Effect Of Isothermal Aging Research Articles

Effects of Thermal Aging on Ductile-to-Brittle Transition Temperature Behavior of Welded A516 Steel

Prolonged high temperature exposure of welded C-Mn steels is likely to cause microstructural changes leading to an inrease in the ductile-to-brittle transition temperature (DBTT) of the welded joint. Consequently, such degrading material properties should be quantified in view of establishing accurate component life prediction model. This study examined effects of isothermal aging on DBTT behavior of the heat affected zone (HAZ) in welded Type A516 Gr 70 steels. Microstructures of the as-received weld region revealed the presence of pearlite and ferrite in the base metal while upper and lower bainite are found in the HAZ and weld metal, respectively. Hardness measures for the weld metal region, HAZ and base steel are 172, 209 and 150, respectively. Aging at 420 oC, 500 hours lowers hardness value of the HAZ by 20 %. A series of Charpy impact tests on V-notched specimens are performed for as-received and thermally aged samples at 420 oC for 500, 800 and 1200 hours. Results showed that the absorbed impact energy displays a sigmoidal variation with test temperatures. DBTT ranges from -60 to 5 oC for HAZ while narrow range from -25 to 12 oC for weld metal region. Absorbed impact energy variations in samples aged for durations up to 800 hours display another saturation level over test temperatures between -30 to 10 oC. Fractographic analysis on HAZ fracture surface indicated brittle fracture at -60 oC while ductile failure dominated at 27.7 oC. A mix-mode fracture mechanism is displayed for test conducted at -38 oC.

Effect of Isothermal Aging and Thermal Cycling on Interfacial IMC Growth and Fracture Behavior of SnAgCu/Cu Joints

The growth behavior of interfacial intermetallic compounds (IMCs) of SnAgCu/Cu soldered joints was investigated during the reflow process, isothermal aging, and thermal cycling with a focus on the influence of these parameters on growth kinetics. The SnAgCu/Cu soldered joints were isothermally aged at 125°C, 150°C, and 175°C while the thermal cycling was performed within the temperature ranges from −25°C to 125°C and −40°C to 125°C. It was observed that a Cu6Sn5 layer formed, followed by rapid coarsening at the solder/Cu interface during reflowing. The grain size of the interfacial Cu6Sn5 was found to increase with aging time, and the morphology evolved from scallop-like to needle-like to rod-like and finally to particles. The rod-like Ag3Sn phase was formed on the solder side in front of the previously formed Cu6Sn5 layer. However, when subject to an increase of the aging time, the Cu3Sn phase was formed at the interface of the Cu6Sn5 layer and Cu substrate. The IMC growth rate increased with aging temperature for isothermally aged joints. During thermal cycling, the thickness of the IMC layer was found to increase with the number of thermal cycles, although the growth rate was slower than that for isothermal aging. The dwell time at the high-temperature end of the thermal cycles was found to significantly influence the growth rate of the IMCs. The growth of the IMCs, for both isothermal aging and thermal cycling, was found to be Arrhenius with aging temperature, and the corresponding diffusion factor and activation energy were obtained by data fitting. The tensile strength of the soldered joints decreased with increasing aging time. Consequently, the fracture site of the soldered joints migrated from the solder matrix to the interfacial Cu6Sn5 layer. Finally, the shear strength of the joints was found to decrease with both an increase in the number of thermal cycles and a decrease in the dwell temperature at the low end of the thermal cycle.

Effect of isothermal aging on the microstructure and properties of as-cast Mg–Gd–Y–Zr alloy

The microstructure of Mg–7Gd–3Y–0.4Zr (wt.%) alloy during isothermal aging at 200°C, 225°C and 250°C was investigated using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction. The mechanical properties of the peak-aged materials were evaluated by using Vickers hardness values, and their corrosion resistance was studied by using immersion tests, salt spray tests and electrochemical measurements in 5% NaCl solutions (wt.%). The results show that owing to the existence of a great many β′ precipitates with a quite small size and uniform distribution in the matrix, optimal mechanical properties and good corrosion resistance in 5% NaCl solutions are obtained when the alloy is aged at 200°C for 120h. The potentiodynamic polarization curves show that the corrosion current densities of the cathodic hydrogen evolution on the alloy peak-aged at 200°C is lower than those for specimens of the alloy aged at 225°C and 250°C, which agrees fairly well with the microstructures of the alloys.

Effect of isothermal aging on room temperature impression creep of lead free Sn–9Zn and Sn–8Zn–3Bi solders

The influence of isothermal aging on the creep behaviour of Sn–9Zn and Sn–8Zn–3Bi solder alloys was studied by impression testing. The tests were carried out under constant impression stress in the range from 90 to 230 MPa at room temperature. Aging affected the microstructure and, thus, the creep behaviour of the materials. The binary Sn–9Zn alloy, with an as cast microstructure characterised by an almost uniform distribution of fine Zn precipitates, was much more creep resistant than the aged condition containing a more sparse precipitate in a softer matrix. In the ternary Sn–8Zn–3Bi alloy, however, isothermal aging enhanced Bi precipitation with almost no change in the distribution and density of Zn particles, the result being an improved creep resistance of the aged material. Irrespective of the processing condition, Sn–8Zn–3Bi showed much lower steady state creep rates than the Sn–9Zn due to both precipitation and a solid solutioning effect of Bi in the Sn matrix. The stress exponents of the as cast and aged conditions were found to be respectively 8·5 and 7·7 for Sn–9Zn and 9·8 and 7·8 for Sn–8Zn–3Bi. These values are in agreement with those determined by room temperature conventional creep testing of the same materials reported in the literature.

Effect of isothermal aging on the mechanical performance of brazed ceramic/metal joints for planar SOFC-stacks

One of the key requirements for the application of planar solid oxide fuel cells is the hermetical sealing of ceramic and metallic stack-components. Gas tightness and associated mechanical integrity of the sealants are therefore key prerequisites for reliable operation over the lifetime of the stack. To withstand mechanical stresses generated by the differences in thermal expansion of the involved materials as well as non-equilibrium thermal conditions upon heating and cooling, joining with metals rather than brittle glass-ceramics is considered to be advantageous. Hence, as one of the joining possibilities for SOFCs of planar design reactive air brazing of the ceramic cell into a metallic frame has gained increasing interest and proofed its suitability in several mid term full scale stack experiments at Forschungszentrum Jülich. Fracture experiments were carried out to characterize the fracture energy and associated failure mechanisms of silver-based reactive air brazes, used for joining the zirconia electrolyte of an anode supported planar cell with a metallic Crofer22APU frame. The specimens were mechanically tested in notched beam bending geometry at room temperature. Mechanical testing was accompanied by in-situ observation with optical and SEM resolution. Discussion of the results focuses on the influence of isothermal aging and associated interfacial reactions on crack path location and mechanical characteristics. State of the art CuO-containing brazes turned out to be outbalanced by a newly conceived CuO-free braze. The new braze did not only display superior delamination resistance in the as-brazed state, combined with an additional pre-oxidation treatment of the steel but also showed improved degradation resistance against isothermal aging.

Effect of isothermal aging on the microstructural evolution of Co–Cr–Mo–C alloy

The microstructural evolution of a Co–28Cr–5Mo–0.3C alloy was investigated during isothermal aging at 850 and 950 °C for 4, 8, 16 and 24 h. The most important structural changes observed as a result of aging, were fcc (metastable) → hcp isothermal martensitic transformation, lamellar-type constituent formation at the grain boundaries and also carbides precipitation in the interior of grains on the stacking faults. According to the obtained results, it was found that isothermal aging at 850 °C resulted in the formation of lamellar-type constituent at the grain boundaries as well as progressive precipitation of carbides on the stacking faults (due to martensitic transformation) in the interior of grains. On the contrary, aging at 950 °C led to the progressive formation of carbides up to 16 h and then lamellar-type constituent formed abruptly at the grain boundaries.

Effect of isothermal ageing on the semi-solid microstructure of rheoprocessed and partially remelted of A390 alloy with 10% Mg addition

The semi-solid microstructure of commercial A390 (Al—17%Si—4.5%Cu—0.5%Mg) hypereutectic Al–Si alloy with an addition of 10% Mg was investigated for two different processing routes: 1) rheocasting after stirring with rotation speed of 260 rpm and 2) partial remelting after fast cooling in a steel mould. The results show that the morphology of α-Al grains becomes globular during isothermal holding time for both cases. However, at the same isothermal condition, the size of the α-Al phase particles for rheocast samples are larger and their morphology are more globular than for the samples examined after the partial remelting process. The microstuctural evolution, size and shape of the primary Mg 2Si as well as the silicon particles during isothermal ageing in the semi-solid region was also investigated for the two processing conditions.

Microstructural evolution during isothermal aging and strain-induced transformation followed by isothermal aging in Co-Cr-Mo-C alloy: A comparative study

The present study was undertaken to investigate the effects of isothermal aging (at 850 °C for 4, 8, 16 and 24 h) and strain-induced transformation (engineering strains of 10% and 20%) followed by isothermal aging (at 850 °C for 4, 8 and 16 h) on the microstructural evolution of a Co-28Cr-5Mo-0.3C alloy. The obtained results showed that isothermal aging at 850 °C resulted in the formation of lamellar-type carbides at the grain boundaries. Moreover, X-ray diffraction analysis indicated that isothermal aging of solution treated specimens at 850 °C for 24 h did not lead to complete fcc phase transformation to hcp one. In contrast with the isothermally aged specimens, applying plastic deformation to the solutionized samples accelerated the completion and saturation of fcc(metastable) → hcp transformation after 8 h aging at 850 °C. In addition, the X-ray diffraction results indicated that implementing isothermal aging of the strain-induced specimens at the higher aging time (16 h) caused the formation of (1 1 1) fcc and (2 0 0) fcc diffraction peaks again. Also, the strain-induced specimens followed by isothermal aging showed higher amount of microhardness as compared with the other specimens aged solely.

Effects of nano-sized Ag reinforcing particulates on the microstructure of Sn-0.7Cu solder joints

Composite solders were prepared by mechanically dispersing different volumes of nano-sized Ag particles into the Sn-0.7Cu eutectic solder. The effects of Ag particle addition on the microstructure of Sn-0.7Cu solder joints were investigated. Besides, the effects of isothermal aging on the microstructural evolution in the interfacial intermetallic compound (IMC) layer of the Sn-0.7Cu solder and the composite solder reinforced with 1vol% Ag particles were analyzed, respectively. Experimental results indicate that the growth rate of the interfacial IMC layer in the Ag particles reinforced composite solder joint is much lower than that in the Sn-0.7Cu solder joint during isothermal aging. The Ag particles reinforced composite solder joint exhibits much lower layer-growth coefficient for the growth of the IMC layer than the corresponding solder joint.

Effects of Isothermal Aging on Tensile Properties and Fracture Behavior of 316L Austenitic Stainless Steel Weld Metal

In this research, variations of tensile properties and fracture behavior of 316L austenitic stainless steel weld metal as a function of aging temperature and time have been investigated. Stainless steel plates were butt-welded using GTAW process. Weld metal tensile specimens were subjected to various aging heat treatments at temperatures of 750 and 850°C for periods of 1 to 100 hours prior to tensile tests at 25 and 500°C. Dissolution of delta-ferrite and formation of sigma phase network during aging resulted in a mild increase in tensile strength and significant reduction in ductility, particularly at 25°C. Although fracture surfaces exhibited ductile features, the dimple morphology and the macroscopic fracture mode were found to be affected by aging. The unaged weld metal exhibited a classic mode of cup and cone fracture, whereas slant and flat types of fracture modes were observed in the aged weld metals, with the slant mode being dominant at 500°C. The slant mode was associated with deformation localization along arrays of primary voids, nucleated at cracked sigma phase particles, oriented at about 45° to loading direction. The transition in the fracture mode is further discussed based on variations in the dimple morphologies and strain hardening exponent.

Effect of Electromigration and Isothermal Aging on the Formation of Metal Whiskers and Hillocks in Eutectic Sn-Bi Solder Joints and Reaction Films

Numerous electronic system failures have been attributed to short circuits caused by metal whiskers that bridged closely spaced circuit elements when they were maintained in a high-current-density environment. Typically, in single-phase interconnect materials, atoms are driven from the cathode to the anode and a compressive stress can build up at the anode end of the stripe, which can induce the formation of whiskers and hillocks. However, the electronic solders used in interconnects are multiphase materials where primary and secondary diffusion atoms/ions exist. In order to accelerate the growth of whiskers and hillocks, a high current density (10 4 A/cm 2 ) combined with high ambient temperatures (80C) was applied to eutectic Sn-Bi solder joints. Metal whiskers and hillocks were observed on the overflowed solder film above the Cu substrate. However, in the absence of the electric field, metal whiskers and hillocks could also be squeezed out from the solder reaction films after several days of isothermal aging (125C), demonstrating that the chemical reaction between Sn atoms and Cu atoms can provide the driving force for the formation and growth of metal whiskers and hillocks.

Effects of prior aging at 288°C in argon environment on time‐dependent deformation behavior of a thermoset polymer at elevated temperature, Part 2: Modeling with viscoplasticity theory based on overstress

AbstractThe viscoplasticity based on overstress (VBO) is augmented to model the effects of prior isothermal aging in an argon environment on the inelastic deformation behavior of PMR‐15 neat resin, a high‐temperature thermoset polymer. VBO is a unified state variable theory with growth laws for three state variables: the equilibrium stress, the kinematic stress and the isotropic stress. A systematic model characterization procedure based on a limited number of well defined experiments is employed to determine the VBO parameters. Experimental findings presented in Part I reveal the equilibrium stress and the kinematic stress to be affected by prior aging. Based on the experimental results, the isotropic stress is developed as a function of prior aging time. In addition, several VBO model parameters are made dependent on prior aging time. Comparison with experimental data demonstrates that the modified VBO successfully predicts the inelastic deformation behavior of the PMR‐15 polymer subjected to prior isothermal aging for up to 2000 h. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Effects of isothermal aging and temperature–humidity treatment of substrate on joint reliability of Sn–3.0Ag–0.5Cu/OSP-finished Cu CSP solder joint

This study examined the effects of isothermal aging and temperature–humidity (TH) treatment of substrate on the joint reliability of a Sn–3.0Ag–0.5Cu (wt.%)/organic solderability preservative (OSP)-finished Cu solder joint. Two types of OSP-finished chip-scale-package (CSP) substrates were used, those subjected and not subjected to the TH test. This study revealed an association between the interfacial reaction behaviors, void formation and mechanical reliability of the solder joint. Many voids were formed at the interface of the OSP-finished Cu joint subjected to the TH test. These voids were caused by the oxidation of the OSP-finished Cu substrate during the TH test. In the shear tests, the shear force of the joint with the substrate not subjected to the TH test was slightly higher than that with the TH test. The mechanical reliability of the solder joint was degraded by voids at the interface.

Effects of Isothermal Aging on the Microstructure and Tensile Behavior of Sn-3.0Ag-0.5Cu-0.2Co Solder

Effects of isothermal aging on the microstructure and tensile behavior of Sn-3.0Ag-0.5Cu-0.2Co (SAC305-0.2Co) solder was explored, and compared with the standard Sn-3.0Ag-0.5Cu (SAC305) solder. The addition of Co resulted in the formation of CoSn2 phase in SAC305-0.2Co solder matrix. The isothermal aging treatment induced microstructural coarsening for both solders. With aging, the eutectic network has been translated into block or granular shape particles. Meanwhile, the outline of the primary Sn phase became less distinct. The tensile test results indicated, under as cast condition, the Co had little effect on the ultimate tensile strength (UTS) of SAC305-0.2Co solder, whereas it suppressed the ductility. After aging, the UTS of SAC305 was obviously decreased, whereas the elongation hardly changed. For the SAC305-0.2Co, the tensile behavior showed two trends: one was the decrease of the UTS, which was similar to that of SAC305; the other was the notable increase of elongation. The examination of the fracture surfaces indicated that isothermal aging could alter the fracture pattern of SAC305-0.2Co solder from brittle to ductile.

Effect of isothermal ageing on microstructure, martensitic transformation and mechanical properties of Ni 53Mn 23.5Ga 18.5Ti 5 ferromagnetic shape memory alloy

The effect of isothermal ageing on the microstructure, martensitic transformation and mechanical properties of Ni 53 Mn 23.5 Ga 18.5 Ti 5 alloy has been investigated. The results show that Ni 3 Ti precipitates are observed in aged samples. The martensitic transformation temperature decreases remarkably with increased ageing time, while the martensitic structure of these alloys is five-layered martensite at room temperature. Additionally, Ti doping can significantly enhance the compressive strength and improve the ductility of Ni–Mn–Ga alloy with the appropriate ageing treatment.

Effect of isothermal aging on phase constitution and mechanical properties of Ti–Cr alloys

In this study, the effect of isothermal aging on phase constitution and mechanical properties of Ti–Cr alloys was investigated by measurements of electrical resistivity and Vickers hardness, X-ray diffraction, optical and scanning electron microscope observations and tensile tests. The obtained results are as follows. In solution treated and quenched state (STQ), hexagonal martensite, α′ was identified at 3Cr and 5Cr alloys, whereas β phase was identified from 5Cr to 20Cr. In 7Cr and 10Cr, athermal ω was also identified. Maximum of HV appeared at 7Cr and then HV decreased with increase of Cr content up to 15Cr. Above 15Cr, HV slightly increased due to solution hardening by Cr addition. On isothermal aging, precipitation of the α phase was delayed by Cr addition. In STQed state, tensile strength is 961 MPa in 10Cr, 988 MPa in 13Cr and 967 MPa in 15Cr, respectively. Elongation is 27.1% in 10Cr, 26.8% in 13Cr and 23.9% in 15Cr, respectively. In 13Cr and 15Cr alloys, nominal stress-nominal strain curves showed work-softening phenomenon after yield stress, whereas S–S curve of 10Cr alloy did not show that phenomenon. In 1173 K−12 ks aged state, tensile specimens of 10Cr and 13Cr were broken with no elongation, while the specimen of 15Cr alloy was broken after about 5% in elongation. It is considered that the reason why aged 10Cr and 13Cr specimen show no elongation is due to precipitation of α.

Effect of Isothermal Aging on the Growth and Morphology of the Intermetallic Compounds Formed at the Solder/Cu Interface of the Lead - Free Solder Joint

The growth and morphology of the intermetallic compounds (IMC) formed at the interface between the solder ( Sn–3.5Ag–0.5Cu ) and the Cu substrate of the lead - free solder joint have been investigated by means of isothermal aging at 125°C. The scalloped Cu6Sn5 intermetallic compound layer was formed at the interface between the solder and Cu substrate upon reflow. The thickness of Cu6Sn5 layer increased with aging time. Cu3Sn appeared between Cu6Sn5 layer and Cu substrate when isothermally aged for 100 hours. Compare to Cu6Sn5 , the thickness of Cu3Sn was rather low, and nearly did not increase with aging time. In this paper, the comparison was made among the Sn-Pb and the Sn-Ag-Cu(SAC) solders which were pre-treated differently before soldering.

Isothermal Aging Effects on the Microstructure, IMC and Strength of SnAgCu/Cu Solder Joint

The formation and evolution of the intermetallic compound (IMCs) between SnAgCu lead-free solder and Cu substrate, after isothermal aging at 150°C for 24, 48, 120, 240 and 480 hours, were studied. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint during aging. The IMC phases were identified by energy dispersive X-ray (EDX). The results showed that IMCs layer of Cu6Sn5 was formed at the interface of solder and Cu during reflowing. With the increase of aging time, the grain size of the interfacial Cu6Sn5 increased and the morphology of the interfacial Cu6Sn5 column was changed from scallop-like to needle-like and then to rod-like and finally to particles. At the same time, the rod-like Ag3Sn phase formed at the interface of solder and the IMCs layer of Cu6Sn5 with the aging time increased. In addition, large Cu6Sn5 formed in the solder with the aging time increased. The tensile strength was measured for the solder joints. The results showed that the tensile strength increases slightly at beginning and then decreases with the aging time. SEM was used to observe the fracture surface and it showed that the fracture position moved from solder matrix to the interfacial Cu6Sn5 IMCs layer with the aging time increased. The weakening of the solder matrix is caused by the coarsening of the eutectic solder structures. The weakening of the interfacial IMCs layer is caused by the evolution of morphology and size of the interface Cu6Sn5 layer.

Effect of Aging on Microstructure and Martensitic Transformation in Ti-Zr-Ni Shape Memory Alloys

Effect of isothermal aging on martensitic transformation temperatures, mechanical properties and microstructure was investigated for a Ni-rich Ti-Zr-Ni shape memory alloy at temperatures ranging from 673 K to 773 K. The aging behaviour was two stage process: the first stage associated with an increase in the Vickers hardness and a decrease in martensitic transformation temperatures and the second stage with a decrease in the hardness and increase in the transformation temperatures. Second stage was also characterized by the appearance of nano-scale precipitates, which has never been reported.

Isothermal aging effects on flex cracking of multilayer ceramic capacitors with standard and flexible terminations

Multilayer ceramic capacitors (MLCCs) are known to experience flex cracking when subjected to bending stresses. An experimental study was conducted to determine the susceptibility to flex cracking of flexible- and standard-termination surface mount MLCCs assembled with lead-free or tin–lead solders and aged at two different temperatures for 200 h. Experimental results showed that MLCCs mounted on printed circuit boards with lead-free solder are less susceptible to flex cracking compared with MLCCs mounted on boards with eutectic tin–lead solder. Two factors which make capacitors assembled with lead-free solder less susceptible to flex cracking were discussed: the lower tensile stresses inside the capacitor body which are a result of the differing elastic–plastic mechanical properties of the solder, and the higher residual compressive stresses after solder reflow assembly which are a result of the higher solidification temperature for the Sn3.0Ag0.5Cu lead-free solder. Flexible-termination MLCCs showed much more resistance to flex cracking in comparison to standard-termination MLCCs assembled with both lead-free and tin–lead solders. Aging at elevated temperatures had little effect on flex cracking susceptibility of MLCCs assembled with tin–lead solder. For MLCCs assembled with lead-free solder, aging at 150 °C increased the susceptibility to flex cracking in comparison with un-aged MLCCs.