A multiscale material model for metallic powder compaction during hot isostatic pressing

The prediction of the distortions during Near-Net-Shape Hot Isostatic Pressing (NNS-HIP) is an intrinsic multiscale problem where the local interactions among particles determine the macroscopic distortions taking place during the sintering and densification of a component. In this work, a multiscale approach is proposed to solve this problem. In particular, a viscoplastic constitutive model capable of predicting macroscopic contractions during a HIP process with high accuracy has been developed, implemented and validated. The macroscopic model incorporates the mechanical behaviour predicted at the meso-scale by means of multiple-particle finite element models (MP-FEM) of an agglomerate of powder particles. The model is validated through the prediction of distortions during HIP of a full scale industrial case. It is concluded that adding the microscopic information of the HIP process to simulate the contractions at the macroscopic level results in a considerable improvement of the accuracy of the predictions

Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface displacements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 ψ XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15

Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface dis-placements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 psi XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15%

Towards highs performance bulk thermoelectric materials with enhanced mechanical properties by Severe Plastic Deformation (SPD)

Nowadays, one of the most promising strategies to produce highly efficient thermoelectric (TE) materials is to reduce the lattice thermal conductivity by introducing phonon scattering centres (such as submicron sized grain boundaries, second-phase nano-particles, and point defects) at different length scales. For highly anisotropic crystals such as Bi2Te3-based thermoelectrics, the combination of nanosized grain structures (to improve phonon scattering) together with strong crystallographic texture (to exploit the anisotropic properties of the crystal) is not readily accessible by the standard high energy ball-milling and powder consolidation techniques. This work presents a novel technique that permits to obtain, simultaneously, highly textured submicron grained structures in Sb2-xBixTe3 thermoelectric material. The severe plastic deformation (SPD) induced by high pressure torsion (HPT) of Sb2-xBixTe3 leads to fully dense disk-shaped samples with stable homogeneous grain sizes of ~100 nm and a strong crystallographic texture with the basal plane preferentially oriented perpendicular to the torsion axis. Such combination has a synergistic effect on the enhancement of the thermoelectric performance. For instance, after HPT processing, Sb1.6Bi0.4Te3 compound displays a good TE performance in a wide range of temperatures and shows a maximum zTRA (i.e. PF measured in-plane and κ out of plane) of ~2 at 375 K, (zTRR~1.5 estimated in-plane). Moreover, HPT improves significantly the hardness of the processed samples, although thetheir strong crystal texture is detrimental for their flexural strength. HPT has also been successfully applied to pure PbTe. The results confirm HPT processing as a promising alternative to spark plasma sintering to process mechanically improved PbTe-based thermoelectric compounds with all-scale hierarchical architectures.Actualmente una de las estrategias más eficaces para procesar materiales termoeléctricos eficientes se basa en introducir defectos cristalinos a diferentes escalas. Es bien sabido que los fonones se dispersan efectivamente mediante defectos tales como las juntas de grano, partículas de segundas fases o defectos puntuales, lo que conlleva una reducción significativa de la conductividad térmica. En el caso de lo los materiales altamente anisótropos, como las aleaciones basadas en Bi2Te3, además de generar estructuras nanométricas es de vital importancia controlar la textura cristalográfica del compuesto ya que las propiedades termoeléctricas son óptimas únicamente en su plano basal. Hasta la fecha no se han obtenido materiales nanocristalinos preferentemente orientados mediante las técnicas de pulvimetalurgia convencionales. Este trabajo presenta una técnica novedosa que permite procesar materiales termoeléctricos de grano ultrafino y altamente texturizados basados en Bi2Te3. La deformación plástica severa (SPD) inducida mediante la torsión bajo alta presión (HPT) permite producir muestras en forma de disco totalmente densas, fuertemente texturizadas (con el plano basal preferentemente orientado perpendicular al eje de torsión) y nanoestructuradas (con un tamaño de grano homogéneo del orden de 100 nm). Debido a la fuerte textura cristalográfica y grano ultrafino obtenidos mediante HPT, tras HPT se mejoran notablemente las propiedades termoeléctricas de los compuestos basados en Bi2Te3. En el caso del compuesto Sb1.6Bi0.4Te3, por ejemplo, se logró una figura de mérito ZTRA (obtenida tras medir el factor de potencia (PF) en el plano y la conductividad térmica (κ)a lo largo del eje axial) de ~2 a 425 K. Además el compuesto muestra un buen rendimiento termoeléctrico en un amplio rango de temperatura. El procesamiento mediante HPT incrementa la dureza y la resistencia a compresión de las muestras, sin embargo la fuerte textura cristalográfica afecta negativamente a la resistencia a flexión. Por último, también se han procesado satisfactoriamente muestras de PbTe puras mediante HPT. Los resultados confirman que el procesamiento mediante HPT es una alternativa viable a la sinterización activada por plasma (SPS), la cual permite fabricar compuestos basados en PbTe que contienen defectos o segundas fases desde la escala atómica hasta la meso-escala

Towards highs performance bulk thermoelectric materials with enhanced mechanical properties by Severe Plastic Deformation (SPD)

Nowadays, one of the most promising strategies to produce highly efficient thermoelectric (TE) materials is to reduce the lattice thermal conductivity by introducing phonon scattering centres (such as submicron sized grain boundaries, second-phase nano-particles, and point defects) at different length scales. For highly anisotropic crystals such as Bi2Te3-based thermoelectrics, the combination of nanosized grain structures (to improve phonon scattering) together with strong crystallographic texture (to exploit the anisotropic properties of the crystal) is not readily accessible by the standard high energy ball-milling and powder consolidation techniques. This work presents a novel technique that permits to obtain, simultaneously, highly textured submicron grained structures in Sb2-xBixTe3 thermoelectric material. The severe plastic deformation (SPD) induced by high pressure torsion (HPT) of Sb2-xBixTe3 leads to fully dense disk-shaped samples with stable homogeneous grain sizes of ~100 nm and a strong crystallographic texture with the basal plane preferentially oriented perpendicular to the torsion axis. Such combination has a synergistic effect on the enhancement of the thermoelectric performance. For instance, after HPT processing, Sb1.6Bi0.4Te3 compound displays a good TE performance in a wide range of temperatures and shows a maximum zTRA (i.e. PF measured in-plane and κ out of plane) of ~2 at 375 K, (zTRR~1.5 estimated in-plane). Moreover, HPT improves significantly the hardness of the processed samples, although thetheir strong crystal texture is detrimental for their flexural strength. HPT has also been successfully applied to pure PbTe. The results confirm HPT processing as a promising alternative to spark plasma sintering to process mechanically improved PbTe-based thermoelectric compounds with all-scale hierarchical architectures.Actualmente una de las estrategias más eficaces para procesar materiales termoeléctricos eficientes se basa en introducir defectos cristalinos a diferentes escalas. Es bien sabido que los fonones se dispersan efectivamente mediante defectos tales como las juntas de grano, partículas de segundas fases o defectos puntuales, lo que conlleva una reducción significativa de la conductividad térmica. En el caso de lo los materiales altamente anisótropos, como las aleaciones basadas en Bi2Te3, además de generar estructuras nanométricas es de vital importancia controlar la textura cristalográfica del compuesto ya que las propiedades termoeléctricas son óptimas únicamente en su plano basal. Hasta la fecha no se han obtenido materiales nanocristalinos preferentemente orientados mediante las técnicas de pulvimetalurgia convencionales. Este trabajo presenta una técnica novedosa que permite procesar materiales termoeléctricos de grano ultrafino y altamente texturizados basados en Bi2Te3. La deformación plástica severa (SPD) inducida mediante la torsión bajo alta presión (HPT) permite producir muestras en forma de disco totalmente densas, fuertemente texturizadas (con el plano basal preferentemente orientado perpendicular al eje de torsión) y nanoestructuradas (con un tamaño de grano homogéneo del orden de 100 nm). Debido a la fuerte textura cristalográfica y grano ultrafino obtenidos mediante HPT, tras HPT se mejoran notablemente las propiedades termoeléctricas de los compuestos basados en Bi2Te3. En el caso del compuesto Sb1.6Bi0.4Te3, por ejemplo, se logró una figura de mérito ZTRA (obtenida tras medir el factor de potencia (PF) en el plano y la conductividad térmica (κ)a lo largo del eje axial) de ~2 a 425 K. Además el compuesto muestra un buen rendimiento termoeléctrico en un amplio rango de temperatura. El procesamiento mediante HPT incrementa la dureza y la resistencia a compresión de las muestras, sin embargo la fuerte textura cristalográfica afecta negativamente a la resistencia a flexión. Por último, también se han procesado satisfactoriamente muestras de PbTe puras mediante HPT. Los resultados confirman que el procesamiento mediante HPT es una alternativa viable a la sinterización activada por plasma (SPS), la cual permite fabricar compuestos basados en PbTe que contienen defectos o segundas fases desde la escala atómica hasta la meso-escala

A microstructure-based constitutive model for pearlite.

Fully pearlitic eutectoid steels have an excellent compromise of mechanical strength and ductility and are widely used for rails, prestressing tendons and high- strength wires. These excellent mechanical properties are a consequence of their particular nanocomposite structure combining thin cementite lamellae (~12% volume fraction) with ferritic lamellae. This complex substructure entails a complex microstructural and property evolution with applied strain that is difficult to model. In this work, a microstructure-based constitutive model for pearlite accounting for both the elastoplastic behaviour and the damage evolution is presented. The original formulation, valid for mesoscopic scales, considers the behaviour of ferrite and cementite separately, assuming that strengthening occurs through the mechanisms acting in ferrite. For its application in macro-scale systems such as wire drawing, a multi-colony homogenization strategy has been applied. For damage, a Continuum Damage Mechanics approach adapted to the features of pearlite has been adopted with the coupling of damage to the mechanical response. The model has been implemented for use in finite element simulations and has been calibrated using experimental data of tensile and torsion tests. Subsequently, the model has been validated, confirming its predictive capabilities across various aspects, including the mechanical response under different stress states, the build-up of internal stresses and the evolution of the microstructure with deformation.Los aceros eutectoides totalmente perlíticos presentan un excelente compromiso de resistencia mecánica y ductilidad, y se utilizan ampliamente para raíles, cables de pretensado y alambres de alta resistencia. Estas excelentes propiedades mecánicas son consecuencia de su particular estructura nanocompuesta que combina finas láminas de cementita (~12% de fracción volumétrica) con láminas ferríticas. Esta compleja subestructura conlleva una compleja evolución microestructural y de propiedades con la deformación aplicada que resulta difícil de modelizar. En este trabajo, se presenta un modelo constitutivo basado en la microestructura de la perlita que considera tanto el comportamiento elastoplástico como la evolución del daño. La formulación original, válida para escalas mesoscópicas, contempla el comportamiento de la ferrita y la cementita por separado, asumiendo que el endurecimiento se produce a través de los mecanismos que actúan en la ferrita. Para su aplicación en sistemas macroscópicos, como el trefilado de alambres, se ha aplicado una estrategia de homogeneización multi-colonia. En cuanto al daño, se ha adoptado un enfoque de la Mecánica del Daño Continuo adaptado a las características de la perlita, con el acoplamiento del daño a la respuesta mecánica. El modelo se ha implementado para su uso en simulaciones de elementos finitos y se ha sometido a un proceso de calibración utilizando datos experimentales de ensayos de tracción y torsión. Posteriormente, se ha validado el modelo, confirmando su capacidad predictiva en varios aspectos, incluyendo la respuesta mecánica bajo diferentes estados de tensión, la acumulación de tensiones internas y la evolución de la microestructura con la deformación

A multiscale material model for metallic powder compaction during hot isostatic pressing

The prediction of the distortions during Near-Net-Shape Hot Isostatic Pressing (NNS-HIP) is an intrinsic multiscale problem where the local interactions among particles determine the macroscopic distortions taking place during the sintering and densification of a component. In this work, a multiscale approach is proposed to solve this problem. In particular, a viscoplastic constitutive model capable of predicting macroscopic contractions during a HIP process with high accuracy has been developed, implemented and validated. The macroscopic model incorporates the mechanical behaviour predicted at the meso-scale by means of multiple-particle finite element models (MP-FEM) of an agglomerate of powder particles. The model is validated through the prediction of distortions during HIP of a full scale industrial case. It is concluded that adding the microscopic information of the HIP process to simulate the contractions at the macroscopic level results in a considerable improvement of the accuracy of the predictions

A microstructure-based constitutive model for eutectoid steels

This work presents a constitutive model for eutectoid steels based on their two-phase lamellar microstructure. The model accounts for the individual behaviour of both ferrite and cementite, with perfect interphase adhesion assumed. It considers anisotropic hardening mechanisms in ferrite derived from the lamellar structure of pearlite while ignoring the crystal structure of either phase. The model also accounts for the evolution of orientation and spacing of lamellae under directional deformation, along with the evolution of internal stress distribution in both phases. Due to its simplicity, the model has very few calibration parameters but is still able to reproduce complex strain paths and loading conditions with excellent accuracy. The model was compared with tensile, compression and torsion tests from a 13-pass wire drawing series (up to drawing strains of 2.7) and reproduced accurately the mechanical response under any loading condition. The robustness of the model lies in the fact that it is able to recreate the evolution of internal stresses built in cementite and ferrite. Such internal stress evolution was confirmed to reproduce accurately the stress partitioning observed in neutron and X-ray diffraction tests reported in literature. Moreover, the model contributes to the understanding of the rapid broadening of cementite diffraction peaks observed during in-situ tensile tests of patented wires

Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface dis-placements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 psi XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15%

Digital image correlation after focused ion beam micro-slit drilling: A new technique for measuring residual stresses in hardmetal components at local scale

A new method has been developed for measuring residual stresses at the surface of hardmetal components with higher spatial resolution than standard X-ray diffraction methods. It is based on measuring the surface displacements produced when stresses are partially released by machining a thin slit perpendicularly to the tested surface. Slit machining is carried out by focused ion beam (FIB). Measurement of the displacement fields around the FIB slit are performed by applying an advanced digital image correlation algorithm based on Fourier analysis with sub-pixel resolution. This method compares SEM images of the same area of the hardmetal surface before and after slitting. The method has been successfully applied to as-ground and femto-laser textured surfaces showing good correlation with the standard sin2 ψ XRD technique. It is concluded that texturing induced by laser pulses in the femtoseconds regime is not perfectly adiabatic, since residual stresses are reduced by 15