Mg-based alloys form a variety of interesting structures including stable and metastable crystalline, stable and metastable quasicrystalline, nanocrystalline and amorphous phases. Many of these phases can be made to coexist by suitable processing leading to an interesting combination of properties. Non-equilibrium processing in combination with suitable heat treatments can be used to control the scale and dispersion of these phases. Further thrust to Mg-based alloys is expected through the development of Mg-based bulk metallic glasses. Magnesium matrix composites are also gaining in prominence. The thesis has been divided into theoretical and experimental parts. The theoretical part focuses on understanding the structure of quasicrystals, rational approximants and related structures. The experimental work involves synthesis, non-equilibrium processing and characterization of specific Mg-based alloys. The structure of quasicrystals and related structures can be understood by working in three dimensions or by projection from higher dimensions. The projection formalism is used to generate quasicrystals and rational approximants in 2D and 3D. Approximants to the Penrose lattice are generated with directions of approximation oriented 90° and 72° apart. Rational approximants to the icosahedral quasilattice are generated and the systematics of lattice-centring in these approximants analysed. Two-dimensional quasiperiodic lattice with 5-fold symmetry, which is periodic along the third dimension, is generated as an approximant to the icosahedral lattice. Approximants are also considered wherein quasiperiodicity is retained along one or two directions. The concept of average lattices can be used to understand diverse structures including vacancy ordered phases (VOP) and orthorhombic approximants to the decagonal phase. VOP which lack incommensurate length scales should be considered as quasiperiodic superlattice (QPSL) approximants rather than as conventional rational approximants and hence have the average lattice scheme built into them. The average lattice approach is further used to unify Kuo’s and Anantharaman's models for orthorhombic approximants to the decagonal quasicrystal. A modified version of Anantharaman's model is also presented. Using the twinned icosahedron model, Robinson and Taylor approximants to the decagonal quasicrystal are generated by the twinning of Mackay and Little approximants to the icosahedral quasicrystal. An indexing scheme based on this model is developed which inherits the merits of the twinned icosahedron model. Further, using cluster of four icosahedra, in a distorted tetrahedral configuration, symmetries of the hexagonal phases, which are related to quasicrystals, are generated. Frank's ratio is brought out as a unifying thread connecting diverse kinds of structures including VOP and hexagonal phases related to quasicrystals, which have pseudocubic symmetry. Experimental work involves the synthesis and characterization of alloys in four systems: a) Mg-Zn-Y, b) Mg-Zn-La, c) Al-Mg-Cu and d) Mg-Cu-Y. Induction melting is used to prepare the alloys and melt-spinning is used as the primary non-equilibrium processing route. The focus in the Mg-Zn-RE systems is in the as cast condition while in the Al-Mg-Cu system it is in the melt-spun condition. Characterization techniques used are XRD, SEM and TEM. In the Mg-Zn-Y system face-centred icosahedral (FC1) phase with quasilattice parameter of 5.21 A is found to coexist with related crystalline phases in the Mg4Zn94Y2 and Mg23Zn5gY9 alloys. A series of crystalline phases with superlattice ordering are seen in the Mg-Zn-Y and Mg-Zn-La systems. These phases with a variety of ordering, many of which display interesting patterns of streaking in the SAD pattern, are related to one-another and to the FCI QC found in the Mg-Zn-Y system. No quasicrystal could be observed in the two alloys investigated in the Mg-Zn-La system with La = 5 and 8 %. Conventional rational approximants were conspicuous by there absence in both the rare-earth containing systems. This is understood in terms of the absence of large clusters in these systems. High Y alloys display a tendency to form nanocrystals in the as-cast condition and amorphous regions are observed in the as-cast alloys with Y > 20 %. Hence, high Y alloys are anticipated to be bulk glass formers. Melt-spinning of the alloys in both the RE containing systems lead to the formation of nanocrystalline regions. The e/a ratio plays an important role in the formation of phases in the Mg-Zn-Y system. An e/a ratio near 2.08 has a stabilising effect on a variety of phases including the FCI quasicrystal, ternary phases related to the quasicrystal and binary phases like YZn12 and Y2Zn17. Formation of quasicrystals in the Al-Mg binary and Mg-Al-Cu ternary seem to be very sensitive to processing conditions and were not observed in the present investigation in the melt-spun alloys. However, β-Al3Mg2 and Mg32(Al,Cu)49 phases with large lattice parameters, which are related to quasicrystals, are observed in as-cast and melt-spun conditions. The Mg32(Al,Cu)49 phase brings out the similarity between this system and the Mg-Al-Zn system. The glass formability of the alloys in the Al-Mg binary and in the Mg-Al-Cu ternary is limited. Except for the formation of amorphous phase in some regions, the alloys were crystalline even when melt-spun at 2800 rpm. The ability to form nanocrystals is also limited in this system as compared to the Mg-Zn-RE systems. Often melt-spun alloys showed a wide range of grain sizes coexisting together.

The ferromagnetic bulk metallic glasses (BMGs) started to be investigated only in the last 10 years.They are difficult to cast, but their properties are uniques. The work deals with casting, mechanical and soft magnetic properties of new Fe-based BMGs. Such alloys can be cast directly in samples with various geometries and they can be use as magnetic parts in different devices.

The glass-forming Ti75Zr10Si15 alloy is regarded as a potential new material for implant applications due to its composition of non-toxic, biocompatible elements and many interesting mechanical properties. The effects of partial substitution of 15 at.-% Ti by Nb on the microstructure and mechanical behavior of the alloy have been investigated. The limited glass-forming ability (GFA) of the Ti75Zr10Si15 alloy results for melt-spun ribbons mainly in nanocomposite structures with β-type nanocrystals being embedded in a glassy matrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of the melt prior to melt-spinning from 1923 K to 2053 K yields a higher amorphous phase fraction for both alloys. A decrease of hardness (H), ultimate stress and reduced Young’s modulus (Er) is observed for Ti60Zr10Nb15Si15 rods as compared to Ti75Zr10Si15 ones. This is attributed to an increase of the fraction of the β-type phase. The melt-spun ribbons show an interesting combination of very high hardness values (H) and moderate reduced elastic modulus values (Er). This results in comparatively very high H/Er ratios of >0.075 which suggests these new materials for applications demanding high wear resistance. The corrosion and passivation behavior of these alloys in their homogenized melt-spun states have been investigated in Ringer solution at 37°C in comparison to their cast multiphase crystalline counterparts and to cp-Ti and β-type Ti-40Nb. All tested materials showed very low corrosion rates. Electrochemical and surface analytical studies revealed a high stability of their passive states in a wide potential range. The addition of Nb does not only improve the glass-forming ability and the mechanical properties but also supports a high pitting resistance even at extreme anodic polarization. With regard to the corrosion properties, the Nb-containing nearly single-phase glassy alloy can compete with the β-type Ti-40Nb alloy. In addition, it has been demonstrated that thermal oxidation could be well applied to Ti75Zr10Si15 and Ti60Zr10Nb15Si15 melt-spun ribbons. Thermal oxidation treatment is one of the simple and cost-effective surface modification methods to improve the surface characteristics of these alloys. In the first tests, ribbon samples of the ternary and the quaternary alloy which were oxidized at 550°C in synthetic air showed suitable fundamental properties for implant applications, i.e. high hardness, good wettability and hydroxyapatite-forming ability after 10 days. All these properties recommend the new glass-forming alloys for application as wear- and corrosion-resistant coating materials for implants. / Die glasbildende Legierung Ti75Zr10Si15 wird wegen ihrer biokompatiblen Zusammensetzung ohne toxische Elemente und auf Grund interessanter mechanischer Eigenschaften als potentielles neues Implantatmaterial betrachtet. Es wurden 15 at.-% Ti durch Nb partiell substituiert und die Effekte auf die Mikrostruktur und die mechanischen Eigenschaften der Legierung untersucht. Auf Grund der eingeschränkten Glasbildungsfähigkeit von Ti75Zr10Si15 bestehen die schmelzgeschleuderten Bänder dieser Legierung hauptsächlich aus Nanokomposit-Strukturen mit β-phasigen Nanokristallen in einer glasartigen Matrix. Die Zugabe von Nb steigert die Glasbildungsfähigkeit. Das Anheben der Überhitzungstemperatur der Schmelze vor dem Schmelzschleudern von 1923 auf 2053 K führt für beide Legierungen zu einem höheren Anteil amorpher Phase. Es wird bei der Legierung Ti60Zr10Nb15Si15 im Vergleich zur Ti75Zr10Si15-Legierung eine Abnahme der Härte (H), Bruchfestigkeit und ein reduzierter E-Modul (Er) beobachtet. Dies wird mit dem Anstieg des beta-Phasenanteils erklärt. Die schmelzgeschleuderten Bänder zeigen eine interessante Kombination aus sehr hoher Härte und moderaten E-Modul Werten (Er). Dies führt zu vergleichsweise sehr hohen H/Er-Verhältnissen von >0,075, wodurch diese Materialien für Anwendungen mit hohen Verschleißanforderungen geeignet sind. Das Korrosions- und Passivierungsverhalten dieser Legierungen in ihrem homogenisierten schmelzgeschleuderten Zustand wurde in Ringer-Lösung bei 37°C untersucht und mit dem gegossenen vielphasigen kristallinen Zustand dieser Legierungen sowie mit cpTi und beta-Typ Ti-40Nb verglichen. Alle untersuchten Materialien zeigten sehr niedrige Korrosionsraten. Elektrochemische Studien und Oberflächenanalysen belegen eine hohe Stabilität der Passivfilme in einem weiten Potentialbereich. Die Zugabe von Niob verbessert nicht nur die Glasbildungsfähigkeit und die mechanischen Eigenschaften, sondern erhöht weiterhin die Lochfraßbeständigkeit, selbst bei stark anodischer Polarisation. Bezüglich der Korrosionseigenschaften konkurriert die Nb-haltige fast einphasige glasartige Legierung mit β-phasigem Ti-40Nb. Weiterhin wurde gezeigt, dass an schmelzgeschleuderten Bändern der Legierung Ti75Zr10Si15 und Ti60Zr10Nb15Si15 eine thermische Oxidation erfolgreich durchgeführt werden konnte. Die thermische Oxidation ist eine der einfachsten und kosteneffektivsten Möglichkeiten der Oberflächenmodifikation um die Eigenschaften der Oberflächen dieser Legierungen zu verbessern. In den ersten Tests zeigten die Bänder-Proben der ternären und der quaternären Legierung, die bei 550°C in synthetischer Luft oxidiert wurden, entsprechende Eigenschaften für Implantat-Anwendungen, d.h. hohe Härte, gute Benetzbarkeit und die Fähigkeit nach 10 Tagen Hydroxylapatit auf der Oberfläche zu bilden. Alle zuvor genannten Eigenschaften machen diese neuen glasbildenden Legierungen zu geeigneten Materialien für die Anwendung als verschleiß- und korrosionsbeständige Beschichtung für Implantate.

No description available.

The aim of this study, which was carried out in two main parts, is to investigate the glass forming ability of Fe-based systems. The first part involves the theoretical modeling to cover the requirement of a predictive model to identify the Fe-based alloy families that have high glass forming ability in the frame of atomistic and thermodynamic approach. The second part involves the experimental investigations to prove the results of the conducted theoretical modeling studies. For this purpose, in the first part, theoretical investigations were performed to identify the third alloying elements that will lead to an increase in the glass forming ability on the base of electronic theory of alloys in pseudopotential approximation for selected Fe- based systems, Fe - (B, Zr, Nb, C, W). In the experimental part, in the frame of the theoretical investigation results, one of the theoretically modeled binary system, and the third alloying elements that were predicted to lead an increase in the glass forming ability of the selected binary system, were determined. As a first step, designated compositions were synthesized by using low grade conventional Fe-B alloy as a raw material by using centrifugal casting technique and copper mold casting method. To compare the results, same compositions were also cast from the high purity elements by using the same technique and method. For the characterization of these cast specimens, DSC, XRD, SEM, EDS and metallographic examination techniques were used. Amorphous structure was successfully obtained in the thin sections of the wedge-cast samples for Fe-B-Nb and Fe-B-W ternary systems.

An investigation of the high-strain-rate mechanical properties, deformation mechanisms, and fracture characteristics of a Zr-based bulk metallic glass (BMG) and its composite with tungsten was conducted through the use of controlled impact experiments and constitutive modeling. The overall objective of this research was to determine the high-strain-rate deformation and failure mechanisms of a BMG and its composite as a function of stress state and strain rate, and describe the mechanical behavior over a range of loading conditions. The research involved performing controlled impact experiments on BMG composites consisting of an amorphous Zr57Nb5Cu15.4Ni12.6Al10 (LM106) with crystalline tungsten reinforcement particles. Monolithic LM106 was also examined to aid in the understanding of the composite. The mechanical behavior of the composite was investigated over a range of strain rates (10^3 s^-1 to 10^6 s^-1), stress states (compression, compression-shear, tension), and temperatures (RT to 600 C) to determine the dependence of mechanical properties and deformation and failure modes (i.e., homogeneous deformation vs. inhomogeneous shear banding) on these parameters. Mechanical testing in the quasi-static to intermediate strain rate regimes was performed using an Instron, Drop Weight Tower, and Split Hopkinson Pressure Bar, respectively. High-strain-rate mechanical properties of the BMG-matrix composite and monolithic BMG were investigated using dynamic compression (reverse Taylor) and dynamic tension (spall) impact experiments performed using a gas gun instrumented with velocity interferometry and high-speed digital photography. These experiments provided information about dynamic strength and deformation modes, and allowed for validation of constitutive models via comparison of experimental and simulated transient deformation profiles and free surface velocity traces. Hugoniot equation of state measurements were performed on the monolithic BMG to investigate the high pressure phase stability of the glass and the possible implications of a high pressure phase transformation on mechanical properties. Specimens were recovered for post-impact microstructural and thermal analysis to gain information about the mechanisms of dynamic deformation and fracture, and to examine for possible shock-induced phase transformations of the amorphous phase.

Les matériaux amorphes sont omniprésents dans la vie quotidienne. Ils comprennent des verres "dures" et "mous". Le systèmes amorphes durs sont généralement considérés comme des matériaux de structure, dont les propriétés et l'utilisation sont comparable à celles des solides cristallins. Les verres mous sont généralement considérés comme des fluides complexes, décrits pour leurs propriétés rhéologiques et les applications correspondantes pratiques. Les matériaux amorphes peuvent soit présenter un comportement de type solide ou de flux en fonction de leur charge mécanique: tous sont des fluides à limite apparente d'élasticité. Leurs limites d'utilisation sont souvent définies par l'apparition de bandes de cisaillement, une forme extrême de localisation vu dans des verres moléculaires ainsi que dans les matériaux granulaires. Il ya maintenant des preuves considérables que ce sont les conséquences de l'existence d'une structure désordonnée au niveau des constituants élémentaires. Les études de la plasticité des solides amorphes, sont encore gênés par l'absence de tout défaut identifiable responsable de la réponse plastique. Il est maintenant reconnu que la plasticité est le résultat net des réarrangements locaux, ou "transformations de cisaillement", impliquant des petits groupes de particules. Ces réarrangements sont thermiquement - activées et sont ubiquitaires dans le processus de relaxation de déformation structurelle des verres à basse température. Malheureusement, ils se déroulent sur des échelles de temps long par rapport à ceux qui sont accessibles aux simulations de dynamique moléculaire. Certains nouveaux outils très prometteurs, cependant, ouvrent la voie vers des algorithmes accélérés pour la simulation de systèmes thermiques. Ils sont basés sur les méthodes numériques développées au cours de ces deux dernières décennies pour déterminer les transitions thermiquement activés dans les systèmes atomiques. Un intérêt particulier ici est la technique d'activation-relaxation (ART). Dans cette étude, nous allons montrer que, même si une recherche exhaustive des points de selle pour des solides désordonnés est impossible, ART peut identifier assez de points de selles pour construire des échantillons statistiquement pertinents, à partir desquelle des distributions stationnaires peuvent être calculées. Le but de cette thèse strictement numérique était de prédire les cinétiques thermiquement activées dans des verres telles que celles rencontrées expérimentalement. La nature de ces événements microscopique qui se produisent dans miscroscopic systèmes désordonnés a été étudiée à la fois sous des contraintes mécaniques et dans des conditions de vieillissement. Nous étudions deux grandeurs décrivant ces événements au sein de l'approximation harmonique de la théorie de l'état de transition, c'est à dire l'énergie d'activation et la fréquence d'attaque. Etant donné que dans la définition d'une fréquence d'attaque la courbure du minimum initial et le point de selle sont présents, nous voulions voir (pour les verres métalliques et verres de silice) s'il y avait une relation entre les fréquences de tentative et les énergies d'activation d'un événement donné. Cette corrélation a été précédemment observée pour un large éventail de phénomènes et est appelé la règle de compensation Meyer-Neldel. Nous aussi tentons de répondre si le simple potentiel BKS sans sommation d'Ewald est capable de reproduire le polyamorphisme observé dans les verres de silice soumis à une compression hydrostatique. Outre les processus activés thermiquement, les analyses structurelles de verres métalliques et de silice ont été réalisées. Les ordres de courte et moyenne portée ont été caractérisées par deux méthodes: pavages de Voronoı pour les verres métalliques, nous fournissant des informations sur les conformations voisines proches, et dans le cas de la silice, les statistiques de distributions de chaînes moléculaires. / Amorphous materials are ubiquitous in everyday life. They comprise "hard" and"soft" glasses. Hard amorphous systems are usually seen as structure materials, with properties and use comparable to those of crystalline solids. Soft glasses are usually seen as complex fluids, described in terms of their rheological properties with the corresponding practical applications (concrete, paints, drilling mud, cosmetic gels, creams or foams, etc). Amorphous materials can either present a solid-like behaviour or flow depending on their mechanical load: all are yield-stress fluids. Their usage limits are often defined by the occurrence of shear-banding, an extreme form of localization seen in molecular glasses as well as in granular materials. There is now considerable evidence that they are consequences of the existence of a disordered structure at the level of the elementary constituents (atoms, particles,...). Studies of plasticity in amorphous solids, are still hampered by the lack of any identifiable defect responsible for the plastic response. It is now acknowledged that plasticity is the net result of local rearrangements, or "shear transformations", involving small clusters of (say a few tens of) particles. These rearrangements are thermally--activated and are ubiquitous processes in the structural relaxation and deformation of glasses at low temperatures. Unfortunately, they take place over timescales long compared to those accessible to direct Molecular Dynamics simulations. Some extremely promising new tools, however, are opening the route towards accelerated algorithms for the simulation of thermal systems. They are based on numerical methods developed over these last two decades to determine thermally activated transitions in atomic systems. Of particular interest here is the Activation-Relaxation Technique (ART), an eigenvector-following method that allows the identification of activated states and paths in the potential energy landscape of atomic systems. In this study, we will show that although an exhaustive search for saddle points in case of disordered solids is unfeasible (because of the exponential number of activated states), ART can identify enough saddles to build statistically relevant samples, from which stationary distributions can be computed. The purpose of this strictly numerical thesis was the prediction of thermally activated kinetics in glasses such as those encountered experimentally. The nature of such miscroscopic events occuring in disordered systems was studied both under mechanical stress and in ageing conditions. We investigate two quantities that describe thermally-activated events within the harmonic approximation of the transition state theory, i.e. activation energy and attempt frequency.Since in the definition of an attempt frequency the curvature of the initial minimum and the saddle point are present, we wanted to see if there was a relation between attempt frequencies and activation energies of a given event in two types of systems: metallic glasses and silica glasses. Such correlation had been observed before for a wide range of phenomena and is referred to as the Meyer-Neldel compensation rule. We also attempt to answer if the simple BKS potential without Ewald summation is able to reproduce polyamorphism observed in silica glasses subject to hydrostatic compression and characterized mainly in terms of coordination numbers. Apart from thermally activated processes, the structural analyses of metallic and silica glasses were performed. The short and medium range orders were characterized using two methods: Voronoi tesselations for metallic glasses, providing us information about near neighbor conformations, and in case of silica, statistics of ring distributions.

Orientador: Prof. Dr. Alejandro Andrés Zúñiga Páez / Coorientador: Prof. Dr. Roberto Gomes de Aguiar Veiga / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Santo André, 2018. / Vidros metálicos (VM) apresentam propriedades mecânicas únicas devido a sua estrutura desordenada (amorfa). Adicionalmente, compósitos de matriz de vidro metálico com nanocristais (fase dispersa) podem apresentar um mecanismo de deformação plástica diferente em relação à dos vidros metálicos monolíticos. O objetivo deste trabalho foi estudar o comportamento mecânico em compressão de compósitos de matriz de VM de Cu¿Zr¿Al contendo nanocristais de diferentes composições e tamanhos usando simulação de dinâmica molecular. Primeiramente, uma caixa de simulação de VM com 6.750.000 átomos, de composição Cu45Zr45Al10 foi produzida pelo método de têmpera. Posteriormente, esta caixa foi usada como matriz para produzir compósitos com as seguintes populações de nanocristais: a) 75 nanocristais de CuZr de 4 nm de diâmetro, b) 1 nanocristal de CuZr de 17 nm de diâmetro, e c) 1 nanocristal de Cu de 17 nm de diâmetro (fração volumétrica ?xada em 2%). Finalmente, todas as amostras (com e sem nanocristais) foram deformadas em compressão com uma taxa de deformação de 108 s-1. As curvas de tensão-deformação mostraram que todas as amostras atingiram a tensão máxima em ~5% de deformação; porém com valores variando entre 2,15 e 2,8 GPa. Observou-se que a variação da tensão máxima atingida dependeu principalmente de dois fatores: da existência de mecanismos de deformação plástica no interior da partícula e do processo de nucleação de bandas de cisalhamento na interfase matriz/nanocristal. Também foi verifcado que uma maior área de interfase matriz/nanocristal gerou uma maior quantidade de bandas de cisalhamento. Adicionalmente, observou-se que a concentração do poliedro icosaédrico (0,0,12,0,0) centrado no átomo de Cu diminuiu em função da deformação e que a região da banda de cisalhamento possuía uma menor concentração de poliedros icosaédricos em relação às demais regiões do vidro metálico. Já as análises do campo de deslocamento atômico mostraram que as regiões centrais das bandas de cisalhamento estavam formadas por vórtices, e que as margens das bandas estavam caracterizadas por movimentações atômicas lineares. Finalmente, a presença de nanocristais in?uenciou o padrão do ?uxo plástico e a estrutura dos vórtices no material. / Metallic glasses exhibit unique mechanical properties due their disordered structure (amorphous). Moreover, glassy-matrix composites with embedded nanocrystals can modify the dynamics of shear banding (plastic deformation mechanism) in relation to the one observed in monolithic metallic glasses. The objective of this work was to study the mechanical behavior in compression of amorphous Cu¿Zr¿Al composites containing nanocrystals of diferent compositions and sizes using molecular dynamics simulation. First, a metallic glass simulation box with 6,750,000 atoms and composition Cu45Zr45Al10 was produced by the quenching method. Afterwards, this box was used as matrix to produce composites with the following populations of nanocrystals: a) 75 CuZr nanocrystals of 4 nm in diameter, b) 1 CuZr nanocrystal of 17 nm in diameter, and c) 1 Cu nanocrystal of 17 nm in diameter (volume fraction set at 2%). Finally, all samples (with and without nanocrystals) were deformed in compression at a strain rate of 108 s-1. The stress-strain curves showed that all samples reached a maximum stress at ~5% strain; but with values varying between 2.15 to 2.8 GPa. It was observed that the maximum stress reached depended mainly on two factors: the existence of plastic deformation inside the nanoparticles and the nucleation of shear bands at the matrix/nanocrystal interface. It was also verifed that a larger area of matrix-nanocrystal interface generated a larger number of shear bands. In addition, it was observed that the concentration of icosahedral polyhedra (0,0,12,0,0) centered in Cu atoms decreased as a function of strain, and that the shear band had a lower concentration of icosahedral polyhedra in relation to other regions of the metallic glass. The analysis of the atomic feld displacement showed that the central regions of the shear bands were formed by vortices, and the margins of the bands were characterized by linear atomic movements. Finally, the presence of nanocrystals infuenced the plastic fow pattern and the structure of the vortices in the material.

No description available.

Indentation hardness, H, has been widely used to characterize the mechanical properties of materials for more than a century because of the following advantages of this technique; (1) it requires small sample and (2) the test is non destructive in nature. Recent technological advances helped in the development of instrumented indentation machines which can record the load, P, vs. displacement, h, data continuously during indentation with excellent load and displacement resolutions. From these, H and the elastic modulus, E, of the indented material can be obtained on the basis of the ‘contact area’ of the indentation at the maximum load. The estimation of true contact area becomes difficult during ‘pile-up’ and ‘sink-in’, commonly observed phenomena while indentation of a low and high strain hardened materials. In order for the better understanding of these phenomena it is important to understand the plastic flow distribution under indenters. It is also important for the prediction of elastic-plastic properties from the P-h data. Recently, there have been considerable theoretical and simulation efforts on this front with a combination of dimensional analysis and finite element simulations. One of the important input parameter for the dimensional analysis is the ‘representative strain’ under the indenter, which is a strong function of the indenter geometry. However there is no comprehensive understanding of the representative strain under the indenter despite several studies till date. One objective of the present thesis is to conduct an experimental analysis of the plastic flow during the sharp indentation. The plastic zone size and shape under conical indenters of different apex angles in a pure and annealed copper were examined by employing the subsurface indentation technique to generate the hardness map. From these isostrain contours are constructed joining the data having similar strain values. The following are the key observations. (1) The plastic strain contours are elliptical in nature, spreading more along the direction of the indenter axis than the lateral direction. (2) The magnitude of the plastic strain in the contact region decreases with increasing the indenter angle. (3) The strain decay in the indentation direction follow a power-law relation with the distance. The estimated representative strains under the indenters, computed as the volume average strain within the elastic-plastic boundary, decreases with increasing indenter angle. We also performed finite element simulations to generate plastic flow distribution under the indenter geometries and compared with the experimental results. The results suggest that the experimental and computed average strains match well. However, the plastic strain contours do not, suggesting that further detailed understanding of the elasto-plastic deformation underneath the sharp indenter is essential before reliable estimates of plastic properties from the P-h curves can be made routinely. The second objective of this thesis is to understand plastic flow in amorphous alloys. It is now well established that plastic deformation in metallic glasses is pressure sensitive, owing to the fundamentally different mechanisms vis-à-vis the dislocation mediated plastic flow in crystalline metals alloys. Early work has shown that the pressure sensitivity of amorphous alloys gets reflected as high constraint factor, C (hardness to yield stress ratio), which sometimes exceed 3.0. In this thesis, we study the temperature dependence of pressure sensitive plastic flow in bulk metallic glasses (BMGs) using C as the proxy for the pressure sensitivity. Experiments on three different BMGs show that C increases with temperature hence the pressure sensitivity. In addition we have carried out finite element simulations to generate P-h curves for different levels of pressure sensitivities and match them with the experimental curves that are obtained at different temperatures. Simulations predict that higher pressure sensitivity index values are required to match the experimental curves at high temperatures confirming that the pressure sensitivity increases with increasing temperature. The fundamental mechanisms responsible for the increase in pressure sensitivity are discussed in detail. Finally we pose a question, is the increase in pressure sensitivity with temperature is common to other amorphous materials such as strong amorphous polymers? In order to answer this question we have chosen PMMA, a strong amorphous polymer. In this study also we have taken C as a proxy to index the pressure sensitivity. Indentation stress-strain curves are constructed at different temperature using spherical indentation experiments. The C values corresponding to different temperatures are determined and plotted as a function of temperature. It is found that C increases with temperature implying that the pressure sensitivity of amorphous polymers also increases with temperature. The micro-mechanisms responsible for the increase in pressure sensitivity are sought.