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Introduction to Alloy Phase Diagrams

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Introduction to Alloy Phase Diagrams ASM Handbook, Volume 3: Alloy Phase Diagrams Copyright © 1992 ASM International® Hugh Baker, editor, p 1.1-1.29 All rights reserved. DOI: 10.1361/asmhba0001123 www.asminternational.org Section 1 Introduction to Alloy Phase Diagrams Hugh Baker, Editor ALLOY PHASE DIAGRAMS are useful to exhaust system). Phase diagrams also are con- terms "phase" and "phase field" is seldom made, metallurgists, materials engineers, and materials sulted when attacking service problems such as and all materials having the same phase name are scientists in four major areas: (1) development of pitting and intergranular corrosion, hydrogen referred to as the same phase. new alloys for specific applications, (2) fabrica- damage, and hot corrosion. Equilibrium. There are three types of equili- tion of these alloys into useful configurations, (3) In a majority of the more widely used commer- bria: stable, metastable, and unstable. These three design and control of heat treatment procedures cial alloys, the allowable composition range en- conditions are illustrated in a mechanical sense in for specific alloys that will produce the required compasses only a small portion of the relevant Fig. l. Stable equilibrium exists when the object mechanical, physical, and chemical properties, phase diagram. The nonequilibrium conditions is in its lowest energy condition; metastable equi- and (4) solving problems that arise with specific that are usually encountered inpractice, however, librium exists when additional energy must be alloys in their performance in commercial appli- necessitate the knowledge of a much greater por- introduced before the object can reach true stabil- cations, thus improving product predictability. In tion of the diagram. Therefore, a thorough under- ity; unstable equilibrium exists when no addi- all these areas, the use of phase diagrams allows standing of alloy phase diagrams in general and tional energy is needed before reaching meta- research, development, and production to be done their practical use will prove to be of great help stability or stability. Although true stable equilib- more efficiently and cost effectively. to a metallurgist expected to solve problems in rium conditions seldom exist in metal objects, the In the area of alloy development, phase dia- any of the areas mentioned above. study of equilibrium systems is extremely valu- grams have proved invaluable for tailoring exist- able, because it constitutes a limiting condition ing alloys to avoid overdesign in current applica- from which actual conditions can be estimated. tions, designing improved alloys for existing and Common Terms Polymorphism.The structure of solid elements new applications, designing special alloys for and compounds under stable equilibrium condi- special applications, and developing alternative Before the subject of alloy phase diagrams is tions is crystalline, and the crystal structure of alloys or alloys with substitute alloying elements discussed in detail, several of the commonly used each is unique. Some elements and compounds, to replace those containing scarce, expensive, terms will be discussed. however, are polymorphic (multishaped); that is, hazardous, or "critical" alloying elements. Appli- Phases. All materials exist in gaseous, liquid, or their structure transforms from one crystal struc- cation of alloy phase diagrams in processing in- solid form (usually referred to as a phase), de- ture to another with changes in temperature and cludes their use to select proper parameters for pending on the conditions of state. State variables pressure, each unique structure constituting a dis- working ingots, blooms, and billets, fmding include composition, temperature, pressure, mag- tinctively separate phase. The term allotropy (ex- causes and cures for microporosity and cracks in netic field, electrostatic field, gravitational field, isting in another form) is usually used to describe castings and welds, controlling solution heat and so on. The term "phase" refers to that region polymorphic changes in chemical elements. treating to prevent damage caused by incipient of space occupied by a physically homogeneous Crystal structure of metals and alloys is discussed melting, and developing new processing technol- material. However, there are two uses of the term: in a later section of this Introduction; the allo- ogy. the strict sense normally used by physical scien- tropic transformations of the elements are listed In the area of performance, phase diagrams give tists and the somewhat looser sense normally used in the Appendix to this Volume. an indication of which phases are thermodynami- by materials engineers. Metastable Phases. Under some conditions, cally stable in an alloy and can be expected to be In the strictest sense, homogeneous means that metastable crystal structures can form instead of present over a long time when the part is subjected the physical properties throughout the region of stable structures. Rapid freezing is a common to a particular temperature (e.g., in an automotive space occupied by the phase are absolutely iden- method of producing metastable structures, but tical, and any change in condition of state, no some (such as Fe3C, or"cementite") are produced matter how small, will result in a different phase. at moderately slow cooling rates. With extremely For example, a sample of solid metal with an rapid freezing, even thermodynamically unstable apparently homogeneous appearance is not truly structures (such as amorphous metal "glasses") a single-phase material, because the pressure con- can be produced. dition varies in the sample due to its own weight Systems. A physical system consists of a sub- in the gravitational field. stance (or a group of substances) that is isolated In a phase diagram, however, each single-phase from its surroundings, a concept used to facilitate field (phase fields are discussed in a following study of the effects of conditions of state. "Iso- Ill section) is usually given a single label, and engi- lated" means that there is no interchange of mass neers often find it convenient to use this label to between the substance and its surroundings: The (a) (b) (c) refer to all the materials lying within the field, substances in alloy systems, for example, might regardless of how much the physical properties of be two metals, such as copper and zinc; a metal the materials continuously change from one part and a nonmetal, such as iron and carbon; a metal Fig. I Mechanical equilibria: (a) Stable. (b) Metas- of the field to another. This means that in en- and an intermetallic compound, such as iron and table. (c) Unstable gineering practice, the distinction between the cementite; or several metals, such as aluminum, 1*2/Introduction to Alloy Phase Diagrams magnesium, and manganese. These substances constitute the components comprising the system and should not be confused with the various quid) phases found within the system. A system, how- 4 ever, also can consist of a single component, such Solid 2 as an element or compound. Liquid / Solidus Phase Diagrams. In order to record and visual- ize the results of studying the effects of state variables on a system, diagrams were devised to show the relationships between the various I Ot phases that appear within the system under equi- Gas librium conditions. As such, the diagrams are variously called constitutional diagrams, equilib- f rium diagrams, or phase diagrams. A single- Temperature component phase diagram can be simply a one- or two-dimensional plot showing the phase changes in the substance as temperature and/or Fig. 2 Schematic pressure-temperature phase diagram pressure change. Most diagrams, however, are two- or three-dimensional plots describing the phase relationships in systems made up of two or more components, and these usually contain beled fields. Stable equilibrium between any two phases occurs along their mutual boundary, and fields (areas) consisting of mixed-phase fields, as Composition B well as single-phase fields. The plotting schemes invariant equilibrium among all three phases oc- A in common use are described in greater detail in curs at the so-called triple point, O, where the three boundaries intersect. This point also is Fig. 3 Schematic binary phase diagram showing mis- subsequent sections of this Introduction. cibility in both the liquid and solid states System Components. Phase diagrams and the called an invariant point because, at that location systems they describe are often classified and on the diagram, all externally controllable factors named for the number (in Latin) of components are fixed (no degrees of freedom). At this point, in the system: all three states (phases) are in equilibrium, but any The Gibbs phase rule applies to all states of changes in pressure and/or temperature will cause matter (solid, liquid, and gaseous), but when the Number of Name of one or two of the states (phases) to disappear. effect of pressure is constant, the rule reduces to: components system or diagrum Univariant Equilibrium. The phase rule says One Unary that stable equilibrium between two phases in a f=c-p+ 1 Two Binary unary system allows one degree of freedom (f= Three Temary 1 - 2 + 2). This condition, called univariant The stable equilibria for binary systems are sum- Four Quatemary equilibrium or monovariant equilibrium, is illus- Five Quinary marized as follows: Six Sexinary trated as lines 1, 2, and 3 separating the single- phase fields in Fig. 2. Either pressure or tempera- Seven Septenary Number of Number of Degrees of Eight Octanary ture may be freely selected, but not both. Once a components ph~es freedom Equilibrium Nine Nonary pressure is selected, there is only one temperature Ten Decinary that will satisfy equilibrium conditions, and con- 2 3 0 Invariant 2 1 Univariant versely. The three curves that issue from the triple 2 l 2 Bivariant point are called triple curves: line 1, representing Phase Rule. Thephase rule, first announced by the reaction between the solid and the gas phases, J. Willard Gibbs in 1876, relates the physical state is the sublimation curve; line 2 is the melting of a mixture to the number of constituents in the curve; and line 3 is the vaporization curve.
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