Oscar Antonio Ruano

Corrosion behaviour of AZ31 magnesium alloy with different grain sizes in simulated biological fluids

The corrosion behaviour of AZ31 magnesium alloy with different grain sizes immersed in simulated body fluids was compared in chloride solution (8 gl(-1)) and in phosphate-buffer solution (PBS). The influence of immersion time was also analyzed. Electrochemical techniques such as open circuit potential, polarization curves, transient currents and electrochemical impedance spectroscopy, complemented with scanning electron microscopy and energy dispersive spectroscopy, were used. Immediately after the immersion in the corrosive media the corrosion resistance was similar for both grain sizes of the AZ31 alloy and higher in NaCl solutions than in PBS. However, this corrosion behaviour was reversed after longer periods of immersion due to the stabilizing of the corrosion products of MgO by P-containing compounds. These P-compounds contribute to a higher level of protection by hindering the aggressive action of chloride ions. The best corrosion behaviour of the AZ31 alloy was obtained for the finest grain alloy associated with the highest transfer resistance value, after long periods of immersion in PBS.

Texture evolution during large-strain hot rolling of the Mg AZ61 alloy

Abstract Grain refinement in a Mg-based AZ61 alloy of initially coarse, recrystallized microstructure was successfully achieved by thermomechanical processing (TMP) consisting of two to three hot-rolling steps with large reductions per pass. Reductions as large as 85% (equivalent to a true strain of ≈1) were achieved without surface cracking. The underlying microscopic mechanisms operative during the TMP that allowed this hcp material to accommodate such large strains per pass were investigated by macro- and microtexture analysis. A significant decrease in the intensity of the initial basal texture was observed after the first pass. This was attributed to rotational dynamic recrystallization, a mechanism by which new recrystallized grains develop, with orientations favourable for basal slip. Upon subsequent passes, basal slip becomes the main deformation mechanism. Simultaneously, grain refinement takes place by continuous dynamic recrystallization. The fine-grained microstructure thus developed showed improved superplastic behaviour in comparison with that of similar alloys processed by more elaborate methods.

Corrosion inhibition of powder metallurgy Mg by fluoride treatments

Pure Mg has been proposed as a potential degradable biomaterial to avoid both the disadvantages of non-degradable internal fixation implants and the use of alloying elements that may be toxic. However, it shows excessively high corrosion rate and insufficient yield strength. The effects of reinforcing Mg by a powder metallurgy (PM) route and the application of biocompatible corrosion inhibitors (immersion in 0.1 and 1M KF solution treatments, 0.1M FST and 1M FST, respectively) were analyzed in order to improve Mg mechanical and corrosion resistance, respectively. Open circuit potential measurements, polarization techniques (PT), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS) were performed to evaluate its corrosion behavior. SECM showed that the local current of attacked areas decreased during the F(-) treatments. The corrosion inhibitory action of 0.1M FST and 1M FST in phosphate buffered solution was assessed by PT and EIS. Under the experimental conditions assayed, 0.1M FST revealed better performance. X-ray photoelectron spectroscopy, energy dispersive X-ray and X-ray diffraction analyses of Mg(PM) with 0.1M FST showed the presence of KMgF(3) crystals on the surface while a MgF(2) film was detected for 1M FST. After fluoride inhibition treatments, promising results were observed for Mg(PM) as degradable metallic biomaterial due to its higher yield strength and lower initial corrosion rate than untreated Mg, as well as a progressive loss of the protective characteristics of the F(-)-containing film which ensures the gradual degradation process.

Deformation of fine-grained alumina by grain boundary sliding accommodated by slip

Creep data from over 40 different polycrystalline alumina materials are reviewed. Most of these studies have attempted to describe the creep data using models based on diffusional creep. In the present paper, however, it is concluded that the dominant deformation mechanism in creep of fine-grained alumina is grain boundary sliding (GBS) accommodated by slip. The slip accommodation process is related to the sequential steps of dislocation glide and climb. When the accommodation process for GBS is that of dislocation climb, the stress exponent is always 2. In this case, the activation energy for creep is either that for oxygen ion diffusion in the lattice or that for oxygen ion diffusion in the grain boundary. When the accommodation process for GBS is that of solute-drag dislocation glide, the stress exponent is 1. For this case, the activation energy is that for solute diffusion at the dislocation site during glide.

Harper-dorn creep in pure metals

Abstract Evidence is presented that Harper-Dora (H-D) viscous creep is a diffusion-controlled, dislocation creep process. The factors influencing H-D creep are the same as those influencing power-law creep, namely dislocation density, subgrain or grain size, stacking fault energy, and elastic modulus. H-D creep has been observed, and is analyzed, for seven different metals: Al, Pb, Sn, α-Zr, α-Ti, α-Fe, and β-Co. In addition, seven metals (α-Fe, Ni, Ag, β-Co, Cu, Mo, and Cr) which were originally believed to be controlled by diffusional (Nabarro-Herring) creep at low stresses, are shown to be controlled by Harper-Dorn creep. An internal stress assisted creep model is presented which correlates data in both the power-law and H-D creep regimes. The model predicts, quantitatively, the conditions under which H-D creep will dominate plastic flow.

Microstructure and high temperature mechanical properties of tin

Abstract The mechanical properties of tin have been studied by tensile tests in the temperature range 293–463 K. Tensile tests were performed for cylindrical samples at a constant strain rate and varying strain rates during deformation. In-situ-tensile tests also were conducted in ribbon-form samples. At the strain rates studied, deformation takes place preferentially by slip, although some scattered twins also were observed at lower temperatures. Strong grain growth occurs at the higher temperatures. Microstructural observations of deformed samples show that dynamic recrystallization is not important in the temperature range investigated. The fracture surface of the cylindrical samples changes from a chisel type of fracture at the lower temperatures to a simple shear type of fracture at the higher temperatures. Both the tensile strength and ductility decrease with increasing temperature. An explanation is given for the loss of ductility at high temperatures. The activation energy for creep, obtained from strain-rate-change tests is 35 kJ mol−1 and the stress exponent is about 6. These values are related to a slip mechanism controlled by pipe diffusion.

Texture evolution during grain growth in annealed MG AZ61 alloy

Abstract The development of new orientations in an extruded Mg based sheet alloy is analysed upon annealing under different conditions. Normal grain growth takes place upon moderate annealing leading to a strong basal texture. Abnormal growth of prismatic { 1 1 2 0 } grains occurs following severe annealing treatments.

Microstructure and fracture properties of an ultrahigh carbon steel–mild steel laminated composite

Abstract A seven layer steel based (mild steel and ultrahigh carbon steel, UHCS) laminated composite was processed by roll bonding. Impact properties were improved in comparison with the UHCS. Delamination plays an important role by deflecting cracks, absorbing energy and imposing the nucleation of new cracks in the next material layer.

Advanced Shape Memory Alloys Processed by Powder Metallurgy

c) the large orientation dependence of the transformation strain, and d) the grain boundary segregation. In order to suppress the intergranular fracture and to improve the ductility of these alloys, the stress concentration in the grain boundaries must be controlled—either by development of high-textured alloys or development of fine grain alloys. [5] The high texture enables the accommodation of stress among adjacent grains and, as a consequence, the stress concentration in the grain boundaries decreases. Alternatively, the stress concentration decrease is due to the grain-size refinement, which produces smaller stress among adjacent grains. Several techniques are suitable for producing the grain-size refinement: addition of other elements in small quantities, [7] melt spinning, [8‐10] and powder metallurgy. [11,12] Finally, powder metallurgy is a relatively new technique in this area, and no attempts to use it in the development of this kind of alloys had been made, in spite of its promising capabilities. [14,15] Thus, taking into account these facts, a new production process of Cu‐Al‐Ni SMA by powder metallurgy is developed in this work. The first part of the present paper contains a detailed description of each stage of the process, whereas the second one is devoted to the characterization of thermomechanical and fracture properties of the obtained materials. Finally, a comparison of these properties with those of single crystals and polycrystals of the same kind of alloys but obtained by classical methods, is also performed.

Deformation mechanisms in an austenitic stainless steel (25Cr-20Ni) at elevated temperature

The creep behaviour at elevated temperature of an austenitic stainless steel (25Cr-20Ni), both with and without antimony additions, has been reanalysed. Formerly, the creep behaviour was interpreted by considering creep mechanisms based on diffusional (Coble) creep and threshold stresses. In the present paper, it is proposed that an alternative mechanism of grain boundary sliding, accommodated by slip in grain boundary mantle regions, can in fact be used to describe more accurately the creep behaviour. Quantitative predictions, based on phenomenological equations for describing creep controlled by grain boundary sliding, are made of the influences of grain size, stress and antimony addition on creep rates, and of the influence of grain size on the activation energy for creep of 25Cr-20Ni stainless steel. Comparison of these predictions with those based on creep models incorporating only diffusional flow are made. Furthermore, the existence of a threshold stress in creep of single-phase, massive materials is strongly questioned.