Investigating the Structure & Properties of Metal Alloys
Embed Size (px)
Transcript of Investigating the Structure & Properties of Metal Alloys
Investigating the Structure & Properties of Metal Alloys
NUCu 150-3 Fe-Cu steel alloy
A Curriculum Project developed by Stephanie Zaucha from Deerfield High School for Northwestern Universitys
Nanoscale Science & Engineering Center through the 2006 Research Experience for Teachers Program
Optical micrograph 1000X Metal alloy visible on microscale
SEM image 4000X Metal alloy visible on microscale
LEAP image Metal atoms visible on nanoscale
Curriculum Outline Guiding Questions
Question One: What is the internal structure of a metal? What does it look like? (The properties of metals are dependent on their crystal structure, and the defects in the structure that can exist.)
a) LAB: The Crystal Structure of Metals b) Follow-up Assignment: Steel Webquest - Part 1
Question Two: What is a metal alloy? How is it made? (Alloys are a combination of two or more metals, which have enhanced properties as a result of their combination.)
c) LAB: Synthesis of an Inexpensive Alloy
Question Three: How are the properties of a metal affected by various heat treatments? (The metal crystal structure can be altered by processing treatments to make them more useful in various applications.)
d) LAB: Investigating the Effects of Various Heat Treatments on the Properties of a Metal e) Follow-up Assignment: Steel Webquest - Part 2
Question Four: How can scientists visualize and study the altered properties of metals? (The metal crystal structure can be observed at the macro, micro, and nanoscale.)
f) ACTIVITY: A Journey Through Size and Scale g) ACTIVITY: Visualizing Metals on the Nanoscale
Curriculum Project Introduction
Developed by: Stephanie Zaucha Graduate Student Mentor: R. Prakash Kolli Principle Investigator: Dr. David N. Seidman Background Scientists have greatly extended the array of materials available for human use. Chemists have learned to modify the properties of matter by physically blending or chemically combining two or more substances. An alloy is a solid solution composed of two or more metals, or of a metal(s) with one or more nonmetals. Metals and alloys are virtually everywhere in our daily lives. Alloys are used to make aircraft engines, automobiles, bridges, buildings and even paper clips. The alloys bronze, brass, and pewter have been used for centuries. The properties exhibited by various metals are directly related to their internal micro and nano structure. When discussing a metals atomic structure (in nanometers nm), metal ions are held together by metallic bonds in which each positive metal ion is attracted to the negatively charged delocalized electrons. Positive ions in a metal can be packed together to form different crystal structures depending on how the ions are layered. When discussing a metals microstructure (in micrometers - m), a grain represents the small crystals that grow around a nucleus in all directions when a molten metal is cooled. Where one grain meets another at the edge is called a grain boundary. However, dislocations can be present, which are defects in the metal lattice structure where a few ions in a layer are missing, causing the neighboring layers to be displaced slightly to minimize the strain. The more grain boundaries there are the more difficult it is for the dislocations to move and for the metal to change shape. The result is that the metal is stiffer and harder. It is also stronger. Most alloys are created to change the elemental metals' physical properties, such as conductivity, density, ductility, hardness, luster, malleability, melting point, tensile strength, and/or chemical properties, such as resistance to corrosion. Alloys often exhibit increased strength and hardness. Precipitation hardening is a process that can be used to affect the physical properties of an alloy, such as steel, by introducing copper-rich precipitates. Various treatments can be used such as cold-working, or heat treatment followed by quenching and tempering a metal. As a result of such treatments, an alloy can demonstrate increased strength and toughness due to the precipitates being formed within its complex microstructure. Many mechanical properties of a precipitation hardened alloy can be analyzed on a macroscopic scale using Tensile testing, Charpy V-notch, and Vickers Hardness analyses. In the 1930s, scientists only had the ability to characterize large (macro) properties of metals as described above; however, now they have the ability to characterize both the structure and chemistry of alloys and their precipitates on a three-dimensional nanoscale. A nanometer is a billionth of a meter, or 10-9 m. This is advantageous to study in light of historical events that involved material failure, such as the 1912 sinking of the Titanic, the 1987 Challenger Disaster, and most recently the 1996 U.S.S. Cole Bombing. The applications of being able to visualize materials on the nanoscale are immense. All modern industries rely on materials to create new products and technologies. By better understanding the atomic-scale structure of these materials and improving their properties to better fit our desired uses for them, we will only continue to make advances in product science and technology.
Motivation for Curriculum Project In the 1930s, scientists only had the ability to characterize large (macro) properties of metals; now with improved technology they have the ability to characterize the microstructure and nanostructure. This is advantageous to study in light of historical events that involved material failure, such as the sinking of the Titanic in 1912 and more recently, the October 12th, 2000 USS Cole Bombing. Scientists are studying various steel alloys in an effort to identify those that are stronger, tougher, and more explosion-resistant without introducing brittleness. Students can also investigate the properties of alloys to better understand how the materials that make up their daily world are studied and improved. As a result, there is historical relevance as well as current applications for studying the properties of steel as a material. The Seidman Group at Northwestern Universitys Center for Atom-Probe Tomography is being partially funded by the Office of Naval Research for this very reason. The following article can be shared with students, or assigned as a reading, to introduce the necessity for studying metal alloys in todays society. McGuinness, P. E. (2006). Redesigning the United States Navy with the atom probe. Scanning the Industry, 28(3), 189. Retrieved August 2, 2006, from
http://www.scanning.org/scanabstracts/SCANNING06/28189.pdf Connection to Curriculum Learning about the crystal structure and properties of metals and chemical bonding are key concepts for any introductory chemistry course. In addition, the ability to describe and manipulate the notion of scale is a core understanding in math and science. This curriculum project, Investigating the Structure and Properties of Metal Alloys, builds upon students knowledge of metals and chemical bonding to introduce relevant applications regarding metal alloys and nanoscale technology. Learning Goal & Objectives The main goal of this curriculum project is to provide students with an opportunity to learn more about the field of materials science and engineering through hands-on experiences with metal alloys. Students will develop an understanding of the relationship between the structure and composition of metals and their observable macroscopic and nanoscale properties. They will discover how these properties determine applications, and gain an appreciation of the historical impact of metals and the role they will play in the future. This goal will be met as a result of addressing four guiding questions, each with its own specific set of objectives. Question One: What is the internal structure of a metal? What does it look like?
At the end of the activity & webquest, students will be able to identify the type of bonding present between metal ions, identify the basic crystal structures that metal atoms form, describe the role of delocalized electrons & positive metal ions in creating a lattice structure, describe imperfections in the crystal structure, including vacancies & dislocations, and illustrate how grains, grain boundaries, and dislocations appear in a metals microstructure.
Question Two: What is a metal alloy? How is it made?
At the end of the lab, students will be able to define what an alloy is and how it is made, explain why alloying a metal is favorable or not, and identify commonly used alloys and in what industries they are used.
Question Three: How are the properties of a metal affected by various heat treatments? At the end of the lab & webquest, students will be able to describe the effects of cold-working and heat treating on the resulting strength of a metal, explain differences in observed strength between elemental and alloyed metals, and evaluate why the properties of the metals are dependent upon their structure.
Question Four: How can scientists visualize and study the altered properties of metals?
At the conclusion of these activities, students will be able to differentiate between exponential & standard notation, determine the length scale between objects of very different sizes, understand the invisibility of the nanoscale to the unaided eye, discuss visual images of atoms, molecules, and cells and their relative sizes, and predict what the internal structure of a metal looks like at the micro and nano scales.
Standards Addressed As a result of the various labs and activities with the curriculum project, the student understandings described below are addressed. This includes stan