Search Results (16)

Fatigue tests were performed on the AZ91 cast alloy to identify the mechanisms of the fatigue crack initiation. In different fatigue regions, different mechanisms were observed. In the low and high cycle fatigue regions, slip markings formation accompanied with Mg₁₇Al₁₂ particles cracking were observed. Slip markings act as the fatigue crack initiation sites. The size and number of slip markings decreased with decreased stress amplitude applied. When slip markings formation was suppressed due to low stress amplitude, particle cracking became more important and the cracks continued to grow through the particle/solid solution interface. The change of the fatigue crack initiation mechanisms led the S-N curve to shift to the higher number of cycles to the fracture, demonstrated by its stepwise character. A lower fatigue limit of 60 MPa was determined at 20 kHz for 2 × 10⁹ cycles compared to the 80 MPa determined at 60 Hz for 1 × 10⁷ cycles. Full article

Pure Mg samples were prepared by powder metallurgy using the cold and hot compacting methods. Cold compacted pure Mg (500 MPa/RT) was characterized by 5% porosity and the mechanical bonding of powder particles. Hot compacted samples (100 MPa/400 °C and 500 MPa/400 °C) exhibited porosity below 0.5%, and diffusion bonding combined with mechanical bonding played a role in material compaction. The prepared pure Mg samples and wrought pure Mg were subjected to corrosion tests using electrochemical impedance spectroscopy. Similar material corrosion behavior was observed for the samples compacted at 500 MPa/RT and 100 MPa/400 °C; however, hot compacted samples processed at 500 MPa/400 °C exhibited longer corrosion resistance in 0.9% NaCl solution. The difference in corrosion behavior was mainly related to the different binding mechanisms of the powder particles. Cold compacted samples were characterized by a more pronounced corrosion attack and the creation of a porous layer of corrosion products. Hot compacted samples prepared at 500 MPa/400 °C were characterized by uniform corrosion and the absence of a layer of corrosion products on the specimen surface. Powder-based cold compacted samples exhibited lower corrosion resistance compared to the wrought pure Mg, while the corrosion behavior of the hot compacted samples prepared at 500 MPa/400 °C was similar to that of wrought material. Full article

Wrought AZ31 magnesium alloy was used as the experimental material for fluoride conversion coating preparation in Na[BF₄] molten salt. Two coating temperatures, 430 °C and 450 °C, and three coating times, 0.5, 2, and 8 h, were used for the coating preparation. A scanning electron microscope and energy-dispersive X-ray spectroscopy were used for an investigation of the surface morphology and the cross-sections of the prepared coatings including chemical composition determination. The corrosion resistance of the prepared specimens was investigated in terms of the potentiodynamic tests, electrochemical impedance spectroscopy and immersion tests in the environment of simulated body fluids at 37 ± 2 °C. The increase in the coating temperature and coating time resulted in higher coatings thicknesses and better corrosion resistance. Higher coating temperature was accompanied by smaller defects uniformly distributed on the coating surface. The defects were most probably created due to the reaction of the Al_xMn_y intermetallic phase with Na[BF₄] molten salt and/or with the product of its decomposition, BF₃ compound, resulting in the creation of soluble Na₃[AlF₆] and AlF₃ compounds, which were removed from the coating during the removal of the secondary Na[MgF₃] layer. The negative influence of the Al_xMn_y intermetallic phase was correlated to the particle size and thus the size of created defects. Full article

The severe shot peening process was applied to the notched specimens from an AW 7075 alloy with the aim to improve fatigue endurance in the very-high cycle fatigue region. To reveal the stress state in the notch vicinity, finite element analysis was performed, simulating the conditions of the used 20 kHz ultrasonic fatigue loading. Modified surface characteristics by the severe shot peening process were analyzed in terms of residual stress distribution measured by X-ray diffraction methods and near-surface microstructural observations by scanning electron microscopy. The applied severe shot peening increased the fatigue limit by 11%; however, the positive effect was recorded only for the loading amplitudes corresponding to the fatigue lifetimes in the range 10⁷–10⁹ cycles. At higher loading amplitudes, the fatigue properties tended to decrease, most likely due to accelerated fatigue crack initiation on the surface damage features created by the peening process and also by rapid residual stress relaxation. Full article

This recent study deals with the optimization of the mechanical performance of Grade 2 and Grade 4 titanium with Conform severe plastic deformation (SPD) processing and subsequent rotary swaging. A comprehensive study of the materials behaviour and characterisation during and after processing is given by (finite element method - FEM) numerical simulation, microscopy methods and mechanical testing. The mechanical and fatigue properties are discussed in terms of texture and microstructure evolution. It is shown that the combination of Conform SPD and rotary swaging is a promising technique for economically reliable, high-gain production of titanium alloys fulfilling requirements for biomedical applications. Such a processing can improve the mechanical properties of the unalloyed titanium to the level of the commonly used Ti-6Al-4V. Full article

The corrosion behavior of duplex Ni-P coatings deposited on AZ91 magnesium alloy was studied. The electroless deposition process of duplex Ni-P coating consisted in the preparation of low-phosphorus Ni-P coating (5.7 wt.% of P), which served as a bond coating and high-phosphorus Ni-P coating (11.5 wt.% of P) deposited on it. The duplex Ni-P coatings with the thickness of 25, 50, 75 and 100 µm were deposited on AZ91 magnesium alloy. The electrochemical corrosion behavior of coated AZ91 magnesium alloy was investigated by electrochemical impedance spectroscopy and potentiodynamic polarization method in 0.1 M NaCl. Obtained results showed a significant improvement in the corrosion resistance of coated specimens when compared to uncoated AZ91 magnesium alloy. From the results of the immersion tests in 3.5 wt.% NaCl, 10% solution of HCl and NaOH and 5% neutral salt spray, a noticeable increase in the corrosion resistance with the increasing thickness of the Ni-P coating was observed. Full article

Single-phase equiatomic five-element high entropy alloy CoCrFeMnNi was prepared by powder metallurgy. Two materials with ultra-fine-grained microstructure were prepared by spark plasma sintering (SPS) of ball-milled powder at two sintering times (5 and 10 min), assigned as HEA 5 and HEA 10, respectively. Basic microstructural and mechanical properties were evaluated. The median grain size of the microstructures was determined to be 0.4 and 0.6 μm for HEA 5 and HEA 10, respectively. The differences in the microstructure led to a significant change in strength and deformation characteristics evaluated at room temperature. The effect of cyclic loading was monitored by three-point bending fatigue test. The results show that even relatively small change in the microstructure causes a significant effect on fatigue life. The fatigue endurance limit was measured to be 1100 MPa and 1000 MPa for HEA 5 and HEA 10, respectively. The detailed fractographic analysis revealed that abnormally large grains, localised in the microstructure on the tensile loaded surface, were a typical fatigue initiation site. The formation of (nano) twins together with dislocation slips caused the crack nucleation because of the cyclic loading. Full article

The use of high strength steels (HSS) allows designing lighter, slenderer and simpler structures with high structural performance. In general, the use of HSS leads to weight reduction of the whole structure, which compensates the higher cost of such a material comparing to the conventional construction steels. Knowledge of the fatigue resistance of material plays the key role during design and maintenance of the bridge structures. This contribution brings a comparison of the fatigue crack growth resistance of S355 J0 steel. Differences in microstructure and the texture of material structure could generally play a role in the fatigue crack growth. This study shows that in the case of studied steel texture of material structure has an influence on material fatigue behavior in Paris’ law regime. Full article

Magnesium alloy QE22 (nominal composition 2 wt % Ag, 2 wt % mixture of rare earth elements, balance Mg) was reinforced with 5 vol % Saffil fibers and 15 vol % SiC particles. The hybrid composite was prepared via the squeeze cast technique. The microstructure of the monolithic alloy and composite was analyzed using scanning electron microscopy. Elastic modulus was measured at room temperature and modeled by the Halpin–Tsai–Kardos mathematical model. The strengthening effect of fibers and particles was calculated and compared with the experimentally obtained values. The main strengthening terms were determined. Fracture surfaces were studied via scanning electron microscope. While the fracture of the matrix alloy had a mainly intercrystalline character, the failure of the hybrid composite was transcrystalline. Full article

Shot peening is a well-known surface treatment method used for fatigue life improvement of cyclically loaded structural components. Since three main variables are considered in the peening process (peening intensity, coverage and peening media type), there is no direct way to choose the best combination of treatment parameters for the best performance, thus it has to be based on experience and laboratory tests. When shot peening is performed with inadequate parameters, or the peening process is not stable in time (decrease of the peening pressure, deterioration of the peening media and so on), it can result in significant degradation of the treated component fatigue properties, what is commonly called as the “overpeening” effect. When a premature fatigue fracture occurs in operation, the fracture surface analysis is usually the most important method of revealing the damage mechanism. This work is aimed at the study of the relation between the shot peening parameters and the fatigue fracture surface character on an AW 7075 aluminium alloy with an objective of identifying marks of overpeening and investigating the fatigue crack initiation mechanism. After performing the tests, it was observed that shot peening with optimized parameters creates a surface layer that is able to change the mechanism of the fatigue crack propagation and improve fatigue strength. On the other hand, using extensive peening parameters decrease the fatigue strength due to the creation of surface cracks and surface layer delamination. Full article

The electrochemical corrosion characteristics of AZ31 and AZ61 magnesium alloys were analyzed in terms of potentiodynamic tests and electrochemical impedance spectroscopy. The influence of the solution composition and material surface finish was examined also through the analysis of corrosion products created on the samples’ surface after electrochemical measurements in terms of scanning electron microscopy using energy-dispersive spectroscopy. Obtained data revealed the differences in the response of the magnesium alloys to enriched Hank’s Balanced Salt Solution—HBSS+ (with Mg²⁺ and Ca²⁺ ions) and Hank’s Balanced Salt Solution—HBSS (without Mg²⁺ and Ca²⁺ ions). Both examined alloys exhibited better corrosion resistance from the thermodynamic and kinetic point of view in the enriched HBSS+. AZ61 magnesium alloy reached higher values of polarization resistance than AZ31 magnesium alloy in both the used corrosion solutions. Phosphate-based corrosion products were characteristic for the AZ31 and AZ61 alloys tested in the HBSS (without Mg²⁺ and Ca²⁺ ions). The combination of phosphate-based corrosion products and clusters of MgO and Mg(OH)₂ was typical for the surface of samples tested in the enriched HBSS+ (with Mg²⁺ and Ca²⁺ ions). Pitting corrosion attack was observed only in the case of enriched HBSS+. Full article

Magnesium with its mechanical properties and nontoxicity is predetermined as a material for biomedical applications; however, its high reactivity is a limiting factor for its usage. Powder metallurgy is one of the promising methods for the enhancement of material mechanical properties and, due to the introduced plastic deformation, can also have a positive influence on corrosion resistance. Pure magnesium samples were prepared via powder metallurgy. Compacting pressures from 100 MPa to 500 MPa were used for samples’ preparation at room temperature and elevated temperatures. The microstructure of the obtained compacts was analyzed in terms of microscopy. The three-point bendisng test and microhardness testing were adopted to define the compacts’ mechanical properties, discussing the results with respect to fractographic analysis. Electrochemical corrosion properties analyzed with electrochemical impedance spectroscopy carried out in HBSS (Hank’s Balanced Salt Solution) and enriched HBSS were correlated with the metallographic analysis of the corrosion process. Cold compacted materials were very brittle with low strength (up to 50 MPa) and microhardness (up to 50 HV (load: 0.025 kg)) and degraded rapidly in both solutions. Hot pressed materials yielded much higher strength (up to 250 MPa) and microhardness (up to 65 HV (load: 0.025 kg)), and the electrochemical characteristics were significantly better when compared to the cold compacted samples. Temperatures of 300 °C and 400 °C and high compacting pressures from 300 MPa to 500 MPa had a positive influence on material bonding, mechanical and electrochemical properties. A compacting temperature of 500 °C had a detrimental effect on material compaction when using pressure above 200 MPa. Full article

The use of zinc-based materials as biodegradable materials for medical purposes is offered as a possible alternative to corrosion-less resistant magnesium-based materials. Zinc powders with two different particle sizes (7.5 µm and 150 µm) were processed by the methods of powder metallurgy: cold pressing, cold pressing followed by sintering and hot pressing. The microstructure of prepared materials was evaluated in terms of light optical microscopy, and the mechanical properties were analyzed with Vickers microhardness testing and three-point bend testing. Fractographic analysis of broken samples was performed with scanning electron microscopy. Particle size was shown to have a significant effect on compacts mechanical properties. The deformability of 7.5 µm particle size powder was improved by increased temperature during the processing, while in the case of larger powder, no significant influence of temperature was observed. Bending properties of prepared materials were positively influenced by elevated temperature during processing and correspond to the increasing compacting pressures. Better properties were achieved for pure zinc prepared from 150 µm particle size powder compared to materials prepared from 7.5 µm particle size powder. Full article

Linear polarization is a potentiodynamic method used for electrochemical characterization of materials. Obtained values of corrosion potential and corrosion current density offer information about material behavior in corrosion environments from the thermodynamic and kinetic points of view, respectively. The present study offers a comparison of applications of the linear polarization method (from −100 mV to +200 mV vs. E_OCP), the cathodic polarization of the specimen (−100 mV vs. E_OCP), and the anodic polarization of the specimen (+100 mV vs. E_OCP), and a discussion of the differences in the obtained values of the electrochemical characteristics of cast AZ91 magnesium alloy. The corrosion current density obtained by cathodic polarization was similar to the corrosion current density obtained by linear polarization, while a lower value was obtained by anodic polarization. Signs of corrosion attack were observed only in the case of linear polarization including cathodic and anodic polarization of the specimen. Full article

New Mg-3Zn-2Ca magnesium alloy was prepared using different processing techniques: gravity casting as well as squeeze casting in liquid and semisolid states. Materials were further thermally treated; thermal treatment of the gravity cast alloy was additionally combined with the equal channel angular pressing (ECAP). Alloy processed by the squeeze casting in liquid as well as in semisolid state exhibit improved plasticity; the ECAP processing positively influenced both the tensile and compressive characteristics of the alloy. Applied heat treatment influenced the distribution and chemical composition of present intermetallic phases. Influence of particular processing techniques, heat treatment, and intermetallic phase distribution is thoroughly discussed in relation to mechanical behavior of presented alloys. Full article