Internal Oxidation of an Ag–1.3 at.% Te Alloy

The internal oxidation of Ag–1.3 at.% Te was studied at 750, 800, and 830°C in pure oxygen (1 atm). The internal oxidation under such high oxygen pressure resulted in formation of two different types of oxide particles and two different fronts of internal oxidation in the internal oxidation zone. The coarser Ag₂TeO₃ particles were formed through the in situ internal oxidation of Ag₂Te particles and the tiny oxide precipitates (most probably also Ag₂TeO₃) were formed through internal oxidation of tellurium from solid solution. Considering the mechanism of internal oxidation, both diffusionless and diffusive modes were found to be present simultaneously in the oxidation of Ag–1.3 at.% Te alloy. These results were examined with regard to the solubility of tellurium in silver, which was found to be 0.1 at.% Te at 750°C and 0.26 at.% Te at 830°C, as well as the presence and dissolution of Ag₂Te particles.     

Effect of Ga on the Oxidation Properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa Solders

The oxidation properties of Sn–8.5Zn–0.5Ag–0.1Al–xGa lead-free solders in the liquid state (250?°C) under O₂ atmosphere were investigated using a thermal gravimetric analyzer. The Ga content of the investigated solders was 0.05–2?wt%. The results indicate that Ga enhances the oxidation resistance of Sn–Zn–Ag–Al solder. Cross-sections of the solder surfaces were examined using focus ion beam milling. The thickness of the oxidation layer, which was about 30–100?nm, increased with increasing Ga content. The oxidation layer was found to be nonuniform at low Ga content. The oxide layers on the surface of solders were investigated using Auger electron spectroscopy and thin-film XRD. The results showed that the oxide layer formed was ZnO. Al and Ga tended to segregate on the surface of the solder.     

Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t^1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag₃Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag₃Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.     

Oxidation Behavior of a PVD-Processed Mg–10.6Zr Alloy

The oxidation behavior in air of a physical vapor-deposited (PVD) Mg–10.6Zr (wt. %) alloy was studied in the 325–450°C temperature range. The oxidation rate of this alloy remains low at temperatures below 375°C. However, at higher temperatures, the alloy experienced extremely high oxidation rates, which can even lead to disintegration of the sample. Oxidation is controlled by fast inward oxygen transport along the open boundaries of the alloy, leading to the formation of cracks throughout the sample, and subsequent formation of a thin MgO at crack interfaces. The MgO layer remains protective while coarsening of zirconium precipitates at the open boundaries does not take place. Thickening of Zr precipitates over a critical size induces impairment of the MgO layer at crack interfaces, facilitating inward oxygen diffusion. The volume increase resulting from the formation of new oxide at open boundaries favors decohesion of open boundaries, leading to accelerated oxidation.     

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27?µm, associated with cooling rates ～2°C?s^?1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70～80% associated with higher UTS (～80?MPa) as compared with the other alloys examined.     

The High Temperature Oxidation Behavior of Mg–Gd–Y–Zr Alloy

The oxidation behavior of pure Mg and Mg–Gd-Y-Zr alloy was studied in O₂ at 300 °C with and without the presence of water vapor. The kinetics curves revealed improved oxidation resistance of Mg–Gd–Y–Zr alloy in O₂, compared with pure Mg. However, when water vapor co-existed with oxygen, the oxidation rate of Mg–Gd–Y–Zr alloy was accelerated; whereas, the oxidation rate of pure Mg was restrained. Detailed XPS analysis of pure Mg oxidized with water vapor revealed that the reduced oxidation rate could be strongly linked with the outer Mg(OH)₂ film. On the contrary, for Mg–Gd–Y–Zr alloy, an incomplete Mg(OH)₂ film was present in the outer region of oxide layer, which can provide a ready pathway for water vapor transport to the inner part of the oxide film and which has little oxidation resistance to water vapor.     

Transient Oxidation of a γ-Ni–28Cr–11Al Alloy

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-Al₂O₃ layer at the oxide/alloy interface. The evolution of such an NiO/Cr₂O₃/Al₂O₃ layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr₂O₃ outer layer and a discontinuous inner layer of Al₂O₃ initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al₂O₃, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr₂O₃ and Al₂O₃ to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al₂O₃ layer.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Electrolyte Optimization of Microarc Oxidation of Magnesium Alloy

MAGNESIUM alloy has many excellent features,suchas low density,high specific strength,specific rigidity,good damping performance,and good electromagneticshielding performance.Therefore,magnesium alloy hasalready been increasingly applied to aviation,automotive and electronic industry.But its corrosionresistance is not so good that its application is greatlylimited.In order to improve the corrosion resistance ofmagnesium alloy,most researchers have used microarcoxidation techniques to acquir…     

Effects of Ga,Sn Addition and Microstructure on Oxidation Behavior of Near-α Ti Alloy

We investigated the effects of adding Ga or Sn, with almost the same Al equivalent, on the oxidation behaviors of near-α Ti alloy with the bimodal structure and lamellar structure. The replacement of Sn with Ga decreased the alloy weight gain during oxidation, suppressed oxide growth, and improved adherence between the oxide and substrate. A lamellar alloy structure showed a lower weight gain during oxidation compared to the bimodal structure. Unlike conventional near-α alloys, recrystallization occurred near the oxide/substrate interface in Ga-modified alloy, which may contribute to the release of stress, improvement of the adherence between the oxide and substrate, and prevention of oxide-scale spallation from the Ga-modified alloy. A possible mechanism for the recrystallization in the Ga-modified alloy was also discussed.     

The Influence of the Alloy Microstructure on the Oxidation Behavior of Ti–46Al–1Cr–0.2Si Alloy

The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe ₂-Ti₃Al phase, being faster for the extrudedmaterial with coarse ₂-Ti₃Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-₂-Ti₃Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance.     

Positron Annihilation Study of High-Temperature Oxidation Behavior of Zr–1Nb Alloy

The oxidation behavior of Zr–1Nb alloys exposed at 873 and 973 K in air was investigated by positron annihilation lifetime spectroscopy together with mass gain, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Mass-gain results showed that during the oxidation process, a transition of the oxidation rate occurred. The transition times of the specimens oxidized at 873 and 973 K were 30 and 6 h, respectively. In the pre-transition stage, the mass-gain curves obeyed the subparabolic law (n?=?2.3), while at the post-transition stage, the mass-gain curves obeyed the linear law. The positron lifetime measurements indicated that in pre-transition stage, the formed oxide scale mainly consisted of a compact layer that only contained small-size vacancy defects. The accumulation of these vacancy defects together with the high compressive stress might cause the breakaway of the oxide layer. During the post-transition stage, the thickness of the porous oxide layer with more and larger-size defects such as voids and pores increased rapidly as increasing the oxidation time. These large-size defects, together with the cracks produced during the transition from protective to breakaway-type oxide, increased the oxygen absorption rate and accelerated the diffusion of oxygen. The formation of cracks in the porous layer was confirmed by SEM examinations.     

Phase equilibria of the Sn–Ag–Ni ternary system and interfacial reactions at the Sn–Ag/Ni joints

There are no previous phase equilibria studies of the Sn–Ag–Ni ternary system, even though the phase equilibria information is important for the electronic industry. The isothermal section of the Sn–Ag–Ni ternary system at 240 °C has been determined in this study both by experimental examination and thermodynamic calculation. Experimental results show no existence of ternary compounds in the Sn–Ag–Ni system, and all the constituent binary compounds have very limited solubilities of the ternary elements. The binary Ni₃Sn₂ phase is very stable and is in equilibrium with most of the phases, Ag₃Sn, ζ-Ag₄Sn, Ag, Ni₃Sn₄ and Ni₃Sn phases. A preliminary thermodynamic model of the ternary system is developed based on the models of the three binary constituent systems without introducing any ternary interaction parameters. This ternary thermodynamic model is used with a commercial software Pandat to calculate the Sn–Ag–Ni 240 °C isothermal section. The phase relationships determined by calculation are consistent with those determined experimentally. Besides phase equilibria determination, the interfacial reactions between the Sn–Ag alloys with Ni substrate are investigated at 240, 300 and 400 °C, respectively. It is found that the phase formations in the Sn–3.5wt%Ag/Ni couples are very similar to those in the Sn/Ni couples.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

Microstructure and mechanical property of Sn–Ag–Cu solder material

Among the lead-free solder materials,Sn-AgCu alloys have many advantages,such as good wetting property,superior interfacial properties and high creep resistance.In this article,the organization and welding performance of Sn-Ag-Cu material were investigated.The surface morphology of the two alloys was observed by stereoscopic microscope and scanning electron microscope(SEM).Chemical constitution was examined by X-ray energy-dispersive spectroscopy(EDS).The mechanical properties of Sn-Ag-Cu solder were evaluated systematically compared with those of Sn-Cu solder.The results show that Sn-Ag-Cu solder based on different solder pads has different welding properties.The thickness of intermetallic compound(IMC) at the interface increases with aging time.For the gold-plated pads,there are a large number of IMC graphic,and in the welding interface,it can reduce the reliability of electrical connection.The Sn-AgCu solder joints show a superior mechanical property over the traditional Sn-Cu solder.The number of dimples decreases and that of cavities increases for Sn-Cu0.7 alloy and the fracture surfaces of Sn-Ag3.0-Cu0.5 alloy have many small size dimples which are homogeneously distributed.     

Water Vapour Effects on Fe–Cr Alloy Oxidation

Isothermal oxidation at 700 °C of binary Fe–Cr alloys containing 9, 17 and 25 wt% chromium was measured using continuous thermogravimetric analysis. All alloys developed thin, protective chromia scales in Ar–20O₂ (vol%). Chromia scale growth on the 17 and 25 Cr alloys was faster in Ar–20O₂–5H₂O and Ar–5O₂–20H₂O. In these gases, the Fe–9Cr failed to form a chromia scale and suffered rapid breakaway oxidation, growing iron-rich oxides instead. A low oxygen potential gas, Ar–10H₂–5H₂O caused chromia scaling on Fe–17Cr and Fe–25Cr, but internal oxidation of Fe–9Cr. Application of Wagner’s criterion for sustaining external scale growth is shown to account satisfactorily for these observations.     

Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and -Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this -Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this -Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.     

Oxidation Behavior of Ni–Cr–Fe-Based Alloys: Effect of Alloy Microstructure and Silicon Content

The oxidation behavior of Ni–Cr–Fe-based alloys in a low oxygen partial pressure atmosphere (H₂–H₂O) was investigated in terms of the effect of alloy microstructure and their silicon content. It was found that the formation and growth kinetics of the oxide scale are rather sensitive to the alloy microstructure and their corresponding Si contents. Oxide ridges were found to form in areas with eutectic structure, while a thin and homogeneous oxide scale formed on austenite matrix. The thicknesses of the oxide ridges and the oxide layer on the austenite matrix were dependent of their corresponding Si contents. The austenite/carbide phase boundaries in eutectic structure can offer fast diffusion paths for metal outward diffusion, which leads to the formation of ridge-like oxide features. The continuous SiO₂ sub-layer formed at the oxide scale/metal interface on the austenitic matrix acted as an effective diffusion barrier to metal outward diffusion, resulting in rather thin and uniform oxide scales.     

Effects of Microstructure and Rare-Earth Constituent on the Oxidation Behavior of Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd Titanium Alloy

The isothermal oxidation behavior in air of high-temperature titanium alloy Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd with bimodal and lamellar microstructures was investigated at 600–800 °C. The results revealed that the alloy with lamellar microstructure has better oxidation resistance than that with bimodal microstructure. The porous oxide scales that form mainly contain TiO₂. A noticeable observation concerns the preferential attack around rare-earth particles, associated rapid oxygen diffusion along the incoherent rare-earth precipitate/matrix interface and cracks formed during oxidation. The resulting internal attack caused fragmentation of rare-earth particles and further oxidation of substrate to form TiO₂ scale with some fine dispersoids of Al₂O₃. Tensile tests showed that the ultimate strength and ductility of the specimens with removed surface were higher than that with a surface scale.