization during deformation, is suitable temperature to make superplasticity at the given strain rate. [doi:10.2320/matertrans.P-M2010811]

Effect of Tool Materials on Dynamic Friction Characteristics and Microstructural Evolution at Elevated Temperature in Extruded AZ31 Magnesium Alloy

The effect of the coating materials of the tools on the dynamic friction characteristics and microstructural evolution with straining was investigated by the ring-typed compressive test at a temperature of 473 K and at a strain rate of 10 � 2 s � 1 in the extruded AZ31 magnesium alloy with an average grain size of 15 mm. The values of the dynamically recrystallized grain size in the samples compressed up to about 45% by using the WC-Co tools with and without the DLC coating were 3.8 mm and 4.8 mm, respectively. The dominant deformation mechanism under all the testing condition for the present alloy was the climb-controlled dislocation creep. The dynamic friction coefficient, m value, for the sample compressed by the WC-Co tool was higher than that done by the DLC tool, even if though there was identical tendency in stress level. The integration degree of the grains within 10 degree from h0001i direction to compressive axis in the sample compressed by the WC-Co tool was larger than that done by the DLC tool. It was concluded that the higher m value could enhance an alignment of the planes perpendicular to the compressive direction to the basal plane even if under same testing condition. [doi:10.2320/matertrans.MBW200914]

Effect of Small Addition of Zinc on Creep Behavior of Tin

Sn-based alloys with high creep resistance are required for soldering applications. This paper describes the effect of solid solution strengthening on the creep resistance of Sn-Zn alloys. The maximum solubility limit of Zn is 0.34 mass% in Sn. The creep behaviors of Sn, Sn0.1 mass%Zn and Sn-0.4 mass%Zn were examined at 298 and 398 K under constant strain rates ranging from 1 � 10 � 4 to 1 � 10 � 2 s � 1 . The creep resistance of Sn was improved significantly by the addition of a small amount of Zn owing to the solid solution strengthening. The creep resistance of Sn-0.4 mass%Zn was at the same level as that of Sn-37 mass%Pb. We obtained a stress exponent of about 7 and an activation energy of 41–45 kJ/mol, which indicates that the creep behavior was climb-controlled dislocation creep controlled by pipe diffusion. The finding that a small addition of Zn improves the creep resistance is useful for developing new Pb-free solders. [doi:10.2320/matertrans.MJ201023]