Formation Of Kirkendall Pores Research Articles

Transient liquid phase sintering in spark plasma sintered tungsten heavy alloys with Nb and Mo additives

In the present investigation spark plasma sintered tungsten heavy alloys of composition 90W-7Ni-3Fe, 84W-7Ni-3Fe-6Mo, and 84W-7Ni-3Fe-6Nb were analyzed to determine the effect of Mo and Nb addition on microstructure and phase formation. The alloys were sintered at temperatures ranging from 1150 °C to 1275 °C and at a fixed sintering pressure of 30 MPa. Nb added alloy exhibited a maximum relative density of 99% at sintering temperature of 1200 °C. Whereas the base alloy and Mo added alloy, exhibited a maximum relative density of 97.9% and 95.9% at 1150 °C respectively. Transient liquid phase sintering was found to play a crucial role in the base alloy and Mo added alloy whereas the Nb added alloy was found to undergo persistent liquid phase sintering. Microstructure characterisation by SEM revealed formation of finer tungsten grains with average grain size ranging from 1 μm to 4.3 μm. Mo addition restricted the tungsten grain growth while Nb addition promoted the tungsten grain growth compared to the base alloy. Phase analysis by XRD revealed the formation of Ni4W intermetallic (at lower temperatures) for the base alloy, NbO2 and NbW intermetallics for Nb added alloy and MoNi rich phase for Mo added alloys. Hardness measured by Vickers hardness method revealed a declining trend in hardness with increase in sintering temperature, owing to tungsten grain growth in all the alloys and along with Kirkendall pore formation and growth for the base alloy and Mo added alloy. Nb addition resulted in maximum hardness of 678 HV1 which is highest ever reported for SPSd WHA with 90 wt% concentration of W.

Enhancing AISI 321 stainless steel's high-temperature oxidation resistance at 600 °C through surface nanocrystallization and pre-oxidation synergy

This study explores the high-temperature oxidation behavior of AISI 321 austenitic stainless steel (ASS) at 600 °C in an Ar-21vol.%O2 atmosphere, considering nanometer-sized surface grain size and pre-oxidation conditions. After initial solution annealing at 1050 °C, AISI 321 ASS samples were water-quenched for homogenized microstructure (SAed samples). Moreover, the surface nanocrystallization of AISI 321 ASS was performed through high-energy shot peening (SNCed samples), reducing the surface grain size to ~75 ± 5 nm. Pre-oxidation occurred at 400 °C for 150 h in Ar-21%O2 for both SAed and SNCed samples. High-temperature oxidation kinetics was analyzed at 600 °C for 150 h using thermal gravimetric analysis (TGA) on pre-oxidized and non-preoxidized SAed and SNCed samples. FESEM/GDOES and GI-XRD revealed that SNCed samples exhibited improved oxidation resistance, showing no Cr depletion in the subsurface oxide layer up to 100 h, and only minor signs after 150 h. Pre-oxidized SNCed samples demonstrated superior oxidation performance, showing no Cr depletion and subsurface Kirkendall pore formation even after 150 h. These findings are coupled with diffusion relationships, and propose potential mechanisms for enhancing the oxidation behavior of pre-oxidized SNCed AISI 321 stainless steel.

Sintering inhibition enables hierarchical porosity with extreme resistance to degradation during redox cycling of Fe-Mo foams

High-temperature (800 ºC) steam-hydrogen redox cycling, relevant to grid-scale energy storage, is studied for iron-based freeze-cast lamellar foams. In contrast to previously studied Fe, Fe-Ni, and Fe-Co foams that rapidly degrade, Fe-25Mo foams feature a much-enhanced structural damage resistance. Utilizing in-situ x-ray diffraction, microscopy, and x-ray tomography, strong sintering inhibition is observed in Fe-Mo foams, creating a hierarchically porous lamellar structure. This leads to (i) wide channels between lamellae, enabling high macroscopic porosity (∼78%) which can accommodate gas flow as well as volumetric expansion without lamellar contact, and (ii) microporosity within lamellae, providing additional free volume to accommodate expansion during oxidation, limiting both swelling of the lamellae and the formation of Kirkendall pores. These combined effects enable a near-complete reversibility of the microstructure during cycling, preventing damage produced via internal lamellar buckling, cracking, contacting and sintering, with a remarkably high porosity (65%) remaining after 50 consecutive redox cycles.

Formation of Kirkendall Pores in Aluminized Steel Sheets and Effect of Si in Aluminizing Bath

Formation of Kirkendall Pores in Aluminized Steel Sheets and Effect of Si in Aluminizing Bath

Development of hybrid electrodeposition/slurry diffusion aluminide coatings on Ni-based superalloy with enhanced hot corrosion resistance

Ni/Co-modified aluminide coatings were prepared on Hastelloy-X superalloy by a combined process of electrodeposition and slurry aluminizing. In this regard, pure layers of Ni and Ni-50wt.%Co were initially applied via electrodeposition process and successive aluminization was carried out by a slurry technique. The microstructural and chemical composition characterization of the specimens revealed that a compact and dense aluminide coating was formed with two-layered structure containing the outer Al-rich b phase and inner interdiffusion zone. Moreover, the presence of pre-electrodeposited layers inhibited the outward diffusion flux of elements from the substrate and effectively suppressed the formation of Kirkendall pores. The hot corrosion studies of the obtained coatings indicated that the addition of a pre-electrodeposited layer could enhance the high-temperature corrosion performance of the coatings when exposed to sulfate salt.

The Cyclic Hot Corrosion Behavior of Pt–Al–7%YSZ Coating

In this study, the microstructure and cyclic oxidation of Pt–Al–7%YSZ thermal barrier coating applied on Rene-80 superalloy were investigated after Type I hot corrosion. The field emission scanning electron microscope and X-ray diffraction were used to evaluate the microstructure and phase identification, respectively. Na2SO4 was added to the surface of the coating for hot corrosion tests. The tests were carried out by the repeated cycles of heating at 900 °C for 30 and 60 min and rapid cooling to 400 °C in 10 min. Before the cyclic hot corrosion test, the specimens were coated with the platinum electroplating, high-activity low-temperature aluminizing, and the thermal spray of 7% yttria-stabilized zirconia (YSZ), respectively. The results showed that the cyclic hot corrosion mechanisms were the basic fluxing of primary thermally grown oxide (TGO), the formation of chromia, spinel and nickel oxides (C.S.N oxides) and Ni–Cr perovskites on the oxide scale. The type I cyclic hot corrosion resistance of Pt–Al–7%YSZ coating decreased with the number of thermal cycles due to a decrease in Al, an increase in Cr concentration, and increase in the volume fraction of C.S.N oxides and Ni–Cr perovskites in the oxide scale. As the number of thermal cycles increased, the oxide scale growth stresses and thermal mismatch stresses increased, leading to TBC degradation by the rumpling and ratcheting mechanisms in the Pt–Al/TGO interface. However, in lower thermal cycles, internal hot corrosion, and the formation of Kirkendall pores at the substrate/Pt–Al interface caused TBC degradation.

Microstructural evolution and mechanical properties of bulk and porous low-cost Ti–Mo–Fe alloys produced by powder metallurgy

A series of low-cost Ti–10Mo-xFe (x = 1, 5, 9 wt%) alloys were fabricated using conventional press and sinter powder metallurgy to investigate the effect of Fe content on their phase stability, sintering response, microstructure and mechanical properties. Phase analyses indicated that all alloys were composed of α, β and intermetallic phases. However, Fe additions increased the proportion of β and intermetallic phases and reduced the propensity of the alloy system to form α phase during slow furnace cooling. Densification of the Ti–10Mo-xFe alloys was subjected to two contradicting effects; a strong sintering response caused by the fast diffusion rate of Fe atoms which promotes densification and formation of Kirkendall pores related to the fast diffusion rate of Fe atoms in conjunction with comparatively slower diffusion of Ti and Mo. Nonetheless, the porosity level of the alloys was less than that of the sintered CP-Ti. Depending on the content of α, β and intermetallic phases, the alloys exhibited varied mechanical properties. It was found that Ti–10Mo–5Fe presented the best combination of mechanical properties including the highest compressive strength (2392 MPa) and strain (43%) and low elastic modulus (91 GPa) superior to the corresponding ones for the commonly used CP-Ti and some other Ti-based alloys. Porous Ti–10Mo–5Fe alloy was also fabricated by addition of 30 and 60 vol% ammonium hydrogen carbonate (NH4HCO3) space holder which generated sintered scaffolds with 25% and 52% porosity, respectively, with an increased effective pore size at higher porosity. This reduced the compressive strengths to 649 MPa and 168 MPa and the elastic moduli to 34 GPa and 16 GPa, respectively. This study demonstrates that Ti–Mo–Fe alloys offer significant savings on raw materials compared to current commercial and many recently developed biomedical Ti alloys. It also shows that porous Ti–10Mo–5Fe scaffolds are promising for hard tissue engineering applications offering mechanical properties which closely match with the human bone and optimal pore sizes essential for bone ingrowth.

Erratum to “Effect of Cr content on interdiffusion and Kirkendall pore formation during homogenization of pack-aluminized Ni and Ni–Cr wires” [Intermetallics 101 (2018) 108–115

Erratum to “Effect of Cr content on interdiffusion and Kirkendall pore formation during homogenization of pack-aluminized Ni and Ni–Cr wires” [Intermetallics 101 (2018) 108–115

Study of the high temperature oxidation and Kirkendall porosity in dissimilar welding joints between FE-CR-AL alloy and stainless steel AISI 310 after isothermal heat treatment at 1150 °C in air

Dissimilar welded joints of Fe-Cr-Al alloy (23Cr-7Al) and AISI 310 austenitic stainless steel (25Cr-20Ni) were studied and characterized, before and after the isothermal heat treatment (IHT) of oxidation at 1150°C for 936h in air. The Fe-Cr-Al alloy was used as the welding consumable and the welding process used was gas-shielded tungsten arc welding (GTAW) with currents of 60, 80 and 130amps, and welding speed 1, 1.1 and 1.3mm/s. In the heat treated specimens, pore formation was observed as well as significant variations of Al contents in all-welded metals and formation of protective layer of alumina-Al2O3 on the oxidized surfaces particularly in the sample 1, welded at 60amps and welding speed of 1mm/s, that showed higher pore index and a greater deflection in the decrease of the Al content. The formation of Kirkendall pores occur due to the rapid diffusion of the aluminum from the all weld metals to the AISI 310 austenitic stainless steel, as was detected by the sample 1 mainly and especially by identifying the chromium content of the dissimilar alloy and the stainless steel.

Microstructure of Bare and Sol–Gel Alumina-Coated Nickel-Base Alloy Inconel 625 After Long-Term Oxidation at 900 °C

Though Ni-based superalloys show a high oxidation and corrosion resistance, coatings can still improve these properties, especially if used at temperatures up to 1000 °C. Here, a coating was prepared by applying a boehmite-sol via dip-coating and a subsequent heat treatment at 600 °C for 30 min. To evaluate the coating, the oxidation behavior of bare and alumina-coated Ni-base alloy Inconel 625 in air at 900 °C was studied for up to 2000 h. Electron microscopy studies of sample surfaces and cross sections showed that (1) in the 3.5–6.3 µm-thick scale formed on the bare alloy, Fe and Ni are located as fine precipitates at the grain boundaries of the chromia-rich scale, (2) Ni and Ti are concentrated to a minor degree at the grain boundaries of the scale, too; and for the coated sample: (3) the only 1.8-µm-thick sol–gel alumina coating slows down the formation of chromia on the alloy surface and reduces the outward diffusion of the alloy constituents. The protective effect of the coating was evidenced by (1) diminished chromium diffusion at grain boundaries resulting in less pronounced string-like protrusions at the outer surface of the coated IN 625, (2) formation of a Cr-enriched zone above the alloy surface which was thinner than the scale on the uncoated sample, (3) lower extension in depth of Cr depletion in the superficial zone of the alloy surface of the coated sample in comparison with that region of the uncoated one, and (4) a narrower zone of formation of Kirkendall pores.

Nucleation and growth of TiAl3 intermetallic phase in diffusion bonded Ti/Al Metal Intermetallic Laminate

A novel nucleation and growth phenomenon for TiAl3 intermetallic phase in Ti/Al diffusion couple is proposed based on diffusion kinetics. The interdiffusion and intrinsic diffusion co-efficients are calculated to make evident of dominant diffusion of Al towards Ti in Ti/Al diffusion couple obtained by solid state diffusion bonding. It was surprising to observe that the diffusion rate of Al was around 20 times higher than Ti with the formation of Kirkendall pores near the Al/TiAl3 interface. With such dominant diffusion of Al towards Ti, the nucleation and growth of TiAl3 intermetallic phase in Ti/Al couple happens mainly at the Ti/TiAl3 interface rather than Al/TiAl3 interface which is evident by the presence of very fine nearly nano-sized TiAl3 nuclei/grains near the Ti/TiAl3 interface. Even though the intermetallic phase is expected to nucleate at Al/TiAl3 interface, the relatively larger TiAl3 grains near that interface depicts grain growth with minimal nucleation. The theoretical calculations on diffusion parameters are in accordance with experimental observations of TiAl3 intermetallic growth phenomenon in Ti/Al system.

Effect of Cr content on interdiffusion and Kirkendall pore formation during homogenization of pack-aluminized Ni and Ni-Cr wires

Using both ex situ metallographic imaging and in situ X-ray tomographic microscopy, we investigate the kinetics of Al- and Ni-interdiffusion during homogenization at 825–1100 °C for surface-aluminized, 50 μm diameter Ni wires with 0, 10 or 20 wt%Cr. Kirkendall pores, which are created due to imbalanced diffusion of atomic species, are not observed at any of the homogenization temperatures in the Cr-free Ni-Al wires, which equilibrate to Ni-rich β-NiAl. By contrast, during homogenization of the aluminized Ni-10Cr and Ni-20Cr wires to β-NiAl(Cr) at 1000 and 1100 °C, numerous Kirkendall pores are created within the wire volume, indicating that the addition of Cr significantly increases the imbalance between the Ni and Al diffusivities. These pores eventually coalesce into a single cavity, with crescent-shape cross-sections and high aspect ratio aligned with the axis of the wires, so that a tubular β-NiAl(Cr) structure is formed. Tomography shows that the Ni-rich β-NiAl(Cr) reaction layer grows radially within the Ni-10Cr and Ni-20Cr wires annealed in situ at 825, 900, and 1000 °C according to a parabolic law. The growth kinetics of this layer increase slightly with increasing Cr content and obey an Arrhenius relationship, from which an activation energy of ∼200 kJ/mol is calculated, in good agreement with literature values for interdiffusion in binary NiAl. The two methods, ex situ metallography and in situ X-ray tomography, are complementary. While tomography very rapidly acquires numerous cross-sectional images showing phase contrast on a single wire, thus replacing interrupted annealing and destructive imaging of multiple samples, metallography has higher spatial resolution and can identify additional phases. In the present case, metallography revealed that α-Cr precipitates form during homogenization of the aluminized Ni-Cr wires due to the limited solubility of Cr in β-NiAl; upon full homogenization, these precipitates re-dissolved in the aluminized Ni-10Cr wires, but remained stable in the Ni-20Cr wires.

Interfacial Microstructure Evolution and Shear Behavior of Au-Sn/Ni-xCu Joints at 350°C

Interfacial reaction and shear behavior of the joints between Au-29Sn (at.%) solder and Ni-xCu (x = 20 at.%, 40 at.%, 60 at.%, and 80 at.%) substrate alloys soldered at 350°C for various durations were investigated in this study. The results show that α(Au) is the common reaction product at the solder/substrate interfaces after a short-time reaction regardless of Cu content. As soldering goes on, another new Ni3Sn2 layer forms at the interface company with ordering of the α(Au) phase, AuCu I/Ni3Sn2 bi-layers formed at the Au-Sn/Ni-20Cu interface, or with AuCu III/Ni3Sn2 bi-layers at the Au-Sn/Ni-40Cu interface. If the content of Cu in the substrate is higher than 40 at.%, periodic layered structure and discontinuous Ni3Sn2 layers appear. In the couple of Au-Sn/Ni-60Cu, AuCu I + AuCu III/Ni3Sn2/α(Au) can be observed while AuCu3/Ni3Sn2/α(Au) forms in the couple of Au-Sn/Ni-80Cu. Shear fracture always occurs in the region near the Ni-20Cu substrate in Au-Sn/Ni-20Cu joints, whereas it appears in the reaction layer for the joint of higher Cu content. The shear strength of Au-Sn/Ni-60Cu and Au-Sn/Ni-80Cu joints achieves about 55 MPa as α(Au) phase forms but decreases remarkably due to pore formation after soldering for a long duration. Whereas, the shear strength of Au-Sn/Ni-40Cu joints can reach 62 MPa as the α(Au) phase forms at an early stage, and maintains above 52 MPa even soldered for a long duration because of the adequate thick α(Au) and AuCu III layer adjacent to substrate provides good bonding. The reason why the soldering joint of Au-Sn/Ni-40Cu possesses higher strength and a better stability exists is that high Ni concentration in α(Au) and the continuous Ni3Sn2 layer inhibit formation of Kirkendall pores.

The effects of ball milling and the addition of blended elemental aluminium on the densification of TiH2 power

Powder production and sintering processes of titanium powder metallurgy are studied in order to reduce the cost of manufacturing titanium parts. The direct sintering of TiH2 simplifies powder processing and helps densification during sintering. The effects of ball milling have been shown to improve the dehydrogenation of TiH2, which during sintering, causes the earlier dissolution of the oxide layer surrounding the solid titanium particles, allowing solid diffusion to occur earlier. The combined effect of reduced particle size and increased dehydrogenation of ball milled TiH2 are studied here during sintering, in order to understand their roles during densification. TiH2 ball milled for various durations is vacuum hot press sintered at 700–900 °C. The density, microstructure and hardness of the fabricated titanium specimens are also studied. The effects of ball milled powder on the introduction of blended elemental aluminium are studied in comparison with a commercial titanium powder. The increase in ball milling duration of TiH2 powders to reduce particle size causes faster densification and dehydrogenation, with a finer uniform microstructure. When using fine ball milled powder with aluminium, the formation of Kirkendall pores through the interaction of titanium and liquid aluminium is worsened.

In situ X-ray tomographic microscopy of Kirkendall pore formation and evolution during homogenization of pack-aluminized Ni–Cr wires

The Kirkendall effect, usually associated with the formation of undesirable fields of micron-size pores, can be used to fabricate hollow nano- and microstructures, where all pores have coalesced into a single cavity. Here, this concept is used with the goal of converting 50 μm diameter Ni-20wt.%Cr wires into Ni–Cr–Al tubes by diffusion of a surface-deposited aluminide coating. The formation and evolution of Kirkendall pores, created by imbalanced diffusion fluxes of Al, Ni and/or Cr, are investigated using both in situ X-ray tomographic microscopy and ex situ annealing studies. The tomography results indicate that the pores nucleate near the interface between the Ni–Cr core and β-NiAl(Cr) reaction layer that develops upon homogenization. They then grow and coalesce near the center of the radial cross-section of the wire. This is followed by a stagnation period until the pores migrate longitudinally due to the influence of a temperature gradient imposed by the laser heating used for annealing. The ex situ experiments are in qualitative agreement with the tomography study with the exception of the secondary pore migration along the wire length, as expected because the ex situ study is conducted under isothermal conditions. Detailed features of the microstructure were discernable only upon metallographic preparation and include Cr-rich precipitates and an α-Cr(Al,Ni) rejection region that formed during coating and homogenization, respectively. In situ X-ray tomographic microscopy is thus demonstrated to be a viable technique to use in conjunction with more conventional ex situ metallographic techniques to conduct interdiffusion and Kirkendall pore studies.

Synthesis of Ti–Ta alloys with dual structure by incomplete diffusion between elemental powders

In this work, powder metallurgical (PM) Ti–Ta alloys were sintered using blended elemental powders. A dual structure, consisting of Ti-rich and Ta-rich zones, was formed due to the insufficient diffusion between Ti and Ta powders. The microstructure, mechanical properties and in vitro biological properties of the alloys were studied. Results indicated that the alloys have inhomogenous microstructures and compositions, but the grain structures were continuous from the Ti-rich zone to the Ta-rich zone. The Ta-rich zone exhibited a much finer grain size than the Ti-rich zone. The alloys had a high relative density in the range of 95–98%, with the porosity increasing with the content of Ta due to the increased difficulty in sintering and the formation of Kirkendall pores. The alloys had a good combination of low elastic modulus and high tensile strength. The strength of alloys was almost doubled compared to that of the ingot metallurgy alloys with the same compositions. The low elastic modulus was due to the residual pores and the alloying effect of Ta, while the high tensile strength resulted from the strengthening effects of solid solution, fine grain size and α phase. The alloys had a high biocompatibility due to the addition of Ta, and were suitable for the attachment of cells due to the surface porosity. It was also indicated that PM Ti–(20–30)Ta alloys are promising for biomedical applications after the evaluations of both the mechanical and the biological properties.

Nanoscale Cellular Structures at Phase Boundaries of Ni-Cr-Al-Ti and Ni-Cr-Mo-Al-Ti Superalloys

The microstructural evolution of Ni-20 pct Cr wires was studied during pack cementation where Al and Ti, with and without prior cementation with Mo, are deposited to the surface of the Ni-Cr wires and subsequently homogenized in their volumes. Mo deposition promotes the formation of Kirkendall pores and subsequent co-deposition of Al and Ti creates a triple-layered diffusional coating on the wire surface. Subsequent homogenization drives the alloying element to distribute evenly in the wires which upon further heat treatment exhibit the γ + γ′ superalloy structure. Unexpectedly, formation of cellular structures is observed at some of the boundaries between primary γ′ grains and γ matrix grains. Based on additional features (i.e., ordered but not perfectly periodic structure, confinement at γ + γ′ phase boundaries as a cellular film with ~100 nm width, as well as lack of topologically close-packed phases), and considering that similar, but much larger, microstructures were reported in commercial superalloys, it is concluded that the present cellular structure solidified as a thin film, composed of eutectic γ + γ′ and from which the γ′ phase was subsequently etched, which was created by incipient melting of a region near the phase boundary with high solute segregation.

Interdiffusion induced defect microstructures of Co 1− xO near confined zirconia particles

Diffusion couples of metal alloys were commonly used to study Kirkendall pore formation and amorphization near the interface of composition discontinuity. In this work, yttria–partially stabilized zirconia (Y–PSZ) powders mixed with Co 1− x O (1:99 molar ratio, denoted as Z 1C 99) were sintered and annealed at 1600 °C for 1–75 h to study interdiffusion-induced defect microstructures of Co 1− x O near confined Y–PSZ particles. Analytical electron microscopic analysis indicated a much larger flux of Co 2+ from Co 1− x O into Y–PSZ compared to the reverse flux of Zr 4+ and Y 3+. In addition, there is a significant inward flux of oxygen from Y–PSZ into Co 1− x O. A net vacancy flux in the opposite direction of ion flux then caused the formation of Kirkendall pores and dislocations in Co 1− x O near corrugated Y–PSZ/Co 1− x O interface. Amorphous regions and {111} faulting were also found in Co 1− x O near Kirkendall pores, which can be rationalized by diffusion-induced amorphization and assembly of cation vacancies along close-packed {111} planes vulnerable to crystallographic shear, respectively. Y–PSZ particles were found to migrate-coalesce as corrugated slabs and rods at Co 1− x O grain boundaries and junctions, respectively, when the composite with more Y–PSZ additive (Z 1C 89) was annealed.

Interdiffusion-Induced Phase Changes of Co1−xO/Zirconia Composites

Co1−xO powders mixed with pure zirconia (ZrO2) or yttira (Y2O3) partially stabilized zirconia (Y-PSZ) were sintered and annealed at 1300°C for 300 h, and the Co1−xO/Y-PSZ composite also at 1600°C for 1–100 h to investigate interdiffusion-induced phase changes. Analytical electron microscopic observations indicated the formation of (Zr,Y)-codoped Co3O4 spinel from intra- and intergranular Co1−xO particles for all the fired composites. The spinel formation above the equilibrium temperature 900°C for pure Co1−xO/Co3O4 pair can be rationalized by the substitution of Zr4+ (<2mol%) and Y3+ (<1mol%) for Co2+ to generate a considerable number of charge- and volume-compensating defects and paracrystalline array of defect clusters in the Co1−xO lattice. The (111) faulting of (Zr,Y)-codoped Co3O4 implies the possible presence of Zinc blende-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane of Co1−xO. About 6mol% Co2+ dissolution of Y-PSZ was found to stabilize the cubic-phase of zirconia upon cooling to room temperature. A relatively larger flux of Co2+ from Co1−xO into Y-PSZ than the opposite flux of Zr4+ and Y3+ causes a net vacancy flux, hence the formation of Kirkendall pores around the Co1−xO particles.

Degradation of TAB outer lead contacts due to the Au-concentration in eutectic tin/lead solder

The influence of the Au-concentration in OLB solder fillets on the contact reliability is shown. Quantitative analysis of the Au content shows a good accordance with the estimated concentrations using the geometrical data. The Au-concentration is the major influence factor on the thermal aging behavior of OLB contacts. A change in the failure mechanism due to Kirkendall porosity is observed if a Au-concentration of 9 wt% in the solder fillet is reached. The pores formed during diffusion at the interface between copper and the ternary intermetallic compound Cu/sub 3/Au/sub 3/Sn/sub 5/ cause a strong degradation of pull forces. For low Au-concentrations, the degradation of pull-forces and the growth of the ternary compounds CuAu/sub 5/Sn/sub 5/ and CuAu/sub 4/Sn/sub 5/ have comparable activation energies in the range of 0.3 eV. For higher Au-concentrations the two effects show different values. The activation energy of pull-force degradation increases significantly up to 0.67 eV, whereas the growth constant of the ternary compound shows a small increase to 0.4 eV. The investigations show evidence of a critical Au-concentration in OLB contacts. Kirkendall pore formation causes higher activation energies for the pull-test degradation compared to the activation energy for the growth rate of the ternary compound. >