Basic knowledge of metal materials

Summary: Metallic materials refer to metal elements or metal elements as the main composition of materials with metal characteristics. It includes pure metals, alloys, metallic materials, intermetallic compounds and special metal materials. (Note: metal oxides (such as alumina) are not metal materials).

1. meaning

The development of human civilization and the progress of society are closely related to metal materials. Following the Stone Age, the bronze age and the iron age are marked by the application of metal materials. Modern, a wide variety of metal materials have become an important material basis for the development of human society.

2. species

Metal materials are usually divided into ferrous metals, non-ferrous metals and special metal materials.

(1) Ferrous metals, also known as iron and steel materials, include industrial pure iron containing more than 90% iron, cast iron containing 2%-4% carbon, carbon steel containing less than 2% carbon, and structural steel, stainless steel, heat-resistant steel, high-temperature alloy, stainless steel, precision alloy and so on. The generalized ferrous metals also include chromium, manganese and their alloys.

(2) Non-ferrous metals refer to all metals and their alloys except iron, chromium and manganese, which are usually divided into light, heavy, precious, semi-metallic, rare and rare earth metals. The strength and hardness of nonferrous alloys are generally higher than those of pure metals, and the resistance is large and the resistance temperature coefficient is small.

(3) special metal materials include structural metallic materials and functional metallic materials for different purposes. Among them are amorphous metal materials obtained by rapid condensation process, quasicrystal, microcrystalline, nanocrystalline metal materials, and other special functional alloys such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, vibration damping and metal matrix composites.

3. performance

Generally divided into two categories: process performance and service performance. The so-called process performance refers to the mechanical parts in the process of manufacturing, metal materials in the set conditions of cold and hot processing performance. The processing capability of metal materials determines its adaptability in forming process. Due to the different processing conditions, the required technological properties are also different, such as casting performance, weldability, malleability, heat treatment performance, cutting processing and so on.

The so-called performance refers to the use of mechanical parts under the conditions, the performance of metal materials, including mechanical properties, physical properties, chemical properties. The performance and service life of metal materials depend on its performance. In the mechanical manufacturing industry, the general mechanical parts are used in normal temperature, atmospheric pressure and very strong corrosive medium, and in the course of use, each mechanical part will bear different loads. The resistance of metal materials to failure under load is called mechanical properties (also known as mechanical properties in the past). The mechanical properties of metallic materials are the main basis for designing and selecting parts. The mechanical properties of metal materials are different under different external loading (such as tensile, compressive, torsional, impact, cyclic loading, etc.). Common mechanical properties include strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit.

Metal material characteristics

1. fatigue

Many mechanical parts and engineering components are subjected to alternating loads. Under the action of alternating load, although the stress level is lower than the yield limit of the material, after repeated stress cycling for a long time, sudden brittle fracture also occurs. This phenomenon is called fatigue of metal materials. The characteristics of fatigue fracture of metallic materials are:

(1) the load stress is alternating.

(2) the loading time is longer.

(3) fracture is instantaneous.

(4) both the plastic and brittle materials are brittle in the fatigue fracture zone. Therefore, fatigue fracture is the most common and dangerous fracture form in engineering.

The fatigue phenomena of metallic materials can be divided into the following categories according to different conditions.

(1) High cycle fatigue: fatigue in which the cycle number of stresses is more than 100,000 under low stress (working stress is lower than the yield limit or even lower than the elastic limit). It is the most common type of fatigue damage. High cycle fatigue is usually referred to as fatigue.

(2) Low-cycle fatigue: refers to the fatigue under high stress (the working stress is close to the yield limit of the material) or high strain conditions, the number of stress cycle in the 100000 ~ 100000 below. Alternating plastic strain plays a major role in this kind of fatigue failure, so it is also called plastic fatigue or strain fatigue.

(3) Thermal fatigue: refers to the fatigue damage caused by the repeated action of thermal stress caused by temperature changes.

(4) Corrosion fatigue: refers to the fatigue failure of machine parts under the combined action of alternating load and corrosive medium (such as acid, alkali, sea water, active gas, etc.).

(5) Contact fatigue: This refers to the contact surface of machine parts, under the repeated action of contact stress, there are pits peeling or surface crushing and peeling, resulting in failure and damage of machine parts.

2. plasticity

Plasticity refers to the ability of metal materials to produce permanent deformation (plastic deformation) without being destroyed under the action of external loads. The length and cross-sectional area of metal materials change when they are stretched. Therefore, the plasticity of metal can be measured by the elongation of length (elongation) and the shrinkage of cross-sectional area (section shrinkage).

The greater the elongation and the shrinkage of the metal material, the better the plasticity of the material.