Wood September 28, 20021DEPARTMENT OF MATERIALS SCIENCEAND ENGINEERINGCARNEGIE MELLON UNIVERSITY27-301Microstructure & PropertiesSeptember-October 2002LABORATORY MANUALExperiment No. 2:Microstructure-Sensitive Mechanical PropertiesWood September 28, 20022IntroductionReadingThere is an enormous literature on this subject. The course text by Porter & Easterling givesa large amount of useful information. Also the text by Tom Courtney, Mechanical Behaviorof Materials, is very helpful.Objective of the ExperimentThe objective of the experiment is to show you, the student, how to manipulatemicrostructures in a given material, and how strong an effect on the properties this can have.This demonstration relies on a classical structural material – steel – whose behavior can belargely understood in terms of the Fe-C phase diagram. Ternary additions have a significanteffect on the phase transformations and this will be explored in more detail at a later point.The reason for choosing the Fe-C system is that it contains a rich variety of phasetransformation behavior. In addition, steel is still a widely used structural material. You as amaterials engineer will be expected to understand and control the properties of materialssuch as steel. Your ability to do this depends on your grasp of the microstructure-propertiesrelationships in this and any other system that you deal with. In addition to the classicalsystem, however, you will also have an opportunity to perform similar experiments onvarious types of wood. These will illustrate some of the key aspects of mechanical behaviorof composite materials and biomaterials.This DocumentThis document contains several sections with general information on mechanical behavior inmetallic systems. Mr. H. Greenberg (former Industrial Internship Coordinator in MSE) isacknowledged for assembling the text. It has been retained in order to provide a basicintroduction to the relevant materials science for this experiment. Students should, however,expect to read additional material.Wood September 28, 20023CHAPTER I - INTRODUCTION1.1 PurposeThe purpose of this manual is to help the student become proficient in experimental skills and to providehim/her with some practical applications of material discussed in lecture courses. In this volume you will findinstructions for the experiments to be performed this term as well as some background material both forworking in the laboratory and for the experiments.In this laboratory course considerable emphasis is placed on the formal technical report. Details for theformat and content of the various sections of a report are presented in the Course Syllabus; please reviewthem each time you write a report for this course .Audio-taped instructions are available on the various stages of metallography. These tapes may be viewedat your convenience in the MSE laboratory; to view the tapes view see Mr. Thomas Gambal for assistance.1.2 Experimental PlanningThe following comments are extracted from Holman†. They are considered significant enough to bereproduced here.“The key to success in experimental work is to ask continually:1. What am I looking for?2. Why am I measuring this?3. Does the measurement really answer any of my questions?4. What does the measurement tell me?These questions may seem rather elementary, but they should be asked frequently throughout theprogress of any experimental program. Some particular questions which should be asked in the initialphases of experiment planning are:1. What primary variables shall be investigated?2. What control must be exerted on the experiment?3. What ranges of the primary variables will be necessary to describe the phenomena under study?4. How many data points should be taken in the various ranges of operation to ensure goodsampling of data, considering instrument accuracy and other factors?5. What instrument accuracy is required for each measurement?6. What safety precautions are necessary if some kind of hazardous material or operation is involvedin the experiment? † J.P. HOLMAN, EXPERIMENTAL METHOD FOR ENGINEERS, 2nd edition, McGraw Hill Book Go., NewYork, 1971.Wood September 28, 200247. What provisions have been made for recording the data?”1.3 Basic ConceptsMaterials science can conveniently be thought of as involving four inter-related concepts: structure properties performance processing1.3.1 StructureStructure can be at several levels starting with the discrete electron energy levels within individual atoms, orthe energy bands associated with an aggregate of atoms (solid, crystalline or not, or liquid but not a gas atnormal pressures - the atoms/molecules are too far apart). Crystal structure is important and results from abalance between packing of atoms or molecules, (sometimes of different sizes) and directionality(anisotropy) associated with certain types of bonding (usually covalent).Defects in the structure such as vacancies, dislocations, grain boundaries and inclusions are important on ascale generally bigger than atomic dimensions. Some of these defects also influence transformations bychanging rates (such as diffusion) or by offering preferred sites for nucleation of a new phase.On an even larger scale, cracks or holes sometimes big enough to be visible to the naked eye can also bepresent. These are important in that they can lead to premature failure.1.3.2 PropertiesThere are many properties which can be of interest depending on the application we have in mind. It isconvenient to make a distinction between those which are:structure-sensitive - e.g. the yield strength of a material depends on such factors as the dislocation density,crystal structure, or grain size andstructure-insensitive - e.g. the modulus, density, or electrical resistivity are (to a very good approximation)independent of imperfections in a given crystal structure. (They do depend however on crystal structure,e.g. whether iron is present as fcc or bcc).In general, we will be interested in properties of the following types:mechanicalelectricalmagneticchemicalWood September 28, 20025optical1.3.3 PerformanceIt is usually fairly simple to select a material that will meet the obvious initial requirements of a serviceapplication. For example, if we are designing a bridge, we need to consider:1.