Phases and the Phase Diagram Pure metallic elements have engineering applications; for example, ultra- high purity copper (Cu) or aluminum (Al) is used

Phases and the Phase Diagram

Pure metallic elements have engineering applications; for example, ultra-high purity copper (Cu) or aluminum (Al) is used to make microelectronic circuitry. In most applications, however, we use alloys. We define an “alloy” as a material that exhibits properties of a metallic material and is made from multiple elements. A plain carbon steel is an alloy of iron (Fe) and carbon (C). Corrosion resistant stainless steels are alloys that usually contain iron (Fe), carbon (C), chromium (Cr), nickel (Ni), and some other elements. Similarly, there are alloys based on aluminum (Al), copper (Cu), cobalt (Co), nickel (Ni), titanium (Ti), zinc (Zn), and zirconium (Zr). There are two types of alloys: single-phase alloys and multiple phase alloys.

PHYSICAL METALLURGY (1) - 2012 R.EQRA

The understanding of phase diagrams for alloy systems is extremely important because there is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram. In addition, phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena

one-component phase diagram (or unary phase diagram) [sometimes also called a pressure–temperature (or P–T) diagram] is represented as a two dimensional plot of pressure (vertical axis) versus temperature ( horizontal axis)

unary phase diagram


Pressure–temperature phase diagram for H2O. Intersection of the dashed horizontal line at 1 atm pressure with the solid–liquid phase boundary (point 2) corresponds to the melting point at this pressure (T 0C). Similarly, point 3, the intersection with the liquid–vapor boundary, represents the boiling point (T 100C).


Schematic unary phase diagram for magnesium, showing the melting and boiling temperatures at one atmosphere pressure. On this diagram, point X is the triple point


BINARY ISOMORPHOUS SYSTEMSPHYSICAL METALLURGY (1) - 2012 R.EQRA

(a) The copper–nickel phase diagram. (b) A portion of the copper nickel phase diagram for which compositions and phase amounts are determined at point B. (Adapted from Phase Diagrams of Binary Nickel Alloys, P. Nash, Editor, 1991. Reprinted by permission of ASM International, Materials Park, OH.)



Figure 10-12


Lever Rule Derivation (mass balance)PHYSICAL METALLURGY (1) - 2012 R.EQRA


Equilibrium Cooling PHYSICAL METALLURGY (1) - 2012 R.EQRA


Schematic representation of the development of microstructure duringthe equilibriumsolidification of a 35wt% Ni–65 wt% Cualloy.


Nonequilibrium CoolingPHYSICAL METALLURGY (1) - 2012 R.EQRA


Schematicrepresentation of thedevelopment ofmicrostructureduring thenonequilibriumsolidification of a 35wt% Ni–65 wt% Cualloy.


Figure 10-17 The change in structure of a Cu 40% Ni alloy during nonequilibrium solidification. Insufficient time for diffusion in the solid produces a segregated structure. Notice the nonequilibrium solidus curve


Nonequilibrium cooling results in an increased temperature range over which liquid and solid are present;

Final solidification occurs at a lower temperature than predicted by the phase diagram;

The final liquid to solidify will be richer in the lower-meltinq-point metal; and since diffusion has not kept pace with crystal growth. There will be a difference in chemical composition from the center to the outside of the grains.

To summarize

The faster the rate of cooling. the greater will be the above effects


Cored structures are most common in as-cast metals

Homogenization

From the discussion of the origin of a cored structure. it is apparent that the last solid formed along the grain boundaries and in the interden dritic spaces is very rich in the lower-melting-point metal. Depending upon the properties of this lower-melting-point metal, the grain boundaries may act as a plane of weakness. It will also result in a serious lack of uniformity in mechanical and physical properties and, in some cases, increased susceptibility to intergranular corrosion because of preferential attack by a corrosive medium. Therefore, for some applications, a cored structure is objectionable


There are two methods for solving the problem of coring

One is to prevent its formation by slow freezing from the liquid, but this results in large grain size and requires a very long time.

The preferred method industrially is to achieve equalization of composition or homogenization of the cored structure by diffusion in the solid state

Diffusion

Diffusion refers to the net flux of any species, such as ions, atoms, electrons, holes and molecules. The magnitude of this flux depends upon the concentration gradient and temperature.


Rate of Diffusion [Fick’s First Law]

The flux J is defined as the number of atoms passing through a plane of unit area per unit time.

Fick’s first law explains the net flux of atoms:

The rate at which atoms, ions, particles or other species diffuse in a material can be measured by the flux J


Composition Profile [Fick’s Second Law]

Fick’s second law, which describes the dynamic, or non-steady state, diffusion of atoms, is the differential equation

Temperature and the Diffusion CoefficientThe kinetics of diffusion are strongly dependent on temperature. The diffusion coefficient D is related

For example


Mechanisms for Diffusion





MECHANICAL PROPERTIES OF ISOMORPHOUS ALLOYS

For the copper–nickel system, (a) tensile strength versus composition, and (b) ductility (%EL) versus composition at room temperature. A solid solution exists over all compositions for this system.



CONGRUENT PHASE TRANSFORMATIONSPHYSICAL METALLURGY (1) - 2012 R.EQRA

(a ) , Using this diagram briefly explain how spreading salt on 0 )32 ( ice that is at a temperature below °C °F can cause the ice to melt.

(b ) At what temperature is salt no longer useful in causing ice to melt?




BINARY EUTECTIC SYSTEMS

The copper–silver phase diagram. [Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 1, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International, Materials Park, OH.]


The lead–tin phase diagram. [Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 3, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by

permission of ASM International, Materials Park, OH .


DEVELOPMENT OF MICROSTRUCTURE IN EUTECTIC ALLOYS

Schematic representations of the equilibrium microstructures for a lead–tin alloy of composition C1 as it is cooled from theliquid-phase region



Schematicrepresentations of theequilibrium microstructuresfor a lead–tin alloy ofcomposition C2 as it iscooled from the liquidphaseregion.



Schematic representations of the equilibrium microstructures for a lead–tin alloy of eutectic composition C3 above and below the eutectic temperature





Schematic representations of the equilibrium microstructures for a lead–tin alloy of composition C4 as it is cooled from the liquid-phase region





Age or Precipitation HardeningAge hardening, or precipitation hardening, is produced by a sequence of phase transformations that leads to a uniform dispersion of nanoscale, coherent precipitates in a softer, more ductile matrix

Step 1: Solution Treatment In the solution treatment, the alloy is first heated above the solvus temperature and held until a homogeneous solid solution α is produced

Step 2: Quench After solution treatment, the alloy, which contains only α in its structure, is rapidly cooled, or quenched. The atoms do not have time to diffuse to potential nucleation sites, so the θ does not form

supersaturated solid solution αss containing excess copper, and it is not an equilibrium structure. It is a metastable structure


Step 3: Age Finally, the supersaturated is heated at a temperature below the solvus temperature. At this aging temperature, atoms diffuse only short distances. Because the supersaturated is metastable, the extra copper atoms diffuse to numerous nucleation sites and precipitates grow. Eventually, if we hold the alloy for a sufficient time at the aging temperature, the equilibrium structure α+θ is produced



Coherent Precipitate Even if we produce a uniform distribution of discontinuous precipitate, the precipitate may not significantly disrupt the surrounding matrix structure (noncoherent)[Figure (a)]. Consequently, the precipitate blocks slip only if it lies directly in the path of the dislocation. When a coherent precipitate forms, the planes of atoms in the crystal structure of the precipitate are related to—or even continuous with—the planes in the crystal structure of the matrix [Figure (b)]. Now a widespread disruption of the matrix crystal structure is created, and the movement of a dislocation is impeded even if the dislocation merely passes near the coherent precipitate. A special heat treatment, such as age hardening, may produce the coherent precipitate.




Nonequilibrium Precipitates during Aging

During aging of aluminum-copper alloys, a continuous series of other precursor precipitate phases forms prior to the formation of the equilibrium phase. At the start of aging, the copper atoms concentrate on {100} planes in the matrix and produce very thin precipitates called Guinier-Preston (GP) zones. As aging continues, more copper atoms diffuse to the precipitate and the GP-I zones thicken into thin disks, or GP-II zones. With continued diffusion, the precipitates develop a greater degree of order and are called θ’. Finally, the stable θ precipitate is produced. The nonequilibrium precipitates—GP-I, GP-II, and θ’ are coherent precipitates. The strength of the alloy increases with aging time as these coherent phases grow in size during the initial stages of the heat treatment. When these coherent precipitates are present, the alloy is in the aged condition. When the stable noncoherent θ phase precipitates, the strength of the alloy begins to decrease. Now the alloy is in the overaged condition. The still provides some dispersion strengthening, but with increasing time, the grows larger and even the simple dispersion-strengthening effect diminishes.


Aging at either 190°C or 260°C is called artificial aging because the alloy is heated to produce precipitation. Some solution treated and quenched alloys age at room temperature; this is called natural aging



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Phases and the Phase Diagram Pure metallic elements have engineering applications; for example, ultra- high purity copper (Cu) or aluminum (Al) is used