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Kirkendall Effect Kirkendall Effect To show that diffusion takes place via a vacancy mechanism Education Level : UG Course Name: Phase transformations and heat treatment LOs for prior viewing : NONE Authors Amol Subhedar (Under Guidance of Prof. M P Gururajan) Learning Objectives After interacting with this Learning Object, the learner will be able to: 1. Explain how vacancy mechanism is responsible for diffusion in Substitutional alloys 2. Explain why the marker shift in Kirkendall effect is apparent Definitions of the components/Keywords: •Vacancies -Vacancies are missing atoms in a crystal 1 structure. 2 •Ring diffusion – A proposed mechanism for diffusion in which three or four atoms in the form of a ring move simultaneously round the ring, thereby interchanging their positions 3 •Direct exchange – A proposed mechanism for diffusion achieved by the interchanging of positions of two adjacent atoms 4 •Vacancy diffusion – Diffusion process aided by vacancies in the lattice 5 Master Layout: 1(A simple Experiment) 1 Step 1: Copper and Brass separated by Molybdenum marker 2 Cu Brass 3 Molybdenum 4 5 Step 1 Description of the activity Audio narration Text to be displayed Draw a green rectangle Consider joint of pure and name it as Cu. Then copper and brass draw a attached red thin separated by insoluble rectangle on right side of marker molybdenum. green block. Name red Brass is alloy of copper strip as Molybdenum. On and zinc. right side of red strip attach a blue rectangle and name it as Brass. animation time : 2 seconds Master Layout: 1(A simple Experiment) 1 Step 2: Sample is heated 2 Heat 3 Cu Brass 4 Molybdenum 5 Step 2 Description of the activity Audio narration Text to be displayed On top of last figure Let us heat the sample. draw two arrow pointing upward and write Heat above them. Animation time : 2 seconds Master Layout: 1(A simple Experiment) 1 Step 3: Length of brass and copper bars are changed 2 Heat Cu Brass 3 Molybdenum 4 Brass Cu 5 Step 3 Description of the activity Audio narration Text to be displayed Next draw an arrow pointing downwards and When this system is draw three rectangles heated it is observed (green red and blue) as that in laboratory shown in slide and mark frame of reference them. Keep position of molybdenum marker red strip same in both remains at original cases. Decrease length position of blue rectangle by 1/3 while length of brass and increase length of part decreases and that green rectangle by 1/3. of copper increases. Animation time : 3 How can this seconds phenomenon be explained? 1 Master Layout 2: Possible mechanisms 2 Step 1: Consider three mechanism 3 Direct exchange Ring diffusion 4 Vacancy diffusion 5 Step 1 Description of the activity Audio narration Text to be displayed Draw three rounded rectangles and name How does differences in them as Direct diffusivity lead to this exchange, Ring Diffusion apparent shift. To explain and Vacancy Diffusion this, let us consider three respectively. mechanisms of diffusion, namely, Direct exchange, Ring diffusion and Vacancy diffusion. Master Layout 3: Direct exchange 1 Step 1: Initial position 2 3 4 5 Step 1 Description of the activity Audio narration Text to be displayed Draw 3X4 array of identical circles. Except Consider a small part of for middle two colour crystal lattice. Yellow and them as black (see red atom will be involved slide). Colour middle in direct exchange circles as yellow and process. red. Master Layout 3: Direct exchange 1 Step 2: Exchange starting 2 3 4 5 Step 2 Description of the activity Audio narration Text to be displayed Move yellow circle towards right and As the Yellow and red upwards and red circle atoms move towards towards left and each other, neighbouring downwards. Ensure that atoms are also red and yellow circles displaced. are diagonally opposite to each other. Animation time : 2 seconds Master Layout 3: Direct exchange 1Step 3: Exchange at intermediate position 2 3 4 5 Step 3 Description of the activity Audio narration Text to be displayed Again move yellow circle Note that displacement towards right and red of neighbour atoms circle towards left. introduces strain on Ensure that red and lattice. yellow circles are diagonally opposite to each other. Animation time : 2 seconds Master Layout 3: Direct exchange 1Step 4: Exchange at final position 2 3 4 5 Step 4 Description of the activity Audio narration Text to be displayed Draw figure as in step Exchange of atoms is 1(current master layout) completed . Red and except switch position of yellow atoms have yellow and red circle. interchanged their Animation time : 2 position. Overall process seconds is energetically demanding. Master Layout 3: Ring Diffusion 1 Step 1: Initial position 2 3 4 5 Step 1 Description of the activity Audio narration Text to be displayed Draw 4X4 array of Consider a small part of identical circles. Except crystal lattice. Yellow, for middle four, colour green , blue and red them as black (see atom will be involved in slide). Colour middle ring diffusion process. circles as yellow, blue green and red . Master Layout 3: Ring Diffusion 1 Step 2: Intermediate position 2 3 4 5 Step 2 Description of the activity Audio narration Text to be displayed Move red , blue , green 4 atoms are involved and yellow circles in in the process. This clockwise direction so transformation also that they form a square. puts lattice into strain Displace neighbour as neighbouring black circles as shown in atoms have to slide. displace. Animation time : 2 seconds Master Layout 3: Ring Diffusion 1 Step 3: Final position 2 3 4 5 Step 3 Description of the activity Audio narration Text to be displayed Draw figure as in first Transformation is step except position of complete. At any original yellow, blue time due to number green and red circle is of atoms involved in taken by respectively the process are 4, red, yellow, blue and therefore is les likely green. to take place. Animation time : 2 seconds Master Layout 4:Vacancy diffusion 1 Step 1: Initial position 2 3 4 5 Step 1 Description of the activity Audio narration Text to be displayed Draw 3x4 array of black Consider small part of circles. As shown in lattice that contains figure mark yellow circle vacancy . and remove circle from next to yellow circle. Master Layout 4:Vacancy diffusion 1 Step 2: Intermediate position 2 3 4 5 Step 2 Description of the activity Audio narration Text to be displayed Shift yellow circle Yellow atom starts towards right. Do not moving towards move rest of black vacancy. Note that this circles. process does not involve Animation time : 2 distortion of lattice. seconds Master Layout 4:Vacancy diffusion 1 Step 3: Final position 2 3 4 5 Step 3 Description of the activity Audio narration Text to be displayed Again shift yellow circle Process of towards right until it transformation is reaches vacant position. complete. Note that Animation time : 2 yellow atom moved seconds towards right while vacancy moved towards left. This process can explain appear ant shift in copper brass interface that we will see in next step Master Layout 4: Interface shift with vacancy 1 diffusion Step 1: Two metals joined 2 3 4 5 Step 1 Description of the activity Audio narration Text to be displayed Draw array of 7x6 blue Assume two metals and red circles having different diffusion separated by a vertical coefficient are dotted line. Remove 3-4 separated by interface. circles from each of blue Metal shown in red and red circles. colour has higher diffusivity than blue coloured metal. Note also presence of vacancies. Dotted line shows the interface Master Layout 4: Interface shift with vacancy 1 diffusion Step 2: 3 red and 1 blue atoms cross diffuse through interface 2 3 4 5 Step 2 Description of the activity Audio narration Text to be displayed Move three red circles to Since red atoms have left of interface and one higher diffusivity, more blue circle to right of number of red atoms will interface pass other side of Animation time : 3 interface than blue seconds atoms. Total number of vacancies and atoms remains constant in the process. More vacancies will be created in the red metal and more vacancies will be depleted in the blue metal. Master Layout 4: Interface shift with vacancy 1 diffusion Step 3: A complete plane of red atoms shifts 2 3 4 5 Step 3 Description of the activity Audio narration Text to be displayed Move four more red As more and more circles to left of interface number of red atoms and two blue circle to cross interface, right of interface eventually entire plane of Animation time : 4 red atom (shown as grey seconds atoms) is shifted to left side. Note that marker remains at same position. This explains the Kirkendall effect Questions 1. Kirkendall effect shows that the mechanism of diffusion is a. Marker shift b. Vacancy diffusion c. Ring mechanism d. Direct exchange 2. The marker shift is Kirkendall effect is real. True or false? 3. If the marker shifts to the right, the direction of motion of vacancies is also to the right. True or false? 4. Kirkendall effect will be seen in interstitial alloys. True or false? 5. Lattice is subjected to least strain in which of following mechanism? a. Vacancy diffusion b. Ring mechanism c. Direct exchange Answers 1. b. Vacancy diffusion. Other mechanism cannot explain Kirkendall effect. 2. False. Marker remains in its original position in the experiment. 3. False. Marker shift will be in direction of mass flux which is in opposite direction of movement of vacancies.
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