Transcript L-5: Thermodynamics of Mixtures (Chapter 7)

FE-3: Review of materials science
for the FE exam
Part 2: Thermal processing, Concrete,
Composite materials
• Based on the following:
• ES 260 text: https://www.wileyplus.com/WileyCDA/Section/Fundamentals-ofMaterials-Science-and-Engineering-An-Integrated-Approach-4th-Edition.id810680.html
• FE free Reference Handbook: http://cbt.ncees.org/fe-reference-handbook/
• FE review manual: http://ppi2pass.com/shop/fe-eit-exam/fe-eit-exam-reviewmaterials/fe-review-manual-ferm3.html
• Practice problems: http://ppi2pass.com/shop/fe-eit-exam/fe-eit-exam-reviewmaterials/fe-practice-problem-package.html
Last revised February 22, 2014 by W.R.Wilcox at Clarkson University.
Thermal Processing
Cooling through a phase transition
• If cool very slowly, get equilibrium phases.
– Must be slow enough that a large amount of diffusion has time to occur in both
old and new phases.
– Final product is that given by the phase diagram, with the new phase(s) being of
uniform composition.
• If cool more rapidly, so that diffusion is small in either the new phase(s), the
old phase, or all of these.
– Still get equilibrium phases, but may take time for transformation to be complete.
– Get composition variation in new phase(s).
• Cool very rapidly by quenching in oil or water to room temperature
– Diffusion doesn't occur.
– Can get high-temperature phase.
– Can get non-equilibrium phase that doesn’t show on phase diagram
• Can then heat back up for some time to allow changes to occur (annealing)
• Examples: transformations starting with austenite containing some interstitial
carbon.
• Rapidly cool austenite of
eutectoid composition to a
given temperature.
• Hold at that temperature and
track the amount converted
to pearlite versus time.
Pearlite
Below about 550oC, get a
different arrangement of
cementite and ferrite, a very
fine structure called
"bainite".
bainite
• Very rapid cooling by quenching in a
liquid produces a non-equilibrium phase,
martensite, which is in the form of
platelets with a body-centered tetragonal
structure containing excess interstitial
carbon. The transformation from
austenite to martensite occurs close to
the speed of sound without diffusion.
Many crystal defects with internal
stresses. Very hard and brittle.
Influence of cooling rate
on structure obtained
In tempering, a martensitic
steel is heated back up to
convert some of the martensite
to ferrite and cementite, thereby
relieving the residual stress and
making the steel tougher. The
time and temperature determine
the amount of conversion and
the final properties.
Martensite
But the cooling rate inside an object
is less than at the surface.
Conduction of heat is not
instantaneous, even in metals.
Copied from FE Manual
Because cooling rate is highest at the surface, the final
hardness is highest at the surface.
• Jominy hardenability test:
• A cylindrical steel bar is
quenched at one end with
room temperature water.
• Hardness is measured
versus distance.
Composite materials
• Typically strong particles or
rods dispersed in a ductile
matrix.
• Get combinations of properties
that cannot be obtained with a
single-phase material.
• Many ways to make, e.g.:
– Precipitation during or after
solidification.
– Mixed in the melt prior to
solidification.
– Wind fibers on a mold, add a
liquid polymer or a monomer
that is polymerized by a
chemical reaction.
• Examples: tires, fiberglass,
concrete, thoria dispersed
nickel.
Properties
Concrete
• Made by mixing Portland cement with fine
aggregate (sand), coarse aggregate (gravel) and
water.
• Hardens over time by formation of crystal hydrates
• Can add reinforcing fibers or rods.
Phases present in cured Portland cement
• Detection of x-rays emitted in a scanning electron microscope enables
analysis of composition (acronyms EDS, EDX, XEDS).
• Image is 256 µm x 200µm.
C 3S
C 2S
C 3A
C4AF
gypsum
K2SO4
lime and/or calcite
silica
periclase (with Mg)
C=CaO, S=SiO2, A=Al2O3, F=Fe2O3 all hydrated