机械制造工程 开卷习题答案.doc

? ? ? ? ? ? ? QUESTIONS ? ? 1.1?Explain the difference between a unit cell and a single crystal. ? Answer (from page # 40): The Smallest group of atoms showing the characteristic lattice structure of a particular metal is known as a Unit Cell. It is the building block of a crystal, and a single crystal can have many unit cells. ? ? 1.2?In tables on crystal structures, iron is listed as having both a bcc and a fee structure. Why? ? Answer (from page # 41): Same metal may form different structures, because of a lower energy requirement at that temperature. For example, iron forms a bcc structure (alpha iron) below 912 °C (1674 °F) and above 1394 °C (2541 °F), but if forms an fcc structure (gamma iron) between 912 °C and 1394 °C. 1.3a. Define anisotropy, and what materials can you think of other than metals that exhibit anisotropic behavior? ? Answer (from page # 42): Because the b/a ratio is different for different directions within the crystal, a single crystal has different properties when tested in different directions. We say that a single crystal is anisotropic. Where “a” is the spacing of the atomic planes and “b” is inversely proportional to the atomic density in the atomic planes. A common example of anisotropy is woven cloth, which stretches differently when we pull it in different directions, or plywood, which is much stronger in the planar direction than along its thickness direction (it splits easily). ? ? 1.4?What effects does recrystallization have on properties of metals? ? Answer (from page # 50): Recrystallization decreases the density of dislocations, lowers the strength, and raises the ductility of the metal. ? ? 1.5 What is strain hardening? And what effects does it have on the properties of metals? ? Answer (