Introduction to Properties of Metals

Transcription

1 Introduction to Properties of Metals Thomas G Stoebe University of Washington Stoebe@uw.edu Copyright Edmonds Community College 2016 This material may be used and reproduced for educational purposes only. Abstract This module provides students with an initial experience with metals and alloys, focusing on common metals and their properties. Students discuss what they know about the metals they use daily, and then categorize them in terms of their properties. An accompanying PowerPoint presentation provides the outline of the discussions, which are followed by a hands-on experiment with paper clips that can be done in the classroom. Depending on the level of a class, this unit can be a major part of a curriculum or simply an introduction to the more detailed units that follow. Goals and Objectives: The student will be able to: Discuss properties of several types of metals and alloys Define several properties of metals Observe metal failures and exploring why they occur. Unit data: 1. Key words: Properties, Metals, Alloys 2. Type of unit/mode of presentation: Guided discussion with follow-up introductory experiment. 3. Time required: 30 to 45 minutes in class 4. Pre-requisite knowledge: None. 5. Target grade levels: All levels, middle school and up. 1

2 Equipment and supplies needed: Access to a PowerPoint projector and computer, Box of paperclips Access to a white board or paper to develop a histogram. For demonstration purposes (optional) Hack saw blade Rubber bands Piece of steel (any heavy piece) Piece of plastic to compare with the steel on density. MatEdU Core Competencies covered: 0.A Demonstrate Good Communication Skills 7.A Illustrate the General Nature of Metals 7.K Compare Thermal, Physical and Other Properties of Materials Instructor Notes: This module introduces students to metals and their properties using the PowerPoint presentation and a simple experiment. Information and dialog is provided in the dialog box on each slide. This dialog is also provided below under Unit Procedure, along with needed background and information for the instructor. Module Procedure: Starting with the PowerPoint slides, develop a discussion as indicated in the slides: 1. Introductory slide 2. What are Metals? As indicated, ask the students to list metals that they know about. The students can do this individually, or the instructor can make a list on the board of the student suggestions. 3. Now ask the students to list properties in whatever way they wish. Keep the list as properties are detailed and categorized on the following slides. 4. Discuss 2 basic properties Density and modulus. a. Density: mass/volume (as an example, compare heavy to light objects metals vs. plastic) 2

3 b. Elastic Modulus: resistance to elastic deformation (a good example would be a hack saw blade, always snaps back to original shape. If stressing a material causes it to deform permanently, that is called Plastic Deformation and is not elastic. Rubber bands are non-linear elastomers officially they are not elastic OR plastic. See the module on Rubber Bands for more information. 5. Mechanical properties are the properties that students most commonly think about. Four basic mechanical properties are listed here. Have students select from their master list the properties that fit this category. 6. Slide 6 lists other types of properties. Have the students add to the list and discuss these types of properties. 7. For metals, what property is the one that is most characteristic? Have the students discuss. The answer is electrical conductivity! Metal conduction varies a lot but is never zero or infinite. Other classes of materials (polymers, plastics, ceramics) generally have very low or zero conductivity (they are used as insulators). Other materials can be in-between like semiconductors, some fibers, etc. 8. Properties depend on metal structure and processing: Structure of metals relates to the arrangement of atoms in a metallic structure. Microstructure relates to the arrangement and shape of single crystal grains within the material. You will learn more on structure in later modules. Production and processing of metals and alloys occurs in different ways, including deformation (mechanical) processing (e.g., forging, drawing, hammering and rolling); chemical processing (alloying, reactions); physical properties based on melting, casting, deposition, and thermal processing by annealing and heat treating. Processing affects microstructure, not crystal structure. 9. Most metals we use are actually alloys which are mixtures of elements. Depending on composition, each alloy will have properties different from the original elements. Steel (iron and carbon) is an obvious example, but aluminum alloys are widely used, as are alloys like brass (copper and zinc), and many others. Most metals we use every day are alloys, not pure metals (exception: copper wire is used in quite pure form for better conduction in household wiring). Have students identify items on their list of metals that are probably alloys. 3

4 10. Here is an everyday example Paper Clips. Paper Clips are usually made from the least expensive steel melted down cars, etc. Properties vary but that is not important for paper clip use as long as they are strong enough for their intended use! 11. Paper Clip Experiment from paper clip module (copy attached for the instructor s information). Here we do only the first experiment from the module (unless the instructor wished to do this in more detail). This slide shows students how to open and bend the paper clip. Review the module before class. You can spend as much or as little time on this experiment as fits your class. Record the class results in a histogram on your board. Basic points to make: This is metal fatigue, which is failure after cyclic loading. The results will vary a lot. This depends on the steel and on the method of bending (rapid or slow) and other variables (refer to the attached module). 12. Summary slide. Three obvious conclusions are listed for the class you can invite them to add to the list. Supporting Materials and References 1. Paper Clip Module (copy attached) Rubber Band module for details on rubber bands, which are not actually elastic Materials Science and Technology Teachers Handbook, Pacific Northwest National Laboratory, 2003, Chapters Available as Featured Textbook under the resources tab at -- Many introductory exercises. 4. Engineering Materials Technology: Structures, Processing, Properties and Selection (5th Edition) by James A. Jacobs, Thomas F. Kilduff, (Hardcover - March, 2004) --This is a good book with applications but the level may be a little high for introductory classes. A listing of other resources is available under the resources tab at 4

5 Acknowledgment: This module draws on many sources from the author's prior teaching experience. The author thanks Mr. Kyle Bates-Green for assistance with the accompanying PowerPoint presentation. The editor also expresses his sincere appreciation to Dr. Wayne L. Elban, Loyola University Maryland, Department of Engineering, for his overall critical review of the full manuscript. The preparation of this module was supported by NSF Advanced Technology Education Grant at Edmonds Community College. Evaluation Packet: Student evaluation questions (discussion or quiz): 1. What property can you use to tell if a material is a metal or a ceramic? 2. Give two examples of mechanical properties for metals. 3. Explain why not everyone s paper clip broke at the same number of bends. 4. If we found some aluminum that is as strong as some steels, what type of aluminum do you think I would be using? Instructor evaluation questions: 1. At what grade level was this module used? 2. Was the level and rigor of the module what you expected? If not, how can it be improved? 3. Did the lab work as presented? Did they add to student learning? Please note any problems or suggestions. 4. Was the background material provided sufficient for your background? Sufficient for your discussion with the students? Comments? 5. Did the lab generate interest among the students? Explain. 6. Please provide your input on how this module can be improved, including comments or suggestions concerning the approach, focus and effectiveness of this activity in your context. Course evaluation questions (for the students) 1. Was the module clear and understandable? 5

6 2. Was the instructor s explanation comprehensive and thorough? 3. Was the instructor interested in your questions? 4. Was the instructor able to answer your questions? 5. Was the importance of materials properties made clear? 6. What was the most interesting thing that you learned This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE # Addendum: Paper Clip Experiment: Metal Properties with Metal Failure Type: Class laboratory exercise Abstract: This experiment introduces students to the properties of metals when they are deformed. In particular, paper clips are bent in a controlled manner until they fail. This type of failure under repeated stress is called fatigue and the fatigue properties of different materials varies with material quality, material source, type of stress and duration of stress. Since paper clips are not high tech materials, they are usually made from cheap steels that have little strength and where variability in quality is not important. Some paper clips have serrations, which will hasten failure as they provide locations for cracks to start. In this experiment the student can experience fatigue and by comparing different types and sizes of paper clips, can develop a simple understanding of the relationship between metal failure, design variables and processing variables of materials. Objectives: Demonstrate the variability of metal failure Compare failure of different products 6

7 Observe relationships between properties and processing of steels Observe differences in properties of metals vs. plastics Grade levels: Middle school to lower division college or technical school classes Time required: As an introductory exercise: 15 minutes for one paper clip per student for a 30-student class 45 minutes for several paper clips per student For more advanced classes: 20 minutes for 2 types of paper clips per student with full discussion 30 minutes for 2 steel clips and 1 plastic clip with discussion on differences MatEd core competencies addressed (most important in bold): 0A 1C 8B 8E 16A Demonstrate good communication skills Demonstrate laboratory skills Apply mechanical testing processes to solid materials Perform tests on metallic materials Distinguish effects of processing and manufacturing variations on materials properties Equipment and supplies needed: One box each of one or more types, sizes and brands of metal paperclips. Standard steel paper clips (two different brands if available) Jumbo steel paper clips (two different brands if available) If available, serrated steel paper clips If available, brass or copper paper clips Plastic paper clips (with the same standard shape as steel paper clips) Easel with paper, or white or chalkboard for recording data 7

8 Instructor background and notes: Bending a metal, as in this exercise, adds defects to the material. As defects accumulate, the metal gets harder and stronger, more difficult to bend. Cracks form, leading to a fatigue failure. It needs to be emphasized that failure occurs because the metal actually gets stronger as it is bent. An incorrect perception is that the metal gets weaker in this circumstance, causing failure. This is never true. Bending also puts energy into the material, causing it to get warmer. In this experiment typically the 180-degree bend breaks the paper clip in less than half the number of bends due to the greater stress on th e bending point. If serrated paper clips are available, the serrations give a point for cracks to start, so the clips will typically break sooner. Differences seen between clips and between brands are usually due to differences in manufacture and differences in the composition of the starting material. Brass, copper paper clips will behave similarly to steels with different numbers of bends. Plastic paperclips will behave differently since the mode of failure will be different, a function of the different structure of plastics. Process: Experiment 1 1. Each student is given two standard paper clips of each type (for middle school grades, using jumbo paper clips is perhaps better as the students can hold the larger clips more easily) 2. Instruct student to take one paper clip and to open it up to look like an S 3. Demonstrate to the students the means for bending to 90 degrees 4. Ask the students to bend the paper clip as demonstrated and to count how many times it can be bent before the paper clip breaks. Count each full cycle (up and back) as one bend. 5. Collect the data and draw a histogram of the results, that is, a histogram of number to bends to failure vs. number of students with each result. 6. Ask if the paper clip got more difficult to bend as the number of bends increased. Have the class discuss why this might be the case. Ask if this is what caused the breakage? 7. Did the paper clip get warmer during bending? What does this indicate? 8. Discuss the differences noted. Ask for ideas as to why there is such a variation in between paper clips. If needed, suggest 8

9 Speed of bending Quality of steel Defects in the metal 9. Repeat the experiment using the second standard paper clip This time have the students bend the paper clip 180 degrees Draw the histogram and discuss the differences 10. Discuss what these results mean in terms of Why there is so much variability between clips and between bending modes Effect of bending stress (90 vs. 180 degrees) on the failure, Possible effect due to variations in metal processin g Possible effects due to variations in quality of the steel Process: Experiment 2 1. Repeat the process with serrated paper clips. Compare to the results of Experiment 1 using a histogram to compare. 2. Discuss differences observed: What effect do the serrations have? Why? What does this mean in terms of the influence of defects on metal properties? 3. Repeat the process if paper clips of different materials and different sizes, if available. 4. Complete the experiment by discussing Possible causes of differences observed between different materials How could one standardize the materials to reduce variability? This is an example of accelerated life testing, used by engineers to determine how much stress a material can take. Why would engineers do this? Further study: (depending on grade level): 1. Further comparisons between paper clips (that is compare standard paper clips to the jumbo paper clips) 2. Internet research on metal fatigue 3. Finding typical composition of steels used for paper clips 4. Research possible causes of differences observed between different materials 9

10 References 1. For other paper clip experiments for kids, see and 2. nairmo68.htm 3. For some history, 4. For a full technical analysis of the bending experiment, see 10