The effect of microstructure and chemistry on the kinetics of passive layer growth and passivity breakdown of some Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and TC21 alloys, was studied. The rate of pitting corrosion was evaluated using cyclic polarization measurements. Chronoamperometry was applied to assess the passive layer growth kinetics and breakdown. Microstructure influence on the uniform corrosion rate of these alloys was also investigated employing dynamic electrochemical impedance spectroscopy (DEIS). Corrosion studies were performed in 0.9% NaCl solution at 37 °C, and the obtained results were compared with ultrapure Ti (99.99%). The different phases of the microstructure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical composition and chemistry of the corroded surfaces were studied using X-ray photoelectron spectroscopy (XPS) analysis. For all studied alloys, the microstructure consisted of α matrix, which was strengthened by β phase. The highest and the lowest values of the β phase’s volume fraction were recorded for TC21 and Ti-Al-Nb alloys, respectively. The susceptibility of the investigated alloys toward pitting corrosion was enhanced following the sequence: Ti-6Al-7Nb < Ti-6Al-4V << TC21. Ti-6Al-7Nb alloy recorded the lowest pitting corrosion resistance (R_pit) among studied alloys, approaching that of pure Ti. The obvious changes in the microstructure of these alloys, together with XPS findings, were adopted to interpret the pronounced variation in the corrosion behavior of these materials.

The effect of long-term thermal exposure and casting superheat on microstructure, topologically close-packed (TCP) phases, γ ′ precipitation and mechanical properties of an experimental Ni-base superalloy were studied. The investigated alloys were produced by investment casting process under two levels of superheat. After solution heat treatment, at 1180°C for 2 h followed by air cooling; the two alloys under investigation were isothermally exposed at 845°C for 24, 200, 1000, and 1500 h. The long-term thermal exposure conditions have a significant impact on the precipitation and morphology of TCP and γ ′ phases. The σ phase precipitated as needle and platelet shapes, whereas the μ phase formed in plate and agglomerated shapes. The μ phase has high concentration of Cr, Mo, W, and Co, while the σ phase has high percentages of Ni and Ti. The μ phase was precipitated after thermal exposure of 200 h in the case of high superheat specimen and after 1000 h in low superheat specimen. The η phase found was also a thick plate-like shape in the microstructure of both alloys in the interdendritic zones. The optimum size and volume fraction of γ ′ precipitates were obtained after being thermally exposed for 200 and 1000 h for high and low superheat alloys, respectively. Consequently, the highest hardness level was achieved at the optimum conditions of γ ′ precipitates in high and low superheat alloys.

Fe–Ga alloys are attractive materials where high mechanical strength, toughness, ductility, and large low-field magnetostriction combine to give unique properties. Adding alloying elements is an effective method to further enhance these properties. In order to integrate these alloys into the operating environments, e.g., micro-robots and magnetic actuators, the corrosion behavior should be addressed. This work analyzed the microstructure, magnetization, hardness, and corrosion properties of Fe₈₁Ga_19−xZ_x (X = 5 at.% of Ni, Mn, or Ti, and 2 at.% Al; separately) alloys. X-ray diffraction (XRD), scanning electron microscope-electron (SEM), vibrating sample magnetometer (VSM), Vickers hardness (HV), and a potentiostat were used for characterization. XRD revealed that the prominent peak belongs to the bcc disorder A2 phase and a small peak for the cubic order L1₂ phase. Fe–Ga–Al alloy got the maximum Ms value, while Fe–Ga–Mn alloy gained the lowest one. However, the Mr and Hc properties for Fe–Ga alloy were distinctly improved by adding Al but slightly affected by doping Mn. Addition of Ti achieved the highest hardness, followed by Ni, Mn, and Al. The microstructure of the different alloys significantly influenced their corrosion behavior. Fe–Ga–Mn alloy with the fine globular grain structure showed the lowest corrosion rate (C R = 0.03 mm/year), whereas Fe–Ga–Al alloy with the coarse longitudinal grains exhibited the highest corrosion rate (C R = 0.19 mm/year).