Thermal treatment effects on high-Mn TWIP steels

This master thesis is going to study and investigate the thermal treatment’s effects on the high Mn-TWIP steels. In order to get these aims some factors can be studied, such microstructures and mechanical properties of high Mn-TWIP steels. The objective of this study are as follows: Study the microstructure evolution of high Mn-TWIP steels thermally treated (i.e. grain size, phase transformation, etc). Study the mechanical properties (hardness, tensile properties and high cycle fatigue) of high Mn-TWIP steels with different microstructures. Thermal fatigue evolution (from 0 up to 75 cycles) at 500°C in order to determine the microstructure evolution and their mechanical properties (harness and tensile properties). In present project, in order to study the microstructure the field emission scanning electron microscopy (FESEM) used. For testing the mechanical properties, Vickers hardness method applied for hardness test, tension condition for tensile test and high cycle fatigue has been used for fatigue properties. Microstructural study indicates that thermal treatment can change the microstructure of the samples, it observed that the grain size changed in various condition. Martensitic and pearlitic transformation occurred under thermal treatments and thermal fatigue conditions respectively. In samples which were thermally fatigued, the concentration of the pearlitic phase increased by increasing the number of cycles. Thermal treatments can have some effects on mechanical properties. Hardness of the materials decrease by increasing the temperature in thermal treatments while thermal fatigue increases the hardness of materials by increasing the number of cycles. Samples which were thermally treated and thermally fatigued showed more brittle behavior compared with the samples which were not thermally treated. It have seen that the voids, TiN and pearlitic phase are the main reasons of fracture under tensile tests. Finally it observed that the life time of the materials can be affected by annealing the samples at 1000°C and normalizing this sample in air

Small-scale assessment of corrosion-induced damage in hardmetals

In this work, the effect of corrosion-induced damage on the mechanical response of hardmetals was evaluated at small-scale level by means of nanoindentation and nanoscratch. Damage was introduced in a controlled way through immersion in acidic solution. It is found that surface degradation associated with corrosion leads to a strong reduction of hardness and elastic modulus, as compared to non-corroded samples. Similarly, significant differences are observed in nanoscratch response, regarding not only width and depth of tracks but also deformation mechanisms developed as contact load is progressively increased. Damage was already evidenced in corroded surfaces at scratching loads one order of magnitude lower than for virgin specimens. Cracking and fragmentation of individual WC grains, together with chipping of at the track edges were the main deformation and fracture micromechanisms identified. Changes in nanoindentation and nanoscratch response and damage scenario are discussed on the basis of the corrosion-induced changes within the intrinsic microstructural assemblage of hardmetals.Postprint (published version

Small-scale mechanical properties of constitutive phases within a polycrystalline cubic boron nitride composite

Micromechanical properties of a polycrystalline cubic boron nitride (PcBN) composite have been assessed by statistical analysis of data gathered from experimental massive nanoindentation. The mechanical study was complemented with electron probe X-Ray microanalysis, aiming to correlate relative B/N ratio and local hardness for individual cBN particles. Best-fit of experimental and deconvoluted data is achieved by considering five mechanically different phases, defined on the basis of chemical nature, TiN/cBN interface presence, ratio between residual imprint dimension and microstructural length scale as well as phase stoichiometry. In-depth local micromechanical and chemical analysis permitted to propose and validate, for the first time, the existence of a correlation between intrinsic hardness and phase stoichiometry for cBN phase. Finally, based on experimental data measured by nanoindentation and analyzed in terms of plastic index, toughness for the PcBN composite studied is estimated to range between 4 and 6¿MPa·vm.Peer ReviewedPostprint (author's final draft

Probing crystalline phases in cubic boron nitride as a function of boron content by massive nanoindentation and microsample testing

Polycrystalline cubic boron nitride (cBN) is a super-hard multiphase composite is extensively used in highly demanding applications, where improved and consistent performances together with high reliability are required. The remarkable mechanical properties of these materials result from a two-fold effectiveness associated with its composite character. On the one hand in terms of composite nature: combination of a brittle cBN particles and a ceramic TiN binder with optimal interface properties, as given by a very low interfacial energy and very good adhesion between cBN and TiN. Information on the small-scale mechanical response mainly for superhard materials is rather scarce in the literature This is particularly true regarding experimental data and analysis on the influence of phase and/or chemical nature and interfacial adhesion on hardness. It is clear that knowledge of these issues is crucial not only to improve the performance of this superhard materials but also to designer of new PCBN systems, which will lead to highly desirable improvements in the cost and time on the materials development cycle. The present work aims to evaluate the boron effect on the cBN particles by doing a systematic micro- and nanomechanical study of the mechanical integrity for different superhard systems, with different binder and reinforcement content. In doing so, different micromechanical approaches are followed: i) Assessment of the micromechanical properties by using the statistical approach, ii) evaluation of the fracture toughness by microcantilever deflection, strength by micropillar compression, and iii) finite element modelling based on 3D FIB tomography is performed by using the acquired micromechanical data in order to correlate micromechanical behavior with macroscopic response of the material. From the obtained results by the statistical method it is found that the boron content strongly modifies the cBN hardness; which produces a modification of this superhard particles being this tetragonal or octhoedrical depending the amount of the boron content dissolved inside the parti

Micromechanical mapping of polycrystalline cubic boron nitride composites by means of high-speed nanoindentation: Assessment of microstructural assemblage effects

Polycrystalline cubic boron nitrides (PcBNs) are composites widely used as cutting tool materials due to their exceptional high hardness and wear resistance. Investigation of their micromechanical properties is key for optimizing PcBN’s performance through microstructural design. Within this context, high-speed nanoindentation is proposed and implemented, for three different PcBN grades, to correlate microstructure with local mechanical properties. A total of 40,000 imprints were performed in each grade. The obtained mechanical maps and data sets are statistically treated following two deconvolution approaches: 1D and 2D Gaussian fitting. The use of high-speed nanoindentation is validated not only by the reliable assessment of the intrinsic mechanical properties of cBN particles, binder and interphase region, but also by the successful mirroring of microstructural assemblage within the mechanical maps attained. Comparison of the results determined from 1D and 2D gaussian representations are in satisfactory agreement. Nevertheless, some difficulties and disparity between them arises when involving fine-grained microstructures.Peer ReviewedPostprint (published version

Effect of lateral laser-cladding process on the corrosion performance of Inconel 625

This study aimed to evaluate the corrosion properties of different samples coated by the laser-cladding method to find the optimal laser parameters. Thereby, potentiodynamic polarization (Tafel) and electrochemical impedance tests were performed to assess the corrosion resistance of coated samples. Consequently, the corrosion morphology of tested samples was inspected by scanning electron microscopy. The results demonstrated that the laser power directly correlates with pitting corrosion and defects on the surface of the samples. Moreover, when molybdenum and chromium ions are increased in the electrolyte solution, the passive and protective layers are more durable, as the ions are sited within the holes and defects, reducing the surface corrosion rate.Peer ReviewedPostprint (published version

Micropillar compression of Ti(C,N)-FeNi cermets: microstructural, processing, and scale effects

The influence of microstructure and processing route on the small-scale mechanical response as well as on the deformation and failure mechanisms of Ti(C,N)-FeNi cermets were investigated by uniaxial compression of micropillars milled by focused ion beam with different sizes. Stress-strain curves were determined and associated deformation mechanisms were observed in-situ using scanning electron microscopy. The appropriate micropillars dimension was assessed, based on the microstructural characteristics of studied cermets, to overcome scale effect issues. A direct relationship was observed between yield strength and ceramic/metal ratio for colloidal samples. Meanwhile, deformation of metallic binder and glide between Ti(C,N)/Ti(C,N) particles were evidenced as dominant mechanisms during the compression for colloidal cermets with 70 and 80 vol% of ceramic phase, respectively. The obtained results illustrate that samples processed from powder attained by colloidal route provide superior mechanical behavior, as compared to that exhibited by specimens shaped following a conventional powder metallurgy one (wet ball-milling/drying).Peer ReviewedPostprint (published version

Micromechanical properties of inorganic multiphase materials

Tesi en modalitat de compendi de publicacionsThis thesis is dedicated to understanding the micromechanical properties of multiphase materials which are indispensable in today’s engineering applications. The mechanical behavior of these materials is dictated by the intrinsic response of each constitutive phase as well as the fashion in which they interact with each other. Therefore, an accurate assessment of both microstructural characteristics and small-scale mechanical properties becomes key for understanding the macroscopic behavior of these materials. Within the above context, the current study is intended to offer a systematic investigation, aiming to assess small-scale mechanical properties of multiphase materials through a protocol based on massive nanoindentation and statistical analysis. It consists of three sequential stages: (1) microstructural characterization, (2) micromechanical evaluation (massive indentation and statistical analysis), (3) correlation between microstructure and mechanical properties using advanced characterization techniques. Microstructural characterization of studied systems was carried out through extensive field emission scanning electron microscopy analysis. This is an essential step for determining testing parameters to be used when implementing massive indentation, particularly penetration depth of performed imprints. Based on the acquired information, massive indentation testing and statistical analysis of experimentally gathered data were implemented to determine the local properties of several unidentified phases. Such data analysis was then complemented by the use of different advanced characterization techniques for deeper inspection of microstructural features. Main goal of this final step was to define the unidentified mechanically distinct phases, based on physically- based correlations between microstructure features and small-scale properties. The proposed and described protocol has been implemented on three different materials: Duplex Stainless Steels (DSS), Polycrystalline cubic Boron Nitride (PcBN) composite and Ti(C,N)-FeNi cermets. They are representative of metal-metal, ceramic- ceramic, and ceramic-metal systems, respectively. Regarding DSS, the influence of the processing route on the local mechanical properties (hardness (H) and elastic modulus (E)) of ¿ and a phases of a DSS was successfully evaluated. Moreover, a novel 2D histogram of hardness and elastic modulus was introduced and validated as an effective tool to correlate microstructure and intrinsic mechanical properties of the constitutive phases of DSSs. PcBN composite consists of cBN particles embedded within a TiN binder. The correlation of relative B/N ratio and local hardness for individual cBN particles was studied, through complementary analysis using electron probe X-ray microanalysis of the data attained using the proposed methodology. The influence of ceramic/metal phase ratio and C addition on the local hardness of Ti(C,N)–FeNi cermets have been assessed. Regarding the small-scale properties of the constitutive phases, the intrinsic hardness of both Ti(C,N) particles and FeNi binder were determined using the suggested testing procedure. It has been proven that the proposed methodology can be considered as a successful testing protocol for determining small-scale mechanical properties (H and E) of the studied multiphase systems. Nevertheless, successful implementation requires careful consideration of testing parameters used, based on microstructural, residual imprint, and plastic flow length scales.Esta tesis esta enfocada en comprender las propiedades micromecánicas de los materiales multifásicos que son indispensables en las aplicaciones de ingeniería actuales. El comportamiento mecánico de estos materiales está dictado por la respuesta intrínseca de cada fase constitutiva, así como por la forma en que actúan entre si. Por lo tanto, una evaluación precisa de las características microestructurales y de las propiedades mecánicas a pequeña escala es clave para comprender el comportamiento macroscópico de estos materiales. Dentro del contexto anterior, el presente estudio pretende ofrecer una investigación sistemática, con el objetivo de evaluar las propiedades mecánicas a pequeña escala de diferentes materiales multifásicos a través de un protocolo basado en nanoindentación masiva y en un análisis estadístico. Esta metodología consta de tres etapas secuenciales: (1) caracterización microestructural, (2) evaluación micromecánica (indentación masiva y análisis estadístico) y la correlación entre microestructura y (3) propiedades mecánicas mediante técnicas avanzadas de caracterización. La caracterización microestructural de los sistemas estudiados se llevó a cabo a través de un análisis de microscopía electrónica de barrido. Este es un paso esencial para determinar los parámetros de ensayo que se utilizarán al implementar las técnicas de nanoindentación masiva, particularmente la profundidad de penetración de las huellas a realizar. Basándose en la información obtenida, se implementaron pruebas de indentación masiva y análisis estadístico de los datos experimentales recopilados para determinar las propiedades locales de varias fases no identificadas. Dicho análisis se complementó con diferentes técnicas de caracterización avanzadas para un análisis más detallado de las características microestructurales. El objetivo principal de este paso final fue definir las fases mecánicamente distintas y no identificadas, teniendo en consideración correlaciones físicas entre las características microestructurales y las propiedades a pequeña escala determinadas. El protocolo propuesto, se implementó en tres materiales diferentes: aceros inoxidables dúplex (DSS), compuestos de nitruro de boro cúbico policristalinos (PcBN) y cermets de Ti(C,N)-FeNi. Estos materiales son representativos de los sistemas metal-metal, cerámica-cerámica y cerámica-metal, respectivamente. En relación a los DDS, se evaluó con éxito la influencia de la ruta de procesamiento en las propiedades mecánicas locales (dureza (H) y módulo de elasticidad (E)) de las fases ¿ y a acero. Adicionalmente, se presentó y validó un nuevo histograma 2D de dureza y el módulo de elasticidad como una herramienta eficaz para correlacionar la microestructura y las propiedades mecánicas intrínsecas de las fases constitutivas de los DSS. El compuesto PcBN formado por partículas de cBN incrustadas dentro de un aglutinante de TiN. Se estudió la correlación de la proporción B/N y la dureza local de las partículas de cBN individuales, atreves de un análisis complementario por medio de microanálisis de rayos X con sonda electrónica de los datos obtenidos mediante la metodología propuesta. Se ha evaluado la influencia de la cantidad relativa de fases (cerámica y metal) y la adición de carbono sobre la dureza local de los cermets de Ti(C,N)-FeNi. Con respecto a las propiedades a pequeña escala de las fases constitutivas, se determinó la dureza intrínseca de partículas de Ti(C,N) y del aglutinante FeNi, mediante el procedimiento de prueba sugerido. Se ha comprobado que la metodología propuesta puede considerarse como protocolo de prueba exitoso para la determinación de propiedades mecánicas a pequeña escala (H y E) de los sistemas multifásicos investigados. Sin embargo, la implementación satisfactoria de esta metodología requiere una consideración minuciosa de los parámetros de ensayo utilizados,Els sistemes multifàsics inclouen un ampli ventall de materials emprats en una gran varietat d’aplicacions industrials. En termes de ciència i enginyeria dels materials, aquests sistemes poden ser descrits com materials compostos amb una microestructura i propietats mecàniques diferents per cadascuna de les fases. Com a conseqüència, la resposta intrínseca de cada constituent, així com la forma en què interactuen, dicten el comportament mecànic d’aquests materials. Per tant, una avaluació precisa i detallada de les característiques microestructurals i de les propietats mecàniques a petita escala emergeix com una acció crítica per comprendre el comportament macroscòpic d’aquests materials. L’adquisició d’aquest coneixement significa poder predir la resposta macroscòpica d’un sistema multifàsic a partir del comportament intrínsec de cadascun dels constituents; i amb això, finalment optimitzar el disseny microestructural d’aquests materials. En aquest context descrit prèviament, es proposa realitzar una investigació sistemàtica amb l’objectiu d’avaluar les propietats mecàniques a petita escala de diferents materials multifàsics a través d’un protocol que es basa en la implementació de tècniques de nanoindentació massiva i, a continuació, l’anàlisi estadístic a partir de les dades experimentals obtingudes. Aquesta metodologia consta de tres etapes seqüencials: (i) caracterització microestructural, (ii) avaluació micromecànica (indentació massiva i anàlisis estadístic), i (iii) correlació entre microestructura i propietats mecàniques mitjançant tècniques avançades de caracterització complementaries. La caracterització microestructural dels sistemes estudiats es va portar a terme a través d’una inspecció extreta i detallada mitjançant microscòpia electrònica d’escombrat. Aquest és un pas essencial per determinar els paràmetres d’assaig que s’utilitzaran per implementar tècniques de nanoindentació massiva, particularment la profunditat de penetració de les impressions a realitzar; i en conseqüència, la càrrega que s’ha d’aplicar i l’espaiat adequat que ha d’existir entre les empremtes. Tenint en compte la informació obtinguda, es van realitzar els assajos de nanoindentació massiva, i tot seguit, l’anàlisi estadístic de les dades experimentals recopilades. El resultat principal d’aquest segon pas, va ser determinar les propietats locals de diferents fases no identificades individualment. Aquests anàlisis de dades es va complementar utilitzant diferents tècniques de caracterització avançada que van permetre una inspecció amb major detall de les característiques microestructurals. L’objectiu principal d’aquest estudi final va ser definir les diferents fases mecànicament no identificades, tenint en consideració correlacions físiques entre les característiques microestructurals i les propietats a petita escala determinades. El protocol proposat i descrit es va implementar en tres materials diferents: acer inoxidable dúplex (DSS), compostos basats en partícules de nitrur de bor cúbic (PcBN) i cermets de tipus Ti(C,N)-FeNi. Aquests materials representen exemples idonis de sistemes multifàsics metall-metall, ceràmic-ceràmic i ceràmic-metall, respectivament. Aquesta tesi doctoral es presenta com un compendi de publicacions científiques en les quals s’aborden diferents objectius específics, utilitzant el protocol d’assaig i anàlisis proposat, per cada sistema multifàsic. Referent al sistema metall-metall, en el primer article es va avaluar satisfactòriament la influència de la ruta de processament en les propietats mecàniques a petita escala (duresa i mòdul elàstic) de les fases austenita i ferrita d’un DSS. Els resultats obtinguts permeten concloure que els efectes de la deformació en fred sobre les propietats a nivell local de les fases austenítica i ferrítica són més pronunciades que els inclosos en altres rutes de processament investigats. Addicionalment, en aquesta part del treball s’introdueix i es valida un nou histograma 2D de duresa i mòdul d’elasticitat com una eina adequada i eficaç per correlacionar la microestructura i les propietats mecàniques intrínseques de les fases constitutives dels DSS. La segona publicació es va centrar en l’extracció de les propietats mecàniques a petita escala d’un compost de PcBN. Aquest material d’extremada duresa està constituït per partícules ceràmiques de cBN immerses en una fase aglutinant de TiN. En aquest article també es va avaluar i comprendre la correlació entre el quocient B/N i la duresa local de partícules de cBN individuals, complementant l’estudi mitjançant microanàlisis de rajos X amb sonda electrònica. Per això, es van definir cinc fases mecànicament diferents, tenint en compte factors diversos tals com la naturalesa química, la presència d’interfases TiN/cBN i l’estequiometria (relació quantitativa entre els continguts de B i N) de les partícules de cBN. Els resultats obtinguts permeten concloure que existeix una relació directa entre la duresa del cBN i la quantitat de N present en cada partícula d’aquesta fase ceràmica. La influència de la quantitat relativa de fases (ceràmica i metàl·lica) i l’addició de carboni sobre les propietats mecàniques locals de cermets de Ti(C,N)-FeNi es van estudiar en el tercer i quart article. A través d’aquest treball es va evidenciar que tant la duresa com el mòdul elàstic dels cermets es relacionen en forma inversament proporcional al quocient metall/ceràmic corresponent a la proporció relativa de les fases presents. Tanmateix, es va arribar a la conclusió que l’addició de carboni resulta en un increment tant de la duresa com del mòdul elàstic dels cermets estudiats. Respecte a les propietats a petita escala de les fases constitutives, es va determinar la duresa intrínseca tant de les partícules de Ti(C,N) com de l’aglutinant metàl·lic FeNi, mitjançant nanoindentació massiva i un posterior anàlisi estadístic de les dades experimentals obtingudes. En aquest sistema, i pel cas particular de l’avaluació de la duresa efectiva de la fase metàl·lica, es va utilitzar un model de pel·lícula fina com una eina d’anàlisis complementaria. Addicionalment, l’estudi dels mecanismes de deformació, dany i fractura en els cermets estudiats va permetre evidenciar l’enduriment de la fase lligant de FeNi, associat directament al constrenyiment exercit per les partícules ceràmiques adjacents. En termes generals, l’estudi realitzat i els resultats obtinguts permeten concloure que la metodologia proposada s’ha implementat i validat amb èxit com a protocol d’assaig per la determinació de propietats mecàniques a petita escala (duresa i mòdul elàstic) dels sistemes mutifàsics investigats. En aquest context, s’ha d’indicar que la implementació satisfactòria d’aquesta metodologia requereix una consideració minuciosa dels paràmetres d’assaig utilitzats, en particular referint-se a la relació entre les escales dimensionals representatives de la plasticitat i les empremtes residuals induïdes respecte a les característiques microestructurals del material estudiat.Postprint (published version

Novel Mechanical Characterization of Austenite and Ferrite Phases within Duplex Stainless Steel

The microstructure and micromechanical properties of the constitutive phases of a particular duplex stainless steel in various processing conditions have been characterized. Hardness (H), elastic modulus (E) and H/E cartography maps were obtained by using a high-speed nanoindentation mapping technique. Small-scale H and E evolution at different processing conditions has been investigated by statistical analysis of a large number of nanoindentations (10,000 imprints per sample). Two mechanically distinct phases, ferrite (α) and austenite (γ), were deconvoluted from this dataset using Ulm and Constantinides’ method, with the remaining values assigned to a third mechanical phase linked to composite-like (containing α/γ interphase boundaries) regions. These mechanical property phase assessments were supplemented by overlaying crystallographic phase maps obtained by electron backscattered diffraction. An excellent correlation between microstructure and small-scale mechanical properties was achieved, especially when considering the ratio H/E

Thermal treatment effects on high-Mn TWIP steels

This master thesis is going to study and investigate the thermal treatment’s effects on the high Mn-TWIP steels. In order to get these aims some factors can be studied, such microstructures and mechanical properties of high Mn-TWIP steels. The objective of this study are as follows: Study the microstructure evolution of high Mn-TWIP steels thermally treated (i.e. grain size, phase transformation, etc). Study the mechanical properties (hardness, tensile properties and high cycle fatigue) of high Mn-TWIP steels with different microstructures. Thermal fatigue evolution (from 0 up to 75 cycles) at 500°C in order to determine the microstructure evolution and their mechanical properties (harness and tensile properties). In present project, in order to study the microstructure the field emission scanning electron microscopy (FESEM) used. For testing the mechanical properties, Vickers hardness method applied for hardness test, tension condition for tensile test and high cycle fatigue has been used for fatigue properties. Microstructural study indicates that thermal treatment can change the microstructure of the samples, it observed that the grain size changed in various condition. Martensitic and pearlitic transformation occurred under thermal treatments and thermal fatigue conditions respectively. In samples which were thermally fatigued, the concentration of the pearlitic phase increased by increasing the number of cycles. Thermal treatments can have some effects on mechanical properties. Hardness of the materials decrease by increasing the temperature in thermal treatments while thermal fatigue increases the hardness of materials by increasing the number of cycles. Samples which were thermally treated and thermally fatigued showed more brittle behavior compared with the samples which were not thermally treated. It have seen that the voids, TiN and pearlitic phase are the main reasons of fracture under tensile tests. Finally it observed that the life time of the materials can be affected by annealing the samples at 1000°C and normalizing this sample in air