辽宁科技学院讲稿.doc

辽宁科技学院讲稿 专业外语 材料成型及控制工程-专业英语阅读 教学内容 备 注 CHAPTER I MATERIALS AND THEIR PROPERTIES 1. 1 Metals and Non-metals Among numerous properties possessed by materials, their mechanical properties, in the majority of cases, are the most essential and therefore, they will be given much consideration in the book. All critical parts and elements, of which a high reliability is required, are made of metals, rather than of glass, plastics or stone. As has been given in Sec 1-l, metals are characterized by the metallic bond; where positive ions occupy the sites of the crystal lattice and are surrounded by electron gas .All non-metals have an ionic or a covalent bond. These types of bond are rigid and are due to electrostatic attraction of two ions of unlike charges. Because of the metallic bond, metals are capable of plastic deformation and self-strengthening upon plastic deformation. Therefore, if there is a defect in a material or if the shape of an element is such that there are stress concentrators, the stresses in these points may attain a great value and even cause cracking. But since the plasticity of the material is high, the metal is deformed plastically in that point, say, at the tip of a crack, undergoes strengthening, and the process of fracture comes to an arrest. This does not occur in non-metals. They are uncapable of plastic deformation and self-strengthening; therefore, fracture will occur as soon as the stresses at the tip of a defect exceed a definite value. These facts explain why metals are reliable structural materials and can not be excelled by non-metallic materials. Words and Terms: Questions: 1) What are the differences in properties between metals and non-metals? 2) Why are metals capable of plastic deformation and self-strengthening? 1. 2 Ferrous Alloys More than 90 % by weight of the metallic materials used by human beings are ferrous alloys. This represents an immense family of engineering materials with a wide range of microstructures an