Chapter 12 Flashcards
Age hardening
A special dispersion-strengthening heat treatment. By solution treatment, quenching,
and aging, a coherent precipitate forms that provides a substantial strengthening effect. (Also
known as precipitation hardening.)
Artificial aging
Reheating a solution-treated and quenched alloy to a temperature below the
solvus in order to provide the thermal energy required for a precipitate to form
Athermal transformation
When the amount of the transformation depends only on the temperature,
not on the time (same as martensitic transformation or displacive transformation).
Austenite
The name given to the FCC crystal structure of iron and iron-carbon alloys
Avrami relationship
Describes the fraction of a transformation that occurs as a function of
time. This describes most solid-state transformations that involve diffusion; thus martensitic transformations
are not described.
Bainite
A two-phase microconstituent, containing ferrite and cementite, that forms in steels that
are isothermally transformed at relatively low temperatures
Bake-hardenable steels
These are steels that can show an increase in their yield stress as a result
of precipitation hardening that can occur at fairly low temperatures (100°C), conditions that simulate
baking of paints on cars. This additional increase leads to better dent resistance
Cementite
The hard, brittle ceramic-like compound Fe3C that, when properly dispersed, provides
the strengthening in steels.
Coherent precipitate
A precipitate with a crystal structure and atomic arrangement that have
a continuous relationship with the matrix from which the precipitate is formed. The coherent precipitate
provides excellent disruption of the atomic arrangement in the matrix and provides excellent
strengthening.
Dihedral angle
The angle that defines the shape of a precipitate particle in the matrix. The dihedral
angle is determined by the relative surface energies of the grain boundary energy of the matrix
and the matrix-precipitate interfacial energy.
Displacive transformation
A phase transformation that occurs via small displacements of
atoms or ions and without diffusion. Same as athermal or martensitic transformation
Ferrite
The name given to the BCC crystal structure of iron that can occur as or . This is not
to be confused with ceramic ferrites, which are magnetic materials.
Guinier-Preston (GP) zones
Clusters of atoms that precipitate from the matrix in the early
stages of the age-hardening process. Although the GP zones are coherent with the matrix, they are
too small to provide optimum strengthening.
Interfacial energy
The energy associated with the boundary between two phases.
Isothermal transformation
When the amount of a transformation at a particular temperature
depends on the time permitted for the transformation.
Martensite
A metastable phase formed in steel and other materials by a diffusionless, athermal
transformation.
Martensitic transformation
A phase transformation that occurs without diffusion. Same as
athermal or displacive transformation. These occur in steels, Ni-Ti, and many ceramic materials.
Natural aging
When a coherent precipitate forms from a solution treated and quenched agehardenable
alloy at room temperature, providing optimum strengthening.
Nitinol
A nickel-titanum shape memory alloy.
Pearlite
A two-phase lamellar microconstituent, containing ferrite and cementite, that forms in
steels cooled in a normal fashion or isothermally transformed at relatively high temperatures
Precipitation hardening
See age hardening.
Shape-memory effect
The ability of certain materials to develop microstructures that, after
being deformed, can return the material to its initial shape when heated (e.g. Ni-Ti alloys).
Smart materials
Materials that can sense an external stimulus (e.g., stress, pressure, temperature
change, magnetic field, etc.) and initiate a response. Passively smart materials can sense external
stimuli; actively smart materials have sensing and actuation capabilities.
Solution treatment
The first step in the age-hardening heat treatment. The alloy is heated above
the solvus temperature to dissolve any second phase and to produce a homogeneous single-phase
structure.
Strain energy
The energy required to permit a precipitate to fit into the surrounding matrix during
nucleation and growth of the precipitate.
Superelastic behavior
Shape-memory alloys deformed above a critical temperature show a large
reversible elastic deformation as a result of a stress-induced martensitic transformation
Supersaturated solid solution
The solid solution formed when a material is rapidly cooled
from a high-temperature single-phase region to a low-temperature two-phase region without the
second phase precipitating. Because the quenched phase contains more alloying element than the
solubility limit, it is supersaturated in that element.
Tempering
A heat treatment used to reduce the hardness of martensite by permitting the martensite
to begin to decompose to the equilibrium phases. This leads to increased toughness
Time-temperature-transformation (TTT) diagram
The TTT diagram describes the time
required at any temperature for a phase transformation to begin and end. The TTT diagram assumes
that the temperature is constant during the transformation.
Widmanstätten structure
The precipitation of a second phase from the matrix when there is
a fixed crystallographic relationship between the precipitate and matrix crystal structures. Often
needle-like or plate-like structures form in the Widmanstätten structure.
T / F - Strengthening Mechanisms - Cold Work/strain hardening
True
T / F - Strengthening Mechanisms - Grain size control
True
T / F - Strengthening Mechanisms - Alloying (solid solution strengthening)
True
T / F - Strengthening Mechanisms - Dispersion strengthening
True
You must ____ the solubility limit for Dispersion strengthening
Exceeding
growth of the precipitates normally occurs by ___-___
diffusion and redistribution of atoms
long-range
the overall rate, or _____, of the transformation process
depends on both nucleation and growth
kinetics
Nucleation is favored by low T (large ________)
undercooling)
Growth is favored by high T (______ driven)
diffusion
The maximum transformation rate is the sum of the
_______ rate and the _______ rate.
nucleation, growth
Converting to a “time” basis results in the “_______
C-curve
Aluminum alloys are commonly _________ strengthened
dispersion
The form and location of the dispersed phase depends
on the “___” taken
path
Interfacial energy influences…
the shape of the dispersed phase
Precipitation Hardening means…
Dispersed particles impede dislocation motion
T / F Requirements for Age Hardening - The alloy system must display decreasing solid
solubility with decreasing temperature.
True
T / F Requirements for Age Hardening - The alloy system must display increasing solid
solubility with decreasing temperature.
False
T / F Requirements for Age Hardening - The alloy must be quenchable
True
T / F Requirements for Age Hardening - The matrix should be relatively hard and brittle,
and the precipitate should
be hard and brittle.
False
T / F Requirements for Age Hardening - The matrix should be relatively soft and ductile,
and the precipitate should
be hard and brittle
True
T / F Requirements for Age Hardening - A coherent precipitate
must form.
True
Manufacturing benefits of an age hardening alloy
Little or no distortion during aging – can machine to shape in the solutionized stated and then age harden!
Use of an age hardened alloy at elevated temperature Must maintain a ….
low enough temperature to not result in aging
Hypoeutectoid is __ 0.77%C |
<
Hypereutectoid is ___ 0.77%C
>
Heat treat martensite to form ______ martensite
tempered
tempered martensite less _____than martensite
brittle
tempering reduces internal ______caused by quenching
stresses
_______ and ______are important in the transformation of oen solid to another
Nucleation, Growth
Dispersion strengthening takes on many forms name 2
–Precipitation strengthening (Age hardening)
–Eutectoid strengthening
Steel benefits from ___-________ structures
non-equilibrium
Isothermal transformation diagrams allow us to understand the _________ times required
transformation
Some transformations are ___________
diffusionless
From an engineering perspective, Martensite is most useful as a precursor to ________ ________
tempered Martensite
