Material Science-Exam II

diffusivity

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dislocation

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ductility

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elasticity

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impurity

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interstitial

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lattice boundary

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mechanical

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strength

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stress concentration

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surface

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grain growth

After recrystallization is complete, the strain-free grains will continue to grow if the metal specimen is left at the elevated temperature; this phenomenon is called grain growth. Grain growth is the increase in average grain size of polycrystalline materials, which proceeds by grain boundary motion.

ductile to brittle transition

At higher temperatures the CVN energy is relatively large, in correlation with a ductile mode of fracture. As the temperature is lowered, the impact energy drops suddenly over a relatively narrow temperature range, below which the energy has a constant but small value; that is, the mode of fracture is brittle.

case harderning

Case hardening by using a carburizing or nitriding process whereby a component is exposed to a carbonaceous or nitrogenous atmosphere at an elevated temperature. A carbon- or nitrogen-rich outer surface layer is introduced by atomic diffusion from the gaseous phase. The case is normally on the order of 1 mm deep and is harder than the inner core of material.

concentration gradient

Concentration gradient is the slope of the concentration profile curve at some specific point. how much of the solute specus you have on one side at the diffusion interface compare to how of the same species you have on other sides.

stable

Cracks in ductile materials are said to be stable (i.e., resist extension without an increase in applied stress).

diffusion flux

Diffusion flux is defined in terms of mass of diffusing species, cross-sectional area, and time.

ductile

Ductile metals typically exhibit substantial plastic deformation with high energy absorption before fracture. Ductile metals are normally tougher than brittle ones.

ductility

Ductility is a measure of the degree to which a material will plastically deform by the time fracture occurs.

recovery

During recovery, some of the stored internal strain energy is relieved by virtue of dislocation motion (in the absence of an externally applied stress), as a result of enhanced atomic diffusion at the elevated temperature.

recrystallization

During recrystallization: A new set of strain-free and equiaxed grains form that have relatively low dislocation densities. The metal becomes softer, weaker, and more ductile.

corronsion

Failure that occurs by the simultaneous action of a cyclic stress and chemical attack is termed corrosion fatigue.

fatigue

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Under these circumstances it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load. (usually brittle like even made by ductile material)

Flick's first law

Fick's second law—diffusion equation for nonsteady-state diffusion (in one direction) For steady-state diffusion, diffusion flux is proportional to the negative of the concentration gradient according to Fick's first law.

unstable

For brittle materials, cracks are unstable—that is, crack propagation, once started, will continue spontaneously without an increase in stress level.

ductile

For ductile metals, two tensile fracture profiles are possible: Necking down to a point fracture when ductility is high, and Only moderate necking with a cup-and-cone fracture profile , when the material is less ductile.

transgranular

For most brittle crystalline materials; This type of fracture is said to be transgranular (or transcrystalline), because the fracture cracks pass through the grains.

nonsteady state diffusion

For nonsteady-state diffusion, there is a net accumulation or depletion of diffusing species and the flux is dependent on time. Concentration of solute atoms of any point in the material changes with time.

fatigue strength

For these materials, the fatigue response is specified as fatigue strength, which is defined as the stress level at which failure will occur for some specified number of cycles (e.g., 10E7 cycles).

Hooke'slaw

Hooke's law relationship between engineering stress and engineering strain for elastic deformation (tension and compression)

moment of inertia

I or J

intergranular

In some alloys, crack propagation is along grain boundaries; this fracture is termed intergranular.

fatigue life

It is the number of cycles to cause failure at a specified stress level, as taken from the S-N plot

yeilding

It is therefore desirable to know the stress level at which plastic deformation begins, or where the phenomenon of yielding occurs.

creep

Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses. Deformation under such circumstances is termed creep. Defined as the time-dependent and permanent deformation of materials when subjected to a constant load or stress, creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. Occur when temperature bigger than 0.4 Tm.

alloys

Most familiar metals are not highly pure; rather, they are alloys, in which impurity atoms have been added intentionally to impart specific characteristics to the material.

carburizing

One way this may be accomplished is by increasing the surface concentration of carbon in a process termed carburizing; the steel piece is exposed, at an elevated temperature, to an atmosphere rich in a hydrocarbon gas, such as methane (CH4).

cup and cone

Only moderate necking with a cup-and-cone fracture profile , when the material is less ductile.

area

Percent cold work dependence on original and deformed cross sectional areas.

Poisson's ratio

Poisson's ratio equal strain transverse over axial strain

stress concentration

Sharp corners may also act as points of stress concentration and should be avoided when designing structures that are subjected to stresses. Fatigue cracks normally nucleate on the surface of a component at some point of stress concentration.

fracture

Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material. Fracture can also occur from fatigue (when cyclic stresses are imposed) and creep (time dependent deformation, normally at elevated temperatures).

shot peening

Small, hard particles (shot) having diameters within the range of 0.1 to 1.0 mm are projected at high velocities onto the surface to be treated. The resulting deformation induces compressive stresses to a depth of between one-quarter and one-half of the shot diameter. To improve fatigue life.

Flick's second law

Solution to Fick's second law for the condition of constant surface concentration (for a semi-infinite solid)

strain hardening

Strain hardening is just the enhancement in strength (and decrease of ductility) of a metal as it is plastically deformed.

Rockwell

The Rockwell tests constitute the most common method used to measure hardness. Rockwell is based on the difference in indentation depth from the imposition of minor and major loads.

activation energy

The activation energy may be thought of as that energy required to produce the diffusive motion of one mole of atoms.

steady state diffusion

The diffusion condition for which the flux is independent of time is known as steady state. No change in the concentration of the diffusing species with time.

diffusion coefficient

The magnitude of the diffusion coefficient is indicative of the rate of atomic motion and depends on both host and diffusing species as well as on temperature.

phase slip

The motion of dislocations in response to an externally applied shear stress is termed slip. Slip plane: crystallographic plane on which dislocation line traverses.

interstitial diffusion

The second type of diffusion involves atoms that migrate from an interstitial position to a neighboring one that is empty. This mechanism is found for interdiffusion of impurities such as hydrogen, carbon, nitrogen, and oxygen, which have atoms that are small enough to fit into the interstitial positions. Faster than substitutional/vacancy process.

modulus of elesticity

The slope of this linear segment corresponds to the modulus of elasticity E. This modulus may be thought of as stiffness, or a material's resistance to elastic deformation.

wear

The wear resistance of a steel gear is to be improved by hardening its surface.

thermal fatigue

Thermal fatigue is normally induced at elevated temperatures by fluctuating thermal stresses; mechanical stresses from an external source need not be present. Environmental effects.

strain

True strain is equal to the natural logarithm of the ratio of instantaneous and original specimen lengths..

stress

True stress is defined as the instantaneous applied load divided by the instantaneouscross-sectional area.

fracture

Two fracture modes are possible: ductile and brittle

yield strength

Yield strength is indicative of the stress at which plastic deformation begins. For most materials yield strength is determined from a stress-strain plot using the 0.002 strain offset technique.

distortion

a change in the shape, size, or position of a place when it is shown on a map

subsitutional

atoms move from one crystal lattice to another 1. need to be enough "activation energy" provided by thermo vibration present 2. there needs to be vacancies

cold working

because the temperature at which deformation takes place is "cold" relative to the absolute melting temperature of the metal, cold working. Most metals strain harden at room temperature. Impact of cold work: yield and tensile strength increase, ductility decrease.

malleability

compressive force and stress (heating up , so metal is easy to bend) As with modulus of elasticity, the magnitudes of both yield and tensile strengths decline with increasing temperature; just the reverse holds for ductility—it usually increases with temperature.

toughness

deform plastically and to adsorb energy in the process before fracture, mostly associated with impact and shock load. (dependent in temperature)- area under stress strain curve.

stiffness

enable a material to endure high stress without great strain, resistance to any deformation type.

fatigue toughness

fracture toughness, a property that is a measure of a material's resistance to brittle fracture when a crack is present.

grain boundaries

grain boundaries act as "barriers" to dislocation motion.

hardness

hardness, which is a measure of a material's resistance to localized plastic deformation (e.g., a small dent or a scratch). Resistance to permanently indenting the surface.

Sharpy-Izod

the Charpy and Izod, were designed to measure fracture toughness and are still used to measure the impact energy. For both Charpy and Izod, the specimen is in the shape of a bar of square cross section, into which a V-notch is machined. The load is applied as an impact blow from a weighted pendulum hammer that is released from a cocked position at a fixed height h. The primary difference between the Charpy and Izod techniques lies in the manner of specimen support. Charpy V-notch (CVN) technique is most commonly used in the United States.

ductility

the measure of degree to which a metal will "plastically" deform by the time fracture occurs. Metal is considered to be ductile if % elongation is bigger than 5%.

diffusion

the migration of atoms from a region of high concentration to a region of low concentration which tends to make the composition more uniform though out "mass transport by random atomic motion".

brittle

there is normally little or no plastic deformation with low energy absorption accompanying a brittle fracture. For brittle fracture, the fracture surface is relatively flat and perpendicular to the direction of the applied tensile load.