Stabilization of metastable phases by pressure and shear

Numerous metallic systems possess allotropic phases that are stable at high temperatures or high pressures and that are endowed with unique properties that cannot be currently exploited because the reverse transformations take place once the pressure is released or the temperature decreases to ambient levels. Devising new processing methods that could lead to the stabilization of such phases would open new avenues for the fabrication of materials with still unforeseeable properties. It has been recently reported that the simultaneous application of pressure and shear by high pressure torsion (HPT) can be utilized to induce bulk allotropic phase transformations and thereby to stabilize metastable phases under ambient conditions. In particular, HPT processing allowed the stabilization of the high pressure phases and β in pure Zr and in Zr–2.5wt.% Nb. Furthermore, the application of shear under pressure led to a drastic reduction of the pressure levels with respect to those required under hydrostatic conditions. Similar results were obtained in pure Ti. HPT has also been recently employed to transform partially stabilized tetragonal zirconia into a monoclinic structure and to stabilize the Si-III and Si-XII phases in a Si (100) wafer. TEM studies have further revealed the occurrence of localized shear-induced phase transformations in HPT processed specimens. Among others, the -BCC to -FCC phase transformation was reported in pearlitic carbon steel leading to a nanocrystalline austenitic structure. Finaly, precipitation of hcp Mg and a low temperature HCP Li-rich phase was observed in single phase BCC Mg–Li alloys. This discussion reviews successful examples of the stabilization of metastable phases by the simultaneous applications of pressure and shear, or by pressure alone. The influence of the processing parameters as well as of microstructure characteristics such as composition, texture, and grain size will be discussed

Estudio de la deformación de aleaciones superplásticas de aluminio mediante análisis de texturas

En este trabajo se estudian los mecanismos de deformación que operan durante la deformación en tracción de algunas aleaciones superplásticas de aluminio. Se ha hecho especial énfasis en la aleación Al-5%Ca-5%Zn y los resultados obtenidos se han extendido a otras aleaciones superplásticas de aluminio: 2090, 8090, Supral 2004, 5083 y 7475. Se han realizado ensayos de tracción a diferentes velocidades de deformación y temperaturas, con el objetivo de analizar la deformación dentro y fuera del rango superplástico. La herramienta principal utilizada para estudiar la evolución microestructural ha sido la determinación de texturas mediante rayos X (macrotextura) y mediante difracción de electrones (microtextura. La aportación más importante de este trabajo de investigación al conocimiento actual es la distinción entre dos clases de evolución microestructural en las aleaciones superplásticas de aluminio. En las que recristalizan de forma continua el deslizamiento cristalográfico opera en respuesta a la tensión aplicada dentro y fuera del intervalo superplástico. En este último caso coexiste con el deslizamiento de fronteras de grano. Las aleaciones que recristalizan de forma continua siguen un esquema de comportamiento tradicional: fuera del rango superplástico el mecanismo predominante es el deslizamiento cristalográfico múltiple y dentro del deslizamiento de fronteras de granoDepto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu

ESTUDIO DE LA DEFORMACIÓN DE ALEACIONES SUPERPLÁSTICAS DE ALUMINIO MEDIANTE ANÁLISIS DE TEXTURAS

In this work the deformation mechanisms which operate in several superplastic aluminum alloys during tensile straining have been investigated by means of texture analysis via x-ray diffraction (macrotexture) and electron diffraction (microtexture). Tensile tests have been performed at different temperatures and strain rates with the aim of studying the deformation behavior outside and within the superplastic regime. Special attention has been paid to the alloy Al-5%Ca-5%Zn (fine-grained alloy, d ≈ 2-3 μm, with 20%vol. of second phase, Al3CaZn) and the main conclusions obtained have been generalized to other superplastic aluminum alloys: 2090, 8090, Supral 2004 and ski5083. The main original contribution of this work is the following: it has been found that in some superplastic aluminum alloys crystallographic slip operates even under optimum superplastic conditions, where this mechanism competes with grain boundary sliding. This behavior seems to be characteristic of alloys which recrystallize in a continuous way. As such, this observation could explain why "continuous" alloys undergo superplastic deformation at relatively high strain rates and are therefore extremely interesting for their potential applications in industry. Alloys which recrystallize in a discontinuous way follow the pattern of behavior traditionally accepted for superplastic alloys.Peer reviewe

Método de obtención de la fase beta estable de un metal puro de transición del grupo IV del sistema periódico y producto obtenible por dicho método

Método de obtención de la fase beta estable de un metal puro de transición del grupo IV del sistema periódico y producto obtenible por dicho método. Se describe un nuevo método para la fabricación controlada de la fase beta de los metales de transición del grupo IV del sistema periódico que es estable en condiciones ambiente (1 atmósfera y 22ºC). El procedimiento comprende una primera etapa de aplicación, sobre una muestra (2) elaborada de la fase alfa del metal de transición considerado, de una presión de compresión de un valor mínimo de entre 1 y 3 GPa, seguida de una segunda etapa de aplicación, manteniendo dicha presión, de una deformación de cizalladura, de valor mínimo no superior a 39.2. El procedimiento descrito permite obtener fase beta estable de metales de transición del grupo IV empleando presiones y cizalladuras de valores sensiblemente inferiores que en el caso de obtención de dicha fase beta estable por medios que no implican aplicación simultánea de presión y cizalladura.Peer reviewedConsejo Superior de Investigaciones Científicas (España), Instituto Madrileño de Estudios AvanzadosA1 Solicitud de patente con informe sobre el estado de la técnic

Micromechanics of fully lamellar TiAl alloys

Fully lamellar gamma titanium aluminides are very promising materials for aerospace applications, due to their increased thrust-to-weight ratios and improved efficiency under aggressive environments at temperatures up to 750 ºC. For that reason, they are projected to replace the heavier Ni- base superalloys currently used for low pressure turbine (LPT) blades manufacturing. However, their ductility is limited due to their inherent anisotropy, associated to the lamellar microstructure. The objective of this work was to study the mechanical response of single colonies of polycrystalline γ-TiAl, as a function of layer thickness and layer orientation, and to relate this mechanical response with the operative deformation mechanisms. With this aim, micropillars with lamellae oriented at 0º, 45º and 90º with respect to the loading direction were compressed at room temperature and elevated temperature. The results revealed a large plastic anisotropy, that was rationalized, based on slip/twin trace analysis, according to the relative orientation of the main operative deformation modes with respect to the lamellar interfaces. Loading at 45º resulted in the activation of soft longitudinal deformation modes, where both the slip plane and the slip direction were parallel to the interfaces, and therefore, little interaction of dislocations with lamellar interfaces is expected. At 0º loading, deformation was mainly accommodated by harder mixed deformation modes (with an oblique slip plane but a slip direction parallel to the lamellar interfaces), although the lamellar interfaces seemed to be relatively transparent to slip transfer. On the contrary, 90º loading represented the hardest direction and deformation was accommodated by the activation of transverse deformation modes, confined to individual lamellae, together with longitudinal modes that were activated due to their softer nature, despite their very small Schmid factors. Finally, a thorough study of pillar size effects revealed that the results were insensitive to pillar size for dimensions above 5 mm. The results can therefore be successfully applied for developing mesoscale plasticity models that capture the micromechanics of fully lamellar TiAl microstructures at larger length scales Additionally, microtensile specimens were also milled out of single colonies and in-situ tested in the SEM, to study the role of interlamellar interfaces on the plastic deformation and fracture under tension. EBSD was used before and after the test to study the role of different type of interfaces (true twin, pseudo twin and order variant) on slip/twin transfer. This study emphasizes the complexity of the micromechanics of fully lamellar TiAl alloys, where the activation of different deformation modes is strongly affected, not only by the lamellar orientation, but also by the character of the interfaces between the different lamellae. References A.J. Palomares, M.T. Pérez-Prado, J.M. Molina-Aldareguia, Acta Mater. 123 (2017) 102-114 A.J. Palomares, I. Sabirov, M.T. Pérez-Prado, J.M. Molina-Aldareguia, Scripta Mater. 139 (2017) 17-2

Superplastic Behavior of a Fine Grained AZ61 Alloy Processed by Large Strain Hot Rolling

A processing route based on conventional hot rolling has been recently developed to induce significant grain refinement in Mg alloys. The simplicity and rapidity of the processing route as well as the fact that conventional rolling is used, may allow it to be put into practice successfully in industry. This method consists of only two to three rolling passes, each producing a large thickness reduction, and intermediate annealings of 5 minutes duration. The resulting microstructure is mainly formed by very fine grains. Tensile tests at different temperatures and strain rates were performed in order to analyse the mechanical behaviour of the processed AZ61 alloy under different testing conditions. Microstructure and texture evolution during deformation were examined by optical microscopy (OM) and X-ray diffraction, respectively. It is shown that the microstructures developed by large strain hot rolling are capable of exhibiting significant superplastic elongations at moderate to low temperatures. Stress exponents close to 2 were measured during deformation under optimum superplastic conditions. Additionally, grains remained equiaxed and a significant decrease in the texture intensity is observed. This is consistent with the predominance of grain boundary sliding as the main deformation mechanism responsible for superplasticityCICYT grant MAT2000-1313 is appreciated. MTP acknowledges a Ramón y Cajal 2001 contract, awarded by the Spanish Ministry of Science and Technology. JAV is thankful to CONICETPeer reviewe

Texture Evolution of AZ31 Magnesium Alloy Sheet at High Strain Rates

In the current contribution the mechanical behaviour at high strain rates of AZ31 magnesium alloy sheet is studied. Uniaxial deformation properties were studied by means of tensile split Hopkinson pressure bar (SHPB) at different temperatures. The influence of the strain rate and temperature on the deformation mechanisms was investigated by means of electron backscatter diffraction (EBSD) and neutron diffraction. It is shown that twinning plays an important role on high strain rate deformation of this alloy, even at elevated temperatures. Significant evidence of prismatic slip as a deformation mechanism is observed, also at warm temperatures, leading to the alignment of directions with the tensile axis and to a spread of the intensities of the basal pole figure towards the in-plane direction perpendicular to the tensile axis. The rate of decrease of the CRSS of non-basal systems is observed to be slower than at quasi-static rates. Secondary twinning and pyramidal slip were also outlined for some conditions. At warm temperatures, in contrast to quasi-static range, a generalized dynamic recrystallization is not observed. Moreover, the activation of rotational recrystallization mechanisms is reporte

Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature

The microstructural evolution of an AZ31 rolled sheet during dynamic deformation at strain rates of ∼103 s−1 has been investigated by electron backscatter diffraction, X-ray and neutron diffraction. The influence of orientation on the predominant deformation mechanisms and on the recovery processes taking place during deformation has been systematically examined. The results have been compared with those corresponding to the same alloy tested quasi-statically under equivalent conditions. It has been found that strain rate enhances the activation of extension twinning dramatically, while contraction and secondary twinning are not significantly influenced. The polarity of extension twinning is even reversed in some grains under selected testing conditions. Significant grain subdivision by the formation of geometrically necessary boundaries (GNBs) takes place during both quasi-static and dynamic deformation of this AZ31 alloy. It is remarkable that GNBs of high misorientations form even at the highest strain rates. The phenomenon of recovery has been found to be orientation dependen

Influence of thermomechanical processing on the grain size, texture and mechanical properties of Mg-Al alloys

El título de la revista en eslovaco es "Kovové Materiály"The work carried out by the authors over the last decade on the processing, microstructural characterization and the mechanical behaviour of Mg alloys is reviewed. In particular, the potential for grain refinement and for the development of specific textures of large strain hot rolling (LSHR), equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) is discussed. The recrystallization and the deformation mechanisms predominant in Mg alloys are analyzed as a function of the grain size and the texture in a wide range of stresses, strain rates and temperatures. Finally, the the feasibility of superplatic forming of Mg alloys was examined, taking into account the influence of factors such as grain size stability and microstructural heterogeneities.The authors acknowledge financial support from CICYT under program MAT 2003/1172Peer reviewe

Influence of thermomechanical processing on the grain size, texture and mechanical properties of Mg-Al alloys

El título de la revista en eslovaco es "Kovové Materiály"The work carried out by the authors over the last decade on the processing, microstructural characterization and the mechanical behaviour of Mg alloys is reviewed. In particular, the potential for grain refinement and for the development of specific textures of large strain hot rolling (LSHR), equal channel angular pressing (ECAP) and accumulative roll bonding (ARB) is discussed. The recrystallization and the deformation mechanisms predominant in Mg alloys are analyzed as a function of the grain size and the texture in a wide range of stresses, strain rates and temperatures. Finally, the the feasibility of superplatic forming of Mg alloys was examined, taking into account the influence of factors such as grain size stability and microstructural heterogeneities.The authors acknowledge financial support from CICYT under program MAT 2003/1172Peer reviewe