Effects of crystallization on corrosion resistance and electron work function of Zr65Al7.5Cu17.5Ni10 amorphous alloys

The effects of crystallization on the electron work function and corrosion resistance of Zr₆₅Al_7.5Cu_17.5Ni₁₀ amorphous alloys have been studied. The single-phase amorphous alloy exhibits a better corrosion resistance and has a higher work function than the partially and fully crystallized alloys with the same composition. The close relationship between corrosion resistance and work function indicates that the Kelvin probe technique can be a powerful tool for characterizing the corrosion behaviour of amorphous alloy on an electronic level.     

GP zone formation without quenched-in vacancies in Al–12 wt% Mg alloy

Dissolution of GP zones, formed in aged Al–12?wt%?Mg alloy at ?15 and 24°C, occurs during primary dilatometric and calorimetric tests, but the effect disappears after secondary tests, at low heating and cooling rates (1–2°C/min). After a?new holding at the above temperatures, GP zones again form and β and β′ phases precipitate. The results suggest the formation of GP zones without the presence of quenched-in vacancies.     

Computer-aided analysis of grain-boundary connectivity of B10 copper-nickel alloys

A numerical calculation method based on the angle in a triple junction composed of a random grain boundary is proposed to predict the connectivity and stability of a grain boundary in a B10 copper-nickel alloy. The grain-boundary connectivity and its effect on corrosion resistance are studied combining computer-aided analysis with electrochemical impedance testing. The results show that the prediction of corrosion resistance using a grain-boundary connectivity numerical method is consistent with immersion experimental results. The B10 alloy exhibited the best corrosion resistance after cold rolling with a 9% reduction rate. The relationship between the grain-boundary characteristics and corrosion resistance is well established using the numerical method to quantify the grain-boundary connectivity. A higher connective frequency and a low proportion of grain-boundary angles between 60° and 180° in the triple junction is detrimental to corrosion resistance of the B10 alloy.     

On the synthesis and characterisation of novel composite-structured high-entropy alloys

A study of novel composite-structured CoCrFeNi₃Si, CoCrFe₂Ni₂Si and Co₂CrFeNi₂Si high-entropy alloys, synthesised by vacuum arc melting, is presented. The designing criteria for the formation of such alloys were based on the enthalpy of mixing and the valence electron concentration. X-ray diffraction analysis revealed that all the alloys were composed of face-centered cubic and intermetallic phases. Scanning electron microscopy, along with elemental analysis, indicated that the face-centered cubic phase mainly contains Co, Cr, Fe and Ni, whereas, the intermetallic compound is Ni-Si rich. All the synthesised high-entropy alloys in the present investigation possess excellent compressive strength along with ductility at room temperature, suggesting significant potential application in the engineering field. Furthermore, enhanced hardness, wear resistance and corrosion resistance were achieved in all the designed high-entropy alloys. The previously given design parameters for the composite-structured high-entropy alloys are in good accord with the current research work.     

High-strength and anti-corrosion of Al-Cu-Mg alloy by controlled ageing process

ABSTRACT

The evolution of strength and corrosion behaviour induced by ageing treatment in Al-Cu-Mg alloy is poorly understood. In this work, correlations among strength, intergranular corrosion and pitting corrosion induced by precipitates were studied by various corrosion performance tests and hardness tests. The susceptibility of intergranular corrosion is decreased and pitting corrosion sensitivity is increased as the ageing temperature and time are increased. Retrogression and re-ageing treatments not only transform the size of the intragranular precipitates from greater than 500?nm to be less than 10?nm but also lead to the discontinuous distribution of grain boundary precipitates. The microstructural changes in the grain interior and grain boundary together promote the Al-Cu-Mg alloys to have high-strength and excellent resistance to intergranular corrosion and pitting corrosion.     

After-effects induced by interactions between hydrogen and the martensite transformation in Ni–Ti superelastic alloy

The effects of dynamic interactions between hydrogen and a stress-induced martensite transformation on the recovery of deteriorated tensile properties by ageing in air at room temperature have been investigated for a Ni–Ti superelastic alloy. A specimen is subjected to single stress-induced martensite and reverse transformations immediately after hydrogen charging. Upon tensile testing, brittle fracture occurs in the latter half of the elastic deformation region of the martensite phase after the stress-induced martensite transformation. Upon ageing before the tensile test, fracture occurs during the stress-induced martensite transformation. In addition, the nano- and micro-morphologies of the brittle outer part of the fracture surface of the specimen are changed by ageing. Thus, the tensile properties markedly deteriorate, rather than recover, by ageing. The present results clearly indicate that dynamic interactions between hydrogen and the stress-induced martensite transformation have serious after-effects on hydrogen embrittlement of Ni–Ti superelastic alloy.     

First interactions between hydrogen and stress-induced reverse transformation of Ni–Ti superelastic alloy

The first dynamic interactions between hydrogen and the stress-induced reverse transformation have been investigated by performing an unloading test on a Ni–Ti superelastic alloy subjected to hydrogen charging under a constant applied strain in the elastic deformation region of the martensite phase. Upon unloading the specimen, charged with a small amount of hydrogen, no change in the behaviour of the stress-induced reverse transformation is observed in the stress-strain curve, although the behaviour of the stress-induced martensite transformation changes. With increasing amount of hydrogen charging, the critical stress for the reverse transformation markedly decreases. Eventually, for a larger amount of hydrogen charging, the reverse transformation does not occur, i.e. there is no recovery of the superelastic strain. The residual martensite phase on the side surface of the unloaded specimen is confirmed by X-ray diffraction. Upon training before the unloading test, the properties of the reverse transformation slightly recover after ageing in air at room temperature. The present study indicates that to change the behaviour of the reverse transformation a larger amount of hydrogen than that for the martensite transformation is necessary. In addition, it is likely that a substantial amount of hydrogen in solid solution more strongly suppresses the reverse transformation than hydrogen trapped at defects, thereby stabilising the martensite phase.     

A combination of Al diffusion and surface nanocrystallization of carbon steel for enhanced corrosion resistance

Surface nanocrystallization is beneficial to the corrosion resistance of passive alloys, but generally has a negative effect on the corrosion behavior of non-passive alloys due to the enhanced surface reactivity. In this study, a combination of Al diffusion treatment and surface nanocrystallization was applied to carbon steel with the aim of exploring an alternative approach to improve the corrosion resistance of non-passive carbon steel. The surface nanocrystallization was achieved by sandblasting and subsequent recovery treatment. The former resulted in severe plastic deformation, while the latter turned high-density dislocation cells into nano-sized grains. The present study demonstrates that the combined Al diffusion and nanocrystallization generated a nanocrystalline Al-containing surface layer on the carbon steel with its surface grain diameter in the range of 10–300 nm. The corrosion resistance of the treated steel was evaluated. It is demonstrated that treated specimens possess increased resistance to corrosion with higher surface electron stability. Surface microstructure of the treated specimens was examined using SEM, AFM, and EDS in order to elucidate the mechanism responsible for the improved corrosion resistance.