Metal Alloys

Phase diagrams for nickel-copper
Liquidus
T (C)
Solidus
Liquid solution
0 100
100 Cupper 0 Nickel
Iron-carbon System
Iron composition
Ferrite, also called alpha ferrite, is a solid solution of bodycentered cubic iron and has a maximum solid solubility of 0.022 percent carbon at a temperature of 727oC.
Eutectoid reaction
At 727oC, a reaction takes place in which austenite is transformed into alpha ferrite (bcc) and cementite, called eutectoid reaction. The structure of eutectoid steel is called pearlite. The microstructure of pearlite consists of alternating layers (lamellae) of ferrite and cementite. Consequently, the mechanical properties of pearlite are intermediate between ferrite (soft and ductile) and cementite(hard and bmpounds
Intermetallic compounds are complex structures in which solute atoms are present among solvent atoms in certain proportions. Thus some intermetallic compounds have solid solubility. Intermetallic compounds are strong, hard, and brittle. Because their high melting points and high strength at elevated temperatures, good oxidation resistance and relatively low density.
• dispersed throughout the structure. Alloying with finely dispersed particles
(second-phase particles) is an important method of strengthening alloys and controlling their properties. In two-phase alloys the second –phase particles present obstacles to dislocation movement, thus increasing strength. • Aggregate structure. In this alloy system, there are two sets of grains, each with its own compositions and properties.
Two-phase systems
Most alloys consist of two or more solid phases and may be regarded mechanical mixtures. Thus we call a system with two solid phases a two-phase system. Each phase is a homogeneous part of the total mass and has its own characteristics and properties.
Phase Diagrams
Alloys solidify over a range of temperature. Solidification begins when the temperature of the molten metal drops below the liquidus; it is completed when the temperature reaches the solidus.
The properties and behaviour of metals and alloys during manufacturing and performance during their service life depend on their composition, structure, and processing history and the heat treatment to which they have been subjected. Important properties such as strength, hardness, ductility and toughness, as well as resistance to wear and scratching, are greatly influenced and modified by alloying elements and by heattreatment processes.
Cementite, which is 100 percent iron carbide(Fe3C), with a carbon content of 6.67 percent. Cementite is a very hard and brittle intermetallic compound and has a significant influence on the properties of steels.
Nonferrous Stainless steel
Precipitation Hardening aging
Structure of alloys
An Alloy is composed of two or more chemical elements, at least one of which is a metal. The majority of metals used in engineering applications are some form of an alloy. Alloying consists of two basic forms: solid solutions and intermetallic compounds.
A phase diagram, also called an equilibrium or a constitutional diagram, shows the relationships among temperature, composition, and the phases present in a particular alloy system. Equilibrium means that the state of a system remains constant over an indefinite period of time.
材料成型过程质量性能控制
Materials Forming process and Quality Control
4. Metal Alloys: Their Structure and Strengthening by heat treatment
余万华 北京科技大学材料科学与工程学院
Introduction
Metal Alloys
Alloying elements
Ferrous Cast irons
Quenching, annealing Tempering normalizing
Solution treatment
Pure metals
Alloys
Phase diagrams Microstructures Solid solutions Substitutional Interstitial
Cast Irons
The term Cast iron refers to a family of ferrous alloys composed of iron, carbon (range from 2.11 percent to about 4.5 percent) and silicon (up to about 3.5 percent). Cast irons are usually classified according to their solidification morphology. • Gray cast iron, or gray iron. • Ductile cast iron, nodular cast iron, or spheroidal graphite cast iron. • white cast iron •Malleable iron •Compacted graphite iron
Solid solutions
When the particular crystal structure of the solvent is maintained during alloying, the alloy is called a solid solution. • Substitutional solid solutions. If the size of the solute atom is similar to that of the solvent atom, the solute atoms can replace solvent atoms and forms a substitutional solid solution. Such as Brass, an alloy of zinc and copper, in which zinc is introduced into the lattice of copper. • Interstitial solid solutions. If the size of the solute atom is much smaller than that of the solvent atom, the solute atom occupies an interstitial positions and forms an interstitial solid solution.