Abstract
. Bulk metallic glasses (BMGs) and their composites (BMGMC) have emerged as competitive materials for structural engineering applications exhibiting superior tensile strength, hardness along with very high elastic strain limit. However, they suffer from a lack of ductility and subsequent low toughness due to the inherent brittleness of the glassy structure which render them to failure without appreciable yielding owing to mechanisms of rapid movement of shear bands all throughout the volume of the material. This severely limits their use in the manufacture of structural engineering parts. Various theories and mechanisms have been proposed to counter this effect. Introduction of secondary ductile phase in the form ofin-situnucleating and growing dendrites from melt during solidification have proved out to be best solution of this problem. Nucleation and growth of these ductile phases have been extensively studied over the last 16 years since their introduction for the first time in Zr-based BMGMC by Prof. Johnson at Caltech. Data about almost all types of phases appearing in different systems have been successfully reported. However, there is very little information available about the precise mechanism underlying their nucleation and growth during solidification in a copper mould during conventional vacuum casting and melt pool of additively manufactured parts. Various routes have been proposed to study this including experiments in microgravity, levitation in synchrotron light and modelling and simulation. In this report, which is Part B of two parts comprehensive overview, state of the art of development, manufacturing, characterisation and modelling and simulation of BMGMCs is described in detail. Evolution of microstructure in BMGMC during additive manufacturing have been presented with the aim to address fundamental problem of lack in ductility along with prediction of grain size and phase evolution with the help of advanced modelling and simulation techniques. It has been systematically proposed that 2 and 3 dimensional cellular automaton method combined with finite element (CAFE) tools programmed on MATLAB® and simulated on Ansys® would best be able to describe this phenomenon in most efficient way. Present part B focuses on methodology by which modelling and simulation can be adopted and applied to describe evolution of microstructure in this complex class of materials.
Highlights
T Introduction of secondary ductile phase in the form of in-situ nucleating and growing dendrites from melt during solidification have proved out to be best solution of this problem
As soon as grain has nucleated, and its growth can be explained by special modified case of CNT for BMGMC (A detailed treatment of modified CNT for BMGMC is given in Appendix A) and its distribution can be explained by Constitutional Supercooling Zone / Interdependence theory, it grows with an interface velocity which is a function of undercooling
Bottom region Glass: The tip of casting is 100% glass. This region is classified as glass and no crystal structure is observed here because cooling rate is maximum here which results in extraction of heat at a very high rate resulting in retaining supercooled liquid state at room temperature
Summary
In use since ancient Roman times [302], modelling and simulation picked up interest and achieved pinnacle in modern day scientific and engineering sectors with the advent of computer technology which came not more than two decades ago. The unique ability of atomistic modelling and simulation is that it uses atomic functions and their variables to generate knowledge about their behaviour under various impulses In both cases, the use of these methods are big help and support in saving time, materials, resources as well as. Despite of these advantages, there are still situations and applications which limits the use of modelling and simulation techniques These include, unavailability of strong efficient computing algorithms (with lesser approximations) needed for the replication of actual real world situations, unavailability of real world experimental data (physical constants and thermo-physical properties) needed to simulate a particular problems, unavailability of more accurate deterministic or non-probability based models using actual situations rather than basing their outcome on statistics. 2.1.3 Modelling and simulation of heat transfer in liquid melt pool – Solidification. The behaviour of a certain metal / alloy in the melt pool can be explained by its cooling curve which is briefly described below
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