Surface film characteristics of Al-Li-Cu-Mg alloys in 0·1 N NaOH

The nature of surface films that form under free corrosion conditions, and their effect on the subsequent polarization behaviour of an Al-1·90Li-1·80Cu-1·00Mg-0·09Zr alloy in 0·1 mol/l NaOH solution at 35°C have been studied. The variation of open circuit potential (OCP) as a function of time is characteristic for the alloy in the electrolyte. It initially changes in the noble direction with the surface of the specimen being enveloped by a black coating, and, later, when the black coating is punctured at some localized regions, the OCP shifts and stabilizes at an active value of −1450 mV vs saturated calomel electrode (SCE). X-ray diffraction analysis of the surface film layer indicates that it consists of essentially lithium aluminum hydroxide at shorter immersion time, and lithium aluminum hydroxide and copper hydroxide after longer immersion time in the electrolyte. The scale morphology as a function of immersion time has also been studied by scanning electron microscopy. Polarization experiments conducted after 2 and 15 h of immersion revealed that the alloy exhibited active-passive type polarization behaviour in both the cases. The polarization behaviour of the specimen immersed for longer times has been explained by considering Cu ennoblement on the surface. It was also observed that the hydride LiAlH₄ forms on the surface regions of the alloy under free corrosion conditions.     

固溶温度对2A96铝锂合金电化学腐蚀性能的影响

为了寻求合适的固溶处理制度,采用电化学测试、金相组织观察、显微硬度测试等手段研究2A96铝锂合金的极化曲线、交流阻抗、开路电位与腐蚀时间的关系及其金相组织和显微硬度。在T6态下,选取5个不同固溶温度处理2A96铝锂合金,固溶处理时间均为60 min。实验结果表明:固溶温度为510 ℃时,合金的自腐蚀电压最小,自腐蚀电流密度最大,即固溶温度为510 ℃时试样最容易被腐蚀,腐蚀速率也最快,此时开路电位随腐蚀时间的增加下降最快;2A96铝锂合金在不同固溶温度下的金相组织晶粒尺寸都比较均匀,晶粒呈等轴状分布;固溶温度为540 ℃时,试样硬度最大,达到236.41 HV。     

On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

High temperature tensile creep behaviour of a directionally-solidified Ni₃Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni₃Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.     

Influence of precipitation on serrated flow in Al-5Zn-1Mg alloy

Abstract

An Al-5Zn-1Mg alloy was aged at three different temperatures, 70 ° C, 150 ° C, and 200 ° C to obtain different states of precipitation. Serrated flow behaviour was studied with progressive aging at these three different aging temperatures. A comparison of serrated flow behaviour for the three different aging condition was made in the peak aged condition. Finally, serrated flow behaviour in Al-5Zn-1Mg alloy was compared with that in Al-Li alloys studied earlier. It is concluded that all the features of serrated flow can be explained only by considering the combined effect of solute - dislocation and precipitate - dislocation interaction.     

Physical metallurgy of aluminum-lithium alloys

The addition of lithium to aluminium reduces the density and increases the elastic modulus; precipitation of the metastableδ′(Al₃Li) phase from supersaturated Al-Li solid solution leads to appreciable increase in strength. The enhanced values for specific modulus and specific strength favour the use of the Al-Li alloys as structural materials for aerospace applications. However the binary alloys suffer from problems of poor ductility and toughness associated with strain localisation (resulting from the ease with whichδ′ particles are sheared during deformation), the presence ofδ′-free zones near grain boundaries and the heterogeneous nucleation of the equilibriumδ phase on the grain boundaries. These problems have been overcome by the development of ternary and quaternary alloys containing copper and magnesium. A small amount (∼0·1%) of zirconium is added to these alloys to improve the recrystallisation characteristics. The properties of alloys developed for commercial exploitation are briefly discussed. An overview of the physical metallurgy of the Al-Li alloys is presented with emphasis on the following features: (i) phase equilibria and precipitation reactions in Al-Li, Al-Cu-Mg, Al-Cu-Li and Al-Mg-Li systems and extension of these results to Al-Li-Cu-Mg alloys, (ii) insoluble particles and their effect on precipitation in the alloys, (iii) microstructural studies on Al-2·3%Li-1·2%Cu-0·7%Mg-0·12%Zr alloy, (iv) lithium depletion during solution treatment, (v) coarsening ofδ′ particles and development of precipitate-free zones near grain boundaries and (vi) microanalysis of the lithium containing phases.     

Effect of beryllium and calcium on aging behaviour of Al–0·75Mg–0·5Si alloy

Abstract

The effect of microadditions of Be and Ca on the aging behaviour of Al–0·75Mg–0·5Si alloy is investigated. It is shown that the addition of 0·1%Be significantly increases the hardening rate and the maximum hardness level attainable when the alloy is aged at various temperatures from room temperature to 300°C, while the addition of 0·2%Ca decreases both the hardening rate and the maximum hardness level attainable. Optical and scanning electron microscopical observations show a significantly higher precipitate density for the Be containing alloy and a slightly lower precipitate density for the Ca containing alloy when compared with the base Al–Mg–Si alloy. The results are consistent with an earlier kinetic study that indicated a Be enhanced nucleation rate for precipitation in the same alloy.

MST/936     

Effect of microstructure on the corrosion behaviour of dental gold alloys

The effect of microstructure on the corrosion behaviour of Au-Cu-11 at % Pd, Au-Cu-11 at % Ag and (Au-Cu-11 at % Ag)-5 at % Pd alloys was studied by anodic potentiodynamic and potentiostatic polarization tests. The single-phase Au-Cu-11 at % Pd alloys had high corrosion resistance and their potentiodynamic polarization curves were simple. The high-nobility Au-Cu-11 at % Ag alloy displayed high corrosion resistance regardless of its microstructure. The two-phase Au-Cu-11 at % Ag and (Au-Cu-11 at % Ag)-5 at % Pd alloys had lowered corrosion resistance compared with the single-phase alloys. Anodic potentiodynamic polarization curves of the two-phase alloys were complex, but they were explained by superimposing the curves of each phase using the principle of additivity. The potential at which the current density sharply increased corresponded with that of each phase, and the value of current density was calculated as the sum of the current density of each phase and taking account of their volume fractions. In the anodic potentiodynamic polarization, there was little galvanic effect between the two phases.     

Influence of alloying elements on thermal conductivity and high temperature strength of copper based alloys

Abstract

The effects of alloying elements and annealing temperature on thermal conductivity and softening behaviour of Cu – 0·1Ag – xP–yMg and Cu – xSn – yTe alloys (all compositions are in wt-%) have been investigated. The Cu – 0·1Ag – xP–yMg alloys, in spite of greater amounts of P and Mg, had a higher electrical conductivity and a higher softening temperature than those of a Cu – 0·1Ag – 0·031P alloy. A Cu – 0·032Sn – 0·023Te alloy had the same levels of electrical conductivity and softening temperature as those of Cu – 0·040Sn. The conductivity and softening temperature of the Cu – 0·032Sn – 0·023Te alloy are comparable with those of the Cu – 0·1Ag – 0·013P alloy currently used for continuous casting mould materials.     

Fe30Mn5Al合金氧化改性层的电化学腐蚀性能

将Fe30Mn5Al合金在800℃空气中循环氧化160h,在氧化层与基体之间获得厚度约为15μm的贫Mn,富Fe铁素体层,研究氧化改性贫Mn层对Fe30Mn5Al合金电化学腐蚀性能的影响。结果表明:在1mol·L-1 Na2SO4溶液中,与原始合金相比,贫Mn层的阳极极化曲线呈自钝化,自腐蚀电位Evs SCE从-750mV提高至-130mV,钝化电流密度ip从310μA/cm2下降至29μA/cm2;电化学交流阻抗谱(EIS)的容抗弧直径及|Z|值增加,相位角平台变宽,利用等效电路Rs-(Rt//CPE)拟合的极化电阻Rt由2.7kΩ·cm2增至9.9kΩ·cm2;贫Mn层比合金基体具有更好的抗蚀性能。     

Corrosion behaviour of β-Ti20Mo alloy in artificial saliva

To evaluate the potential of β-Ti20Mo alloy as a dental material, we tested its corrosion behaviour in artificial saliva in comparison to that of cp-Ti. Open-circuit potential (E_OC), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used as electrochemical methods to characterize the corrosion behaviour of Ti20Mo alloy and cp-Ti, respectively. Corrosion current and passive current densities obtained from the polarization curves showed low values indicating a typical passive behaviour for Ti20Mo alloy. The EIS technique enabled us to study the nature of the passive film formed on the binary Ti20Mo alloy at various imposed potentials. The Bode phase spectra obtained for Ti20Mo alloy in artificial saliva exhibited two-time constants at higher potential (0.5 V, 1.0 V), indicating a two-layer structure. According to our experimental measurements, Ti20Mo alloy appears to possess superior corrosion resistance to that of cp-Ti in artificial saliva.     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

Production of Si55 Al20 Fe10 Ni5 Cr5 Zr5 bulk amorphous alloy by hot pressing

Abstract

The temperature dependence of the relative density was examined for a Si₅₅ Al₂₀ Fe₁₀ Ni₅ Cr₅ Zr₅ alloy which was produced by hot pressing of the amorphous powder during heating up to various temperatures under a critical pressure of 1·5 GPa through a heating/pressing cycle. The density of the compacts increased with increasing temperature and reached a maximum near the crystallisation temperature of 698 K. The relative density of the compacts reached 98·3% at the critical condition of 1·5 GPa and 687 K. The hardness value of the bulk specimen was 940 HV(10 gf, 0·098 N), corresponding to that of the amorphous ribbon. Exposure to higher temperatures resulted in the precipitation of the crystalline phase. The present results indicate that Si based alloys can be produced in a compact form with a low fraction of voids by hot pressing the amorphous powder.     

Effects of pH on the electrochemical behaviour of titanium alloys for implant applications

The electrochemical behaviour of two commercial titanium alloys Ti-6Al-4 V (ASTM F136) and Ti-13Nb-13Zr (ASTM F1713) was investigated in Ringer physiological solution at two pH values (5.5 and 7.0). The corrosion properties were examined by using electrochemical techniques: Potentiodynamic anodic polarization, cyclic polarization and electrochemical impedance spectroscopy (EIS). The electrochemical corrosion properties of both alloys at different conditions were measured in terms of corrosion potential (E _corr), corrosion current density (i _corr) and passivation current density (i _pass). Equivalent electrical circuits were used to modulate EIS data, in order to characterize alloys surface and better understanding the pH effect on the interface alloy/solution.     

Corrosion behavior of bulk amorphous Zr55Al10Cu30Ni5−xPdx alloys

Bulk amorphous Zr₅₅Al₁₀Cu₃₀Ni_5−xPd_x (x=0, 5at.%) alloys were produced by copper mould casting. The microstructure of samples was characterized by X-ray diffraction (XRD). The corrosion resistance of the bulk amorphous alloys was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The Pd-free alloy showed an active–passive transition by anodic polarization in 3.5% NaCl solution, it was spontaneously passive and had a wide passive region with significantly low passive current density. The 5% Pd amorphous alloy shows a single active state even with a limited passive region. X-ray photoelectron spectroscopy (XPS) analysis revealed that the spontaneous passive films formed on the alloys after immersion in 0.6 M NaCl solution for 72 h were composed of the oxidation of Zr, Al and Cu element. The Pd-free and 5% Pd alloys show excellent corrosion properties in 1 M NaOH solution.     

Influence of cerium content on the corrosion behavior of Al-Co-Ce amorphous alloys in 0.6 M NaCl solution

The effect of cerium content on the corrosion behavior of Al-Co-Ce amorphous alloys in 0.6 M NaCl solution was investigated by cyclic polarization, Mott-Schottky and X-ray photoelectron spectroscopy techniques. Results indicated that the open circuit potential of Al-Co-Ce amorphous alloys displayed a decreased tendency with the increase in Ce content, and the amorphous alloy with 4 at.% Ce presented both the lowest passive current density and donor density indicating the best corrosion resistance, while adding excess Ce led to the reduced corrosion resistance of Al-Co-Ce alloys. Furthermore, it was found that a low Ce content is beneficial to the formation of a more protective passive film on Al-Co-Ce amorphous alloys, and the corrosion inhibition reactions of Al-Co-Ce alloys in 0.6 M NaCl solution were changed with the increase in Ce content and the detailed reasons were discussed.     

Effect of grain size and indium addition on tensile characteristics of Al–1Si alloy

Abstract

The effect of grain size and indium addition on the workhardening characteristics of Al–1Si (wt-%) alloy has been investigated at room temperature (RT). The samples were preaged at different temperatures in the range 523–623 K. The yield stress, the fracture stress, the fracture time and the linear workhardening coefficient generally decreased with increasing temperature and/or grain size, while the fracture strain and dislocation slip distance increased. The yield and fracture stresses for different grain sizes at different temperatures were found to be linearly related to grain diameters. Indium addition caused general increase for all the measured strength parameters. As concluded from transmission electron microscope (TEM) investigations, In addition to Al–Si alloy may retard the coarsening of Si particles. The energies activating the operating fracture mechanisms were found to be 79·6±0·4 and 32·4±0·4 kJ mol^?1 for alloys Al–1Si and Al–1Si–0·2In respectively. This suggests a value of 47·2 kJ mol^?1 as a binding energy between Si and In atoms in Al matrix.     

Effect of carbon on hot deformation behaviour and microstructural evolution of near-α titanium alloy Ti–5·6Al–4·8Sn–2Zr–1Mo–0·35Si–0·7Nd

Abstract

The hot deformation behaviour and microstructural evolution of a near-α titanium alloy (Ti–5·6Al–4·8Sn–2Zr–1Mo–0·35Si–0·7Nd) containing 0·06%C or 0·3%C with bimodal or Widmanstätten starting microstructures were investigated using isothermal compression test at strain rates of 0·01–10 s^?1 in the α+β or β regions. In the α+β region, both alloys exhibited continuous flow softening. The globularisation of transformed β structure or the recrystallisation of globular α phase took place, which was more remarkable in the 0·3%C alloy. In the β region, both alloys exhibited steady-state flow behaviour. Dynamic recrystallisation of the β phase occurred in the 0·06%C alloy, while was absent in the 0·3%C alloy. Due to the solution hardening of carbon atoms for the phases and the pinning effect of the carbides on grain boundary, the apparent activation energies of the 0·3%C alloy are higher than those of the 0·06%C alloy in the corresponding α+β or β phase regions.     

Influence of silicon and magnesium on grain refinement in aluminium alloys

Abstract

Grain refinement in Al–Si alloys with silicon contents in the range of 0·2–30 wt-% has been studied in detail with conventional as well as higher level additions of a Al–5Ti–1B master alloy. A poisoning effect was observed with Al–Si alloys containing ≥7 wt-%Si and the extent of poisoning increased with an increase in the silicon content. Silicon improves the grain refining behaviour of aluminium when added in small quantities (0·2%). Magnesium can counteract the poisoning effect of silicon. The optimum level of magnesium required to overcome the poisoning effect depends on the silicon content of the alloy. Higher level additions of a grain refiner could overcome the poisoning effect of silicon and the level required to achieve good grain refinement is a function of the silicon and magnesium contents of the alloy. The present paper also reports the influence of degasser and melt temperature on the grain refining response of Al–Si alloys.     

Effect of aging treatment on microstructure and creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys

Abstract

The present study was undertaken to investigate the influence of aging temperature on the creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys at testing temperatures ranging from 333 to 363 K under constant stress of 7·80 MPa. The steady state creep rate was found to increase continuously with increasing aging temperature at all testing temperatures. Results show that addition of Bi to the binary Sn–Ag solder alloy led to a significant increase in the strength and improvement in the creep resistance. The activation energy for the creep process of Sn–Ag and Sn–Ag–Bi solder alloys was found to have an average value of 36 and 45 kJ mol^?1 respectively. This might be characterised by diffusion of Ag in Sn. The microstructure of the aged samples for both alloys examined by X-ray diffraction measurements supported the improvement in the creep resistance for Sn–Ag alloy by adding a small trace of Bi.     

Electrochemical behaviour of Ti–25Nb–3Mo–3Zr–2Sn biomedical alloy in different simulated physiological environments

The electrochemical corrosion behaviour of biomedical Ti–25Nb–3Mo–3Zr-2Sn (TLM) alloy was investigated in various simulated body fluids at 37±0·5°C utilising potentiodynamic polarisation and current–time curves. The Ti–6Al–4V (TC4) alloy was also investigated to make a comparison. The different simulated body fluids comprised of 0·9%NaCl saline, Hank’s and Ringer’s solution were employed. The effect of heat treatment on the electrochemical behaviour of the TLM alloy was also considered. It was discovered that all the test specimens were passivated once immersed into the simulated body fluids. It was also found that the TLM alloy has poorer corrosion resistance in Hank’s solution, due to the chemical composition of the Hank’s. After different heat treated, the TLM alloy had different phases and microstructure, and the corrosion behaviour of the TLM alloy was different. In this study, after the heat treatment of 760°C/1 h/AC+550°C/6 h/AC, the TLM alloy had better corrosion resistance. Owing to the corrosion resistance of the TLM alloy was influenced by numerous factors, such as microstructure and the chemical composition of electrolyte, the corrosion behaviour of the TLM alloy is complex. By comparing with the corrosion behaviour of the TC4 alloy, the TLM alloy has poorer corrosion resistant than the TC4 alloy under the same conditions. But the current–time curves of the TLM alloy were more stable than these of the TC4 alloy with further experiments, because of the more passivation film on the surface of the TLM alloy.