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Metallic materials refer to a broad category of materials that primarily consist of metallic elements or are predominantly composed of metallic elements, exhibiting characteristic metallic properties. This encompasses various types such as pure metals, alloys, intermetallic compounds, and special metallic materials. (Note: Metal oxides, such as alumina, are not classified as metallic materials.)
Vol.1 Significance
The development of human civilization and the progress of society are closely related to metallic materials. The Copper Age and the Iron Age, which followed the Stone Age, are both marked by the application of metallic materials. In modern times, a wide variety of metallic materials have become an important material basis for the development of human society.
Vol.2 Categories
Metal materials are typically classified into 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 with an iron content exceeding 90%, cast iron with a carbon content of 2-4%, carbon steel with a carbon content of less than 2%, and various structural steels, stainless steels, heat-resistant steels, high-temperature alloys, precision alloys, etc., for different applications. In a broader sense, ferrous metals also encompass chromium, manganese, and their alloys.
(2) Non-ferrous Metals: This category refers to all metals and their alloys except iron, chromium, and manganese. They are commonly divided into light metals, heavy metals, precious metals, semi-metals, rare metals, and rare earth metals. Non-ferrous alloys generally exhibit higher strength and hardness compared to pure metals, along with greater electrical resistance and a smaller temperature coefficient of resistance.
(3) Special Metallic Materials:** This category encompasses structural and functional metallic materials for diverse applications. It includes amorphous metallic materials obtained through rapid solidification processes, as well as quasicrystals, microcrystalline materials, nanocrystalline materials, etc. Additionally, it covers special functional alloys with properties such as stealth, hydrogen resistance, superconductivity, shape memory, wear resistance, and vibration damping, along with metal matrix composites.
Vol. 3 Performance
Generally, the properties of metal materials are categorized into two types: processing performance and application performance. Processing performance refers to the characteristics exhibited by the metal material under specified cold and hot working conditions during the manufacturing process of mechanical parts. The quality of the processing performance determines the adaptability of the material to be shaped during manufacturing. Due to different processing conditions, the required processing performance varies, including casting performance, weldability, forgeability, heat treatability, and machinability.
The term "service performance" refers to the properties exhibited by metallic materials under specific operating conditions. This encompasses a range of characteristics, including mechanical, physical, and chemical properties. The quality of these service properties dictates the material's applicability and lifespan. Within the mechanical manufacturing industry, components typically operate under normal temperature and pressure conditions, often in environments that are not highly corrosive. During operation, these components are subjected to various loads. The ability of a metal to resist failure under load is defined as its mechanical properties (previously also referred to as mechanical performance). These properties serve as the primary basis for both component design and material selection.
Different types of external loads (e.g., tensile, compressive, torsional, impact, cyclic loads) necessitate distinct mechanical properties in the chosen metal. Commonly considered mechanical properties include：Strength、Plasticity、Hardness、Impact toughness、Fatigue strength、Fatigue limit。
Properties of metallic materials
Vol. 1 Fatigue
Many mechanical components and engineering structures operate under alternating loads. Under the influence of these cyclic loads, even when stress levels remain below the material's yield strength, sudden brittle fractures can occur after a prolonged period of stress reversals. This phenomenon is known as metal fatigue.
Characteristics of metal fatigue failure:
1. Alternating Load Stress: The applied load creating stress fluctuates over time.
2. Extended Load Duration: The load acts on the material for a significant period.
3. Instantaneous Fracture: The failure occurs suddenly and without warning.
4. Brittle Fracture Zone: Regardless of whether the material is ductile or brittle, the fracture zone in fatigue failures exhibits brittle characteristics.
Due to these factors, fatigue failure is one of the most common and dangerous forms of failure encountered in engineering applications.
The fatigue phenomena in metallic materials can be categorized into the following types based on varying conditions:
#1 High-Cycle Fatigue
This refers to fatigue under low stress conditions where the working stress is below the material's yield strength, often even below the elastic limit. The stress cycle count typically exceeds 100,000 cycles. High-cycle fatigue is the most common type of fatigue failure and is often simply referred to as "fatigue."
#2 Low-Cycle Fatigue:
This type of fatigue occurs under high stress conditions where the working stress approaches the material's yield strength, or under high strain conditions. The stress cycle count is typically between 10,000 and 100,000 cycles. Due to the significant role of alternating plastic strain in this type of fatigue failure, it is also known as plastic fatigue or strain fatigue.
#3 Thermal Fatigue:
This refers to fatigue failure caused by the repeated action of thermal stresses resulting from temperature fluctuations.
#4 Corrosion Fatigue:
This type of fatigue failure occurs when machine components are subjected to the combined effects of alternating loads and corrosive media such as acids, alkalis, seawater, or reactive gases.
#5 Contact Fatigue:
This phenomenon refers to the surface damage on contact areas of machine parts under the repeated action of contact stress. This damage manifests as pitting, spalling, or surface crushing, ultimately leading to component failure.
Vol. 2 Plasticity
Plasticity refers to the ability of a metal material to undergo permanent deformation (plastic deformation) under an applied load without fracturing.** When a metal material is subjected to tensile stress, both its length and cross-sectional area change. Therefore, the plasticity of a metal can be measured by two indicators: elongation (percentage elongation) and reduction in area (percentage reduction in area).
The greater the elongation and reduction in area of a metal material, the better its plasticity, indicating that the material can withstand greater plastic deformation without failure.** Generally, metal materials with an elongation greater than 5% are called ductile materials (such as low-carbon steel), while those with an elongation less than 5% are called brittle materials (such as gray cast iron).Materials with good plasticity can undergo plastic deformation over a larger macroscopic range. During plastic deformation, the metal material is strengthened due to work hardening, thereby increasing its strength and ensuring the safe use of the part.In addition, materials with good plasticity can be easily processed by certain forming processes, such as stamping, cold bending, cold drawing, and straightening. Therefore, when selecting metal materials for mechanical parts, certain plasticity indices must be met.
Vol. 3 Durability
Main Forms of Corrosion in Building Metals:
(1) Uniform Corrosion:
This type of corrosion affects the metal surface evenly, causing a consistent reduction in thickness across the entire section. The corrosion rate, typically measured as the average annual thickness loss, serves as a key indicator of corrosion performance. Steel structures exposed to the atmosphere usually experience uniform corrosion.
(2) Pitting Corrosion:
This form of corrosion manifests as localized pits or holes on the metal surface. The occurrence of pitting corrosion is influenced by the nature of the metal and the surrounding environment. It is particularly prevalent in media containing chloride salts. The maximum pit depth is commonly used as an evaluation metric for pitting corrosion. In the context of pipelines, pitting corrosion is a significant concern.
(3) Galvanic Corrosion:
When dissimilar metals come into contact, a difference in their electrical potentials can lead to galvanic corrosion. This type of corrosion is driven by the flow of electrons between the metals.
(4) Crevice Corrosion:
Localized corrosion can occur within crevices or other shielded areas on the metal surface. This is often caused by variations in the composition and concentration of the medium within these confined spaces compared to the exposed surfaces.
(5) Stress Corrosion Cracking:
The combined presence of a corrosive environment and high tensile stress can lead to stress corrosion cracking. This phenomenon involves the formation and propagation of microscopic cracks on the metal surface, which can ultimately result in sudden failure. High-strength steel reinforcements (wires) embedded in concrete are susceptible to this type of damage.
Vol. 4 Hardness
Hardness signifies a material's resistance to the indentation of a harder object on its surface. It is a crucial mechanical property indicator for metallic materials. Generally, higher hardness correlates with better wear resistance. Common hardness measurements include Brinell, Rockwell, and Vickers hardness.
Brinell Hardness (HB):A hardened steel ball (typically 10mm diameter) is pressed onto the material's surface under a specific load (generally 3000kg). After a set duration, the load is removed, and the Brinell hardness value (HB) is calculated as the ratio of the load to the indentation area. The unit for HB is kgf/mm² (N/mm²).
Rockwell Hardness (HR): When HB exceeds 450, or the sample is too small, Rockwell hardness testing is employed instead of Brinell. This method utilizes a diamond cone with a 120° angle or a hardened steel ball with a diameter of 1.59mm or 3.18mm. The indenter is pressed into the material's surface under a specific load, and the material's hardness is determined from the indentation depth. Different scales with varying indenters and total test forces exist, each denoted by a letter after the Rockwell hardness symbol (HR). Commonly used scales include HRA, HRB, and HRC, with HRC being the most prevalent.
HRA: Employs a 60kg load and a diamond cone indenter, suitable for extremely hard materials like cemented carbides.
HRB: Employs a 100kg load and a 1.58mm diameter hardened steel ball, suitable for softer materials like annealed steel and cast iron.
HRC: Employs a 150kg load and a diamond cone indenter, suitable for very hard materials like hardened steel.
Vickers Hardness (HV): A diamond pyramid indenter with a 136° angle is pressed into the material's surface under a load up to 120kg. The Vickers hardness value (HV) is calculated by dividing the surface area of the indentation by the applied load.Hardness testing is the simplest and most convenient mechanical test. To substitute certain mechanical property tests with hardness tests, a relatively accurate conversion relationship between hardness and strength is necessary in production. Experience shows that various hardness values of metallic materials have an approximate corresponding relationship with their strength values. This is because hardness is determined by the initial and continued resistance to plastic deformation. The higher the material's strength, the higher its resistance to plastic deformation, leading to a higher hardness value.
Properties of metallic materials
The performance of metallic materials determines their applicability and the rationality of their application. The properties of metallic materials are mainly divided into four aspects: mechanical properties, chemical properties, physical properties, and technological properties.
Vol.1
Mechanical Properties
Stress:
Stress is defined as the force acting on a unit area within an object. Stress caused by external forces is called working stress, while stress balanced within the object without external forces is called internal stress (e.g., tissue stress, thermal stress, residual stress remaining after processing).
Mechanical Properties:
The ability of a metal to resist deformation and fracture under the action of external forces (loads) at a certain temperature is called the mechanical properties (also known as mechanical performance) of the metal material. Metal materials can bear various forms of loads, which can be static or dynamic, including tensile stress, compressive stress, bending stress, shear stress, torsional stress, as well as friction, vibration, impact, etc. Therefore, the main indicators to measure the mechanical properties of metal materials include the following.
1.1 Strength
This describes a material's maximum ability to resist deformation and fracture under external forces. It can be categorized into tensile strength limit (σb), flexural strength limit (σbb), and compressive strength limit (σbc). Since the deformation and fracture of metallic materials under external forces follow certain patterns, tensile tests are typically used for measurement. This involves preparing metal specimens of specific dimensions and subjecting them to tension in a tensile testing machine until they fracture. The main strength indicators measured include:
(1) Tensile Strength:
The maximum stress a material can withstand under external forces before it breaks. It generally refers to the ultimate tensile strength under tensile forces, denoted as σb, as shown in the highest point (b) of the tensile test curve. The commonly used unit is megapascal (MPa). The conversion relationships are as follows: 1 MPa = 1 N/m² = (9.8)^-1 kgf/mm² or 1 kgf/mm² = 9.8 MPa.
(2) Yield Strength Limit:
When the external force applied to a metal material sample exceeds its elastic limit, the sample undergoes significant plastic deformation even though the stress remains constant. This phenomenon is called yielding, meaning that when the material withstands external force to a certain extent, its deformation no longer remains proportional to the force and evident plastic deformation occurs. The stress at which yielding occurs is called the yield strength limit, denoted by σs. The corresponding point on the tensile test curve is called the yield point (point S). For materials with high plasticity, a distinct yield point appears on the tensile curve. However, for materials with low plasticity, there is no clear yield point, making it difficult to determine the yield strength limit based on the external force at the yield point. Therefore, in tensile testing methods, the stress at which the gauge length on the sample exhibits 0.2% plastic deformation is typically defined as the conditional yield strength limit, denoted by σ0.2. The yield strength limit indicator can be used as a design basis for parts that require no significant plastic deformation during operation. However, for some critical components, a lower yield strength ratio (σs/σb) is also considered to enhance their safety and reliability, although this results in lower material utilization.
(3) Elastic Limit:
When subjected to external forces, a material undergoes deformation. However, if it can revert to its original shape upon removal of the force, this ability is termed elasticity. The maximum stress a metallic material can withstand while maintaining elastic deformation is known as the elastic limit. This corresponds to point "e" on the tensile test curve and is represented by σe, measured in megapascals (MPa):
σe = Pe / Fo，where Pe is the maximum external force that maintains elasticity (or the load at the material's maximum elastic deformation).
(4) Elastic Modulus:
Within the elastic limit of a material, the elastic modulus (E) is the ratio of stress (σ) to strain (δ), represented in units of megapascals (MPa): E = σ/δ = tan α. Here, α is the angle between the o-e line (within the elastic deformation range) on the tensile test curve and the horizontal axis o-x. The elastic modulus reflects the rigidity of a metallic material (rigidity refers to the ability of a metallic material to resist elastic deformation under stress).
1.2 Plasticity
The maximum ability of a metallic material to undergo permanent deformation under external force without fracture is called plasticity. It is usually characterized by the elongation (δ) and the reduction of area (ψ) of a specimen during a tensile test.
Elongation (δ): This is expressed as a percentage and calculated using the formula: δ = [(L1 - L0) / L0] x 100%. In this formula, L1 represents the final gauge length after fracture when the two broken ends are fitted together, and L0 represents the original gauge length of the specimen.
It's important to note that elongation values can differ for the same material depending on the specimen's dimensions (diameter, cross-sectional shape - e.g., square, circular, rectangular) and gauge length. Therefore, specific notations are used to indicate these parameters. For example, the elongation measured on a commonly used circular cross-section specimen with an initial gauge length five times its diameter is denoted as δ5, while δ10 signifies the elongation with a gauge length ten times the diameter.
Reduction of Area (ψ): This is also expressed as a percentage and calculated using the formula: ψ = [(F0 - F1) / F0] x 100%. Here, F0 represents the original cross-sectional area, and F1 represents the minimum cross-sectional area at the necked region of the fracture.
For practical purposes, in the case of the commonly used circular cross-section specimens, the reduction of area can be calculated through diameter measurements using the formula: ψ = [1 - (D1 / D0)^2] x 100%. In this formula, D0 represents the original diameter of the specimen, and D1 represents the minimum diameter at the necked region after fracture.
Higher values of both δ and ψ indicate better plasticity of the material.
1.3 Toughness
The ability of a metallic material to resist fracture under impact loading is called toughness. It is usually measured by impact testing, which in
