Engineering Metals and Alloys
Introduction
 Metals are the most important engineering materials.
 Metals have properties that satisfy a wide variety of design
requirements.
 The properties of metals can be enhanced through heat treatment
 The technological and commercial importance of metals results from the
following general properties possessed by all of the common metals:
 High stiffness and strength. Metals can be alloyed for high rigidity,
strength, and hardness; thus, they are used to provide the structural
framework for most engineered products.
 Toughness. Metals have the capacity to absorb energy better than other
classes of materials.
 Good electrical conductivity. Metals are conductors because of their
metallic bonding that permits the free movement of electrons as charge
carriers.
 Good thermal conductivity. Metallic bonding also explains why metals
generally conduct heat better than ceramics or polymers.
 Metals are converted into parts and products using a variety of
manufacturing processes:
 Cast Metal, in which the initial form is a casting;
 Wrought Metal, in which the metal has been worked or can be worked
(e.g., rolled or otherwise formed) after casting; better mechanical properties
are generally associated with wrought metals compared with cast metals;
 Powdered Metal, in which the metal is purchased in the form of very small
powders for conversion into parts using powder metallurgy techniques.
 Metals are classified into two major groups:
 Ferrous: those based on iron (Fe), steels and cast irons;
 Nonferrous: all other metals, aluminum, copper, titanium…etc.
Alloys and Phase Diagrams
Alloys
 Although some metals are important as pure elements (e.g., gold, silver,
copper), most engineering applications require the improved properties
obtained by alloying.
 Through alloying, it is possible to enhance strength, hardness, and other
properties compared with pure metals.
 Alloy; is a metal composed of two or more elements, at least one of
which is metallic.
 Solid solutions;
 Intermediate phases.
 Solid Solutions; is an alloy in which one element is dissolved in another
to form a single-phase structure. The solvent or base element is metallic,
and the dissolved element can be either metallic or nonmetallic.
Figure 1 shows two forms of solid solutions: (a) substitutional solid solution, Brass is an
example, in which zinc is dissolved in copper and (b) interstitial solid solution, Steel is
an example, in which carbon dissolved in iron.
 The term phase describes any homogeneous mass of material, such as a
metal in which the grains all have the same crystal lattice structure.
 Intermediate Phases; when the amount of the dissolving element in the
alloy exceeds the solid solubility limit of the base metal, a second phase
forms in the alloy.
Phase Diagrams
 Phase diagram; is a graphical means of representing the phases of a metal alloy
system as a function of composition and temperature at atmospheric pressures.
This type of diagram is called a binary phase diagram.
 The best way to introduce the phase diagram is by example showing in Figure 2
that presents one of the simplest cases, the Cu–Ni alloy system is a solid solution
alloy throughout its entire range of compositions ..
Figure 2 shows phase diagram for the copper– nickel alloy system.
 Determining Chemical Compositions of Phases
 Draw a horizontal line at the temperature of interest.
 The points of intersection between the horizontal line and the
solidus and liquidus indicate the compositions of the solid and
liquid phases present, respectively.
 Construct the vertical projections from the intersection points to
the x-axis and read the corresponding compositions.
 Example 1: Determining Compositions from the Phase Diagram
Suppose one wants to analyze the compositions of the liquid and solid phases
present in the copper-nickel system at an aggregate composition of 50%
nickel and a temperature of 1260oC.
 Solution:
A horizontal line is drawn at the given temperature level as shown in Figure 2.
The line intersects the solidus at a composition of 62% nickel, thus indicating
the composition of the solid phase. The intersection with the liquidus occurs at
a composition of 36% Ni, corresponding to the analysis of the liquid phase.
 Determining Amounts of Each Phase
 This is done by the inverse lever rule:
(1) using the same horizontal line as before that indicates the
overall composition at a given temperature, identifying the
distances as CL and CS, respectively (see Figure .2);
(2) The proportion (amount) of liquid phase present is given by
(3) The proportion (amount) of solid phase present is given by
 Example 2: Determining Proportions (amounts) of Each Phase
Determine the proportions of liquid and solid phases for the 50% nickel
composition of the copper–nickel system at the temperature of 1260oC.
 Solution:
Using the same horizontal line in Figure 2 as in previous Example 1, the
distances CS and CL are measured as 10 mm and 12 mm, respectively.
Liquid phase = (62-50)/(62-36) = 0.46 (46%)
Solid phase = (50-36)/(62-36) = 0.54 (54%)
 The Lead (Pb) - Tin (Sn) Alloy System
Figure 3 Shows phase diagram for the lead - tin alloy system.
 Tin–lead alloys have traditionally been used as solders for making
electrical and mechanical connections.
 The phase diagram exhibits two solid phases, alpha (α) and beta
(β). The (α) phase is a solid solution of tin in lead, and the (β)
phase is a solid solution of lead in tin.
 Between these solid solutions lies a mixture of the two solid
phases, (α+β).
 Pure tin melts at 232oC, and pure lead melts at 327oC.
 Eutectic alloy has a particular composition of 61.9%Sn in an alloy
system for which the solidus and liquidus are at the same
temperature of 183oC, which is the lowest melting point for an
alloy system.