Cu-rich Intermetallics Research Articles

A dual-precipitation strategy for simultaneously enhancing the strength and ductility of cost-effective medium Mn steels at low temperatures

A dual-precipitation strategy has been utilized to simultaneously improve the strength and ductility of the cost-effective medium Mn steels for cryogenic applications. When the temperature decreases from room temperature (RT) to liquid nitrogen temperature (LNT), the dual-precipitation state, including NiAl and Cu-rich intermetallics in ferrite and austenite respectively, exhibits a simultaneous increase in the yield strength (YS, to ∼1160 MPa) and tensile elongation (TEL, to ∼40 %), and a significantly higher secondary strain hardening. On the one hand, the NiAl precipitates can strengthen the ferrite matrix and provide dislocation storage capacity for strain accommodation. On the other hand, the Cu-rich precipitates can stabilize the austenite matrix, activate the formation of microbands (MBs) and delay the transformation-induced plasticity (TRIP) effect until large strains. Therefore, multistage cooperative precipitation hardening, microband-induced plasticity (MBIP) and TRIP give rise to comprehensive strength-ductility interactions, which are comparable to those of the best-performing expensive cryogenic materials.

A Study of the Localized Ceria Coating Deposition on Fe-Rich Intermetallics in an AlSiFe Cast Alloy.

Corrosion inhibiting conversion coating formation is triggered by the activity of micro-galvanic couples in the microstructure and subsequent local increase in pH at cathodic sites, which in the case of aluminium alloys are usually intermetallics. Ceria coatings are formed spontaneously upon immersion of aluminium alloys in a cerium conversion coating solution, the high pH gradient in the vicinity of intermetallics drives the local precipitation of ceria conversion compounds. Cu-rich intermetallics demonstrate a highly cathodic nature and have shown the local precipitation reaction to occur readily. Fe-rich intermetallics are, however, weaker cathodes and have shown varied extents of localized deposits and are in focus in the current work. Model cast Al-7wt.%Si alloys have been designed with 1 wt.% Fe, solidified at different cooling rates to achieve two different microstructures, with big and small intermetallics, respectively. Upon subjecting the two microstructures to the same conversion coating treatment (immersion in a 0.1 M CeCl3 solution) for a short period of 2 h, preferential heavy deposition on the boundaries of the big intermetallics and light deposition on the small intermetallics was observed. Based on these observations, a mechanism of localized coating initiation at these Fe-rich intermetallic particles (IM) is proposed.

The role of microstructure and cathodic intermetallics in localised deposition mechanism of conversion compounds on Al (Si, Fe, Cu) alloy

Cerium-based conversion coating formation is triggered by a local rise in pH at cathodic sites produced by the oxygen reduction reaction. Therefore, size, morphology distribution and electrochemical potential of those sites play a crucial role. While the deposition reaction is sensitive to both immersion bath concentrations and underlying substrate microstructure, only the former has been widely investigated. This research attempts to fill the gap by studying the effect of controlled microstructure variables like the cathodic intermetallics' geometry and spatial distribution on the conversion compound deposit initiation. A controlled cast Al alloy was synthesised for this study and consisted of distinct cathodic phases: Cu-rich intermetallics, Fe-rich intermetallics and Si particles. The localised deposition preferentially formed only on strong cathodic Cu-rich intermetallics. Size (surface area) of the Cu-rich intermetallic correlated linearly with the deposited area over it in terms of lateral and z-direction spread. The pH gradient occurring from the oxygen reduction near an IM is very local and does not affect pH gradients of a neighbouring IM. When immersion time is increased from 0.5 h to 1 h, the percentage of Cu-rich IM covered with conversion coating increases. Big Cu-rich IM activate faster for deposition reaction than small Cu-rich IM.

Friction stir welding and explosive welding of aluminum/copper: process analysis

ABSTRACTThe 6082 aluminum alloy was welded to copper-DHP by friction stir welding and explosive welding. The effect of each welding process on the microstructural evolution, the intermetallic phases distribution, and the mechanical behavior of both types of welds was analyzed and compared. The microstructural changes proved to be much more expressive in friction stir welding due to the larger area under plastic deformation, the stirring and mixing of the alloys, the longer time under high temperature, and the longer interaction times between the base materials during welding. As explosive welding process is much faster, it avoids extensive microstructural changes and significant interaction of the materials, reducing the intermetallic volumes and their distribution along the interface. The friction stir welds presented Cu-rich intermetallics while the explosive welds presented Al-rich intermetallics. For alloys that can easily form brittle intermetallic phases, excessive interaction during the welding process leads to a very poor mechanical behavior of the joints.

Mechanism to improve the reliability of copper wire bonding with palladium-coating of the wire

Pd-coating of Cu (PCC) bond wires has been widely employed to improve the reliability of Cu wire bonding technology for microelectronics. Description of the underlying mechanism, however, is lacking in literature. We provide an interpretation of how the improvement is induced. After an un-biased highly accelerated stress test (uHAST), the most Cu-rich intermetallics (IMC) were Cu3Al2 under the bare Cu bond wire, and predominantly (CuPdx)Al under the PCC wire. The Pd-coating therefore had suppressed the formation of (CuPdx)3Al2. The suppression stemmed from the improvement of the thermodynamic stability of (CuPdx)Al, which also decreased the kinetics of phase transformation toward (CuPdx)3Al2. The cathode/anode area ratio of (CuPdx)Al is lower than that of Cu3Al2. The corrosion rate of (CuPdx)Al therefore becomes lower, and the reliability of the wire bonding increases. Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to similarly improve the corrosion resistance. Furthermore, the use of Al wires on Al alloy pads will implement a monometallic system across the bonding interface which can also enhance the corrosion resistance. The underlying correlation between the atomic ratio of (Inert Element)/(Active Element) of the IMC and the corrosion rate of the IMC revealed here is pertinent to the research in other fields including the corrosion of alloys/dealloying and nanoporosity, and the making of functional materials with nanoporosity for applications such as catalysis and advanced energy technologies.

Quality index and hot tearing susceptibility of Al–7Si–0.35Mg– x Cu alloys

Abstract The effects of Cu addition (0.5%, 1%, 1.5%, 2%, and 3%, mass fraction) on the quality index (Qi) and hot tearing susceptibility (HTS) of A356 alloy were investigated. According to the results, Cu addition up to 1.5% increases the Qi by almost 10%, which seems to be due to its solid solution strengthening and dispersion hardening effect of Cu-rich Al2Cu and AlMgCuSi compounds. However, further addition of Cu (up to 3%) decreases the Qi by almost 12%, which is likely due to the reduction of tensile strength and elongation caused by increased volume fraction of brittle Cu-rich intermetallics and microporosities in the microstructure. It is also found that Cu increases the HTS of A356 alloy measured by constrained rod casting method. According to the thermal analysis results, Cu widens the solidification range of the alloy, which in turn, decreases its fluidity and increases the time period during which the mushy-state alloy is exposed to the hot tearing susceptible zone. SEM examination of the hot tear surfaces in high-Cu alloys also demonstrates their rough nature and the occurrence of interdendritic/intergranular microcracks as convincing evidences for the initiation of hot tears in the late stages of solidification in which there is not enough time for crack healing.

Development of in-Situ Al-Si/CuAl₂ Metal Matrix Composites: Microstructure, Hardness, and Wear Behavior.

In the present work, in-situ metal matrix composites were fabricated through squeeze casting. The copper particles were dispersed with different weight percentages (3%, 6%, 10%, and 15%) into Al-12% Si piston alloy. Also, heat treatments were performed at 380 °C and 450 °C for holding times of 6 and 18 h. The microstructures, X-ray diffractometer (XRD) pattern, hardness, and wear characteristics were evaluated. The results showed that these copper particles have reacted with the aluminum under all of the aforementioned processing conditions resulting in the formation of fine copper aluminide intermetallics. Most of the intermetallics were CuAl2, while AlCu appeared in a small ratio. Additionally, these intermetallics were homogenously distributed within the alloy matrix with up to 6% Cu addition. The amounts of those intermetallics increased after performing heat treatment. Most of these intermetallics were CuAl2 at 380 °C, while the Cu-rich intermetallics appeared at 450 °C. Increasing the holding time to 18 h, however, led to grain coarsening and resulted in the formation of some cracks. The hardness of the resulting composite materials was improved. The hardness value reached to about 170 HV after heat treating at 380 °C for 8 h. The wear resistance of the resulting composite materials was remarkably improved, especially at lower additions of Cu and at the lower heat treatment temperature.

Fabrication of In-Situ Al–Cu Intermetallics on Aluminum Surface by Friction Stir Processing

In-situ Al–Cu intermetallic metal matrix composites (MMCs) surface layer has been fabricated on an A 1050-H24 aluminum plate by friction stir processing (FSP). Pure Cu powder of \({5{\rm \mu}}\)m in size was packed into a groove cut on the aluminum plate and covered with an aluminum sheet. A FSP tool of square probe shape, rotated at a rate of 750–1500 rpm, was plunged into the plate through the cover sheet and the groove and moved along the groove at traveling speed of 0.83 and 1.66 mm/s. Double and triple passes were applied. After the surface MMCs were fabricated on the Al plate, the type and homogeneity of the distributed particles inside the Al matrix have been evaluated from the macro-/microstructure and hardness distribution. As a result, it was found that the copper particles reacted with the aluminum matrix at almost all the processing conditions even during the first pass. Also, at a traveling speed of 1.66 mm/s, most of the resulted intermetallics were CuAl2 at a rotation speed of 750 rpm, while the Cu-rich intermetallics (Al–Cu and Al4Cu9) appeared at rotation speeds of 1000 and 1500 rpm. Moreover, when the traveling speed was decreased to 0.83 mm/s, the amounts of the resulted intermetallics were increased. At a rotation and traveling speeds of 750 rpm and 0.83 mm/s, respectively, the nugget zone matrix consisted of nano-sized CuAl2 intermetallics distributed in aluminum which increase the hardness to almost five times of the aluminum matrix adjacent to the nugget zone.

The Role of Chromium (III) in the Corrosion Inhibition of AA2024-T3 By Trivalent Chromium Process Coatings

High strength aluminum alloys (e.g., AA2024-T3) are widely used as aircraft structural components. The inclusion of two major alloying elements, Cu and Fe, make the metal more susceptible to localized damage in the form of pitting and intergranular corrosion. The susceptibility of the alloy to localized corrosion is due to the fact that galvanic cells form between the Cu- and Fe-rich intermetallic phases, sites that support relatively rapid oxygen reduction reaction kinetics, and the aluminum, which undergoes dissolution. The intermetallic phases are more noble than the surrounding aluminum matrix leading to the localized pitting or dissolution of the aluminum near the intermetallics. Inhibiting this localized corrosion is essential for achieving long performance lifetime of these structural components. Conversion pretreatment coatings are one component of a multilayer coating system that is typically used to protect aerospace aluminum alloys from corrosion. The multi-layer coating system consists of a conversion coating, a primer and an organic topcoat. The focus of current research in our group is on the application of alternate, more environmentally-friendly conversion coatings that serve as suitable replacements for the currently-used chromate conversion coatings (CCC). The corrosion inhibition provided by the multilayer coating system depends on the chemical composition, physical structure and thickness and uniformity of the conversion coating. The need for some level of corrosion inhibition by the conversion coating arises when there is coating breakthrough, in other words a scratch through the coating system that exposes some bare metal. Conversion coating thicknesses are generally in the 50-200 nm range and they provide corrosion inhibition, at least in part, by providing a barrier to oxygen and water/electrolyte accessibility to the metal surface. The trivalent chromium process (TCP) conversion coating is an environmentally-friendly potential replacement for the currently used chromate conversion coating. TCP is regarded as one of the leading drop-in replacements for CCC. Prior work has shown that the TCP coating inhibits corrosion on aluminum alloys, in part, by serving as a barrier layer. A key question with regard to the mechanism of corrosion inhibition is the role of Cr(III) or the transiently formed Cr(VI) that has been shown to form in the coating. We conducted studies using a trivalent chromium conversion coating from Henkel Corp., named Alodine® T5900™ , and a formulation of the same coating devoid of Cr(III). The formation, structure and corrosion inhibition of the coatings was investigated on AA2024-T3. The coating was formed by immersion on a polished, degreased and deoxidized specimen. These conversion coatings are about 100 nm thick when formed by immersion and consist of a biphasic structure with a hydrated zirconia (ZrO2·nH2O) top layer and a fluoroaluminate (e.g., KxAlF3+x) interfacial layer. Electrochemical methods were used to assess the degree of corrosion inhibition. Despite having the same physical structure, the conversion coating containing the Cr(III) exhibited a polarization resistance 10x greater than that for the coating devoid of it. Polarization curves revealed that the TCP coating provided cathodic inhibition. Rotating disk voltammetry was to assess the effect of the coating on the oxygen reduction reaction (ORR) kinetics. The results demonstrated that the TCP coating, as formed, inhibits the ORR. The reduced rate of oxygen reduction is attributed to (i) the formation of a poorly conducting layer on the Cu-rich intermetallics that blocks sites for O2 chemisorption and serves as a barrier to electron transfer and (ii) the formation of diffisusion barrier layer for O2 to the reaction sites.

Microstructures and tensile properties of Al/2024–Al4Sr composite after hot extrusion and T6 heat treatment

Microstructures and tensile properties of Al-2024 matrix composite containing 10wt% Al4Sr intermetallic have been studied after extrusion process and T6 heat treatment. Microstructural examinations were assessed by the use of optical microscopy, scanning electron microscopy (SEM) and X-ray diffractometry. The results showed that changing the matrix from Al to 2024 alloy reduces the length of primary Al4Sr particles and improves ultimate tensile (UTS) values considerably from 206MPa to 321MPa which is based on the formation of Cu-rich intermetallics. Applying extrusion process and T6 heat treatment was found to be very effective in enhancing tensile properties of Al/2024–10Al4Sr composite. In current research, UTS and elongation values were found to be 381MPa and 13.6%, introducing a tough composite. Moreover, the study of fracture surfaces of the hot-extruded composites specimens revealed that ductile mode of fracture is dominant.

Use of the Scanning Vibrating Probe for in-Situ Monitoring of SCC Via the Coupling Current in 5000 Series Al Alloys

When as-received or sensitized AA5083 are immersed in an electrolyte, a growing crack creates a galvanic couple with the external surfaces according to the differential aeration hypothesis. Monitoring the resulting “coupling current” yielded “real-time” information about the status of a stress corrosion crack. As a result of this study, we have determined that the scanning vibrating electrode technique (SVET) can be used to monitor the coupling current flowing through the solution from the crack to the external surfaces. Preliminary feasibility studies were performed to map the current/potential in larger galvanic couples designed to simulate Cu-rich intermetallics dispersed in an aluminum matrix. A novel horizontal fracture mechanics apparatus, designed to apply a known stress intensity, was employed to reveal the onset of subcritical crack growth. In addition to the preliminary studies, we successfully designed a setup to allow the typical fracture mechanics testing to occur with the SVET. We report on the progress that was made in designing a customized four-point bend, fracture mechanics device that allows for simultaneous loading and electrochemical mapping. Spatially resolved coupling current/potential maps indicate that both the notch and the crack-tip become and remain anodic for a considerable duration of the crack growth process. Furthermore, there are peaks in the coupling current associated with the onset of subcritical crack growth.

Characterization of intermetallic compounds in Cu–Al ball bonds: Mechanical properties, interface delamination and thermal conductivity

In high power automotive electronics copper wire bonding is regarded as the most promising alternative for gold wire bonding in 1st level interconnects and therefore subjected to severe functional requirements. In the Cu–Al ball bond interface the growth of intermetallic compounds may deteriorate the physical and mechanical properties. The layer growth and properties of these intermetallic compounds are crucial in the prediction of the long term behavior. To mimic the growth of intermetallic compounds during and after copper ball bonding, diffusion couples of aluminium and copper were annealed at 225–500°C and chemically analyzed by SEM/EDS. Also five separate intermetallic compounds were melted together from the pure elements and aged in evacuated quartz ampoules for 240h at 500°C. In this work values for the indentation Young’s modulus, load independent hardness, indentation fracture toughness, volumetric densities, interface delamination and thermal conductivity are presented. It can be concluded that the Cu-rich intermetallics Cu9Al4 and Cu3Al2 are less sensitive to fracture and have lower average densities than the other intermetallic compounds. The volumetric decrease during formation causes internal stress. Interfacial delamination initiates in the Al-rich intermetallics (CuAl, CuAl2) and propagates easily into other intermetallic layers. The Cu9Al4–Cu s.s. interface is also found to be susceptible for delamination fracture. The thermal conductivity for 3 intermetallics is much lower than for pure copper or aluminium and in the range of 26–87W/m−1K−1, where Cu3Al2 layer has the lowest thermal conductivity (26–33W/m−1K−1).

Mechanistic Evaluation of SCC in Sensitized and Unsensitized Specimens of AA5083 Using Localized Probing Techniques

When as-received or sensitized AA5083 are immersed in an electrolyte, a growing crack creates a galvanic couple with the external surfaces according to the differential aeration hypothesis. Monitoring the resulting “coupling current” yields information about the mechanisms of crack advance. The purpose of this study is to determine if the scanning vibrating electrode technique (SVET) can be used to monitor the coupling current flowing through the solution from the crack to the external surfaces. If successful, this method, which maps potential as a function of probe position via a scanning vibrating probe (SVP), can be used to visualize and quantify the coupling current emanating from a growing stress corrosion crack in sensitized and unsensitized aluminum alloy samples to yield more localized information about the crack growth process. Preliminary feasibility studies were performed to map the current/potential in larger galvanic couples designed to simulate Cu-rich intermetallics dispersed in an aluminum matrix (as in the Cu-rich particles in AA2024 and Mg-rich intermetallics in AA5083). A fracture mechanics apparatus, designed to apply a known stress intensity, was employed to reveal the onset of subcritical crack growth. The final goal is to combine the typical fracture mechanics testing with the SVET and acoustic emissions. We report on the progress that has been made in designing a customized four-point bend, fracture mechanics device that allows for simultaneous loading and electrochemical mapping. Future work will report data gathered from in-situ electrochemical and acoustic emissions testing on pre-cracked AA5083 specimens.

The effect of Cu addition and solution heat treatment on the microstructure, hardness and tensile properties of Al–15%Mg2Si–0.15%Li composite

This study has been carried out to investigate the effect of Cu addition (0.1–3wt%) on the microstructure, hardness and tensile properties of Al–15%Mg2Si–0.15%Li in-situ metal matrix composite before and after solutionizing heat treatment. It was found that the presence of Cu at higher concentrations (>1wt%) results in the formation of hard Cu-rich intermetallics segregated in cell boundaries which intensifies ultimate tensile strength (UTS) values, but reduces elongation percentage (El%). Further results revealed that solution heat treatment partially dissolves different types of intermetallics which leads to proper distribution of such hard particles and ductility enhancement. The study of the fracture surfaces of the heat treated specimens via scanning electron microscopy (SEM) revealed fine dimples which are the main characteristic of ductile fracture mode.

The Formation, Structure, Electrochemical Properties and Stability of Trivalent Chrome Process (TCP) Coatings on AA2024

We report on the formation, structure, electrochemical properties and stability of trichrome process coatings (TCP) on AA2024-T3. The coating is 50–100 nm thick and forms over most of the alloy surface. It consists of hydrated zirconia (ZrO2•2H2O) and its formation under open circuit conditions is driven by an increase in the interfacial pH caused by (i) dissolution of the oxide layer and (ii) oxygen reduction mainly at the Cu-rich intermetallics. The coating appears biphasic with a hydrated zirconia overlayer and a fluoroaluminate interfacial layer (KxAlF3 + x). Cr(III) oxide is coprecipitated with the hydrated zirconia with Cr-rich regions in and around pits. Some anodic and cathodic protection is provided by physically blocking Al-rich sites (oxidation) and Cu-rich IMCs (reduction). This is evidenced by a 10 × greater polarization resistance, Rp, for the TCP-coated alloys, and suppressed anodic and cathodic currents (air saturated 0.5 M Na2SO4, room temperature) in potentiodynamic polarization scans. In the short-term (4-h immersion), the coating is stable, as evidenced by unchanging passivation performance. While no Cr(VI) species were detected in the coating immediately after formation, Raman spectroscopy revealed consistent evidence for transient formation of Cr(VI) species in the coating after immersion in different air-saturated electrolyte solutions from 1–14 days.

Sedimentation of Cu-rich intermetallics in liquid lead-free solders

This study investigated the behavior of Cu-containing intermetallic compounds (IMCs) in liquid Sn–Ag and Sn–Zn solders. Experimental results show that for the intermetallics investigated, Cu–Sn and Cu–Zn compounds, the occurrence of settling was dominated by the crystalline temperature of IMCs, buoyancy due to difference in densities, and dissolution potential for the compounds into the liquid. The complete dissolution of Cu–Zn compounds, which took place in the Sn–Zn solders when the Cu content exceeded a critical value, might be ascribed to the depletion of Zn in the melt.

Effects of the dissolved oxygen in Ti films on Ti reactions in Cu/Ti/SiO2/Si system upon annealing

The reactions of Cu/Ti/SiO2 structures at temperatures ranging from 200 to 700 °C have been studied for various Ti thicknesses. X-ray and Rutherford backscattering spectroscopy (RBS) analyses were used to identify the reaction products resulting from Ti reactions in Cu/Ti/SiO2 systems and the oxygen composition in the unreacted Ti, and revealed a correlation between the oxygen concentration in Ti films and the sequences of the Ti reactions. The reaction products initially formed, at around 300 °C, were a series of Cu–Ti intermetallics (Cu3Ti/CuTi) at the Cu–Ti interface with the oxygen dissolved in the Ti moving from the compounds into the remaining unreacted Ti. At 500 °C, the Cu3Ti was converted into Cu-rich intermetallics, Cu4Ti, which grew at the expense of the CuTi due to the increased oxygen content in the Ti. In addition, the outdiffusion of Ti, to the Cu surface, and the Ti–SiO2 reactions caused an abrupt increase in the oxygen content in the Ti layer, which placed thermodynamic restraints on further Ti reactions. Furthermore, thinner Ti layers showed a higher increased rate of oxygen accumulation for the same consumption of Ti, which led to significantly reduced Ti consumption. The diffusion barrier properties of SiO2 for Cu metallization decreased with an increasing Ti thickness.

Plasma-treated triazole as a novel organic slow-release paint pigment for corrosion control of AA2024-T3

Triazole is effective for corrosion control of AA2024-T3 which is well known to contain Cu-rich intermetallics. However, it cannot be used as a paint pigment because of its high water-solubility. In this research, we present a novel method to modify water-soluble triazole particles using a plasma polymerization technique. Time-of-flight secondary ion mass spectrometry (TOFSIMS) confirmed the presence of an ultrathin polymer film on the particles. The plasma-treated triazole was used as a paint pigment in a water-based epoxy coating. Using electrochemical impedance spectroscopy (EIS) techniques and immersion tests, it was demonstrated that the encapsulated water-soluble inhibitor could slowly release into the environment to protect a metal as needed. This technique is a feasible and promising method to promote the replacement of chromate pigments in paints.

Effects of Ti Thickness on Ti Reactions in Cu/Ti/SiO2 System upon Annealing

The reactions of <TEX>$Cu/Ti/SiO_2$</TEX> structures at temperatures ranging from 200 to <TEX>$700^{\circ}C$</TEX> have been studied for various Ti thicknesses. The reaction products initially formed, at around <TEX>$300^{\circ}C$</TEX>, were a series of Cu-Ti intermetallics (<TEX>$Cu_3$</TEX>Ti/CuTi) with the oxygen dissolved in the Ti moving from the compounds into the remaining unreacted Ti. At <TEX>$500^{\circ}C$</TEX>, the <TEX>$Cu_3$</TEX>Ti was converted into Cu-rich intermetallics, <TEX>$Cu_4$</TEX>Ti, which grew at the expense of the CuTi due to the increased oxygen content in the Ti. In addition, the outdiffusion of Ti, to the Cu surface, and the <TEX>$Ti-SiO_2$</TEX> reactions, caused an abrupt increase in the oxygen content in the Ti layer, which placed thermodynamic restraints on further Ti reactions. Furthermore, thinner Ti layers showed a higher increasing rate of oxygen accumulation for the same consumption of Ti, which led to significantly reduced Ti consumption. The <TEX>$SiO_2$</TEX> film under the Ti diffusion barrier was more easily destroyed with increasing Ti thickness.