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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Microstructural characterization of dissimilar welds between Incoloy 800H and 321 Austenitic Stainless Steel

In this work, the microstructural character of dissimilar welds between Incoloy 800H and 321 Stainless Steel has been discussed. The microscopic examination of the base metals, fusion zones and interfaces was characterized using an optical microscope and scanning electron microscopy. The results revealed precipitates of Ti (C, N) in the austenitic matrix along the grain boundaries of the base metals. Migration of grain boundaries in the Inconel 82 weld metal was very extensive when compared to Inconel 617 weldment. Epitaxial growth was observed in the 617 weldment which increases the strength and ductility of the weld metal. Unmixed zone near the fusion line between 321 Stainless Steel and Inconel 82 weld metal was identified. From the results, it has been concluded that Inconel 617 filler metal is a preferable choice for the joint between Incoloy 800H and 321 Stainless Steel. - Highlights: • Failure mechanisms produced by dissimilar welding of Incoloy 800H to AISI 321SS • Influence of filler wire on microstructure properties • Contemplative comparisons of metallurgical aspects of these weldments • Microstructure and chemical studies including metallography, SEM–EDS • EDS-line scan study at interface.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Interconnection of thermal parameters, microstructure and mechanical properties in directionally solidified Sn–Sb lead-free solder alloys

Considerable effort is being made to develop lead-free solders for assembling in environmental-conscious electronics, due to the inherent toxicity of Pb. The search for substitute alloys of Pb–Sn solders has increased in order to comply with different soldering purposes. The solder must not only meet the expected levels of electrical performance but may also have appropriate mechanical strength, with the absence of cracks in the solder joints. The Sn–Sb alloy system has a range of compositions that can be potentially included in the class of high temperature solders. This study aims to establish interrelations of solidification thermal parameters, microstructure and mechanical properties of Sn–Sb alloys (2 wt.%Sb and 5.5 wt.%Sb) samples, which were directionally solidified under cooling rates similar to those of reflow procedures in industrial practice. A complete high-cooling rate cellular growth is shown to be associated with the Sn–2.0 wt.%Sb alloy and a reverse dendrite-to-cell transition is observed for the Sn–5.5 wt.%Sb alloy. Strength and ductility of the Sn–2.0 wt.%Sb alloy are shown not to be affected by the cellular spacing. On the other hand, a considerable variation in these properties is associated with the cellular region of the Sn–5.5 wt.%Sb alloy casting. - Graphical abstract: Display Omitted - Highlights: • The microstructure of the Sn–2 wt.%Sb alloy is characterized by high-cooling rates cells. • Reverse dendrite > cell transition occurs for Sn–5.5 wt.%Sb alloy: cells prevail for cooling rates > 1.2 K/s. • Sn–5.5 wt.%Sb alloy: the dendritic region occurs for cooling rates < 0.9 K/s. • Sn–5.5 wt.%Sb alloy: tensile properties are improved with decreasing cellular spacing.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The correlation of local deformation and stress-assisted local phase transformations in MMC foams

Cellular structures are of growing interest for industry, and are of particular importance for lightweight applications. In this paper, a special case of metal matrix composite foams (MMCs) is investigated. The investigated foams are composed of austenitic steel exhibiting transformation induced plasticity (TRIP) and magnesia partially stabilized zirconia (Mg-PSZ). Both components exhibit martensitic phase transformation during deformation, thus generating the potential for improved mechanical properties such as strength, ductility, and energy absorption capability. The aim of these investigations was to show that stress-assisted phase transformations within the ceramic reinforcement correspond to strong local deformation, and to determine whether they can trigger martensitic phase transformations in the steel matrix. To this end, in situ interrupted compression experiments were performed in an X-ray computed tomography device (XCT). By using a recently developed registration algorithm, local deformation could be calculated and regions of interest could be defined. Corresponding cross sections were prepared and used to analyze the local phase composition by electron backscatter diffraction (EBSD). The results show a strong correlation between local deformation and phase transformation. - Graphical abstract: Display Omitted - Highlights: • In situ compressive deformation on MMC foams was performed in an XCT. • Local deformation fields and their gradient amplitudes were estimated. • Cross sections were manufactured containing defined regions of interest. • Local EBSD phase analysis was performed. • Local deformation and local phase transformation are correlated.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure and mechanical properties of a novel rapidly solidified, high-temperature Al-alloy

Rapid solidification (RS) processing, as a production method, offers a variety of unique properties based on far-from-equilibrium microstructures obtained through rapid cooling rates. In this study, we seek to investigate the microstructures and properties of a novel Al-alloy specifically designed for high temperature mechanical stability. Synthesis of, AlFe{sub 11.4}Si{sub 1.8}V{sub 1.6}Mn{sub 0.9} (wt.%), was performed by two approaches: rotating cup atomization (“shot”) and melt spinning (“flake”). These methods were chosen because of their ability to produce alloys with tailored microstructures due to their inherent differences in cooling rate. The as-solidified precursor materials were microstructurally characterized with electron microscopy. The results show that the higher cooling rate flake material exhibited the formation of nanocrystalline regions as well additional phase morphologies not seen in the shot material. Secondary dendritic branching in the flake material was on the order of 0.1–0.25 μm whereas branching in the shot material was 0.5–1.0 μm. Consolidated and extruded material from both precursor materials was mechanically evaluated at both ambient and high (300 °C) temperature. The consolidated RS flake material is shown to exhibit higher strengths than the shot material. The ultimate tensile strength of the melt spun flake was reported as 544.2 MPa at room temperature and 298.0 MPa at 300 °C. These results forecast the ability to design alloys and processing approaches with unique non-equilibrium microstructures with robust mechanical properties at elevated temperatures. - Highlights: • A novel alloy, AlFe{sub 11.4}Si{sub 1.8}V{sub 1.6}Mn{sub 0.9} was fabricated by rapid solidification. • Room temperature yield strength exceeded 500 MPa. • Elevated temperature (300 °C) yield strength exceeded 275 MPa. • Forging, after extrusion of the alloy resulted in microstructural coarsening. • Decreased strength and ductility was identified as a result of forging.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Microstructure and mechanical behavior of direct metal laser sintered Inconel alloy 718

In this paper, we investigate microstructure and quasi-static mechanical behavior of the direct metal laser sintered Inconel 718 superalloy as a function of build direction (BD). The printed material was further processed by annealing and double-aging, hot isostatic pressing (HIP), and machining. We characterize porosity fraction and distribution using micro X-ray computed tomography (μXCT), grain structure and crystallographic texture using electron backscattered diffraction (EBSD), and mechanical response in quasi-static tension and compression using standard mechanical testing at room temperature. Analysis of the μXCT imaging shows that majority of porosity develops in the outer layer of the printed material. However, porosity inside the material is also present. The EBSD measurements reveal formation of columnar grains, which favor < 001 > fiber texture components along the BD. These measurements also show evidence of coarse-grained microstructure present in the samples treated by HIP. Finally, analysis of grain boundaries reveal that HIP results in a large number of annealing twins compared to that in samples that underwent annealing and double-aging. The yield strength varies with the testing direction by approximately 7%, which is governed by a combination of grain morphology and crystallographic texture. In particular, we determine tension–compression asymmetry in the yield stress as well as anisotropy of the material flow during compression. We find that HIP lowers yield stress but improves ductility relative to the annealed and aged material. These results are discussed and critically compared with the data reported for wrought material in the same condition. - Highlights: • Microstructure and mechanical properties of DMLS Inconel 718 are studied in function of build direction. • Inhomogeneity of microstructure in the material in several conditions is quantified by μXCT and EBSD. • Anisotropy and asymmetry in the mechanical response are determined by tension and compression testing.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Correlative characterization of primary particles formed in as-cast Al-Mg alloy containing a high level of Sc

Primary particles formed in as-cast Al-5Mg-0.6Sc alloy and their role in microstructure and mechanical properties of the alloy were investigated using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD) and tensile testing. It was found that primary particles due to a close orientation to matrix could serve as the potent heterogeneous nucleation sites for α-Al during solidification and thus impose a remarkable grain refinement effect. Eutectic structure consisted of layer by layer of ‘Al{sub 3}Sc + α-Al + Al{sub 3}Sc + ⋯’ and cellular-dendritic substructure were simultaneously observed at the particles inside, indicating that these particles could be identified as the eutectics rather than individual Al{sub 3}Sc phase. A calculating method, based on EBSD results, was introduced for the spatial distribution of these particles in matrix. The results showed that these eutectic particles randomly distributed in matrix. In addition, the formation of primary eutectic particles significant improved the strength of the Al-Mg alloy in as-cast condition, which is ascribed to the structural evolution from coarse dendrites to prefect fine equiaxed grains. On the other hand, these large-sized particles due to the tendency to act as the microcrack sources could cause a harmful effect in the ductility of Al-Mg-Sc alloy. - Highlights: •Primary particles exhibit an ‘Al{sub 3}Sc + α-Al + Al{sub 3}Sc + ⋯’ multilayer feature with a cellular-dendritic mode of growth. •EBSD analyses the mechanism of grain refinement and the distribution of primary particles in α-Al matrix. •A computational method was presented to calculate the habit planes of primary particles.

36 MATERIALS SCIENCE↗
DOE OSTI2016

In-situ EBSD study of deformation behavior of retained austenite in a low-carbon quenching and partitioning steel via uniaxial tensile tests

Through using in-situ electron back-scattered diffraction and uniaxial tensile tests, this work mainly focuses on the deformation behavior of retained austenite (RA) in a low-carbon quenching and partitioning (Q&P) steel. In this paper, three different types of RA can be distinguished from different locations, respectively, RA grains at the triple edges, twinned austenite and RA grains positioned between martensite. The results have shown that grains at the triple edges and twinned austenite could transform easily with increasing strain, i.e. are less stable when compared with RA grains distributed between martensite that could resist a larger plastic deformation. Meanwhile, the strain leads to rotations of RA grains distributed at the triple edges and between martensite. Moreover, RA grains with a similar orientation undergone similar rotations with the same true strain. These RA grains rotated along a specific slip plane and slip direction and the grain rotation is taken as a significant factor to improve the ductility of steel. In addition, grain sizes of RA decreased gradually with an increase of true strain and smaller (0–0.2 μm) grains were more capable of resisting the deformation. According to kernel average misorientation (KAM) analysis, it can be found that strain distribution is preferentially localized near martensite–austenite phase boundaries and in the interior of martensite. The average KAM values increased continuously with increasing true strain. - Highlights: •The in-situ and ex-situ tensile specimens differ to some extent in mechanical properties. •Retained austenite grains at the triple edges and twinned austenite transformed easily at the early stage of true strain. •Film-like retained austenite grains only rotated prior to the transformation during straining. •Retained austenite grains having with a similar orientation experienced similar rotations during the same true strain.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Improving precipitation hardening behavior of Mg−Zn based alloys with Ce−Ca microalloying additions

The precipitation hardening behavior of newly developed Mg−Zn−Ca−Ce alloys, with modified texture and improved ductility, is studied to delineate the microstructural characteristics that lead to effective hardening upon ageing treatments. Advanced electron microscopy and atom probe techniques are used to analyze the structural characteristics in relevance to the hardening potential. It has been found that the formation of a new basal precipitate phase, which evolves from a single atomic layer GP zone, and is finely distributed in both under-aged and peak-aged microstructures, has a significant impact in the improvement of the hardening response compared with the base Mg−Zn alloys. It has also been found that the β′{sub 1} rod precipitates, commonly formed during ageing treatments of Mg−Zn alloys, have their size and distribution significantly refined in the Ca−Ce containing alloys. The role of alloy chemistry in the formation of the fine basal plate GP zones and the refinement in β′{sub 1} precipitation and their relationships to the hardening behavior are discussed. It is proposed that Ca microalloying governs the formation of the GP zones and the enhancement of hardening, particularly in the under-aged conditions, but that this is aided by a beneficial effect from Ce. - Highlights: • Ce−Ca microalloying additions improve hardening in Mg−Zn, over Ce or Ca alone. • Improved hardening is due to refined β′{sub 1} rods, and fine basal plate precipitates. • Atom probe tomography identifies Ca in both β′{sub 1} and the fine basal plates. • The fine basal plates originate as ordered monolayer GP zones with 1:1 Zn:Ca (at.%). • With ageing GP zones become more Zn-rich and transform to the fine basal plates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Characterization of yttrium-rich precipitates in a titanium alloy weld

The yttrium-rich (Y-rich) precipitates that form in the fusion zone (FZ) of a Ti–5Al–1Sn–1Zr–1V–0.8Mo (wt.%) alloy, or Ti-5111, gas-tungsten arc welds (GTAW) were characterized. The filler metal was modified by a small concentration of Y in order to refine the microstructure and thus improve the FZ ductility. A high number density of nanoscale Y-rich precipitates were characterized in the weld FZ by atom probe tomography (APT) and scanning transmission electron microscopy (STEM). - Highlights: •A high number density of nanoscale precipitates were observed in the FZ matrix. •The nanoscale precipitates are enriched in yttrium. •Oxygen and sulfur are also present in the Y-rich precipitates and their interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tensile and high cycle fatigue behaviors of high-Mn steels at 298 and 110 K

Tensile and high cycle fatigue behaviors of high-Mn austenitic steels, including 25Mn, 25Mn0.2Al, 25Mn0.5Cu, 24Mn4Cr, 22Mn3Cr and 16Mn2Al specimens, were investigated at 298 and 110 K. Depending on the alloying elements, tensile ductility of high-Mn steels either increased or decreased with decreasing temperature from 298 to 110 K. Reasonable correlation between the tendency for martensitic tranformation, the critical twinning stress and the percent change in tensile elongation suggested that tensile deformation of high-Mn steels was strongly influenced by SFE determining TRIP and TWIP effects. Tensile strength was the most important parameter in determining the resistance to high cycle fatigue of high-Mn steels with an exceptional work hardening capability at room and cryogenic temperatures. The fatigue crack nucleation mechanism in high-Mn steels did not vary with decreasing tempertature, except Cr-added specimens with grain boundary cracking at 298 K and slip band cracking at 110 K. The EBSD (electron backscatter diffraction) analyses suggested that the deformation mechanism under fatigue loading was significantly different from tensile deformation which could be affected by TRIP and TWIP effects. - Highlights: •The resistances to HCF of various high-Mn steels were measured. •The variables affecting tensile and HCF behaviors of high-Mn steels were assessed. •The relationship between tensile and the HCF behaviors of high-Mn steels was established.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effect of cooling rates on the structure, density and micro-indentation behavior of the Fe, Co-based bulk metallic glass

The experiments demonstrate that ductility of the samples of bulk metallic glass (BMG) with the same chemical composition increased with decreasing sample size. It is shown that microhardness and density increases with decreasing the cooling rate. The fracture morphology of rods after compressive fracture were different on the cross section. Two characteristic features of the compressive fracture morphologies of metallic glasses (MGs) were observed in samples: smooth region and the vein pattern. Many parallel shear bands were observed on the deformed specimen with ϕ = 2 mm in diameter. The results provide more understanding on the relationship among the cooling rate, structure and micro-indentation behavior of the Fe-Co-based BMGs. - Highlights: •Fracture morphology and micro-indentation behavior is studied. •The smaller BMG sample exhibits the larger plasticity. •Microhardness and density increase with decreasing the cooling rate. •Formation of shear bands has been reported in deformed specimens. •Structure and mechanical properties of BMGs can be controlled by the cooling rate.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Microstructures and deformation mechanisms of Cr26Mn20Fe20Co20Ni14 alloys

Highlights: • The single-phase FCC alloy as the annealing temperature exceeding 1015 °C • The dual-phase structure as the annealing temperature lower than 1000 °C • The σ phases grow primarily at grain boundaries. • The single-phase alloy deforms via dislocations and twins. • In-situ tensile experiment reveals twinning at the crack tip. - Abstract: We investigated microstructures and tension properties of Cr{sub 26}Mn{sub 20}Fe{sub 20}Co{sub 20}Ni{sub 14} alloys that were produced by arc-melting and subsequent thermal-mechanical processing—hot-rolling → cold-rolling → annealing at different temperatures ranging from 500 °C to 1100 °C for 1 h → air cooling. Microstructure characterization shows the single-phase solid solution with face centered cubic structure as the annealing temperature exceeding 1015 °C while the dual-phase structure (Cr-rich σ phase and FCC solid solution) as the annealing temperature lower than 1000 °C. The σ phases grow primarily at grain boundaries and act as precipitation strengthening units to enhance strain hardening rate. The single-phase solid solution deforms plastically via dislocations and twins. Twin boundaries associated with deformation twinning impede dislocation motion, enhancing the strain hardening capacity. In-situ tensile experiment in transmission electron microscope further reveals that twinning occurs at the crack tip and facilitates the deflection of crack propagation towards twinning shear direction. This work implies that a strength-ductility combination can be realized by coupling precipitation strengthening and twinning-induced plasticity in multiple principle elements alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hydrogen induced mechanical degradation in tungsten alloyed steels

Highlights: • The effect of H charging on tungsten alloyed steels is assessed by tensile tests. • The HE resistance increased for the Q&T condition compared to the as-Q state. • H trapping and absorption are determined by H characterization methods. • H diffusivity was higher in Q&T condition due to dislocation density decrease. • A correlation between the degree of HE and the amount of mobile H was established. - Abstract: The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W{sub 2}C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A → B → C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Microstructure, texture and tensile behavior of pulsed electrodeposited Ni–Al composites produced using organic additive-free sulfamate bath loaded with Al nanoparticles

Highlights: • Foils of fine grained Ni with Al nanoparticles reinforcement are produced by pulsed electro-codeposition. • Ni–Al composite foils exhibit increased compressive residual stresses with increasing Al content. • Well-dispersed Al nanoparticles refine Ni matrix microstructure and induces significant texture changes. • Ultrafine grained Ni-Al composites exhibit improved hardness and tensile strength. - Abstract: Nickel and Ni–Al composites with varying Al nanoparticle content were produced by pulsed electrodeposition using an organic additive-free sulfamate bath. The as-deposited Ni–Al composites exhibited increased compressive residual stresses with increasing Al nanoparticles. Detailed microstructural investigation revealed that Al codeposition led to significant refinement of the columnar morphology and grain structure of Ni matrix along with a texture change from strong <001> to <011> fiber texture parallel to the growth direction. The as-deposited Ni with bimodal grain structure displayed good tensile strength and ductility. The microhardness and tensile strength of the Ni deposits were increased initially with Al content and then decreased at higher particle content. Particle induced grain refinement and texture changes could be related to change in the growth of upcoming Ni nuclei on to the surface of well-dispersed Al nanoparticles. Significant grain refinement and lesser internal stresses along with good particle distribution and relatively harder <011> fiber texture at lower particles concentration collectively contributed to the observed higher strengths in Ni–Al composites as compared to Ni deposits. There was substantial change in fracture morphology from spherical dimples to flat regions with increasing Al content due to agglomeration of nanoparticles at higher particle loading.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Effect of high content of manganese on microstructure, texture and mechanical properties of magnesium alloy

Highlights: • Extremely refined Mg-3.0Mn alloys are prepared by conventional extrusion process. • Mn addition can effectively weaken the basal texture of the alloys. • Strength, yield anisotropy and ductility have been obviously improved. - Abstract: In the present investigation, microstructure, texture and mechanical properties of Mg-3Mn alloys fabricated by hot extrusion process with the extrusion temperatures of 250 °C and 300 °C were investigated. The results revealed that the samples exhibited the refined microstructure and weakened basal texture. Refined microstructure of the samples was attributed to the dynamic precipitates suppressing the recrystallization grain growth during extrusion process. Weakened basal texture of the samples was explained as a result of particle stimulated nucleation (PSN) promoted by coarsened Mn precipitates altering the texture of extruded profiles during recrystallization by orienting the c-axis of grains at an angle that favors basal slip activity. As a result, the samples exhibited the high tensile elongation and good yield strength at room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ameliorated mechanical and thermal properties of SiC reinforced Al matrix composites through hybridizing carbon nanotubes

Highlights: • The carbon nanotubes were hybridized into SiC reinforced Al based composites. • Hybridization of CNTs leads to the uniform dispersion of SiC in the Al matrix. • Hybridization of CNTs enhances the strength and ductility of the composites. • CNTs can help to decrease the coefficient of thermal expansion of the composites. - Abstract: By hybridizing carbon nanotubes (CNTs) into SiC reinforced Al matrix composites, a novel composite was fabricated to achieve excellent mechanical properties and low coefficient of thermal expansion simultaneously. The study demonstrated that the CNTs can help to acquire the uniform dispersion of SiC particles in the Al matrix, because of their interaction during ball milling and hot rolling processes. In addition, the results revealed that such design optimizes both the mechanical and thermal properties, due to homogeneous microstructure as well as tight and uniform interfaces induced by the addition of CNTs. Strengthening mechanism and Sideridis's model were used to discuss the effects of the CNTs on the mechanical and thermal properties. This study opens up an unexplored avenue for simultaneous improvement of the mechanical and thermal properties of metal matrix composites.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Dynamic shear localization of a titanium alloy under high-rate tension characterized by x-ray digital image correlation

Highlights: • First systematic error analyses for XDIC • First XDIC study of Ti-6Al-4V under dynamic tension • Deformation mode transition induces dynamic ductility decrease. • Dynamic shear localization due to rate-dependent microvoid coalescence process. - Abstract: Dynamic and quasi-static tension experiments are conducted on Ti-6Al-4V alloys, with in situ, synchrotron-based, high-speed, x-ray phase contrast imaging implemented to characterize the dynamic deformation and fracture process of Ti alloys at the Advanced Photon Source. X-ray digital imaging correlation (XDIC) is applied for strain field mapping. The size distribution of x-ray speckles are quantified via a morphological analysis, with a mean of ∼20 μm. Systematic error analyses of displacement and strain field measurements are firstly conducted for XDIC, and demonstrate that the displacement and strain errors can be controlled below 0.01 pixel and 0.1%, respectively. Mesoscale strain characteristics measured via XDIC are consistent with and reveal mechanisms for the bulk-scale stress–strain responses. Under dynamic tension, a sharp transition to strain softening occurs when the bulk strain exceeds about 0.04, which leads to a lower dynamic fracture strain (0.08) than the quasi-static one (0.1). The corresponding strain filed mapping demonstrates that shear deformation localizations grow and coalesce rapidly into a narrow shear deformation band under dynamic loading, while tensile deformation and necking progresses gradually under quasi-static loading. Scanning electron microscopy shows that void nucleation occurs mainly at the interface of α and β phases for both quasi-static and dynamic tension. However, microvoids coalesce preferentially along colony boundaries or the boundary α phase under quasi-static tension, but along the maximum shear stress direction (across colonies) under dynamic tension. Fractography of recovered samples also shows consistent features.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the formation mechanism of solid-solution Cu-to-Cu joints in the Cu/Ni/Ga/Ni/Cu system

Highlights: • The mechanism of Cu/(Ni,Cu,Ga)/Cu solid-solution joint formation is elaborated. • Ga is the dominant diffusion species in the Cu/Ni/Ga/Ni/Cu bonding system. • Loose and columnar Ni{sub 3}Ga{sub 7} layer is formed by the (001)-textured Ni/Ga reactions. • Cu significantly accelerates the growth of Ni{sub 3}Ga{sub 7} layer by enhancing Ga diffusion. • Supersaturated Cu in Ni{sub 2}Ga{sub 3} phase promotes the formation of the final fcc phase. - Abstract: Copper (Cu)-to-Cu interconnection is crucial in electronic packaging for applications ranging from three-dimensional integrated circuits to die-attachment in wide band-gap devices. Solid-solution phases are innate with better ductility and electrical and thermal conductivity over intermetallic compounds, which are brittle, electrical resistant, and unstable; yet are commonly seen in electronic joints. The desired solid-solution Cu-to-Cu joints have recently been demonstrated using transient-liquid-phase bonding with gallium (Ga) as the filler material to bond Cu substrates with electroplated nickel (Ni) as under-bump-metallurgy. The Cu/Ni/Ga/Ni/Cu couples can fully transform into the Cu/face-centered cubic (fcc)-(Ni,Cu,Ga)/Cu solid-solution joints. However, why and how the solid-solution joints were formed remains unclear. Here we proposed the formation mechanism of the fcc-(Ni,Cu,Ga) solid-solution joints based on five sets of Cu-Ni-Ga couples, namely the Ni/Ga, Ni/Cu/Ga, Ni/Ni/Ga, (Cu,Ni)/Ga, and Cu/Ni/Ni{sub 3}Ga{sub 7}/Ni{sub 3}Ga{sub 7}/Ni/Cu reactions. The roles of each element in the Cu-Ga-Ni interactions as well as the reaction progressions are elaborated.

36 MATERIALS SCIENCE↗