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The first oxide crystals formed during the early oxidation stages are considered to be the critical stage for the corrosion resistance of metallic materials against oxidants. The corrosion resistance of Cr-containing alloys is generally linked to Cr-rich oxide layers; however, their applicative performance varies remarkably. Classical models addressing the formation of these layers fail to account for the associated dynamics. We present evidence that Cr-containing Fe-, Co-, Ni-base and high-entropy alloys exhibit a parallel phenomenon during the initial transient stage: two-layered, multicompositional oxide grains with Cr always occupying the inner layer adjacent to the bulk. We propose that oxide growth occurs at a single location which allows to consistently incorporate all presented results. Oxide layer growth occurs inside the oxide grains at the transition to the Cr enrichment, independent of the initial oxide crystal structure. A potential multi-scale model for oxidation requires implementing the presented results which emphasizes the dominance of kinetic contributions during the early stages of oxidation.

Co-based alloys are promising to offer high-temperature corrosion resistance under extreme environmental conditions. Here, we report a novel approach that exploits the characteristics of the oxidation scale by making the specimens ground at three different levels to compare the effects of surface roughness in the presence and absence of external pressure. At a constant pressure of 2.5 MPa, the influence of surface roughness on the oxidation behaviour at an elevated temperature of 1050°C for 430 h has been investigated via scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer and X-ray diffraction. The results have revealed that for rough surfaces the thickness of the oxide scale with Ra = 76.0 nm is significantly higher compared to a smooth surface with Ra = 13.0 nm. Furthermore, the oxide scale on the rough surface has some micro defects and protrusion compared to smooth surfaces. Besides that, under the applied external pressure and surface roughness, more pressure- and roughness-induced cracks and discontinuity have been observed on the oxide scale. [ABSTRACT FROM AUTHOR]

The effect of multiple recasting of commercial Co-Cr base dental alloy (Dentalit-C) on its structural characteristics and ion release rates in artificial saliva has been investigated. The number of recasting was once, four and eight times. The as-cast microstructure became more uniform as the number of recasting was increased whereby macro and microhardness and electrical conductivity of the castings decreased. Metallic ion release in artificial saliva was monitored in vitro over the period of 22 weeks at the temperature of 37 0C and pH 2.3. Only Co and Fe were evidenced in artificial saliva and their release rates are measured. The ion release rates increases with time. The ion release rate of Fe was larger than Co. The ion release rates increases substantially by increasing the number of recasting. [Projekat Ministartsva nauke Republike Srbije, br 45012 i br. 172019]

Porous Co-base nano-composites were successfully prepared by the combination of different techniques. Scanning electron microscopy, optical microscopy, X-ray diffraction, compression and cell culture tests were used to characterise the fabricated samples and milled metal powder. The results of X-ray diffraction analysis show that induced strain during milling and compression test, and high temperature during sintering process lead to HCP–FCC phase transformation. Lamellar carbides, weaker interparticle bonding and phase transformation can lead to the brittle behaviour of the samples during compression test and also fluctuations observed in peak region in compressive stress–strain curves. The elastic modulus and compressive strength were measured to be from 7.3 to 63 MPa and 0.58 to 2.3 GPa for samples with porosities ranging from 58.9 to 38.1%, respectively. Cell cultures show good adhesion of cells on the surface and with each other and also reveal that cells grow on the surface and inside the pores. [ABSTRACT FROM PUBLISHER]

In this study, the fabrication and magnetic characterization of amorphous/nanocrystalline Co 70 B 20 Si 5 Fe 4 Mo 1 alloy by means of mechanical alloying (MA) and melt spinning (MS) process have been investigated. The produced samples were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), vibrating sample magnetometer (VSM) and scanning electron microscopy (SEM). The results showed that fully amorphous phase can only be formed by means of melt spinning processes, and the final product during mechanical alloying is the combination of nanocrystalline (Co HCP + Co FCC (about 10 nm)) and amorphous phases. The produced melt spun amorphous ribbons exhibit soft magnetic properties with the coercivity and saturation of magnetization in the range of 1.5–2 Oe and 90–120 emu/g, respectively. [ABSTRACT FROM AUTHOR]

In this study, two types of blended alloys were processed by plasma transferred arc (PTA) hardfacing to form coatings. In the first case, a pre-alloyed Cu-Sn alloy and a pre-alloyed Co-base alloy were blended at a weight ratio of 85:15 (Cu-Sn/Co-base). In the second case, the pre-alloyed Cu-Sn and Co-base alloys were mixed with the molybdenum (Mo) element at a weight ratio of 81:15:4 (Cu-Sn/Co-base/Mo). Metastable liquid phase separation formed in the Cu-Sn/Co-base and Cu-Sn/Co-base/Mo coatings. The Co-rich spheroids as the minor precipitating alloy particles distributed on the Cu-rich matrix. The Co-rich spheroids consisted of α(Co) and M 7 C 3 phases, and the Cu-rich matrix included α(Cu, Sn) and Cu 10 Sn 3 phases. The relative content of α(Co) + M 7 C 3 phases in the Cu-Sn/Co-base/Mo coating was higher than that in the Cu-Sn/Co-base coating. The numbers of the Co-rich spheroids in the cross section of the Cu-Sn/Co-base coating increased with increasing the distance from the substrate, and an obvious macroscopic segregation could be found in the coating close to its substrate. When Mo as additive was further added, the numbers of the Co-rich spheroids in the cross section of the coating increased and the macroscopic segregation could not be seen when compared to that in the Cu-Sn/Co-base coating. The wear rate of the Cu-Sn/Co-base and Cu-Sn/Co-base/Mo coatings was lower than that of the Cu-Sn coating, and a distinct improvement of the wear rate could be obtained in the Cu-Su/Co-base/Mo coating, attributing to mainly the increasing hardness of the coatings by sequence of the Cu-Sn coating, Cu-Sn/Co-base coating, and Cu-Sn/Co-base/Mo coating. The friction coefficient of the coatings has an increasing trend by sequence of the Cu-Sn coating, Cu-Sn/Co-base coating, and Cu-Sn/Co-base/Mo coating, but an obvious difference could not be seen in the coatings. [ABSTRACT FROM AUTHOR]

Abstract: In orthopedic surgery, different self-mating metal couples are used for sliding wear applications. Despite the fact that in mechanical engineering, self-mating austenitic alloys often lead to adhesion and seizure in biomedical engineering, the different grades of Co-base alloys show good clinical results, e.g., as hip joints. The reason stems from the fact that they generate a so-called tribomaterial during articulation, which consists of a mixture of nanometer small metallic grains and organic substances from the interfacial medium, which act as a boundary lubricant. Even though stainless steel also generate such a tribomaterial, they were ruled out from the beginning already in the 1950s as “inappropriate”. On the basis of materials with a clinical track record, this contribution shows that the cyclic creep characteristics within the shear zone underneath the tribomaterial are another important criterion for a sufficient wear behavior. By means of sliding wear and torsional fatigue tests followed by electron microscopy, it is shown that austenitic materials generate wear particles of either nano- or of microsize. The latter are produced by crack initiation and propagation within the shear fatigue zone which is related to the formation of subsurface dislocation cells and, therefore, by the fact that an Ni-containing CrNiMo solid solution allows for wavy-slip. In contrast to this, an Ni-free CrMnMo solid solution with further additions of C and N only shows planar slip. This leads to the formation of nanosize wear particles and distinctly improves the wear behavior. Still, the latter does not fully achieve that of CoCrMo, which also shows a solely planar-slip behavior. This explains why for metallurgical reasons the Ni-containing 316L-type of steels had to fail in such boundary lubricated sliding wear tribosystems. [Copyright &y& Elsevier]

DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both θ-Al2O3 and α-Al2O3 at 900 and 1000°C but only α-Al2O3 at 1050 and 1100°C. [ABSTRACT FROM AUTHOR]

• The activation energy of oxidation can be from the kinetic data of weight gain. • The oxidation reaction rate can be further expressed from the activation energy. • This expression can be favorable for knowing the oxidation behavior of superalloys. This presentation deals with the activation energy of oxidation for a practical Co-base superalloy suitable for turbine blades. The kinetic data of weight gain were measured to get the corresponding rate constants of oxidation for this high temperature alloy oxidized at 900 °C and 1000 °C in air, and the activation energy of oxidation has been derived for this alloy at high temperatures of 900~1000 °C. According to this, the specified relations can be further achieved to express the oxidation reaction rate for this superalloy at different temperatures in air. These would be favourable for knowing the oxidation behaviour of the superalloy at high temperatures in air, and evaluating the service life to get the safe application for the superalloy. [ABSTRACT FROM AUTHOR]