Isothermal Aging Research Articles

Designing the heat treatment process for Mg-10Gd-4Y-xZn-0.6Zr (wt.%) alloy: A study on the transformation of microstructure and properties

Heat treatment of Mg10Gd-4Y-xZn-0.6Zr (x = 0, 1, 2 wt%) alloys has been optimized based on thermodynamic calculations and experiments. The phase transformations during the heat treatment and their effect on the mechanical properties of the alloys were discussed. Using thermodynamic calculations, three solution treatment temperatures of 475 °C, 500 °C, and 525 °C were determined, and the phase transformation under these temperatures were calculated and characterized, the results aligned well with experimental results. The optimal solution treatment technology was determined to be 500 °C for 18 h. After solution treatment, experimental alloys were subject to isothermal aging at 225 °C, the peak aging time for the experimental alloys was established. The microstructure under peak-aged state of the alloy was characterized, revealing a co-existed β’ and β1 phases. The GWZ1041K alloy, after peak aging treatment, exhibited optimal mechanical properties, with a tensile strength, yield strength, and elongation of 392.3 MPa, 249.3 MPa, and 5.7%, respectively. The study further analyzed the reasons for the microstructure transformation during the heat treatment and their influence on mechanical properties.

Morphology and kinetics of nanoheterogeneities in Fe81Ga19–Tb alloy: A small-angle neutron scattering study

The formation of nanoprecipitates in the Fe81Ga19 alloy doped with Tb (∼0.1 at. %) was studied by small-angle neutron scattering (SANS). Measurements at room temperature for several samples pre-aged at a fixed temperature in the range of (300–700)°C revealed a dilute system of precipitates with a characteristic size at nanoscale (<1000 Å) growing with an increase in ageing temperature. In situ measurements during isothermal ageing at 300 °C for 3 h revealed fast (order of 10 min) formation of precipitates. SANS analysis is consistent with the results of neutron diffraction experiments, which suggest the microstructure of this alloy as a matrix of disordered atomic structure (A2 phase) with embedded precipitates of the structurally and magnetically ordered D03 phase.

Effects of lattice misfit of γ/γ′ phases on hydrogen embrittlement behavior in Ni-based single crystal superalloy

The effect of γ/γ′ phase lattice misfit on hydrogen embrittlement (HE) behavior was fundamentally investigated by utilizing a Ni-based single crystal CMSX-4 superalloy with a simple microstructure that can exclude the H-trapping effects of grain boundary, c-vacancy, and misfit dislocation. Increasing isothermal aging time increased γ′ precipitate size while maintaining its volume fraction and a fully coherent interface. The magnitude of negative lattice misfit between the γ and γ′ phases increased from −0.08 to −0.22% according to γ′ precipitate coarsening, resulting in a well-developed tensile-strain field within the γ′ precipitate. The increased tensile-strain field enhanced activation energy for H-desorption in the γ′ precipitate from 31.9 to 35.1 kJ/mol. Therefore, H is preferentially distributed along the γ/γ′ interface within the γ′ precipitate even before deformation. H basically promotes the slip planarity through the H-enhanced localized plasticity (HELP) mechanism in Ni-based single crystal alloys, in which slip is the dominant deformation behavior. As the superalloy possesses a larger negative lattice misfit, H becomes trapped as diffusible-state at the tensile-strain field of the γ/γ′ interface, resulting in cuboidal brittle fracture along the {001} planes. In this process, the HELP mechanism facilitated H localization at the γ/γ′ interface. Thus, the HE behavior distinctly transitioned from HELP to an interaction between HELP and H-enhanced decohesion mechanisms (HEDE) with an increase in the magnitude of the lattice misfit. The HE behavior was investigated by correlating microstructural characterization, H-trapping behavior, and crystallographic fracture mechanism analysis.

Microstructure and Shear Properties Evolution of Minor Fe-Doped SAC/Cu Substrate Solder Joint under Isothermal Aging

Microstructure and Shear Properties Evolution of Minor Fe-Doped SAC/Cu Substrate Solder Joint under Isothermal Aging

Role of Cu microstructure during isothermal aging of Cu/Sn/Cu micro solder joints

Role of Cu microstructure during isothermal aging of Cu/Sn/Cu micro solder joints

Continuous heating aging characteristics of Al-Cu-Mg-Ag alloy based on stress relaxation method

Clarifying the aging characteristics of aluminum alloys during continuous heating is crucial for formulating isothermal and non-isothermal aging (continuous heating aging) processes. This paper investigates the stress relaxation characteristics of an Al-Cu-Mg-Ag alloy under three-point bending during continuous heating. Taking into account the mechanical properties and precipitate analysis, it demonstrates the feasibility of using the stress relaxation method to determine the peak aging temperature during continuous heating. This approach is efficient, requires minimal sample, and yields continuous data. The peak aging temperature of the sample increases from 260 °C to 305 °C when the heating rate increases from 1 to 3 °C/min with the same initial load (3 N). The peak aging temperature is around 280 ℃, when the initial load increases from 1 N to 9 N with the same heating rate (2 °C/min). Under low temperatures (25 ℃∼ 200 ℃), the sample displays the presence of GP zones, and a few Ω precipitates during the continuous heating process of 2 ℃/min. As the temperature increases, the GP zones dissolve and transform into the Ω precipitates. A large number of Ω precipitates appear in the matrix in a continuous precipitation manner when the temperature reaches 235 °C. The precipitation strengthening effect is strongest at 280 °C, and the Ω precipitates begin to coarsen as the temperature exceeds 280 °C.

Precipitate evolution and mechanical properties of Si-modified A286 alloy aged at 510 °C

Si-modified A286 alloy was designed to be compatible with liquid lead-bismuth eutectic (LBE) metal. The addition of Si resulted in a distinct microstructure compared to A286 alloy. In this study, we investigated the microstructure and mechanical properties of the experimental alloy after 510 °C isothermal aging for 5000 h. The results showed the presence of two types of G-phase in the alloy before aging: one with a core-shell morphology, the other with a chain-like morphology. After long-term aging, the Si-modified A286 alloy maintained the same refined grains as in the SAT(solid solution and aging treatment) specimen, with minimal changes observed in the two G-phases even after 5000 h of aging. However, a newly precipitated submicron spherical-like G-phase was observed on the grain boundaries after 500 h of aging, leading to a significant decrease in impact toughness. These spherical-like G-phases gradually coarsened with increasing aging time and eventually stabilized after 2000 h. The reduction in impact toughness in the Si-modified A286 alloy was attributed to the precipitation of the spherical-like G-phase.

MECHANICAL PROPERTIES OF MELT-QUENCHED PB-BA-SN ALLOYS FOR GRIDS OF LEAD ACID BATTERIES

Mechanical properties and structure of specimens of melt-quenched lead alloyed with Ba and Sn for batteries grids have been studied. It has been shown that during isothermal aging at 80°C alloys do show no tendency to recrystallize and lose strength. The results of studies of the phase composition have allowed us to propose a mechanism of alloy strengthening during the ageing process. Strengthening of the Pb–Ba–Sn alloys proceeds due to precipitation hardening and formation of Pb3Ba precipitates that further transform to inclusions of Sn5Ba phase. Barium has no modification effect on crystallization processes of the alloys. The grains of lead-based solid solution become finer because more crystallization nuclei form under higher overcooling.

Impact of Vanadium Presence on the Mechanical and Microstructural Characteristics of a Dilute Al–Sc–Zr–Si Alloy During Isothermal Aging

In this work, we studied precipitation tendency and resulting ambient and elevated temperature properties in low-V (Al-0.06Sc-0.06Zr-0.18Si-0.05V at.%), high-V (Al-0.06Sc-0.06Zr-0.18Si-0.10V at.%), and free-V (Al-0.06Sc-0.06Zr-0.18Si at.%) alloys subjected to isothermal aging at 300 °C and 400 °C. Scanning transmission electron microscopy revealed that V incorporated into Al3(Sc, Zr) nano-precipitates during aging, forming Al3(Sc, Zr, V) while retaining the spherical morphology and L12 order. Analysis showed that V delays precipitation kinetics by interacting with vacancies, requiring longer aging for peak strength. Both V alloys demonstrated increased peak microhardness around 50–100 MPa relative to the free-V alloy after extended aging. V also reduced precipitate coarsening rates during overaging. However, creep testing exhibited no improvements to threshold stress, indicating V did not expand the precipitate/matrix lattice mismatch needed for enhanced creep resistance. The threshold stress changed from 10.2 MPa in the free-V alloy to 10.4 MPa and 10.1 MPa in the low-V and high-V alloys, respectively. Instead, V localization in the interface of nano-precipitates with matrix contributed more stability. The precipitation strengthening mechanisms in all alloys were studied using established models for L12-reinforced systems. The results showed that coherency and ordering strengthening are the primary active mechanisms.

The Effect of Isothermal Aging on the Intermetallic Growth between SN100C Lead-Free Solders and ENIG Surface Finish

The selection of substrate material depends on solder joint requirements and will influence intermetallic compound (IMC) layer formation and reliability of the solder joints. Besides, using different solder materials can also affect the microstructure of IMCs formed during soldering and isothermal ageing process. This study investigated the effect of the IMC formation and microstructure evolution during reflow soldering and isothermal ageing using SN100C lead-free solders and ENIG surface finish. The characterization of the IMC formed during both reflow soldering and isothermal ageing in terms of the type, morphology, and thickness is then analyzed by using the Optical Microscope (OM), Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX). The result reveals that there are two IMC formed at an interface between SN100C and ENIG which are (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 where (Ni, Cu)3Sn4 grows beneath (Cu, Ni)6Sn5. The thickness of the IMC formation in the SN100C/ENIG solder joint is also directly proportional to the ageing duration, indicating that the longer the time of the IMC exposed to high temperature affects the thickness of the IMC.

Hierarchical nanostructure stabilizing high content coherent nanoprecipitates in Al-Cr-Fe-Ni-V high-entropy alloy

High-density coherent nanoprecipitates have been widely introduced into the design of new structural materials to achieve a superior strength-ductility balance. However, the thermal instability of nanostructures limits their fabrication and application. In this study, we investigated the temporal evolution of nanoprecipitates in coherent nanoprecipitation-strengthened Al0.5Cr0.9FeNi2.5V0.2 high-entropy alloy during isothermal aging. When annealed at 600 °C for more than 100 h, we found that its nanoprecipitates were invariably stable, with no obvious changes occurring in terms of morphology and distribution. The excellent stability was mainly attributed to the restricted state of interface migration and diffusion owing to the hierarchical nanostructure. The Cr-enriched nano-lamellar BCC phase divided the Cr-depleted FCC(L12) matrix, forming barriers to long-range diffusion and resulting in a kinetically slow coarsening rate. As the nano-lamellar BCC phase spheroidized as the aging temperature increased to 700 °C, the diffusion barriers were destroyed. Remarkable coarsening occurred after that, which further verified the significant effect of the nano-lamellar BCC phase on the microstructural stability. These results provide a paradigm for designing alloys stabilized via hierarchical nanostructure, achieving good strength-ductility synergy while excellent thermal stability.

Dual heterogeneous structure enabled ultrahigh strength and ductility across a broad temperature range in CrCoNi-based medium-entropy alloy

Developing alloys with exceptional strength-ductility combinations across a broad temperature range is crucial for advanced structural applications. The emerging face-centered cubic medium-entropy alloys (MEAs) demonstrate outstanding mechanical properties at both ambient and cryogenic temperatures. They are anticipated to extend their applicability to elevated temperatures, owing to their inherent advantages in leveraging multiple strengthening and deformation mechanisms. Here, a dual heterostructure, comprising of heterogeneous grain structure with heterogeneous distribution of the micro-scale Nb-rich Laves phases, is introduced in a CrCoNi-based MEA through thermo-mechanical processing. Additionally, a high-density nano-coherent γ' phase is introduced within the grains through isothermal aging treatments. The superior thermal stability of the heterogeneously distributed precipitates enables the dual heterostructure to persist at temperatures up to 1073 K, allowing the MEA to maintain excellent mechanical properties across a wide temperature range. The yield strength of the dual-heterogeneous-structured MEA reaches up to 1.2 GPa, 1.1 GPa, 0.8 GPa, and 0.6 GPa, coupled with total elongation values of 28.6 %, 28.4 %, 12.6 %, and 6.1 % at 93 K, 298 K, 873 K, and 1073 K, respectively. The high yield strength primarily stems from precipitation strengthening and hetero-deformation-induced strengthening. The high flow stress and low stacking fault energy of the dual-heterogeneous-structured MEA promote the formation of high-density stacking faults and nanotwins during deformation from 93 K to 1073 K, and their density increase with decreasing deformation temperature. This greatly contributes to the enhanced strain-hardening capability and ductility across a wide temperature range. This study offers a practical solution for designing dual-heterogeneous-structured MEAs with both high yield strength and large ductility across a wide temperature range.

Investigation of isothermal aged Sn-3Ag-0.5Cu/Sn58Bi-Co hybrid solder joints on ENIG and ENEPIG substrate with various mechanical performances

The stability of solder joints is of utmost critical in electronic equipment. The integration of packaged devices in these systems often leads to the generation of heat, which might have a significant impact on how long the equipment lasts when exposed to high temperatures. It is imperative to perform an isothermal aging test on the solder joints in order to replicate the extended use of these joints. This research looked into how the mechanical characteristics and IMC growth of hybrid solder joints were affected by isothermal aging over 240 h, 480 h and 720 h. The reliability of solder joint under isothermal aging condition was improved by adding Pd coating and Co element. The results revealed that Pd coating over ENIG (Electroless Nickel Immersion Gold) pad reduced the growth rate of IMC in SAC305/SnBi hybrid solder joints during isothermal aging. ENIG solder joint strength dropped by 57.66% after 720 h of isothermal aging, and practically all fractures were IMC fractures. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) shear strength lowered 31.01%, IMC fracture mode 20%, strong reliability at high temperature. IMC development was suppressed in SAC305/SnBi-Co hybrid solder joints by higher Co content. After 720 h isothermal aging, SAC305/SnBi-Co solder joints had better shear strength than non-Co joints. According to fracture morphology, SAC305/SnBi-Co hybrid solder joints reliability improves owing to the change of IMC morphology and grain refining.

Mechanical Properties of Doped Solder SAC-Q for High Strain Rate Testing at Extreme Surrounding Temperatures for 6 Months of Isothermal Aging

Abstract In various industries, including automobile industry, oil and gas, aerospace, and medical technology, electronic components are subjected to significant strain during shocks, vibrations, and drop conditions. Electronic components of this type are frequently subjected to extreme temperatures ranging from –65 °C to 200 °C. These electronic equipment are frequently subjected to strain rates of 1 to 100 sec−1 in sensitive conditions. SAC-Q, SAC-R, Innolot, and other doped SAC solder alloys have recently been introduced to electronic components and packages. SAC-Q refers to the addition of Bi to the Sn-Ag-Cu composition. The mechanical characteristic results and statistics for lead-free solder alloys are particularly important for increasing electronic package stability and high-temperature storage and strain rates. Furthermore, the mechanical characteristics of solder alloys can be dramatically altered owing to thermal aging, which causes microstructure alteration. For high-temperature aging for longer durations and testing at extremely low to very high working temperatures, SAC-Q solder alloy data is not available. The SAC-Q solder material was tested and studied at working temperatures ranging from −65 °C to 200 °C, with strain rates of up to 75 sec−1, for this study. A comparison was also done with the SAC305 solder, which had been thermally aged and tested under identical conditions. Isothermal aging specimens were kept at 100 °C for up to 180 days following manufacturing and reflowing when tensile studies were conducted at various operating temperatures. This study develops and describes stress–strain curves for a wide variety of strain rates and test temperatures. In addition, the test results and data collected were matched to the Anand viscoplasticity model, and Anand constants were calculated by estimating high strain rate behavior throughout a wide range of operating temperatures and stress levels. In addition, for BGA package assembly with PCB, FE analysis was performed for drop/shock events. For varied aging circumstances of SAC-Q solder ball joints, hysteresis stress–strain curves and plastic work density curves are generated.

Atomic structure of clusters and GP-zones in an Al-Mg-Si alloy

The structures of atomic clusters and GP-zones preceding precipitation in an Al-0.70Mg-0.85Si-0.15Fe-0.25Mn (wt. %) alloy have been investigated by annular dark field scanning transmission electron microscopy imaging and density functional theory calculations. One analysed condition consisted of one-month natural aging, and another was in a pre-baked state after 24 h aging at 90 °C. To quantify the effect of clusters and GP-zones on microstructure development during a subsequent isothermal artificial aging at 185 °C, hardness evolution was measured, and precipitate statistics was performed at the peak hardness corresponding to 5 h aging. Several types of clusters and GP-zones have been observed and based on structure and visual appearance named 'Disordered Frank-Kasper', 'Square', 'Binocular', and 1β", 2β", 3β", 4β". The first four are most common in the natural aged condition and are associated with a well-known influence on the initial hardness evolution. The last three are fractions of the hardening β" phase and refer to the number of structural units they contain. These GP-zones have an increased density in the pre-baked condition, leading to a rapid increase in hardness during artificial aging and to a finer microstructure of high-density small needle precipitates at peak hardness.

Epoxy-Printed Circuit Board Interfacial Fracture Reliability Under Three-Point and Four-Point Bend Loading After Sustained Elevated Temperature Exposure

Abstract The survivability and reliability of commercial electronic components under very high thermo-mechanical loads are improved using underfilling and potting methods. Potting protects from operating conditions such as moisture, water, or corrosive agents. Furthermore, potting offers damping against shock and vibrations, heat dissipation, and structural support. Being one of the most cost-efficient methods, potting greatly increases reliability and therefore reduces costs for replacements and repairs. It also addresses trapped hot air issues better than other restraint technologies. In potted electronic assemblies, interfacial delamination at the epoxy and printed circuit board (PCB) interface has been one of the major failure modes. Interfacial delamination happens at the epoxy/PCB interface under dynamic shock loads, which leads to failures at the solder interconnects of the electronic components. Sustained operation and storage at elevated temperatures change the interfacial characteristics at the epoxy/PCB interface. This research is focused on interfacial failure mechanics at epoxy/PCB interfaces with high-temperature isothermal aging. In the selection of the epoxy potting material and the reliability assessments of the supplemental restraint systems, fracture parameters such as steady-state strain energy release rate stress and fracture toughness are critical. Rectangular beam specimens of the epoxy/PCB interface are fabricated for different potting compounds, and the fracture behavior is studied under quasi-static monotonic three-point and four-point bend loads. One of the main differences between three-point and four-point bend loading is that the maximum bending stress occurs at the midpoint under the point of loading of the specimen in three-point bending, whereas the peak stress is distributed over the section of the specimen between the loading points in four-point bending. Four different potting compounds with diverse properties have been studied. The recommended curing schedule from the manufacturer has been chosen and followed for all the potting compounds. The epoxy/PCB interfacial samples are aged at a high temperature of 100 °C for 30–180 days. Damage has been assumed to happen at the epoxy/PCB interface under dynamic loads. The critical load of crack initiation for the epoxy/PCB interface has been determined from the experimental findings, and it is used in the computation of fracture toughness values. The fracture toughness values are compared for the various epoxy/PCB systems based on the number of days of thermal aging and the method of flexure testing. A cohesive zone model has been constructed for predominantly mode-I delamination with four-point bend stress to predict the interfacial delamination behavior at the epoxy/PCB interfaces. It has been assumed that the bulk material is linear elastic during the bending load. The cohesive zone has been modeled at the interface, where the interfacial fracture has been assumed to occur. The fracture behavior in the simulation is predicted based on the fracture parameters determined through the experiment. The computed cohesive zone parameters are unique to the interfaces, and they can be used across various applications with the same epoxy/PCB interface to predict interfacial delamination and to select a more suitable potting material.

Influence of carbon nanotubes on the morphology of Cu6Sn5 in Cu/(Sn–Ag–Cu) solder joints

This study delves into the intricate interaction between multi-walled carbon nanotubes (MWCNTs) and the Sn–Ag–Cu solder system, highlighting its relevance in lead-free soldering applications. Reflow and isothermal aging induce the formation of intermetallic layers at the joint interface, including Cu6Sn5, Cu3Sn, and irregularly shaped Ag3Sn particles. The addition of MWCNTs leads to the flattening of Cu6Sn5 grains, restraining their growth and potentially improving mechanical strength. Notably, MWCNTs exhibit a relatively high affinity with Ag3Sn. Grain size distribution analysis indicates that MWCNTs reduce particle size, effectively suppressing intermetallic compound growth. Over time, grain size increases due to simultaneous coarsening and growth during isothermal aging. In the growth kinetics analysis of intermetallic compounds in SAC305-MWCNTs solder joints, stable behavior is observed for Cu3Sn growth, driven by solid-state diffusion between Cu6Sn5 and Cu. In contrast, Cu6Sn5 exhibits significant variations in growth behavior with temperature changes. Furthermore, the evaluation of activation enthalpy for Cu3Sn growth in the Cu/(SAC305-MWCNTs) and Cu/SAC305 diffusion couples reveals similar n and k values of the power function for the layer growth, indicating a comparable rate-controlling process for Cu3Sn growth. The presence of MWCNTs in the solder does not significantly influence the rate-controlling process or activation enthalpy for Cu3Sn growth, contrasting with the observed effects on Cu6Sn5 growth. This study provides valuable insights into the interactions within the SAC305-MWCNTs solder system, offering significant implications for lead-free soldering applications.

The enhanced mechanical properties of laser powder bed fusion Al-Mn-Sc alloy via non-isothermal aging

ABSTRACT The post-heat treatment of laser powder bed fusion (LPBF) Sc-modified Al alloy is a remarkably necessary method to improve mechanical properties. In this work, the LPBF Al-Mn-Sc alloys with a high yield strength of 586 ± 4 MPa and excellent elongation of 14.5% were obtained by a novel and efficient non-isothermal aging (NIA) treatment at the heating rate of 2 °C/min from 30°C to 350°C. The precipitation behaviour and mechanical properties of the LPBF Al-Mn-Sc alloys after NIA treatment were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), hardness test as well as tensile test. The activation energy and Avrami exponent were calculated through DSC results with various heating rates. A non-isothermal aging kinetic model was applied to evaluate the NIA process referring to isothermal aging. The excellent mechanical property was analysed by revealing the strengthening mechanism of LPBF Al-Mn-Sc alloys after NIA.

Strong composite SnAgCu solder joints with internal aligned carbon fibers by magnetic field

Lightweight carbon fibers with high mechanical performance are referred to as an ideal reinforcement for an extensive variety of composite materials. However, the addition of carbon fibers into electronic interconnection solders is extremely limited so that the expected reinforcement in the mechanical properties of the composite solder joints has not been completely realized. Here, we used magnetic fields to align Ni-coated carbon fibers in the Sn-3.0Ag-0.5Cu solder vertically with the Cu substrate and prepared the oriented composite solder joints. We achieved a further improvement in the shear performance of composite solder joints (∼58 MPa) with a small addition of Ni-coated carbon fibers (0.5 wt%), which is 25% higher than that of pristine joints. It is found that these vertically oriented carbon fibers changed the propagation path of shear cracks and thus increased the shear resistance. Ni-coated carbon fibers also promoted nucleation of interfacial Cu6Sn5 in the solder joints during reflow, but prevented the formation of brittle Cu3Sn phase during isothermal aging, thus ensuring the high shear strength of composite solder joints after isothermal aging. This success provides a new pathway to fabricate a strong composite solder joint with a limited addition of reinforcements by aligning their orientation.

The Effect of Deformation and Isothermal Heat Treatment of a 5005 Aluminum Alloy

In the aluminum industry, forming is an important process step that introduces dislocations in the material. To investigate the effect of dislocation retention after ageing on 6xxx-series alloys, a non-heat-treatable 5005 alloy was selected to measure the change in mechanical properties due to dislocation annihilation during dynamic recovery. However, the isothermal ageing treatment led to an unexpected and significant increase in mechanical properties after deformation. Increases in yield strength of 120% and tensile strength of 50% compared with the as-received material were achieved. However, this caused a significant decrease in elongation properties. The deformation start temperature did not have any impact on the final mechanical properties. TEM analysis attributed the increase in mechanical properties to an increased precipitation and dislocation density compared with the undeformed reference material. The precipitates are located along dislocation lines, showing that the solute elements are preferentially segregating to dislocations and precipitating. The precipitates were typical for the Al–Mg–Si(–Cu) system; therefore, the low amounts of Si and, to a lesser extent, Cu were responsible for the precipitation hardening in the 5005 alloy.