Isothermal Aging Process Research Articles

Dissimilar copper-aluminum joint processed by low-temperature nickel electroplating

This study proposed a low-temperature method to form joints between copper (Cu) and aluminum (Al) by electroplating nickel (Ni) at temperatures below 50°C. Both the Cu/Ni and Al/Ni interfaces were characterized by transmission electron microscopy (TEM). The Cu/Ni interface was an approximately 60-nm-wide layer of an unlimited substitutional solid solution of Cu and Ni, whereas a 20-nm-wide layer of a BCC-structured AlNi intermetallic compound (IMC) was observed at the Al/Ni interface on the Ni side. The joined samples were heat-treated at 200°C for durations of 5h to 1000h, and the joint strength did not degrade after isothermal aging. No further growth of the intermetallic compound was observed at the Al/Ni interface during the isothermal aging process.

Influence of Second Reflow on the Intermetallic Compound Growth with Different Surface Finish

The formation and growth of the intermetallic were frequently discussed since lead free solder took place replacing the lead solder. However, the effect of multiple reflow process on the intermetallic morphology that was subjected to aging still needs further investigation. Thus, this study aimed to investigate the effect of second reflow towards the intermetallic compound formation and growth. Two types of surface finishes were used such as Immersion Tin (ImSn) and Electroless Nickel Immersion Gold (ENIG). Both test boards were reflowed once with Sn-3Ag-0.5Cu at the temperature of 225 °C and soaking for 8 seconds. Then, they were reflowed again at the same temperature for 25 minutes prior to an isothermal aging process for 250, 500, 1000 and 2000 hours at the temperature of 150 °C. The ProgRes C3 IM7200 Optical Microscope and ImageJ were used for the microstructural study, which includes morphology and thickness. Results indicated that IMC thickness formed between solder and ImSn surface finish increased significantly with 1.28 µm incremental when exposed to the second reflow. Whereas the IMC thickness of ENIG surface finish was increased for up to 0.15 µm. In addition, ENIG showed higher activation energy as compared to ImSn.

Microstructures and mechanical properties of homogenization and isothermal aging Mg–Gd–Er–Zn–Zr alloy

The effects of homogenization and isothermal aging treatment on the mechanical properties of Mg–12Gd–2Er–1Zn–0.6Zr (wt%) alloy were investigated. The precipitated long-period stacking order (LPSO) structure and the aging precipitation sequence of the conditioned alloys were observed and analyzed, respectively. The results indicate that the 14H-LPSO structure occurs after the homogenization treatment and the β′ phase forms after the isothermal aging process. These two independent processes could be controlled by the precipitation temperature range. The significant increase in the elongation of the as-cast alloy after homogenization treatment is attributed to the disappearance of the coarse primary Mg5(Gd, Er, Zn) phase and the presence of the 14H-LPSO structure. The precipitation sequence of the investigated alloy is α-Mg(SSS)/β′′(D019)/β′(cbco)/β. Furthermore, the yield tensile strength (YTS) and ultimate tensile strength (UTS) values of the isothermal aging alloy have a great improvement, which could be attributed to the high density of the precipitated β′ phase. With the aging time prolonging, the β′ phase is precipitated in the alloy. Both the 14H-LPSO structure and the β′ phase coexist in the alloy after aging for 84 h at 498 K. The layer structures of 14H-LPSO are obviously observed along [11 $$\overline{2}$$ 0]Mg direction, which is shown in the high-resolution transmission electron microscopy (HRTEM) image. The 14H-type structure displays a stacking order with a period of c = 3.67 nm, which does not change during the isothermal aging process.

Microstructural evolution of aged heat-resistant cast steel following strain controlled fatigue

The paper presents the results of research on the microstructure of high-chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel after the isothermal ageing process and fatigue process. The fatigue process was performed at room temperature and elevated temperature (600°C), with the value of total strain amplitude ɛac amounting to 0.25% and 0.60%. Microstructural tests of GP91 cast steel were carried out by means of high-resolution transmission electron microscope. Quantitative study performed by means of TEM included the characteristics of changes in the dislocation substructure and morphology of M23C6 carbides. Performed research has shown that the microstructure of the examined cast steel after ageing is characterized by partly remaining lath microstructure with numerous precipitations of the MX and M23C6 type, as well as the Laves phase. It has been shown that the fatigue test at room temperature contributes to the process of dislocation strengthening of the examined cast steel. The increase of fatigue test temperature influences the degree of increase in the matrix softening. The degree of softening of the cast steel microstructure at elevated temperature depends also on the value of strain amplitude εac. The softening process of the examined cast steel was connected with the decrease of dislocation density and increase of subgrains.

Optimal Ni(P) thickness design in ultrathin-ENEPIG metallization for soldering application concerning electrical impedance and mechanical bonding strength

ENEPIG with various thicknesses of submicron Ni(P) deposit (Ultrathin-ENEPIG) was adopted to evaluate the electrical impedance behavior, microstructural comparison, and high-speed impact resistance prior to and after isothermal aging process. The results show that when extending the aging time to 1000h, the high-speed impact energy of ultrathin-ENEPIG with 0.31μm Ni(P) layer deteriorated significantly. In contrast, mechanical performance maintained well for ultrathin-ENEPIG with 0.18μm Ni(P) deposit. The loss of adhesion might be attributed to the Kirkendall voids resulting from the unbalanced diffusion for Sn and Cu in Ni3P crystalline. In ultrathin-ENEPIG with 0.18μm Ni(P) deposit, on the other hand, the migration of Sn in Ni2Sn1+xP1−x was faster than in Ni3P. Besides, secondary-(Cu,Ni)6Sn5 layer slowed down the out-diffusion of Cu. Thereby, the unbalanced diffusion can be eliminated. The optimization of the Ni(P) deposit thickness selection in ultrathin-ENEPIG would be discussed and proposed based on the observations of the electrical behavior, joint strength evolution, and interfacial thermal stability.

Microstructure Evolution of Mg-10Gd-2Y-0.5Zn-0.3Zr Alloy during Isothermal Forging Process

The isothermal forging process of a bracket and its microstructure evolution of Mg-10Gd-2Y-0.5Zn-0.3Zr alloy have been investigated in the present study. The results show that the bracket with thin-web and high-rib is well formed through modifying corners and adding an active damping block into male die. Amounts of lamellae and particles distribute uniformly on the matrix after the isothermal forging process and ageing process. The isothermal forging process has an obvious effect on the precipitation behaviour of secondary phases, while it did not change the grain size greatly. With the increase of ageing time, more secondary phases precipitate from α-Mg matrix until 60h. The optimal ultimate tensile strength and elongation of the peak-aged alloy are 382MPa and 4.03%, respectively. The combined effects of LPO and β′ phases contribute to the high strength of the peak-aged alloy.

Influence of TiO2 nanoparticles on IMC growth in Sn–3.0Ag–0.5Cu–xTiO2 solder joints during isothermal aging process

The influence of TiO2 nanoparticles on the growth of intermetallic compound (IMC) between Sn–3.0 wt% Ag–0.5 wt% Cu–x wt% TiO2 (x = 0, 0.02, 0.05, 0.1, 0.3, and 0.6) composite solder and the Cu substrate during isothermal aging process at temperatures of 120, 150, and 190 °C has been investigated in this study. Scanning electron microscopy was used to observe the microstructural evolution of the solder joints and measure the thickness of IMC layer. The IMC phases were identified by energy-dispersive X-ray spectroscopy and X-ray diffractometry. Results show that two intermetallic layers, Cu6Sn5 and Cu3Sn phase are formed at the interface and the morphology of the Cu6Sn5 phase transforms from scallop-type to layer-type in Sn–3.0Ag–0.5Cu–xTiO2 solder joints. The addition of nano-TiO2 has a strong influence on the growth of overall IMC layers, and the thickness of overall IMC layers rapidly increase with aging temperature and time. The growth rates and activation energies of the IMC growth of six solder alloys were determined. Results reveal that, for three different ageing temperatures, the growth rates of overall IMC layers decrease with an increase in nano-TiO2 proportion. The activation energies for the growth of overall IMC layers range from 48.34 to 63.61 kJ/mol. Adding nano-TiO2 to Sn–3.0Ag–0.5Cu solder could evidently increase the activation energy of overall IMC layers, reduce the atomic interdiffusion rate, and thus inhibit excessive growth of overall IMC layers.

Analytical Solutions and a Numerical Approach for Diffusion-Induced Stresses in Intermetallic Compound Layers of Solder Joints

Intermetallic compounds (IMC) play a key role in the mechanical reliability of solder joints. The present work investigates the diffusion-induced stress developed in the Cu pad/IMC/solder sandwich structure during a solid-state isothermal aging process. An analytical model and a numerical approach are proposed to predict the stress. The model consists of a Cu6Sn5 layer sandwiched between a Cu pad and a solder layer, and it is assumed that the diffusivity of the Cu atoms is much greater than that of the Sn atoms. We use the Laplace transformation method to obtain the distribution of the Cu atoms concentration. The diffusion-induced stress is determined analytically by the volumetric strain resulted from the effect of the atomic diffusion. It is found that the Cu6Sn5 layer is subjected to compressive stress due to the Cu atoms diffusion. As the diffusion time is long enough, the diffusion-induced stress shows a linear relationship with the thickness of the Cu6Sn5 layer. A finite element approach to calculate the diffusion-induced stress is proposed, and it is compared and validated by the analytical solution. The results show that the proposed approach can give a well estimation of the diffusion-induced stress in the Cu6Sn5 layer, and is also efficient in predicting the diffusion-induced stress in the structures with more complex geometry. The distribution of the Cu atoms concentration and the diffusion-induced stress in the model with a scallop-like or flat-like Cu6Sn5/solder interface are calculated by the numerical approach. The results show that the interfacial morphology of the Cu6Sn5/solder has great influence on the evolution of the Cu atoms concentration, and the diffusion-induced stress in the Cu6Sn5 layer with the scallop edge is less than that with the flat edge.

Effect of Aging Temperature on the Intermetallic Compound (IMC) Formation of Sn-0.7Cu/Si<sub>3</sub>N<sub>4</sub> Composite Solder

Nowadays, excessive growth intermetallic formation becomes the major issue in electronic packaging industry. The investigation on the effect of aging temperature to the intermetallic compound (IMCs) growth formation for Sn-0.7Cu/1.0-Si3N4 was studied. Isothermal aging process was carried out for 24 hours, with 5 difference aging temperature from 50°C up to 150°C. It is found that the Cu-Sn IMCs which appear after reflowed process, has grew rapidly when aging temperature was increased up to 125°C and started to reduced after 150°C aging temperature.

Laboratory Investigation on the Feasibility of Light-Oil Autoignition for Application of the High-Pressure Air Injection (HPAI) Process

Whether thermal effects have a contribution on oil recovery mechanisms in the high-pressure air injection (HPAI) process in a light-oil reservoir has confused many people over several decades. The argument is due to the lack of convictive evidence from both direct laboratory and simulation studies. This paper is the prelusion for the subsequent study for a better understanding of the influence of thermal effects on oil production. In a previous study, thermogravimetry/differential thermogravimetry (TG/DTG) and differential thermal analysis (DTA) tests were employed to investigate the kinetic parameters and exothermic behavior of Keke Ya light crude oil (Tarim Basin, China) and oil + cuttings, showing that Keke Ya light crude oil as well as oil + cuttings exhibited very low activation energy and favorable exothermic behavior. In this study, we develop a thermal effect monitoring device, which is similar to the isothermal oxidation tube, and we have detected the obvious exothermic phenomenon of Keke Ya light crude oil in porous media in the isothermal aging test at reservoir conditions (80 °C and 16.7 MPa). The witness of the combustion (or high-temperature) process is exhibited, which is expressed in specific characteristics made up of the self-heating behavior, low hydrogen/carbon (H/C) ratio of 1.50 from gas composition analysis, and generated precipitation coverings (halite crystalloid) in the core section surface through scanning electron microscopy (SEM) analysis. This study provides new insight that thermal effects should exist during the light-oil isothermal aging process in porous media and proposes a laboratory analysis method to understand the feasibility of light-oil autoignition at the oil-bearing reservoir during application of HPAI.

Microstructure Evolution During the Aging at Elevated Temperature of Sn-Ag-Cu Solder Alloys

In this work, binary Sn-3.5%Ag and Sn-0.7%Cu and ternary Sn-3.5%Ag-0.9%Cu eutectic alloys has been used to investigate the dynamic of microstructure transformation during isothermal artificial aging process at 180¼C. The microstructure was investigated using optical microscopy (OM), scanning electron micrograph (SEM) and EDX (energy dispersive X-ray microanalysis) and followed by Differential Scanning Calorimetry technique (DSC). In the as-cast condition the binary eutectic alloys have a structure composed of Sn primary phase crystals surrounded by an irregular eutectic structure composed of intermetallic sheets of Ag3Sn and Cu6Sn5 in an Sn matrix while the microstructure of the ternary alloy shows the following phases: β-Sn surrounded by Sn-Ag binary eutectic and Sn-Ag-Cu ternary eutectic. After 48hours of isothermal aging at 180°C both alloys show a noticeable change in their microstructures. In binary alloys a larger grain structure is formed, Sn dendrites coarsen, and in the ternary alloy the intermetallic phases Ag3Sn Cu6Sn5 are randomly distributed in a Sn matrix. There is a coarsening of Sn dendrites and Ag3Sn phase produced by the growth mechanism of Ostwald (Ostwald Ripening).

Particle-Size-Grouping Model of Precipitation Kinetics in Microalloyed Steels

The formation, growth, and size distribution of precipitates greatly affects the microstructure and properties of microalloyed steels. Computational particle-size-grouping (PSG) kinetic models based on population balances are developed to simulate precipitate particle growth resulting from collision and diffusion mechanisms. First, the generalized PSG method for collision is explained clearly and verified. Then, a new PSG method is proposed to model diffusion-controlled precipitate nucleation, growth, and coarsening with complete mass conservation and no fitting parameters. Compared with the original population-balance models, this PSG method saves significant computation and preserves enough accuracy to model a realistic range of particle sizes. Finally, the new PSG method is combined with an equilibrium phase fraction model for plain carbon steels and is applied to simulate the precipitated fraction of aluminum nitride and the size distribution of niobium carbide during isothermal aging processes. Good matches are found with experimental measurements, suggesting that the new PSG method offers a promising framework for the future development of realistic models of precipitation.

Effect of intermetallic compounds on fracture behaviors of Sn3.0Ag0.5Cu lead-free solder joints during in situ tensile test

In this paper, in situ tensile tests under various amounts of deformation were performed on Sn3.0Ag0.5Cu lead-free solder joints subjected to multi-reflow and isothermal aging processes by using a scanning electron microscope. Microstructure evolution and deformation behavior of the solder joints were observed. Effects of the intermetallic compound (IMC) Cu6Sn5 on fracture behaviors of the solder joints were investigated. Results showed that the Sn3.0Ag0.5Cu lead-free solder joints contained only a few Sn grains, and the sequence and degree of plastic deformation varied for the different grains in the same solder joint due to the strong anisotropic properties of Sn grains. Further experiments revealed that plastic deformation occured primarily in the form of slip bands in the solder joints during the in situ tensile test. Various fracture modes including intergranular and phase boundary fractures were observed. The fracture behaviors of solder joints were significantly affected by morphologies and distributions of the Cu6Sn5 IMCs. It was found that Cu6Sn5 particles located at the grain boundaries are apt to become crack sources, and that the long rod shaped Cu6Sn5 were easily broken. However, spherical Cu6Sn5 hardly deformed during the tensile test, resulting in dynamic recrystallization. In this case, fracture occured at the sub-grain boundaries.

Compositional Variations between Different Generations of γ′ Precipitates Forming during Continuous Cooling of a Commercial Nickel-Base Superalloy

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.

Failure mode analysis of lead-free solder joints under high speed impact testing

Using an Instron micro-impact system, this study investigates the failure characteristics of 96.5Sn–3Ag–0.5Cu lead-free solder joints aged at either room temperature or 125°C, respectively, and then impacted at shear rates of up to 1m/s. Four types of failure mode are identified, namely M1: interfacial fracture with no residual solder left on the pad; M2: interfacial fracture with residual solder left on the pad; M3: solder ball fracture; and M4: substrate fracture. The experimental results reveal that the solder specimens fail in different failure modes at the same impact speed. The transition from ductile to brittle failure occurs at an impact speed of around 0.5m/s. At an impact speed of 0.7 (±0.05)m/s or more, over 70% of the specimens fail in the M1 or M2 modes under all of the testing conditions. The isothermal aging process is found to reduce the interfacial strength, and hence the percentage of M3 and M4 mode failures reduces significantly. Overall, the experimental results suggest that the failure mode distribution obtained in high speed impact tests performed at 0.5m/s provides a feasible component-level quality assurance index.

Glassy dynamics in thin films of polystyrene

Glassy dynamics was investigated for thin films of atactic polystyrene by complex electric capacitance measurements using dielectric relaxation spectroscopy. During the isothermal aging process the real part of the electric capacitance increased with time, whereas the imaginary part decreased with time. It follows that the aging time dependences of real and imaginary parts of the electric capacitance were primarily associated with change in volume (film thickness) and dielectric permittivity, respectively. Further, dielectric permittivity showed memory and rejuvenation effects in a similar manner to those observed for poly(methyl methacrylate) thin films. On the other hand, volume did not show a strong rejuvenation effect.

Phase transformation and microstructural changes during ageing process of an Ag–Pd–Cu–Au alloy

Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α 1 phase as a matrix, the Cu–Pd α 2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α 1 and Cu–Pd α 2 phases were transformed into the Ag-rich α ′ 1 and Cu 3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α 1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α 1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α 2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

Investigation of sagging phenomenon observed in laser reflow soldering right-angled solder joints during isothermal aging or thermal cycle process

This paper presents a detailed analysis of sagging phenomenon happened in right-angled solder joints formed by laser reflow soldering. When the solder joints were subjected to 125 °C isothermal aging or thermal cycle process, in which the temperature varied from −40 to 125 °C, sink steps formed near the interface of solder and pad with 4.0, 3.0 or 0.9 μm thickness of Au surface finish, and this phenomenon was called as sagging. The sink steps were larger for pads with thicker Au surface finish, and sagging phenomenon was more serious for Sn–37Pb solder joints than that of Sn–2.0Ag–0.75Cu–3.0Bi and Sn–3.5Ag–0.75Cu solder joints. Moreover, evolution of the sink steps were faster in thermal cycle process than that in aging porcess for Sn–37Pb solder joints. Through SEM and EDX analysis, it was supposed that the emergence of sagging phenomenon was caused by the formation and evolution of AuSn x intermetallic components (IMCs) and/or CuSnAu phases at the interface of solder and pad, and the stress cumulated in the solder joints during thermal cycle process may have great effects on the evolution speed of the IMCs.

Phase Transformation and Microstructural Changes during Ageing Process of an Ag-Pd-Cu-Au Alloy

Abstract Age-hardening behaviour and the related phase transformation and microstructural changes during isothermal ageing process were studied to elucidate the age-hardening mechanism of an Ag-based dental casting alloy composed of Ag–Pd–Cu–Au–Zn, Ir and In by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and energy dispersive spectroscopic microanalysis (EDS). In the hardness test at 350 and 400 °C, the hardness of the solution-treated specimen began to increase and reached a maximum value with increasing ageing time, and subsequently the hardness decreased gradually. By considering XRD results and SEM observations together, the solution-treated specimen consisted of three phases, the Ag-rich α1 phase as a matrix, the Cu–Pd α2 phase and the CuPd β phase with a CsCl-type as particle-like structures. By ageing the solution-treated specimen, the Ag-rich α1 and Cu–Pd α2 phases were transformed into the Ag-rich α ′ 1 and Cu3Pd α ′ 2 phases, respectively. The CuPd β phase with a CsCl-type was not changed apparently during the ageing process. From the results of the hardness test, XRD study, SEM observations and EDS analysis, it could be derived that the hardness increased by the diffusion and precipitation of the Cu-rich phase from the Ag-rich matrix during the early stage of phase transformation of α1 into α ′ 1 and that the progress of coarsening of the Cu-rich precipitates with an entanglement structure caused the hardness decrease during the later stage of phase transformation of α1 into α ′ 1 . The particle-like structures composed of the Cu–Pd α2 and the CuPd β phase with a CsCl-type contributed little to the hardness increase which occurred in the early stage of aging process.

Prediction of Sigma Phase Precipitation in Super Duplex Stainless Steel Weldments

In order to predict the sigma (σ) phase embrittlement in multipass weldment of duplex stainless steels, the precipitation behaviours of the σ phase in the base metal and the weld metal were investigated using super duplex stainless steels (SAF2507 and DP3W) and a conventional duplex stainless steel (NAS64). In all the steels, the amount of the σ phase precipitated during an isothermal ageing process sigmoidally increased with the length of the ageing time. The kinetics of σ phase precipitation in the base metal and the weld metal could be expressed approximately by the Johnson-Mehl type equation, however, the initiating time of σ phase precipitation in the weld metal was longer than that in the base metal. The precipitation of the σ phase in the DP3W base metal was much delayed and depressed compared with the SAF2507 and NAS64 base metals. Increasing the amount of the σ phase drastically decreased the absorbed impact energies of aged steels. They fell to as low as 1/4 of the unaged values when the amount of the σ phase exceeded 5%. The precipitation of the σ phase in duplex stainless steels during multiple thermal cycle process were predicted using the additivity rule. We predicted that the largest amount of the σ phase and the most significant degradation of impact toughness would occur in the base metal region (HAZ) for peak temperatures of 1 100–1 300 K in multiple thermal cycle.