Anisotropy of ultrafine-lamellar and nanolamellar pearlitic structures revealed by in-situ micro compression testing

Applying severe plastic deformation to pearlitic steels leads to a transformation of the random ultrafine-lamellar (ufl) colony structure to a nanolamellar (nl) composite. The distinct reduction of the interlamellar spacing generates a high-strength steel with a strength of up to 3.7 GPa, which can readily be produced. Additionally, the ferrite and cementite lamellae align in a preferential direction giving rise to an anisotropic mechanical response. The aim of this work is to determine the material anisotropy in terms of flow stress and deformation behavior in the nl state and to compare it with its ufl state. Thus, micron sized samples are fundamental in order to fit inside a single colony of a distinct lamellae alignment. In addition, these small dimensions ensure a homogeneous structure within the nl samples. The compression setup on the other side enables to characterize the deformation behavior up to large strains, since early failure as a consequence of necking is prevented. Hence, micro compression experiments are an established tool for characterizing deformation mechanisms of fine-scaled materials and they further allow to link the deformation characteristics observed in the scanning electron microscope (SEM) to the mechanical data. In this work anisotropic mechanical properties could be successfully measured by micromechanical testing of pillars consisting of single ufl pearlite colonies and nl pearlitic structures for the first time. For both lamellae spacings three different types of micro pillars were focused ion beam milled, with the lamellae being aligned parallel, normal and inclined with respect to the loading direction. Comparing the stress-strain curves and the deformation characterstics of the ufl and nl micro pillars, it could be revealed that not only the interlamellar spacing but also the loading direction of the lamellae have a significant influence on the materials behavior. Especially, it was found that the yield point of the material is mainly controlled by the interlamellar spacing, whereas the lamellae orientation governs the hardening capability, the critical stress for the onset of localized deformation and thereby also the strength in a subsequently arising plateau regime. Furthermore, it could be shown that the global failure and deformation characteristics vary depending on the lamellae alignment. Finally it should be noted, that the anisotropy of the hardening behavior in the ufl and nl pearlite is different

Structural instabilities during cyclic loading of ultrafine-grained copper studied with micro bending experiments

The cyclic mechanical properties and microstructural stability of severe plastically deformed copper were investigated by means of micro bending experiments. The ultrafine-grained structure of OFHC copper was synthesized utilizing the high pressure torsion (HPT) technique. Micron sized cantilevers were focused-ion-beam milled and subsequently tested within a scanning electron microscope in the low cycle fatigue regime at strain amplitudes in the range of 1.1 − 3.2 ∗ 10−3. It was found that HPT processed ultra-fine grained copper is prone to cyclic softening, which is a consequence of grain coarsening in the absence of shear banding in the micro samples. Novel insights into the grain coarsening mechanism were revealed by quasi in-situ EBSD scans, showing i) continuous migration of high angle grain boundaries, ii) preferential growth of larger grains at the expense of adjacent smaller ones, iii) a reduction of misorientation gradients within larger grains if the grain structure in the neighborhood is altered and iv) no evidence that a favorable crystallographic orientation drives grain growth during homogeneous coarsening at moderate accumulated strains, tested here

Deformation behavior of bulk metallic glasses produced via Severe Plastic Deformation and the influence of a second phase

Over the last years bulk metallic glasses (BMG) have been strongly investigated as their mechanical properties are very promising especially in terms of their high yield strength and high elastic strain. However, a major drawback is their complicated production depending strongly on dimensions and chemistry. A promising technique to overcome these drawbacks is using a severe plastic deformation process, e.g. high pressure torsion (HPT), where the production can be started with metallic glass powders, which are generally much easier to fabricate. For this route, the powder is consolidated and then the powder particles are welded together by applying a high shear deformation. The produced specimen remain fully amorphous and no porosity is detectable after sufficient deformation [1]. To improve the mechanical properties of the BMG, the used Zr-based metallic glass powder is mixed with a crystalline Cu-powder or a Ni-based metallic glass powder to achieve a metal/metallic glass composite or a metallic glass/ metallic glass composite, respectively. Due to the small amounts of produced material, conventional macroscopic characterization methods, like compression or tension tests can hardly be used to analyze the overall mechanical properties. Therefore, in this work different micromechanical testing methods, such as nanoindentation, in-situ SEM micropillar compression, and finally in situ TEM picoindentation were carried out to investigate the deformation behavior under ambient but also non-ambient conditions. Using nanoindentation, the hardness and the Young’s Modulus was determined for two HPT-deformed BMGs with different composition. Additionally, high temperature nanoindentation experiments up to 350 °C were conducted to determine not only the temperature dependent hardness and the Young’s modulus but also to study the change in thermally activated processes during deformation via nanoindentation strain rate jump tests. It was found, that nanoindentation hardness is in good accordance to the macroscopic Vickers results. Increasing the temperature, hardness decreases slightly, while the modulus increases. The shear band formation is also dependent on the deformation temperature, since the extent of stair case formation in the load-displacement curves changes. Overcoming 300°C, the material becomes extremely ductile showing a strong strain rate sensitivity. Further, the uniaxial mechanical response of Zr-based BMG was examined in-situ in SEM using FIB prepared micropillars. The microcompression experiments revealed a strength of more than 2 GPa. Steps in the stress-strain curve suggest shear band formation, which could also be confirmed by the in-situ recorded SEM images Finally the influence of the second materials phase was investigated via TEM in-situ picoindentation, where a wedge shaped indenter was pressed in a TEM lamella. The load-displacement curve show a similar stair case behavior as seen during nanoindention and microcompression, indicating shear band formation. The shear band formation could be also observed in the TEM micrographs

Fracture mechanics of microsamples

Fracture mechanics has been developed to avoid catastrophic failures of structures. It is nowadays the basic tool for damage tolerant design. From materials science point of view the development of materials with improved fracture resistance has become an important topic. The developed standards to determine the fracture mechanics parameters are adapted to sample sizes from centimetre to meter. The standards are designed to generate materials specific data and not size dependent parameters. The transferability of this testing procedure to dimensions of micrometers is important but not straight forward. The fracture mechanical properties of materials in small dimensions have become an important research area in materials science. A driving force is the growing industrial importance of micro-electronic and micro-electromechanical systems and new down sized devices, for example for medical applications. The load bearing capacity and life time of such micro sized components are determined by the mechanical properties of the material with the corresponding dimensions. Another reason for the development of mechanical tests with samples in the micro range is the evaluation of individual properties of microstructural elements like grain boundaries or individual phases, which have typical dimensions in the micrometer and submicrometer regime. The main goal of the paper will be devoted to: - limits of the application of fracture mechanic tests to microsized samples, - when do fracture mechanic parameters remain size independent and when not, - what can we learn from micromechanical tests about fracture of materials in general, - can we solve fracture problems with microsamples which cannot be generated from conventional fracture mechanics tests

Bulk metallic glass composites: microstructural influences on mechanical properties

Bulk metallic glasses (BMGs) have been strongly investigated as they show on the one hand interesting mechanical properties as high strength and good wear resistance but on the other hand limited ductile deformability. Bulk metallic glass composites (BMGCs) are very promising to overcome and improve the properties by clever combination of different phases. However, a major drawback is the limited choices of phase combinations in common fabrication routes and lack of major microstructure adjustability. A promising technique to overcome these drawbacks is severe plastic deformation process, e.g. high pressure torsion (HPT), where the production can be started with metallic glass powders. For this route, the powder is consolidated and deformed by applying a high shear deformation to bulk samples. Therefore, it is possible to produce fully amorphous specimens [1], but also composites containing two different amorphous phases or an amorphous and a crystalline one [2]. Please click Additional Files below to see the full abstract

Prediction of the mechanical behaviour of pearlitic steel based on microcompression tests, micromechanical models and homogenization approaches

In this paper, results from microcompression tests on pearlitic steel pillars are used to determine properties of cementite and ferrite. Pillars with different orientation of the cementite lamellae have been tested to distinguish model parameters of cementite and the ferrite in two different micromechanically based models of pearlite. Both models are based on the assumption that the yielding is primarily caused by shear of the ferrite between the cementite lamellae. In the first of these models the cementite and ferrite are modelled individually. The second model is a mesomodel of a cementite lamella together with the surrounding ferrite. Based on these micromechanically based models different homogenization approaches are adopted to obtain the macroscopic behaviour of pearlitic steel. During deformation of the pearlitic steel anisotropy evolves which is assumed to be governed by the re-orientation of the cementite lamellae during the deformation. The most fundamental homogenization approach that is studied is a 3D grain structure where the fluctuating displacement field within the grain structure is solved by using Finite Element Method (FEM). The re-orientation of the cementite lamellae is governed by the deformation of the grain structure. In the investigated analytical homogenization approaches the re-orientation is assumed to follow the areal affine assumption where the normals of the cementite lamellae are convected with the macroscopic deformation gradient. Numerical results for the different models and homogenization approaches, when subjected to simple shear loading, are given and comparisons of stress–strain response are shown

A comparison of homogenization approaches for modelling the mechanical behaviour of pearlitic steel

In this paper different homogenization approaches are adopted for two micromechanical based models of elasto-plasticity in pearlitic steel. Both models are based on the assumption that the yielding is primarily caused by shear of the ferrite between the cementite lamellae. The orientation distribution of the cementite lamellae determines the macroscopic anisotropy characteristics of pearlitic steel. Properties of the cementite and the ferrite are determined from microcompression tests where the orientation of the cementite lamellae is varied. This is done for both of the micromechanical based models. The first of these models is a micromodel where cementite and ferrite are modeled individually. The second model is a mesomodel where a homogenization approach of a cementite lamella together with the surrounding ferrite is proposed. The anisotropy evolution is assumed to be governed by the re-orientation of the cementite lamellae during the deformation. The most fundamental model that is studied is a 3D grain structure where the fluctuating displacement field within the grain structure is solved by using Finite Element Method (FEM). The re-orientation of the cementite lamellae is governed by the deformation of the grain structure. In the analytically homogenized models the re-orientation is assumed to follow the areal affine assumption where the normals of the cementite lamellae are convected with the macroscopic deformation gradient. Numerical results for the different models, when subjected to simple shear loading, are given and comparisons of stress-strain response are shown

A comparison of homogenization approaches for modelling the mechanical behaviour of pearlitic steel

In this paper different homogenization approaches are adopted for two micromechanical based models of elasto-plasticity in pearlitic steel. Both models are based on the assumption that the yielding is primarily caused by shear of the ferrite between the cementite lamellae. The orientation distribution of the cementite lamellae determines the macroscopic anisotropy characteristics of pearlitic steel. Properties of the cementite and the ferrite are determined from microcompression tests where the orientation of the cementite lamellae is varied. This is done for both of the micromechanical based models. The first of these models is a micromodel where cementite and ferrite are modeled individually. The second model is a mesomodel where a homogenization approach of a cementite lamella together with the surrounding ferrite is proposed. The anisotropy evolution is assumed to be governed by the re-orientation of the cementite lamellae during the deformation. The most fundamental model that is studied is a 3D grain structure where the fluctuating displacement field within the grain structure is solved by using Finite Element Method (FEM). The re-orientation of the cementite lamellae is governed by the deformation of the grain structure. In the analytically homogenized models the re-orientation is assumed to follow the areal affine assumption where the normals of the cementite lamellae are convected with the macroscopic deformation gradient. Numerical results for the different models, when subjected to simple shear loading, are given and comparisons of stress-strain response are shown