When a small amount of Li is added into Al to create an Al-Li alloy, the two elements typically do not mix homogeneously. Instead, most of the material is nearly-pure Al, while some small regions are Li-rich.2 (These small regions are called precipitates and they are responsible for giving Al-Li alloys much better mechanical properties relative to pure Al. Well learn more about the alloying process and about precipitates later in the course.) The Li-rich regions have the chemical formula Al3Li and belong to the cubic crystal system.2,3 In the unit cell, the Li atoms are located at [000], while the Al atoms are located at [ 1 2 1 2 0], [ 1 2 0 1 2 ], and [0 1 2 1 2 ].

(a) Draw the unit cell of Al3Li.

(b) Which of the crystal structures from Callister Chapter 3 does this resemble? Why do we not call it that structure?

(c) Given that the lattice parameter is 0.401 nm, what is the density of Al3Li?

(d) Pretend that we made a mistake in the alloying process and added so much lithium that the entire material is converted to randomly-oriented regions of Al3Li. Sketch this situation schematicallyno need to draw the atoms or unit cells just label the [100] direction of each region. What word do we use to describe this microstructure?

When a small amount of Li is added into Al to create an Al-Li alloy, the two elements typically do not mix homogeneously. Instead, most of the material is nearly-pure Al, while some small regions are Li-rich.2 (These small regions are called precipitates and they are responsible for giving AlLi alloys much better mechanical properties relative to pure Al. We'll learn more about the alloying process and about precipitates later in the course.) The Li-rich regions have the chemical formula Al3Li and belong to the cubic crystal system. 2,3In the unit cell, the Li atoms are located at [000], while the Al atoms are located at [21210],[21021], and [02121] (a) Draw the unit cell of Al3Li. [10] (b) Which of the crystal structures from Callister Chapter 3 does this resemble? Why do we not call it that structure? [5] (c) Given that the lattice parameter is 0.401nm, what is the density of Al3Li ? [5] (d) Pretend that we made a mistake in the alloying process and added so much lithium that the entire material is converted to randomly-oriented regions of Al3Li. Sketch this situation schematically-no need to draw the atoms or unit cells; just label the [100] direction of each region. What word do we use to describe this microstructure? [5]