Room Temperature Aging Research Articles

Gray Iron Aging Effects As Measured Using NDT and Microstructural Analysis

Gray (flake graphite) iron is well known to age at room temperature after casting, as evidenced by increasing strength and hardness over time, but there are conflicting theories/beliefs regarding the cause(s) of such aging response(s). Some researchers have concluded that those aging responses result from slow precipitation of submicroscopic nitrogen precipitates that are modified by interaction with titanium,1–7 but both of those elements are also known to have significant effects on graphite and matrix morphology.8–10 The results of this study correlate the logarithmic room temperature aging responses as measured by pitch and catch ultrasonic velocity testing to quantified metallography metrics and mechanical properties in providing an alternate explanation for the aging responses of gray iron. Since the positive logarithmic aging curves are characteristic of natural decay phenomena of which stress relief is an example, stress relief is a viable alternate explanation for gray iron aging effects.

Stress-induced phase transformation and room temperature aging in Ti-Nb-Fe alloys

Room temperature deformation behavior of Ti-17Nb-1Fe and Ti-17Nb-2Fe alloys was studied by synchrotron X-ray diffraction and tensile testing. It was found that, after proper heat treatment, both alloys were able to recover a deformation strain of above 3.5% due to the Stress-induced Martensite (SIM) phase transformation. Higher Fe content increased the beta phase stability and onset stress for SIM transformation. A strong {110}β texture was produced in Ti-17Nb-2Fe compared to the {210}β texture that was observed in Ti-17Nb-1Fe. Room temperature aging was observed in both alloys, where the formation of the omega phase increased the yield strength (also SIM onset stress), and decreased the ductility and strain recovery. Other metastable beta Ti alloys may show a similar aging response and this should draw the attention of materials design engineers.

Long term reliability study and life time model of quantum cascade lasers

Here, we present results of quantum cascade laser lifetime tests under various aging conditions including an accelerated life test. The total accumulated life time exceeds 1.5 × 106 device hours. The longest single device aging time was 46 500 hours without failure in the room temperature aging test. Four failures were found in a group of 19 devices subjected to the accelerated life test with a heat-sink temperature of 60 °C and a continuous-wave current of 1 A. Failure mode analyses revealed that thermally induced oxidation of InP in the semi-insulating layer is the cause of failure. An activation energy of 1.2 eV is derived from the dependence of the failure rate on laser core temperature. The mean time to failure of the quantum cascade lasers operating at a typical condition with the current density of 5 kA/cm2 and heat-sink temperature of 25 °C is expected to be 809 000 hours.

STUDY OF THE FRIEDEL–CRAFT CO-CURING OF ETHYLENE–PROPYLENE–DIENE RUBBER AND STYRENE–BUTADIENE RUBBER

ABSTRACTPolymer blends are mixtures of at least two macromolecular species, polymers, and/or copolymers. A good blend should have strong interphases between different parts of the constituent polymers. To improve adhesion and miscibility of EPDM and SBR in their blends, a Lewis acid, AlCl3, was used to form EPDM-g-SBR copolymer through Friedel–Craft reactions. The effects of blend AlCl3 content, the diene monomer content of the EPDM, the EPDM–SBR weight ratio in the blend, the room temperature aging of the blend, and the type of the oil in the blend on cross-link reactions were studied. The results showed that an increase in AlCl3 content, up to 2 phr in the formulation, was beneficial to ΔTorque (difference between minimum and maximum torque in cure trace) and cross-link density (CLD) values of the compounds. The viscosity of the blends played a key role on AlCl3 curing of the compounds. As a general rule, the ΔTorque and CLD values tended to increase with diene monomer content of the EPDM. A high reduction in ΔTorque values was observed after 3 months of aging at room temperature. The oil incorporation was beneficial to cure parameters in the following order: oleic acid, paraffin oil, no oil, and aromatic oil, respectively. The EPDM–SBR weight ratios of 50:50 and/or 60:40 were demonstrated to be desired blend ratios.

Elastic aging from coexistence and transformations of ferroelectric and antiferroelectric states in PZT

Materials undergoing antiferroelectric/ferroelectric (AFE/FE) transitions are studied for possible applications that exploit the large volume, charge, and entropy differences between the two states, such as electrocaloric cooling, energy storage, and electromechanical actuators. Though certain compositions of PbZr1−xTixO3 codoped with La and Sn may withstand millions of electrically induced AFE/FE cycles, in other cases few thermally induced cycles and room temperature aging may cause noticeable changes in the material properties. This is particularly evident in the elastic moduli, which at room temperature can become as much as four times softer. In order to get more insight into the mechanisms involved in such elastic aging and full recovering with mild annealing at 600–800 K, the effect of La doping on PbZr0.954Ti0.046O3 is studied with anelastic measurements. Complete suppression of the time dependent phenomena is found after the transformation of the intermediate FE phase into incommensurate AFE by 2% La doping. This is discussed in terms of disappearance of the stress and electric fields at the FE/AFE interfaces, in the light of the thermally activated anelastic relaxation processes that are observed at high temperature and are due to mobile defects, presumably O vacancies.

Effect of the Solution Annealing and Chemical Passivation Followed by Aging on the Corrosion of Shell Mold Cast CF8 Stainless Steel

The effects of solution annealing and passivation of shell mold cast CF8 stainless steels on Elbow pipe fittings with 2-month room temperature aging have been studied using a corrosion technique. The resistance of corrosion increased with 2-month room temperature aging combined with solid solution annealing and chemical passivation. The mode of corrosion was deeply related to the δ-ferrite content, permeability, and passivation. The corrosion probability decreased as both the δ-ferrite content and the permeability decreased. Therefore, it is considered that δ-ferrite content and passive film of Cr2O3 play an important role in corrosion resistance of CF8 Elbow pipe fittings due to the long-term aging with solid solution annealing and chemical passivation. This result shows that the corrosion resistance of CF8 fittings can be enhanced by the solid solution annealing and chemical passivation. Decreased ferrite phases and permeability improve IGC resistance in CF8 steel.

Structural transition in sputter-deposited amorphous germanium films by aging at ambient temperature

The structure of amorphous Ge (a-Ge) films prepared by sputter-deposition and the effects of aging at ambient temperature and pressure were studied by pair-distribution-function (PDF) analysis from electron scattering and molecular dynamics simulations. The PDFs of the as-deposited and aged samples for 3–13 months showed that the major peaks for Ge-Ge bonds decrease in intensity and broaden with aging for up to 7 months. In the PDFs of a-Ge of molecular dynamics simulation obtained by quenching liquid at different rates, the major peak intensities of a slowly cooled model are higher than those of a rapidly cooled model. Analyses on short- and medium-range configurations show that the slowly cooled model includes a certain amount of medium-range ordered (MRO) clusters, while the rapidly cooled model includes liquid-like configurations rather than MRO clusters. The similarity between experimental and computational PDFs implies that as-deposited films are similar in structure to the slowly cooled model, whereas the fully aged films are similar to the rapidly cooled model. It is assumed that as they undergo room-temperature aging, the MRO clusters disintegrate and transform into liquid-like regions in the same matrix. This transition in local configurations is discussed in terms of instability and the non-equilibrium of nanoclusters produced by a vapor-deposition process.

Reactions in Electrodeposited Cu/Sn and Cu/Ni/Sn Nanoscale Multilayers for Interconnects.

Miniaturization of electronic devices has led to the development of 3D IC packages which require ultra-small-scale interconnections. Such small interconnects can be completely converted into Cu-Sn based intermetallic compounds (IMCs) after reflow. In an effort to improve IMC based interconnects, an attempt is made to add Ni to Cu-Sn-based IMCs. Multilayer interconnects consisting of stacks of Cu/Sn/Cu/Sn/Cu or Cu/Ni/Sn/Ni/Sn/Cu/Ni/Sn/Ni/Cu with Ni = 35 nm, 70 nm, and 150 nm were electrodeposited sequentially using copper pyrophosphate, tin methanesulfonic, and nickel Watts baths, respectively. These multilayer interconnects were investigated under room temperature aging conditions and for solid-liquid reactions, where the samples were subjected to 250 °C reflow for 60 s and also 300 °C for 3600 s. The progress of the reaction in the multilayers was monitored by using X-ray Diffraction, Scanning Electron Microscope, and Energy dispersive X-ray Spectroscopy. FIB-milled samples were also prepared for investigation under room temperature aging conditions. Results show that by inserting a 70 nanometres thick Ni layer between copper and tin, premature reaction between Cu and Sn at room temperature can be avoided. During short reflow, the addition of Ni suppresses formation of Cu3Sn IMC. With increasing Ni thickness, Cu consumption is decreased and Ni starts acting as a barrier layer. On the other hand, during long reflow, two types of IMC were found in the Cu/Ni/Sn samples which are the (Cu,Ni)6Sn5 and (Cu,Ni)3Sn, respectively. Details of the reaction sequence and mechanisms are discussed.

Influence of heat treatment on microstructure and mechanical properties of as-cast Mg–8Li–3Al–2Zn–xY alloy with duplex structure

In this study, the microstructure evolution and mechanical properties of as-cast Mg–8Li–3Al–2Zn–0.5Y alloy during solid solution treatment from 300°C to 450°C were firstly investigated, and then the effect of Y content (from 0.5wt% to 1.5wt%) on the age softening of solid solution treated Mg–8Li–3Al–2Zn alloy was also analyzed. The results show that the as-cast Mg–8Li–3Al–2Zn–0.5Y alloy mainly consists of α-Mg, β-Li, Al2Y and AlLi phases. With the increase of solid solution temperature from 300°C to 450°C for as-cast Mg–8Li–3Al–2Zn–0.5Y alloy, the AlLi phase is decomposed and the atoms of Al and Li dissolve into the matrix gradually. The hardness, yield strength and ultimate tensile strength of the tested alloy are increased dramatically; however, the ductility is relatively decreased. Meanwhile, the room temperature aging treated Mg–8Li–3Al–2Zn–0.5Y has age hardening during initial time, and the addition of Y is helpful to the thermal stability of Mg–8Li–3Al–2Zn alloy. In addition, the fracture behavior and strengthening mechanism of the studied alloys were also investigated systematically.

Impact of the B2 ordering behavior on the mechanical properties of a FeCoMo alloy

A Fe - 25at% Co - 9at% Mo alloy can be hardened by nm-sized (Fe, Co)7Mo6 µ-phase precipitates which is accomplished by solution annealing in the austenite region followed by rapid quenching to room temperature and subsequent aging below the austenite transition temperature. In overaged condition the Mo-content in the remaining matrix drops towards zero and, therefore, the matrix consist of 71at% Fe and 29at% Co. The binary Fe-Co system shows a disorder-order, A2↔B2 transition at a critical ordering temperature between 25at% and 72at% Co. It is expected that the remaining matrix of an overaged Fe - 25at% Co - 9at% Mo alloy also exhibits such an ordering reaction. It will be demonstrated that the formation of a B2 ordered FeCo phase can be delayed or completely prevented by rapid quenching from temperatures above the critical ordering temperature. This has a strong impact on the mechanical properties of this alloy which have been studied by means of tensile, impact toughness and hardness testing. The evidence for a disorder-order transition in this alloy has been given by neutron diffraction as well as high resolution transmission electron microscopy.

Coupled diffusion-deformation multiphase field model for elastoplastic materials applied to the growth of Cu6Sn5

A coupled diffusion-deformation, multiphase field model for elastoplastic materials is presented. The equations governing the evolution of the phase fields and the molar concentration field are derived in a thermodynamically consistent way using microforce balance laws. As an example of its capabilities, the model is used to study the growth of the intermetallic compound (IMC) Cu6Sn5 during room-temperature aging. This IMC is of great importance in, e.g., soldering of electronic components. The model accounts for grain boundary diffusion between IMC grains and plastic deformation of the microstructure. A plasticity model with hardening, based on an evolving dislocation density, is used for the Cu and Sn phases. Results from the numerical simulations suggest that the thickness of the IMC layer increases linearly with time and that the morphology of the IMC gradually changes from scallop-like to planar, consistent with previous experimental findings. The model predicts that plastic deformation occurs in both the Cu and the Sn layers. Furthermore, the mean value of the biaxial stress in the Sn layer is found to saturate at a level of −8 MPa to −10 MPa during aging. This is in good agreement with experimental data.

Microstructural Evolution of SAC305 Solder Joints in Wafer Level Chip-Scale Packaging (WLCSP) with Continuous and Interrupted Accelerated Thermal Cycling

Four high-strain design wafer level chip scale packages were given accelerated thermal cycling with a 10°C/min ramp rate and 10 min hold times between 0°C and 100°C to examine the effects of continuous and interrupted thermal cycling on the number of cycles to failure. The interruptions given two of the samples were the result of periodic examinations using electron backscattered pattern mapping, leading to room temperature aging of 30 days–2.5 years after increments of about 100 cycles at several stages of the cycling history. The continuous thermal cycling resulted in solder joints with a much larger degree of recrystallization, whereas the interrupted thermal cycling tests led to much less recrystallization, which was more localized near the package side, and the crack was more localized near the interface and had less branching. The failure mode for both conditions was still the same, with cracks nucleating along the high angle grain boundaries formed during recrystallization. In conditions where there were few recrystallized grains, recovery led to formation of subgrains that strengthened the solder, and the higher strength led to a larger driving force for crack growth through the solder, leading to failure after less than half of the cycles in the continuous accelerated thermal cycling condition. This work shows that there is a critical point where sufficient strain energy accumulation will trigger recrystallization, but this point depends on the rate of strain accumulation in each cycle and various recovery processes, which further depends on local crystal orientations, stress state evolution, and specific activated slip and twinning systems.

Influence of organic fillers on surface tension of phenol-formaldehyde adhesives

Organic fillers derived from biomass waste streams are commonly formulated with phenol-formaldehyde resoles (PF) in the production of veneer-based wood composites such as plywood and laminated veneer lumber. The surface tensions of PF/filler formulations were studied as a function of filler types: flours of walnut shell (Juglans regia), red alder (Alnus rubra) bark, and corn cob (Zea mays) furfural production residue. Surface tensions were measured using the drop weight method (with the Harkins–Brown and the Lee–Chan–Pogaku corrections), and also the drop shape method. In these non-Newtonian liquids, viscosity effects on surface tension were determined over a 10-fold range in shear rate. Viscosity effects were minor or negligible, but measurements at the lowest shear rates were considered most reliable. All fillers reduced PF surface tension by 17–25% with effects greatest in alder bark and walnut shell. For all fillers, room temperature aging resulted in further reductions in surface tension. Surface tension reductions roughly correlated to the chemical compositions of the fillers, and probably resulted from the release of surface active compounds extracted from the fillers in the alkaline PF medium.

Improvement of stability of ZnO/CH3NH3PbI3 bilayer by aging step for preparing high-performance perovskite solar cells under ambient conditions

The room-temperature aging method was first introduced to ZnO-based perovskite solar cells to improve the stability of ZnO/CH3NH3PbI3 bilayer. The prepared devices with a maximum efficiency of 9.4% exhibited promising stability under ambient conditions.

Differences in the accumulation of ion-beam damage in Ni and NiFe explained by atomistic simulations

Following low-dose irradiation with a 3 MeV beam of Au ions, i.e. less than one displacement per atom, a lower number of defects were experimentally observed in NiFe than in pure Ni. At higher doses, more damage is observed in NiFe than in pure Ni. Also, at these high doses, defect structures are observed deep in the material, far from the region where ions are implanted, more so in Ni than in NiFe. In this study, these experimental results are explained using atomistic modeling. Sequences of overlapping displacement cascades with intervening defect aging are simulated. Evidence is provided that nanosecond aging at 900 K can be used as a surrogate for long-time, room-temperature aging. Then, using this procedure, it is shown that the low defect diffusivity of NiFe leads to less aggregation and recombination events between each displacement cascade in a given volume than in Ni. Variations in the local defect chemistry in NiFe produces a broad spectrum of defect formation energy, leading to the trapping of defects at energetically favorable sites: this explains the low defect diffusivity. Also, this low diffusivity explains why, at high dose, defects in NiFe do not propagate as deeply in the material than in pure Ni.

Thermal cycling induced room temperature aging effect in Ni-rich NiTi shape memory alloy

This work shows that the characteristics of B19′ martensite transformation in Ni-rich NiTi alloys are rather stable during aging at room temperature after quenching from 1273K. However, if transformation cycles between austenite and martensite are performed, the B19′ martensite transformation can be strongly suppressed following aging at room temperature. As the NiTi elements normally experience cyclic transformation in applications, this phenomenon has to be taken into account as a factor for the functional degradation of NiTi alloys. Possible mechanisms of the uncovered aging effect are discussed.

Alloy design strategies for sustained ductility in Mg-based amorphous alloys – Tackling structural relaxation

Mg-based metallic glasses are promising candidates for use in light-weight, high-specific-strength applications, but their low fracture toughness and rapid room-temperature structural relaxation have so far prevented their deployment. In this study we present various design strategies for tackling structural relaxation via studying several Mg-based amorphous alloy systems during room-temperature aging. Our investigations show that relaxation kinetics is strongly affected by solute content, solute type and glass transition temperature Tg, and an increase in solvent content generally results in lower relaxation enthalpy, faster relaxation rates, prolonged ductility and a reduction of Tg, which increases the alloys' susceptibility to room-temperature crystallization. The results also show that chemical bonding and specific topologies contribute to relaxation kinetics, where alloys with greater mixing enthalpies and greater differences in atomic radii between the constituents exhibit greater relaxation enthalpies and relaxation rates. In conclusion, we present design concepts towards enhancing long-term ductility in Mg-based glasses via a delicate balancing of their chemistry and topology.

Elastic-modulus enhancement during room-temperature aging and its suppression in metastable Ti–Nb-Based alloys with low body-centered cubic phase stability

Changes in the elastic properties during room-temperature aging (RT aging) of metastable Ti–Nb-based alloy single crystals with low body-centered cubic (bcc)-phase stability were investigated. The elastic stiffness components of Ti–Nb–Ta–Zr alloys with different Nb concentrations were measured by resonant ultrasound spectroscopy during RT aging; the results revealed that shear moduli c′ and c44 were increased by RT aging. In the alloy with the lowest Nb concentration, i.e., with the lowest bcc phase stability, shear moduli c′ and c44 were enhanced by the largest amount. The increase rates were ∼5% for 1.1 × 107 s (127 days), whereas the bulk modulus was hardly changed by aging. In Ti–Nb–Ta–Zr–O alloys with different oxygen concentrations, shear moduli c′ and c44 of the alloy with the lowest oxygen concentration increased most significantly. Moreover, the electrical resistivity of Ti–Nb–Ta–Zr and Ti–Nb–Ta–Zr–O alloys was increased by RT aging. Importantly, the enhancements of shear moduli and electrical resistivity were suppressed by increases in the bcc-phase stability (i.e., increase in the Nb concentration) and oxygen concentration; these factors are known to suppress ω (hexagonal) phase formation. However, transmission electron microscopy (TEM) observations revealed that only a diffuse ω structure—an ω-like lattice distortion—was formed after RT aging. On the basis of alloying element effects, TEM observations, and analysis of the changes in elastic properties by using a micromechanics model, it was deduced that the enhancements of shear moduli and electrical resistivity were possibly caused by the formation of a diffuse ω structure.

Characterization of a New and Complete Lead-Free Thick Film Resistor System for the Hybrid Circuit Market

The move to lead-free thick film technology, including resistors does not necessarily mean a move away from performance. There have been many recent developments in the area of lead-free glasses, thick film dielectrics, and conductor materials. However, the development of lead-free resistor materials has been slower to enter the market. This slower evolution is due, in part, to the need to develop a series of discrete compositions that are compatible with each other and cover a wide range of resistance values. There can be 8–10 individual members in such a series. The primary goal of this development was to achieve resistor performance on par with, or superior to an existing and popular premium lead based resistor system. This paper describes a new series of lead-free thick film resistor compositions that cover a complete range of decade values from 1 Ohm/square to 10 MegOhms/square. Each member is compatible and blendable with adjacent members to cover all possible resistivity needs. This system provides excellent environmental stability of laser trimmed resistors, tight TCR gap control, a wide processing latitude, and minimal termination effects with a wide range of lead-free conductor metallurgies including Au, Ag, Ag/Pt, and Ag/Pd. In this paper, we report the results of a characterization study that includes 1000 hours of laser trim stability in 85°C/85%RH, thermal ageing at 150°C, room temperature ageing, and thermal cycle stability at −50°C +150°C. Electrical properties are presented and include Power Handling, Quan-tech Noise, and Electrostatic Discharge stability. In addition, this paper investigates and documents: resistor length effects, fire and refire sensitivity effects, thickness behavior, and the use of lead-free encapsulation.

Simulation of Natural Aging in Al-Mg-Si Alloys

Natural aging during storage of Al-Mg-Si alloys at room temperature can significantly reduce the maximum strengthening potential (T6) during artificial aging and, therefore, is a key topic in aluminium research and industry. Many different strategies to understand and reduce the negative effect of natural aging have been investigated during the last decades, including analysis of different thermal pre-treatments and considering the effect of different microalloying elements. From these investigations, the vacancy evolution and the formation of clusters containing Mg and Si were found to be the governing aging mechanisms behind natural aging. In this work, we present a model to simulate and predict the behavior of these alloys when subjected to room temperature aging after solutionizing and demonstrate the effects of different thermal routes and chemical composition variations. In the implemented model, the evolution of excess quenched-in vacancies and the effect of solute vacancy traps are considered. Special emphasis is placed on co-cluster formation and its contribution to strengthening. The thermokinetic software MatCalc is used for the simulations and the results of the simulations are validated by experimental investigation.