Overview

Metallic materials refer to substances composed of metal elements or primarily consisting of metal elements and possessing metallic properties. This category includes pure metals, alloys, intermetallic compounds, and special metallic materials.
(Note: Metal oxides, such as aluminum oxide, are not classified as metallic materials.)

Significance

The development of human civilization and societal progress are closely linked with metallic materials. The Copper Age and the Iron Age, which followed the Stone Age, are notable for the widespread use of metals. In modern times, the vast array of metallic materials forms an essential material foundation for human development.

Categories

Metallic materials are generally divided into three major categories: ferrous metals, non-ferrous metals, and special metallic materials.

1. Ferrous Metals
        Also known as iron and steel materials, this category includes industrial      pure iron (over 90% iron), cast iron (2%–4% carbon), carbon steel (less      than 2% carbon), as well as various structural steels, stainless steels,      heat-resistant steels, high-temperature alloys, precision alloys, etc. In      a broader sense, ferrous metals may also include chromium, manganese, and      their alloys.

2. Non-Ferrous Metals
        This category refers to all metals and their alloys except iron, chromium,      and manganese. Non-ferrous metals are typically divided into light metals,      heavy metals, precious metals, semimetals, rare metals, and rare earth      metals. Non-ferrous alloys usually have higher strength and hardness than      pure metals, with higher electrical resistance and smaller resistance      temperature coefficients.

3. Special Metallic Materials
        These include metallic materials with specific structural or functional      applications. Examples include amorphous metals produced by rapid      solidification, quasicrystals, microcrystalline and nanocrystalline      metallic materials. There are also special functional alloys for stealth,      hydrogen resistance, superconductivity, shape memory, wear resistance,      vibration damping, and more, as well as metal matrix composites.

Properties

Metallic materials possess two major types of properties: process performance and service performance.

• Process Performance
       This refers to how the material behaves under specified cold or hot      working conditions during manufacturing. Good process performance ensures      the material can be efficiently shaped or formed. Depending on the process      method, requirements include casting performance, weldability,      forgeability, heat treatability, and machinability.

• Service Performance
       This refers to how the material performs under actual operating      conditions. It includes mechanical, physical, and chemical properties.      These properties determine the service range and life span of a material.      Most mechanical parts work under normal temperature and pressure, without      highly corrosive environments, and must bear varying types of loads. The mechanical properties — a      material’s ability to resist failure under load — are the most critical      for design and material selection. Depending on the type of loading (e.g.,      tension, compression, torsion, impact, cyclic loads), different mechanical      properties are prioritized.

Common mechanical properties include:

• Strength

• Ductility

• Hardness

• Impact toughness

• Fatigue resistance

• Endurance limit

Fatigue

Many machine components operate under cyclic loads. Even when the stress is below the material's yield limit, repeated cycles over time may cause sudden brittle fracture — a phenomenon known as metal fatigue. Characteristics of fatigue failure include:

1. Alternating load stress

2. Long duration of load application

3. Sudden and instantaneous failure

4. Brittle failure, regardless of      material ductility

Fatigue is among the most common and dangerous failure modes in engineering. Fatigue can be classified as follows:

1. High-Cycle Fatigue

Occurs under low stress (below yield or even elastic limit) with more than 100,000 stress cycles. This is the most typical type of fatigue failure, often referred to simply as “fatigue.”

2. Low-Cycle Fatigue

Occurs under high stress (close to yield strength) or high strain, with fewer than 10,000–100,000 cycles. Also known as plastic fatigue or strain fatigue, since cyclic plastic deformation plays a key role.

3. Thermal Fatigue

Caused by thermal stress due to cyclic temperature variations.

4. Corrosion Fatigue

Fatigue failure resulting from the combined effect of cyclic loads and corrosive environments (e.g., acids, alkalis, seawater, reactive gases).

5. Contact Fatigue

Occurs at the contact surfaces of machine parts under repeated contact stress, causing pitting or surface spalling.

Plasticity

Plasticity refers to a material's ability to undergo permanent deformation without fracture under external load. When a metal is stretched, its length increases and cross-sectional area decreases. Therefore, plasticity is commonly measured by:

• Elongation

• Reduction of area

The higher these values, the better the material’s plasticity — i.e., its ability to sustain large plastic deformation without breaking. Metals with elongation greater than 5% are generally considered ductile (e.g., low-carbon steel), while those with less than 5% are considered brittle (e.g., gray cast iron).

Good plasticity allows a metal to undergo significant deformation while strengthening during the process (work hardening), improving strength and ensuring safe use. It also enables forming processes such as stamping, cold bending, drawing, and straightening. Thus, adequate plasticity is crucial when selecting metallic materials for mechanical parts.

Durability

Common types of corrosion in construction metals include:

1. Uniform Corrosion
        Surface corrosion occurs evenly, thinning the cross-section. The corrosion      rate is often evaluated by the average thickness loss per year. Steel generally      undergoes uniform corrosion in atmospheric conditions.

2. Pitting Corrosion
        Localized point corrosion forming pits. Influenced by metal properties and      medium composition (e.g., chloride-rich environments), with maximum pit      depth as a key measure. Piping corrosion often focuses on pitting.

3. Galvanic Corrosion
        Occurs at the junction of dissimilar metals due to potential differences.

4. Crevice Corrosion
        Arises in confined spaces or gaps due to local chemical concentration      differences.

5. Stress Corrosion Cracking (SCC)
        Caused by the combined action of tensile stress and corrosive media,      forming microcracks that may lead to sudden fracture. High-strength steel      wires in concrete are susceptible to SCC.