ME 306 Exam 2

4 Strategies for strengthening metals 2. Use a Solid Solution Alloy

Adding a second metal in solid solution increases strength, since metal atoms of different type and/or size help block dislocation movement

Facts about strengthening metal alloys

Adding another metallic element, even if it goes into solid solution, will strengthen the metal, smaller grain size will increase the strength of a metal, lots of small precipitate particles will cause more strengthening that a smaller number of large particles, assuming the total amounts in weight percentage are the same, and cold working generally causes an increase in strength, with very pure metals seeing smaller increases.

What microstructure forms when Austentine is quenched

Martensite

Fatigue Failure

Accounts for a large percentage of mechanical failures

4 Strategies for strengthening metals 4. Cold Work the Metal

Cold working produces large numbers of new dislocations, and these block each other. Dislocation movement is impeded and strength increases

When austentine goes below to eutectoid line of 727C what does it convert to?

Converts to Coarse Pearlite when Austentine is unstable

The recrystallization temperature of a cold worked metal

Decreases as the amount of cold working increases

Mechanism by which elastomers elastically deform

Elastomers consist of amorphous, crosslinked chains that are highly twisted, kinked, and coiled. When stress is applies, the chains untwist, unkink, and uncoil. These deformations are elastic (recoverable) and result in large deformation under relatively low stress (low modulus of elasticity). The behavior also tends to be nonlinear. When stress is released, the chains snap back to their original condition, unless the crosslinks are broken during excessive deformation.

Fatigue failure is not a particularly common type of mechanical failure

False

For metals, isomorphous binary phase diagrams are the most commonly encountered diagrams

False

In general, equilibrium phase diagrams are excellent predictors of results for actual materials heat processing, since equilibrium is usually established very quickly at elevated temperatures

False

In the iron-iron carbide system, cementite (Fe3C) is a true equilibrium compound

False

Once a metal is cold worked, the mechanical properties are permanently changed and cannot be restored back to their original pre-cold-worked values

False

There is no correlation between hardness of a metal and its tensile strength

False

As heat treating temperature increases, the transformation rate

For austentine to pearlite decreases, re crystallization of cold worked brass increases, and the precipitation strengthening of aluminum alloys increases

In order to cold form sheet metal into fabricated shapes, what heat treating schedule would you use and when would you do the cold working?

Heat the alloy to above 500C, probably 500-550C and hold long enough to get 100% a-phase. Quench to RT to freeze in the a-phase. Do any cold working and forming needed, since the alloy will be ductile with all a-phase. Then heat so the alloy is in the a+theta two-phase region. This will probably be in the range of ~250-300C. Hold long enough to get the desired theta-phase size distribution. Higher temperatures will produce faster precipitation, and we need to prevent "over-aging", which will occur if the allot is held too long or at too high a temperature.

Why do brittle materials show a range of fracture strength values and why is this of importance in engineering design?

In brittle materials, fracture occurs via the propagation of cracks or flaws when the stress concentration at the crack tip exceeds some critical stress value. When larger or sharper flaws are present, fracture will occur at lower applied stress, since the stress concentration at the crack tip will be larger. When smaller or blunter flaws are present, there will be less stress concentration, and failure will occur at higher applied stresses.

Mechanisms by which semicrystalline polymers plastically deform

Plastic deformation involves the tilting and deformation of the crystalline blocks to align with the applied stress. The Crystalline blocks then begin to separate into smaller, disengaged blocks of material. These blocks become further separated and disengaged from each other until the blocks are separate entities bonded by the remaining amorphous strands. Failure occurs when these strands are broken.

4 Strategies for strengthening metals 3. Use Precipitation Strengthening

Precipitated particles, whether a second phase in the alloy system or insoluble particles like SiC added to the metal, block dislocation movement

The mechanisms by which semicrystalline polymers elastically deform

Semicrystalline polymers consist of crystalline regions bounded by amorphous chains or polymer strands. In the elastic portion of deformation, amorphous strands stretch elastically, which can provide significant deformation at modest applied stress. The crystalline blocks also stretch elastically and become thicker or longer in the stress direction by stretching the bonds in the crystalline lattice.

4 Strategies for strengthening metals 1. Decrease grain size

Smaller grain size produces more grain boundary area, and grain boundaries inhibit dislocation movement, increasing strength.

If a steel alloy having either pearlitic or bainitic microstructures is heated to, and left at, a temperature below the eutectoid for a sufficiently long period of time this microstructure will form into

Spherodite

Importance of Impact Energy vs. Temperature for 8 plain carbon steels as a function of temperature.

The decrease in impact energy (toughness) is called the ductile-to-brittle transition temperature (DBTT), and low carbon steels are seen to have very high impact energies at temperature above -50C. Medium carbon steels have much lower impact energies and the DBTT is around room temperature and the transition is much less abrupt. Higher carbon steels do not have high impact energies at any temperature and thus exhibit low toughness. We know that higher carbon content produces higher strength and lower ductility, but these curves do not directly tell us that. We can infer lower ductility from the fact that toughness is lower. When we need high toughness, we will need to select lower carbon steels, but this compromises strength.

Why might there by significant scatter in the fracture strength of ceramic materials? And why does fracture strength for a ceramic increase with decreasing specimen size

The main reason there is scatter in the fracture strength data is that there are a variety of flaw sizes that can be present. Some samples will have larger flaws, which will result in lower strength values, implying that not all samples will have identical strength values, since there will be variability in the maximum flaw sizes in the samples. As a result when the population of samples is large, there will be a significant greater probability of there being some abnormally large flaws in some samples, and therefore the strength distribution curve will have a skewed or non-normal distribution of shape. There will be more low strength samples than high strength samples. In addition, physically smaller samples are less likely to have large flaws that would decrease their strength. Physically large samples are more likely to have large flaws. Short chains or ropes, for example, are less likely to have a very weak link and therefore generally fail at higher applied loads than do long chains or ropes.

Which of the following apply to the Titanic Disaster

The prime cause of the sinking was a collision with an iceberg

What two fracture mechanisms are exhibited by brittle polycrystalline materials?

The two fracture mechanisms are intergranular and intragranular or transgranular fracture. Intergranular fracture occurs allong grain boundaries and thus, between grains. Intragranular or transgranular fracture occurs right through the grains.

A slip system is the combination of slip planes and slip directions on those planes

True

Recrystallization of a metal occurs more rapidly at higher temperatures

True

The crsytalline structure of a metal influences whether or not a ductile-to-brittle transition behavior with decreasing temperature occurs

True

The stress level at fatigue failure is usually much lower than the failure stress for a static load

True

Process of Recrystallization of a heavily cold worked metal, and what happens to microstructure, strength, and ductility.

When a metal has been heavily cold worked, it will be stronger and less ductile. Much of the ductility can be restored by recrystallizing the microstructure. By heating to an elevated temperature, new grains can be nucleated and grown. This results in a brand new microstructure and reduces the the dislocation density. This decreases strength and increases ductility. At higher temperatures, nucleation and growth is more rapid, and one needs to exceed a "recrystallization temperature" to cause this change.