Transcript Document

Metals – Phase diagram
Property
high stiffness, better toughness, good electrical
conductivity, good thermal conductivity
Why metals have these nice properties
- structures at atomic level
Fig.1.1
1
Metals – Phase diagram
Ways to change the structure
temperature, alloying, chemistry, mechanical
Pure metals and their Alloys
- Gold, silver, and copper may exist in applications
as their pure form, but most of metals are alloyed.
- An alloy is a metal comprised of two or more
elements, at least one of which is metallic. Two
main categories of alloys are: (1) solid solutions and
(2) intermediate phase.
2
Metals – Phase diagram
Solid solutions: one element dissolved in another to
form single-phase solution
Phase-Any homogeneous mass, metal with grains
having same lattice structure
Types: Substitutional and Interstitial
Fig.1.2
- Solid solution alloy structure stronger and harder
3
Metals – Phase diagram
Conditions for substitutional solid solutions possible:
(1) The atomic radii of the two elements similar
(2) Their lattice types must be the same
(3) The lower valency metal becomes the solvent
(4) Their chemical affinity is small
Example: BRASS (ZINC in COPPER)
4
Metals – Phase diagram
Interstitial solid solution:
Atoms of dissolving element fit into vacant spaces
between base metal atoms in lattice structure
- Solute atoms small compared to Solvent atoms
Example:
Carbon dissolved in Iron to form STEEL
5
Metals – Phase diagram
Intermediate phases:
• Every element has a limit for its solubility of
another element
• When element A completely dissolved into
another element B, the whole system is one
phase of that solid solution.
6
Metals – Phase diagram
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.
Intermediate phase
Its properties are between
two pure elements
Here, the system has two elements (A,B) and two
phases: intermediate phase and solid solution (A,B)
7
Metals – Phase diagram
Phase diagram
A means to represent the phase or status of a metal
alloy system with respect to (1) composition and (2)
temperature
P =
f (T, C)
(a) Amount of dissolving element A and amount of
solvent element B ?
(b) Amount of phase 1 and amount of phase 2 ?
8
Metals – Phase diagram
Fig. 1.3 is a copper-nickel alloy system
Fig.1.3
9
Metals – Phase diagram
The following things are known from Fig.1.3
(1) Pure copper melts at 1981 F
(2) Pure nickel melts at 2651 F
(3) The system is a solid solution throughout
(4) Below solidus line – solid
(5) Above liquidus line – liquid
(6) Between, two phases: solid and liquid
10
Metals – Phase diagram
- The overall composition of the alloy (i.e., amount of
copper and amount of nickel) is given by its position
along the horizontal axis.
-The compositions of the liquid and solid phases are
not the same (Cu-Ni ratio in Solid phase NOT
EQUAL to Cu-Ni ratio in Liquid phase)
- Ex 1. 50 % copper-nickel and at 1260 deg C, see
Fig.1.3. Find the compositions of the solid and liquid.
Solution: draw a horizontal line, which intersects the
liquidus and solidus lines, respectively, see Fig.1.3.
11
Metals – Phase diagram
Obtain: 62 % Ni in solid, 36 % Ni in liquid
When reduce temperature at 50-50 point to the solidus line,
we obtain: 50% Ni in solid and 26% Ni in liquid.
The is the result of the assumption made in the phase
diagram, i.e., the equilibrium state; there is sufficient time
given (for diffusion) to the whole system to meet that
which is indicated by the intersection point along the
liquidus.
In practice, there is a situation called “segregation” when
the liquid freezes
12
Metals – Phase diagram
Segregation:
The first liquid to solidify has a composition that is rich in
the metal element with the higher melting point. Then, as
additional metal solidified, its composition is different
from that of the first metal to freeze.
13
Metals – Phase diagram
Tin-lead system – a more complicated phase diagram,
see Fig. 1.4.
Fig.1.4
14
Metals – Phase diagram
New features (Fig.1.4):
(1) Presence of two new solids:  and 
(2) Eutectic point, which has the lowest melting point
(3) Pure tin and lead have the highest melting point; any
of their alloys melt at lower temperature.
Ex 2: determine the compositions in two corresponding
phases for the aggregate composition, 25 %, at
temperature 500 F.
15
Metals – Phase Diagram for Iron and Carbon
1. Iron – Ferrous Metals, Fe
Iron + Carbon = Alloys: Steel and Cast iron
BCC structure
2. Iron – Carbon phase diagram (Fig.2.1)
• Pure iron, melting point 2802 F
• From room temp. to melting, several solid phases
transforms: α -> γ -> δ
16
Metals – Phase diagram for Iron and Carbon
FCC
Eutectic
BCC
Eutectoid
Fe3C
2.1%
0.022%
Handouts 1
17
Metals – Phase diagram for Iron and Carbon
• Iron:
pure iron (99.99 %),
ingot iron (some carbon, 0.1 % impurities),
wrought iron (3% slug with low carbon).
• Solubility of carbon in iron will depend on solid phases
of iron: Ferrite 0.02%; Austenite 2.1 %;
• Steel: 0.02 – 2.1 %;
Cast iron: 2.1-4 %
• Cementite, Fe3C: hard and brittle
• Carbon: an element increasing strength; Fe is soft.
18
Metals – Phase diagram for Iron and Carbon
3. Steel
- Steel is an alloy of iron that contains carbon ranging
by weight between 0.02% and 2.11%
-It often includes other alloying ingredients as well:
manganese, chromium, nickel, molybdenum
- classification of steels:
plain carbon, low-alloy, stainless, tool
19
Metals – Phase diagram for Iron and Carbon
3. Steel
plain carbon steel: low-carbon (<0.2%), mediumcarbon (0.2%< and <0.5%), high-carbon (>0.5%).
Low-alloy steel: Cr, Mn, Mo, Ni, V
Stainless steels: highly alloyed steels, Cr 15%, Ni
Tool steel: highly alloyed steels designed for use as
industrial cutting tools, dies, and molds.
20
Metals – Nonferrous metals
1. Metals
Ferrous and nonferrous
2. Nonferrous
Aluminum, copper, magnesium, nickel, titanium, and
zinc and their alloys.
21
General feature of non-ferrous metals:
(1) Strength is not as good as the steel
(2) Corrosion resistance and/or strength-to-weight
ratios higher
(3) Lower electrical resistance – copper
(4) Higher thermal conductivity – aluminum
(5) Lower melting point – Zinc (die casting)
22
 Super alloys
- Substantial amount of 3 or more metals rather than
one base metal + alloying elements
 Processing of metals
- Shaping- casting, forming, material removal
- Assembly
- Finishing process- electroplating, painting
- Property Enhancement- Cold working, Heat
Treatment
23