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The microstructure variation and oxidation behavior of a hot-rolled and quenched Ti-24Al-11Nb (at%) intermetallic alloy with and without protective Ni-20Cr (at%) coating were studied in the air at a cyclic annealing temperature of 600 °C and 900 °C. The phase transformations monitored up to 120 h of alloy oxidation and oxidation products were examined using different experimental techniques. The oxidation kinetics was determined by recording the mass gain vs. time data and oxidation activation energy was evaluated. It was found that the annealing temperature and deposited coating significantly affect the scale formation and growth. The alloy showed better oxidation resistance at 600 °C, irrespective of the protective coating application. The higher temperature promoted the formation of thicker and multi-layered scale predominantly composed of Nb-doped TiO2 and Al2O3 oxides, which cracked and spalled causing oxidation rate for an order of magnitude higher than that at 600 °C. The presence of AlN decelerated the oxide scale growth, which obeyed a parabolic rate law. It was also noted that a two-phase α2+β microstructure changed to a greater extent. With increasing temperature, the α2→β phase transformation occurred more intensive and new α2′′ and O-Ti2AlNb phase appeared. The Ni-20Cr coating reduced the oxidation rate at both temperatures and improved the scale-spallation resistance. The compact Cr2O3 layer formed on the coated alloy led to the preferential Al2O3 formation, increasing its amount in mixture with Nb-enriched TiO2 oxide. Thin TiN layer detected beneath the oxide scale also supported the formation of slow-growing Al2O3, leading to a decrease in the oxidation rate. The external scale suppression contributed to the greater diffusion zone enrichment, resulting in the faster O-Ti2AlNb formation. The experimentally observed O-Ti2AlNb formation was confirmed by ab initio modeling. Furthermore, additional structures were predicted and studied using first-princip

Wear damage behavior of new orthopedic Ti-13Nb-13Zr (mass %) alloy with martensitic microstructures developed by different thermo-mechanical treatments were examined during sliding in simulated physiological solution. The results obtained for this alloy processed by cold and hot rolling were compared with that of the standard Ti-6Al-4 V (mass %) alloy heat-treated to produce martensitic microstructure. The block-on-disk sliding friction and wear tests were conducted in Ringer’s solution. The friction coefficient, wear rate, and wear damage mechanisms were determined over a range of normal loads (20–60 N) and linear sliding speeds (0.26–1.0 m/s). The surface damage morphology revealed the presence of corrosive, abrasive, and adhesive wear in all investigated materials. The friction and wear properties were found to be dependent on the predominant wear damage mechanism and applied load/sliding speed combination. In the case of a Ti-13Nb-13Zr alloy, the adhesive wear contributes to the higher wear rate and better friction behavior. This cold-rolled alloy, having a martensitic microstructure with the lowest hardness, shows the lowest friction coefficient values due to high material transfer from the alloy surface. The lowest friction resistance is observed for the Ti-6Al-4 V alloy with the highest hardness since the dominant wear damage mechanism is abrasion resulting in the low wear rate and material transfer.

effect of initial microstructure on the mechanical properties and fracture mode of the Ti-6Al-4V ELI alloy subjected to the high-pressure torsion (HPT) processing at room temperature and 500 °C was investigated. In this purpose, the four different microstructures (fully lamellar, martensitic, equiaxed and globular microstructures) were developed by the proper heat treatments. The results showed that the application of the HPT processing provided substantial microstructural refinement, independent on the alloy initial microstructure. As a consequence, the alloy hardness, tensile strength and ductility significantly increased. At the same time, the elastic modulus of the HPT-processed alloy is almost 2 times lower as compared to that of the alloy in the initial heat treated state. The obtained improvements strongly depend on the alloy initial microstructure and HPT processing temperature. It was found that the initial martensitic microstructure is beneficial to improve the hardness, tensile and fracture properties. The HPT processing at 500 °C enhanced them more effectively. After the warm HPT processing, alloy with the initial martensitic microstructure exhibited exceptional hardness (455 MPa), high ultimate tensile strength (1546 MPa), high elongation to failure (18.8%) and low elastic modulus (78.6 GPa). Although the alloy with initial fully lamellar microstructure displayed the lowest elastic modulus (68 GPa), the ultrafine and homogeneous (α + β) two-phase microstructure produced by the HPT processing of the alloy with initial martensitic microstructure offered the best combination of the strength-ductility balance and strength-to-elastic modulus ratio (19.8 × 10−3). The complex fracture process, involving transgranular quasi-cleavage and ductile dimple fracture mode, corresponded to the changes in ductility.

The electrochemical behaviour of the Ti-13Nb-13Zr and Ti-6Al-4V ELI alloys with martensitic microstructures was investigated by polarization and electrochemical impedance spectroscopy (EIS) in Ringers solution. The impedance spectra were interpreted by a two time-constants equivalent circuit. Both investigated alloys showed high corrosion resistance, but the thin and uniform passive film on the Ti-6Al-4V ELI alloy surface was more protective. The inner barrier and outer porous layer were highly resistant and capacitive. However, thicker and more porous passive film on the Ti-13Nb-13Zr alloy surface may be beneficial for osteointegration. The suitable thermomechanical processing improved the corrosion resistance of Ti-13Nb-13Zr alloy. (c) 2014 Elsevier Ltd. All rights reserved.

The effects of microstructural morphology on the fracture behavior of Ti-6Al-4V ELI (extra-low impurity) alloy in two different heat treatment conditions were examined. Alloy was solution treated above (beta ST) and below (alpha + beta ST) beta transus temperature followed by furnace cooling (FC) in order to obtain the fully lamellar and equiaxed microstructures. Tensile and fracture toughness tests were conducted. The crack tip opening displacement (CTOD) and strain distribution near the crack tip were measured on the compact tension (CT) specimen surface by digital stereometric method. The crack propagation resistance (CTOD-R) curves were developed by applying the modified normalization method and critical CTOD values were determined. To identify the microstructural length scale controlling the fracture resistance of this alloy, the crack propagation path and fracture surface morphology were evaluated. It was found that the reduction in the characteristic microstructural dimension of an order of magnitude and significant change in the alpha phase aspect ratio contribute to drastic increase in the tensile properties and decrease in the crack initiation and propagation resistance. The fully lamellar microstructure displays slightly better biocompatibility because of the lower elastic modulus and superior fracture resistance. The enhanced crack propagation resistance of this microstructure is associated with the larger propensity for crack tip tortuousity, due to the coarser microstructural dimensions (lamellar colony size vs. primary alpha grain size). The difference in the crack propagation modes affects the shape and size of the actual crack tip strain distribution. These results were discussed correlating the complex multiple fracture mechanisms with the stress state in two microstructures. (C) 2013 Elsevier Ltd. All rights reserved.

Wear and corrosion behaviour of cold-rolled Ti-13Nb-13Zr alloy, with martensitic microstructure, and Ti-6Al-4V ELI alloy, in martensitic and two-phase (alpha + beta) microstructural conditions, was studied in a Ringers solution. The wear experiments were performed at room temperature with a normal load of 40 N and sliding speeds 0.26. 0.5 and 1.0 m/s. The corrosion behaviour was studied at 37 degrees C using open circuit potential-time measurements and potentiodynamic polarization. It was found that Ti-13Nb-13Zr alloy has a substantially lower wear resistance than Ti-6Al-4V ELI alloy in both microstructural conditions. Surface damage extent increases with sliding speed increase and is always smallest for martensitic Ti-6Al-4V ELI alloy with highest hardness. Both alloys exhibit spontaneous passivity in Ringers solution. Corrosion potential values are similar for all three materials. However, Ti-13Nb-13Zr and martensitic Ti-6Al-4V ELI alloys show improved corrosion resistance comparatively to Ti-6Al-4V ELI alloy with (alpha + beta) microstructure. Martensitic Ti-6Al-4V ELI alloy possesses the best combination of both corrosion and wear resistance, although its corrosion resistance is found to be slightly higher than that of the Ti-13Nb-13Zr alloy. (C) 2010 Elsevier Ltd. All rights reserved.

The article deals with the relations between microstructural parameters, microscopic fracture mechanisms, and resultant plane-strain fracture toughness, K(IC), of hot-forged 7000 alloy plates in overaged condition. Three industrially produced alloys with the (Zn + Mg + Cu) content of similar to 11.5 mass% and different contents of (Fe + Si) impurities (from 0.23 to 0.37 mass%) were used. The variation in alloy purity generated differences in the geometric characteristics of coarse intermetallic phase particles, exerting significant effect on fracture behavior of these alloys. To evaluate the fracture mechanism, characterization of the broken K(IC) test specimen surfaces was performed using scanning electron microscopy. Three micromechanisms governing fracture process were identified. The extent of primary voiding at coarse intermetallic phase particles increases systematically with the increase of (Fe + Si) level, whereas the extent of intergranular/transgranular fracture decreases. This observation was confirmed by a quantitative description of fractures by means of a profilometric method. It was found that morphological parameters of the fracture profiles clearly differentiate between different fracture modes and offer conclusive proof that coarse voiding at fractured intermetallic phase particles dominates fracture at low alloy purity. A higher fraction of coarse voiding is shown to result in an overall reduction of fracture toughness.

The aim of this study is to compare the tribological behaviour of novel orthopaedic implant alloy Ti-13Nb-13Zr with that of the standard Ti-6Al-4V ELI alloy, available in four different microstructural conditions produced by variations in the heat treatments. The friction and wear tests were performed by using a block-on-disc tribometer in Ringers solution at ambient temperature with a normal load of 20-60 N and sliding speed of 0.26-1.0 m/s. It was found that variations in microstructures produced significant variations in the wear resistance of Ti-6Al-4V ELI alloy. The wear losses of materials solution treated (ST) above the beta transus temperature are significantly lower compared with those of materials ST in the (alpha + beta) phase field and are almost insensitive to applied load and sliding speed. Wear loss of the (alpha + beta) ST Ti-6Al-4V ELI alloy continuously increased as applied load was increased and was highest at the highest sliding speed. The Ti-6Al-4V ELI alloy in all microstructural conditions possesses a much better wear resistance than cold-rolled Ti-13Nb-13Zr alloy. Friction results and morphology of worn surfaces showed that the observed behaviour is attributed to the predominant wear damage mechanism.

To identify the most important parameters of multiscale microstructural features influencing the fracture modes and resistance to damage, detailed microstructural and fractographic analysis of overaged 7000 alloy plates are performed using the broken plane-strain fracture toughness, K-Ic, test specimens. The geometric characteristics of differently sized second-phase particles are changed by the compositional variations. It was found that the fracture process involves three main micromechanisms. The dominant fracture mode changes with alloy purity, leading to fracture toughness degradation. Quantitative description of fractures by profilometry confirmed that crack initiation and propagation is fostered by the coarse Fe- and Si-rich particles. (C) 2009 Elsevier B. V. All rights reserved

An attempt has been made to model the plane-strain fracture toughness, KIc, in Al-Zn-Mg-Cu alloy forgings subjected to overageing. The proposed model, based on the multiple micromechanisms, reveals the quantitative relations between fracture toughness, fraction of all fracture modes and microstructural parameters associated with multiscale-sized second-phase particles and precipitate-free zones. The new model is validated by the present quantitative data of microstructural and fractographic analysis performed along with mechanical tests on hot-forged plates in T73 condition. The relevant parameters changed by the compositional variations were determined in two orientations. It was found that the predicted KIc values represent the tendency of fracture toughness change well. The new model provides better agreement for the case of dominant transgranular fracture mode.

The effects of microstructural parameters associated with the coarse intermetallic phases particles and precipitates on the fracture toughness of the overaged 7000 alloy forgings are investigated. Detailed microstructural and fractographic analysis together with fracture toughness tests are carried out using three alloys with different (Fe + Si) contents. The fracture mechanisms are identified and area fractions of different fracture modes are assessed. The data are then quantitatively correlated to plane-strain fracture toughness, K-Ic, and the bulk microstructural attributes estimated via image analysis. A multiple micromechanism-based model is developed, which accurately describes the dependence of K-Ic on individual microstructural parameters. (C) 2007 Published by Elsevier Ltd.

Micromechanisms controlling the fracture damage in 7000 alloy forgings are studied for T73-type heat treatments. Insight into the fracture modes are obtained by varying the microstructural attributes associated with the coarse intermetallic (IM) particles and precipitates. To identify the fracture micromechanisms, extensive fractographic analysis by scanning electron microscopy (SEM) observation of the broken plane-strain fracture-toughness test specimen surfaces are carried out. The relative fractions of the various fracture modes as a function of microstructural features are quantitatively determined. The relationships between these data and values of fracture toughness are then established. It was found that multiple-fracture micromechanisms occur and the fracture toughness degradation is consistent with a change in the dominant fracture mode governed by variations in the alloy purity. (C) 2006 Elsevier B.V. All rights reserved.

The Analysis of variance (ANOVA) method has been used to illustrate the implementation of adaptive numerical (AN) techniques for the prediction of the precipitation behaviour of commercial materials. Case studies involving the analysis of a-phase precipitation kinetics in duplex stainless steel (DSS) produced by sand casting and age hardening of 2219 alummium alloy microalloyed with Ge and/or Si are presented. For each alloy, complex datasets comprised the results obtained from heat treatment trials on a range of commercial processing conditions, so that a single and combined effect of various parameters can be determined. This enabled an estimate of the most influential parameters to be made, providing an effective means of commercial alloy development and process optimisation. (c) 2005 Elsevier B.V. All rights reserved.