Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

Age hardening and creep resistance of cast Al–Cu alloy modified by praseodymium

The effects of praseodymium on age hardening behavior and creep resistance of cast Al–Cu alloy were investigated. The results indicated that praseodymium facilitated the formation of the θ′ precipitates during the age process and improved the hardness of the Al–Cu alloy. Besides, praseodymium resulted in the formation of the Al₁₁Pr₃ phase in the grain boundaries and among the dendrites of the modified alloy. Because of the good thermal stability of Al₁₁Pr₃ phase, it inhibits grain boundary migration and dislocation movement during the creep process, which contributes to the improvement in the creep resistance of the modified alloy at elevated temperatures.     

Transient and steady state creep of age-hardenable Al–5 wt% Mg alloy during superimposed torsional oscillations

An experimental study has been conducted to assess the effect of imposed small torsional oscillations on the creep behavior of Al–5 wt% Mg alloy during phase transformation. A series of tensile tests were conducted on the material with different levels of torsional oscillations and with different aging and matrix temperatures. The carefully designed experiments prove that increasing the shear strain amplitude of torsional oscillations and/or testing temperatures resulted in an increase of both transient and steady state creep parameters. The changes in work-hardening behavior of the samples with aging temperatures were rationalized in view of type, size, and distributions of phases existing in the original matrix. The results clearly show that the mechanism operating in the creep process was the precipitate–dislocation intersection. The microstructure of the samples studied was investigated by transmission electron microscopy.     

Gap-filling of Cu–Al alloy into nanotrenches by cyclic metalorganic chemical vapor deposition

In this work, Cu–Al alloy thin films with lower values of electrical resistivity than that of an Al-free Cu thin film were produced by cyclic metalorganic chemical vapor deposition (MOCVD), followed by thermal annealing of the Cu/Al multilayer formed, with controlled Cu and Al precursor delivery times. The Ru-coated SiO₂ trench with the opening width of 50 nm and aspect ratio of 1:6.7 could be completely filled by the Cu–Al alloy. The Ru/SiO₂ trench, filled conformally and voidlessly by the Cu–Al (0.7 at.%) alloy, showed no presence of intermetallic compounds.     

Hydrogen-induced gas porosity formation in Al–4.5 wt% Cu–1.4 wt% Mg alloy

The effects of low (0.067 cm³/100 g) and relatively high (0.19 and 0.27 cm³/100 g) initial melt hydrogen concentration, solidification processing conditions, and grain refining on the formation of hydrogen-induced gas porosity in Al–4.5 wt% Cu–1.4 wt% Mg alloy have been quantitatively investigated. The study was conducted with unidirectionally cooled laboratory-size ingots solidified at 0.2–37 K/s. An optical microscope-based image analyzer and precision density measurement based on the Archimedes’ principle were used to quantify the characteristics of the hydrogen-induced porosity in the ingots. Predictably, increase in melt hydrogen concentration and decrease in solidification rate increased the amount of porosity and average pore size. However, the effect of solidification rate was greater at the very low melt hydrogen concentration (0.067 cm³/100 g). These results are consistent with reported effects of solidification rate and melt hydrogen content on porosity formation in other aluminum alloys. Addition of grain refiner slightly increased the amount of porosity and the average pore size, especially at solidification rates above 1 K/s.     

Mechanisms governing cyclic fracture in an Al–Cu–Li alloy

Abstract

The fracture behaviour of a peak-aged, partially recrystallized Al–4·5Cu–1·21Li–0·51Mn–0·20Cd alloy has been investigated as a function of strain amplitude, stress intensity, and environment. It was found that the failure was predominantly intergranular separation, regardless of the environment, stress intensity, or strain amplitude, and that the fracture behaviour was influenced mostly by intrinsic microstructural features, rather than the nature of the environment. The shearable nature of matrix strengthening precipitates, large recrystallized grains, and precipitate-free zones along the high-angle grain boundaries aid in localizing the deformation, resulting in low-energy intergranular fracture. The iron- and silicon-rich intermetallic precipitates in the alloy promote void nucleation following fracture of the particle. A model is proposed which suggests the need for high stresses and strains for the initiation and spontaneous growth and coalescence of microvoids. The mechanisms of fracture behaviour of the alloy are discussed in terms of several concurrent processes involving strength of the material, intrinsic microstructural effects, deformation behaviour, state of stress, and strain.

MST/497     

The significance of grain boundary sliding in the superplastic Zn–22 % Al alloy processed by ECAP

A Zn–22 % Al eutectoid alloy was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~0.8 μm. Tensile testing at 473 K showed superplastic characteristics with a maximum elongation of ~2230 % at a strain rate of 1.0 × 10^?2 s^?1. The significance of grain boundary sliding (GBS) was evaluated by measuring sliding offsets at adjacent grains from the displacements of surface marker lines in samples pulled to elongations of 30 % at a series of different strain rates. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic ductility. There were relatively large offsets at the Zn–Zn and Zn–Al interfaces, but smaller offsets at the Al–Al interfaces. Analysis shows the results are affected by the presence of agglomerates of similar grains which are present after ECAP processing and specifically by the increased fraction of Al–Al boundaries. The experimental results are in excellent agreement with the predictions of a deformation mechanism map depicting the flow behavior in the Zn–22 % Al alloy, and the results confirm the importance of GBS as the dominant mechanism of flow in superplasticity after processing by ECAP.     

The effect of cooling rate on α-phase ordering in Cu-12.4wt% Al alloy

The formation of a superlattice in the-phase of Cu-12.4wt% Al alloy was studied during cooling. Specimens cooled at different rates were examined using electron microscopy and differential thermal analysis. The superlattice structure formed was described by means of theD-parameter which determines the position of superspots in the reciprocal lattice. Variation of theD-parameter with cooling rate has a linear form, hence it may be concluded that the superlattice in the-phase is formed as a transitional structure. The relationship between theD-parameter and enthalpy suggests that the latter can be taken as a measure of superstructure development.     

Tensile creep behavior of Sn–Ag–Cu–Ni multicomponent lead-free solder alloy

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10^?4–10^?8 s^?1, when the normalized stress, σ/E, is in the range of 10^?4–10^?3. Based on the Dorn equation, the apparent stress exponent (n _a), threshold stress (σ _th), and activation energy of creep (Q _C) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag₃Sn and (Cu,Ni)₆Sn₅ IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.     

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Enhancement of creep resistance and thermal behavior of eutectic Sn–Cu lead-free solder alloy by Ag and In-additions

The eutectic Sn–0.7Cu solder alloy is widely used in electronic packaging in which the creep property of the solder joint is essential to meet the global demand for longer operating lifetime in their applications. In this study, the influence of Ag and In additions on tensile creep behavior and thermal properties of bulk eutectic Sn–Cu solder alloy is reported. Results show that addition of Ag and In resulted not only in the formation of new Ag₃Sn and γ-SnIn₄ intermetallic compounds (IMCs), but also in the refinement of grain size of Sn–0.7Cu solder from ∼0.50 to ∼0.15 μm. Accordingly, the creep properties of the Ag or In-containing solder alloys are notably improved. The creep strain rate increases and creep lifetime decreases as the applied stress level and temperature increase. Room and elevated-temperature creep rate of bulk Sn–Cu solder was reduced by 521.0% after Ag addition, but for In addition the reduction was about 200.7%. These differences are attributed to the presence of new Ag₃Sn and γ-SnIn₄ precipitates and their rules in classical dispersion strengthening as a separate phases. Moreover, the eutectic temperature of Sn–0.7Cu is decreased from 227.4 to 217.8 and 224.0 °C with the addition of Ag and In, respectively.     

The behaviour of double oxide film defects in Al–4.5 wt% Mg melt

The possibility of bonding of the two layers of a double oxide film defect when held in a liquid Al–4.5 wt% Mg alloy was investigated. The defect was modelled experimentally by maintaining two aluminium oxide layers in contact with each other in an Al–4.5 wt% Mg liquid alloy at 750 °C from 2 min to 16 h. Any changes in the composition and morphology of these layers were studied by SEM, EDX and XRD. The results showed that in contrast to previous studies reported in the literature on Al–0.3 wt% Mg in which the two layers bonded to each other after a holding time of 5 h, no bonding took place between the two oxide layers even after a holding time of 16 h. Based on the comparison between the two studies, it was concluded that a transformation involving rearrangement of atoms at the interface between the two oxide layers is essential for the bonding to take place between the two oxide layers. This criterion could be used to predict the bonding behaviour of oxide film defects when held in different liquid aluminium alloys, or when subjected to a HIPping process.     

Tribological behaviour of hot extruded Al–Cu–Mg–Ag alloy reinforced by TiB2 particles

In the present investigation, tribological behaviour of the hot extruded Al–Cu–Mg–Ag (matrix) alloy and the effect of Ti and TiB₂ addition in matrix alloy have been studied. Hot extrusion was introduced to eliminate cast defects like porosity, voids and micro cracks. Addition of Ti and TiB₂ particles increased the hardness of the matrix by grain refinement and dispersion hardening, respectively. It has been observed that the increase in hardness had significantly improved the wear resistance of the material. Detail study of the wear surfaces and debris were carried out to understand the wear mechanism of the samples. It revealed a complex mechanism of micro-cutting, plastic deformation, abrasion and delamination of the wear samples.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Dependence of electrical and thermal conductivity on temperature in directionally solidified Sn–3.5 wt% Ag eutectic alloy

Sn–3.5 wt% Ag alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) and a temperature gradient (G = 3.3 K/mm) in a Bridgman-type growth apparatus. The variations of electrical resistivity (ρ) with temperature in the range of 293–476 K for the directionally solidified Sn–3.5 wt% Ag eutectic alloy was measured. The measurements indicate that the electrical resistivity of the directionally solidified Sn–Ag eutectic solder increases with increasing temperature. The variations of thermal conductivity of solid phases versus temperature for the same alloy was determined from the Wiedemann-Franz and Smith-Palmer equations by using the measured values of electrical conductivity. From the graphs of electrical resistivity and thermal conductivity versus temperature, the temperature coefficient of electrical resistivity (α _TCR ) and the temperature coefficient of thermal conductivity (α _TCT ) for the same alloy were obtained. According to experimental results, the electrical and thermal conductivity of Sn–Ag eutectic solder linearly decrease with increasing the temperature. The enthalpy of fusion (ΔH) and the change of specific heat (ΔC _P ) during the transformation at the studied alloy were determined from heating curve during the transformation from eutectic solid to eutectic liquid by means of differential scanning calorimeter (DSC).     

Synthesis of Sn–Ag binary alloy powders by mechanical alloying

Sn–Ag binary powders of 2–5 wt%Ag were synthesized by mechanical alloying. Structural evolutions, morphologies, particle size distributions and melting points of the milled Sn–Ag powders were studied. The results show that the milled Sn–Ag powders consist of a supersaturated solid solution of Ag in Sn, Sn(Ag), and Ag₃Sn. During ball milling, Sn, Ag particles in the Sn–3.5Ag powders are deformed, overlapped and cold-welded together to form the Sn/Ag composite particles with a lamellar structure, and then the composite particles are fractured into small spherical particles. When increasing the Ag content from 2 to 5 wt%, the average particle sizes of the 60 h milled Sn–Ag powders are changed from 2.2 to 5.7 μm, and the morphologies of them are changed from spherical shape to irregular shape, respectively. It indicates that the cold-welding and agglomeration of the Sn–Ag powders increases with the Ag content during MA. The melting point of the 60 h milled Sn–3.5Ag powders was detected to be 224.23 °C, near to the eutectic point of the Sn–Ag binary system (221 °C).     

Effect of cyclic loading on subsequent creep behaviour and its implications in creep–fatigue life assessment

Abstract

Evaluation of creep–fatigue failure is essential in design and fitness evaluation of high-temperature components in power generation plants. Cyclic deformation may alter the creep properties of the material and taking cyclic effects into account may improve the accuracy of creep–fatigue failure life prediction. To evaluate such a possibility, creep tests were conducted on 316FR and modified 9Cr–1Mo steel specimens subjected to prior cyclic loading; their creep deformation and rupture behaviours were compared with those of as-received materials. It was found that creep rupture life and elongation generally decreased following cyclic loading in both materials. In particular, the rupture elongation of 316FR in long-term creep conditions drastically decreases as a result of being cyclically deformed at a large strain range. Use of creep rupture properties after cyclic deformation, instead of those of as-received material, in strain-based and energy-based life estimation approaches brought about a clear improvement of creep–fatigue life prediction.     

The influence of adhesive on the Al alloy in laser weld bonding Mg–Al process

Laser weld bonding is a new welding technology, being used to join Mg–Al alloys. The penetration depth of LWB Mg–Al joint was larger than that in simply laser welding joint in same welding parameters. The temperature at the edge of the Al fusion zone in LWB Mg–Al joint was higher than that in laser welding joint, which was measured through the thermal couples. The laser-introduced plasma in LWB Mg–Al process is observed by the high-speed camera, which is different from that in laser welding process. The surface temperature and state of the Al alloy were changed because of the addition of the adhesive, thus the laser absorptive of Al alloy was increased in LWB process, comparing with that in laser welding process; and the decomposition of the adhesive would make a depression in the Al fusion zone, which would be beneficial to the formation of keyhole welding in LWB Mg–Al joint.