Memory Metals(Lesson Plan)

(Nitinol, A Shape Memory Alloy)

Suggested Grade Level9-12

Standard Statements

3.1.10 EDescribe patterns of change in nature, physical, and man-made systems.

3.2.10 BApply process knowledge and organize scientific and technological phenomena in varied ways.

3.4.10 AExplain concepts about the structure and properties of matter.

3.6.10 CApply physical technologies of structural design, analysis and engineering, personal relations, financial affairs, structural production, marketing, research, and design to real world problems.

3.8.10 BAnalyze how human ingenuity and technological resources satisfy specific human needs and improve the quality of life.

Content Objectives

Students will know that

  1. A shape memory alloy “remembers” its original shape after being deformed.
  2. Nitinol is an alloy of nickel and titanium.
  3. Scientific work involves human curiosity and imagination and is affected by social priorities and by the individuals involved.
  4. Nitinol can undergo phase changes while in the solid state, and these phases are temperature dependent.
  5. When Nitinol is in one phase(austenite), it is rigid and superelastic; when Nitinol is in another phase(martensite), it is soft and easily deformable.
  6. Macroscopic changes are changes you can see; microscopic changes are changes you can’t see.
  7. The temperatures at which Nitinol changes phase depends on the amount of nickel and titanium present in the alloy.
  8. Nitinol is used in a myriad of commercial applications.
Process Objectives

Students will be able to

  1. Devise aprocedure to determine which metal is Nitinol among a set of unknown metals.
  2. Make the distinction between macroscopic and microscopic changes by observing Nitinol’s behavior in hot and cold water.
  3. Determine the relationship between percent composition and transformation temperatures by examining Excel data from a manufacturer of Nitinol.
  4. Devise and carry out a procedure to estimate the percent composition of nickel in a sample of Nitinol.
  5. Change the parent shape of a Nitinol wire by holding it over a candle or burner flame.
Assessment Strategies
  1. Evaluation of completed student handout.
  2. Class discussion of key ideas throughout the activity.
Materials

Per class:

  • CD from Nitinol Devices & Components (NDC)
  • Computer with sound
  • Projector (optional)

Per group:

  • Nitinol wire
  • Pieces of wire of 3-4 othermetals (brass, steel, copper, etc.)
  • Beaker (100- or 250-mL)
  • Water
  • Ice
  • Hot Plates
  • Bunsen burner or candle
  • Matches or flame strikers
  • Tongs, forceps, or pliers (students should have 2 pair of one of these)
  • Plastic bowl or other container (large enough to accommodate the entire length of the Nitinol wire)
  • Thermometer

Per student:

  • Photocopies of the two Wire Material Data Sheets (se508 wire data; sm495 wire data)
Procedures
Part 1(Homework Assignment)
  1. Have the students read about the history / discovery of Nitinol wire and answer questions in theirhandout as a homework assignment.
Part 2(1, 40-min Class Period)
  1. Distribute all of the materials. Don’t identify the metals at this time.
  2. Using what they learned about Nitinol’s discovery, students follow directions in their handout to ascertain which of their metal wire samples could be Nitinol. Once they reach a conclusion, ask the students to share their findings during a class discussion. Encourage students to clearly articulate how they reached their conclusions.
Part 3(2, 40-min Class Periods)
  1. Assist the students in summarizing the reading from Part 1 by going over the discussion questions in their handout. Note the emphasis on the nature of science.
  2. Have students read a short paragraph that briefly explains phase changes within a solid. Help the students interpret the paragraph, and then introduce Nitinol’s two solid phases – austenite and martensite – as the rigid and soft phases, respectively.
  3. Show the short video from the CD that animates and describes the difference between austenite and martensite.
  4. Explain the difference between macroscopic and microscopic changes.
  5. In light of the video clip and explanation, students will deform their Nitinol wire into any shape, place it in hot water, and then observe it spring back to its original shape. By answering questions in their handout, students are led to realize that the obvious macroscopic change is evidence of a microscopic phase change to austenite that cannot be seen. Students will take the wire out of the hot water without deforming it, place it into ice water, and then observe the wire as it cools down. Students should notice that the wire stays in its original shapeand realize that even though nothing happens macroscopically, a microscopic phase change to martensite still occurred.
  6. Explain shape-memory, super-elasticity, and stress-induced martensite formation.
Part 4(1, 40-min Class Period)
  1. Tell students that there are many different types of Nitinol. By adjusting its composition slightly, manufacturers can adjust the temperatures at which nitinol exhibits shape-memory and super-elasticity.
  2. Direct students’ attention to the Material Data Sheets and the Excel data found in their handout. Point out the symbols Ms, Mf, As, and Af and discuss what they mean. Tell the students that the modulus of elasticity is a measure of stiffness (the higher the value, the stiffer the material).
  3. Students look atMaterial Data Sheets for two types of Nitinol wire. They compare the moduli of elasticity, Af temperatures, and weight percent compositions of the two types of Nitinol. Studentsthink about relationships between composition and these properties. They decide which type of Nitinol would be in the martensite (soft) phase at room temperature and which would be in the austenite (rigid) phase at room temperature.
  4. Students look at data from the Excelspreadsheet that contains percent composition of nickel for nine types of Nitinol along with their corresponding transformation temperatures. The students determine a relationship betweenpercent composition of nickel and the temperature at which Nitinol is austenitic and martensitic. Students observe how very small changes in nickel composition result in substantial changes in the properties of Nitinol.
Part 5(1, 40-min Class Period)
  1. Ask students to devise a procedure to predict the composition of their Nitinol wire sample. Emphasize that this determinationcannot be exact but will only be a reasonable prediction.
  2. The students carry out and then share their results during a class discussion.
Part 6(30-min)
  1. Students try re-setting the parent shapeof their Nitinol sample by holding it into a desired shape with tweezers or pliers over a candle or burner flame. They see if their wire remembers this shape by deforming it and then placing it in hot water.
  2. Students reflect on the shape memory and super-elastic properties of Nitinol they have observed. Students think about how quickly and forcefully the deformed Nitinol wire resumed its parent shape when placed in hot water. As a class, they discuss how these properties could be useful in every-day life. They think about places where engineers might use this metal to solve problems.
  3. Students read about Nitinol’s uses.

Memory MetalsLesson Plan1

MemoryMetals(Teacher Notes)

(Nitinol, A Shape Memory Alloy)

General Lesson Notes
  • Warnings. After being deformed and then placed in very hot water, Nitinol may change shape very quickly and forcefully. Students should use caution and be aware of splashing hot water.
  • Alloys. An alloy is a mixture of two or more metals. Examples of common alloys are brass (copper-zinc), bronze (copper-tin), and steel (iron-carbon).
  • Shape Memory Alloys. Also known as “memory metals” or “smart metals,” shape memory alloys (SMAs) “remember” their original shape after being deformed. An SMA will return to its trained (previously set) shape when it is heated to a specific temperature. Although a variety of SMAs exist (copper-aluminum-nickel, copper-zinc-aluminum, iron-manganese-silicon, etc.), the nickel-titanium ones are considered to be the most useful.
Part 1 Notes
  • Nitinol. (This explanation pertains to Question #1 parts a) and b)) Nitinol (more correctly, NiTiNOL or NiTinol) is the name for the family of SMAs which contain nickel and titanium. Nitinol is an acronym for “Nickel Titanium Naval Ordnance Laboratory,” after where its shape-memory behavior was discovered. Nitinol contains roughly equal amounts of nickel and titanium atoms.
  • The discovery of Nitinol and the nature of science. (This explanation pertains to Question #1 parts c) through i) in the Student Handout) It is recommended that the reading and questions in Part 1 be assigned for homework. The questions that accompany the reading are intended to help students think about aspects of the nature of science found within the context of Nitinol’s historical discovery. It would be advantageous to make these connections explicit, however, during a class discussion. The fact that William Buehler was working on a project to develop metallic materials for the nose cone of a U.S. Navy missile when he stumbled across nickel-titanium alloys demonstrates how science is affected by social priorities and the cultural contexts in which it occurs (question c). You may need to explain to students that the U.S. Navy Polaris was a missile and that the U.S. Naval Ordnance Laboratory is a place where military weapons and equipment are developed. The fact that Dr. Harold Margolin and his colleagues had looked at nickel-titanium alloys prior to Buehler but did not notice anything particularly unique about them shows how science involves subjectivity (question d). The reason why Buehler began looking at newly developed alloys for the Navy missile project was mainly was because he was bored (question e)! This not only speaks to the fact that science is subjective (another person might not have been bored at all), but also more importantly that science is a creative endeavor. Science has a subjective component because individual commitments, beliefs, interests, knowledge, and experiences influence what scientists observe (and don’t observe), how investigations are carried out, and how observations are interpreted. Buehler’s curiosity compelled him not only to think of a new way of solving a problem, but also to listen to the sound the nitinol bars were making as he dropped them on the concrete floor of his lab (question f). Curiosity also got the best of Dr. Muzzey when he decided to heat the demo piece of nitinol wire with his pipe lighter, only to make the capstone discovery of nitinol’s unique shape memory property (question g). Curiosity, imagination, and creativity abound in science (question h)! The fact that many people were integral to the scientific discovery of nitinol demonstrates how science is NOT an autonomous activity (question i).
Part 3 Notes
  • Video of nitinol. The video on compact disc that accompanies this activity was purchased from a commercial company that manufactures nitinol wire (Nitinol Devices and Components). Additional CDs can be obtained by visiting their website and emailing a customer service representative. Also included on the CD are several scientific papers that discuss research on nitinol’s applications. The animations on the video are particularly helpful in understanding how nitinol works. It is recommended that the video be played and then projected onto a screen for the entire class to view. However, if projecting it with sound is not possible, you could have it playing on a stationary computer and have students take turns viewing it.
  • Martensite and Austenite Phases. (This explanation pertains to Question #5 in the Student Handout) Nitinol displays two distinct crystal structures in the solid phase – martensite and austenite. The martensite is the “soft” phase, and the austenite is the rigid phase. The particular crystal structure that Nitinol possesses at any given time depends on its temperature and the amount of force (stress) to which it is being subjected.
  • Shape-memory. (This explanation pertains to Question #10 in the Student Handout) A metal’s ability to return to its original shape after being deformed. Nitinol exhibits shape-memory when it returns to its original shape upon heating. Different types of nitinol will exhibit shape-memory at different temperatures.
  • Super-elasticity. (This explanation pertains to Question #10 in the Student Handout) A metal’s ability to be deformed to a large extent without permanent damage or fatigue. Nitinol’s ability to spring back after being deformed in the austenite phase is about 10 times greater than that of stainless steel. Said differently, Nitinol can be bent more than stainless steel without permanent damage. The reason why nitinol is able to exhibit super-elasticity is because of its ability to transition into its easily deformable martensite phase under large stresses and strains (see explanation of stress-induced martensite formation below.)
  • Stress-induced martensite formation. (This explanation pertains to Question #10 in the Student Handout) When Nitinol isin its austenite (rigid) phase but is subjected to large amount of stress from an externally applied force, it can transition into the martensite (easily deformable) phase without requiring cooler temperatures. This is known as stress-induced martensite formation. When the stress is released, martensite transforms back into the rigid austenite phase, and the metal springs back to its original shape. Stress-induced martensite formation allows Nitinol to be super-elastic, which means that Nitinol can undergo a large amount of deflection (bending) or displacement (stretching) to accommodate large loads without being permanently deformed. It also means that as Nitinol bends or stretches during continued, increased loading, it experiences very little increase in stress (see plateau in stress / strain curve). The reason for this isthat Nitinolcan temporarily exist in its easily deformed martensite phase during loading.
  • Shape-memory vs Super-elastic Nitinol. (This explanation pertains to Question #10 in the Student Handout) Some Nitinol is called “super-elastic” and some is called “shape-memory.” This distinction (made primarily by manufacturers) can be confusing. All Nitinol exhibits both super-elastic and shape-memory behavior, but the exact composition of Nitinol dictates the temperatures where these properties exist. Typically, super-elastic Nitinol is Nitinol that is in its austenite (rigid) phase at room temperature (and therefore must be cooled to temperatures below 0oC to reach its soft phase). Shape-memory Nitinol is in its martensite (soft) phase at room temperature (and therefore must be heated to reach its austenite / memory phase). Manufacturers often abbreviate these distinctions as “SM” (“shape-memory”) and “SE” (“super-elastic”).
Part 4 Notes
  • Transformation Temperatures. (This explanation pertains to Question #13 in the Student Handout) Because the transformations between crystal structure occur over temperature ranges, scientists use the following naming conventions:
  • As = austenite start temperature (where Nitinol starts transforming to austenite during heating)
  • Af = austenite finish temperature (where Nitinol has finished transforming to austenite during heating)
  • Ms = martensite start temperature (where Nitinol starts transforming to martensite during cooling)
  • Mf = martensite finish temperature (where Nitinol has finished transforming to martensite during cooling)

Martensite exists at lower temperatures and is the phase in which Nitinol can be easily deformed into any shape. As long as the temperature does not change, Nitinol will remain deformed. When Nitinol is heated, it transforms into the austenite phase, where it “remembers” the shape it had before it was deformed. In the austenite phase, Nitinol is rigid and does not deform easily. Students should observe this when they attempt to deform their Nitinol wire while it is in hot water.

  • Composition. (This explanation pertains to Questions #14 and #15 in the Student Handout) Manufacturers adjust the exact composition of Nitinol in order to change the temperatures at which shape memory and super-elasticity are exhibited. Other small amounts of elements (e.g., Co, Fe, V, etc.) can also be added for this purpose. Small differencesin nickel and titanium composition between different types of Nitinol can produce fairly significant changesin the transformation temperatures. Students will observe this inanalyzing the data from the spreadsheet and from thematerial data sheets for two types of Nitinol wire sold by one manufacturer. As nickel composition increases, the temperatures decrease.
Part 5 Notes
  • Determining percent composition of nickel. (This explanation pertains to Question #16 in the Student Handout) In Part 5, students are looking for a temperature range. The easiest one to do is the range of temperatures at which their Nitinol wire is austenitic because water can easily be heated to various temperatures. By some systematic trial and error, students should be able to observe where the wire begins to be austenitic (where it slowly begins returning to its original shape) and where the wire immediately snaps back to its original shape (austenite finish temperature). Remind the students that this determination will not be exact, but only an approximation.
Part 6 Notes
  • Re-setting Nitinol. In Part 6, students attempt to change the set shape of their nitinol wires. When nitinol is heated to a temperature which is significantly higher than its transition temperature, it can be re-set into a different parent shape. Remind the students not to touch their wire immediately after they take it out of the flame, as it will burn them.
  • Uses. (This explanation pertains to Question #19 in the Student Handout) Although Nitinol has been around for over 40 years, at first it was used almost exclusively by the military. Research on commercial applications for Nitinol was initially hindered by its high cost and manufacturing difficulties. The arrival of Frederick E. Wang (a native of Taiwan) to NOL in 1962 helped to accelerate research because he was able to provide an understanding of the unique crystal behaviors of these alloys. Nitinol’s first commercial application was Raychem Corporation’s “shrink-to-fit” pipe couplers that were used to join hydraulic tubes in F-14 fighter planes. Another early commercial application was orthodontic arch wires for braces, which exploited Nitinol’s superelastic property to exert uniform pressure on the teeth as it tried to return to its unbent shape.[1] There have been a plethora of other applications, only some of which are listed below. More extensive information on Nitinol’s uses can be found on the CD archiving the Nitinol Devices and Componentswebsite.

Other Uses