Lecture 6 - CMCs and MMCs Flashcards
If you have a new material, which industries is the best to focus on?
Military, medical, sports since they are willing to pay.
Why is it hard to introduce a new composite to the automotive industri?
Because of the need of mass production.
Why do the transportation industri want composites?
Less weight which gives more fuel savings and improved accelerations.
Why do the wind energy want composites?
The lower the weight the more power can the turbine produce.
What properties does PMC, MMC and CMC have that the material by it self cannot attain alone?
PMC: Increased modulus, yield and tensile strength, creep resistance
MMC: Increased yield and tensile strength, creep resistance.
CMC: Increased fracture strength.
Which 3 industries has the largest growth of composites?
Automotive/transportation, Wind energy and Aerospace.
Comparing MMC and PMC, what are the main disadvantages and advantages?
MMC are more expensive than PMC and the conventional materials they are replacing.
MMC have advantageous properties over PMCs (Operates in a wider range of temperature, do not absorb moisture, have better electrical and thermal conductivity and are resistant to radiation damage) but they are also difficult to fabricate.
MMC can be used up to 1000°C only CMC can be used above that.
what does inert mean and why is that important in composite manufacturing?
Inert means that the materials are chemically compatible and this is important to prevent severe reactions between the components.
What is CTE?
Coefficient of thermal expansion
For CMC CTE can be a problem, why?
The CTE is low, compared to its reinforcement. which can lead to cracks.
which are the typical MMC matrix materials?
Lighter metals: Aluminium, Magnesium, Titanium,
High Temperature applications: Nickel, Cobolt
Övrigt: Intermetallics, Superalloys
Which are the most common lightweight metals used?
Aluminium: is the most common metal matrix material, due to its low density (2.7 g/cm3), its high processability (associated with the low melting temperature of 660°C), and its high ductility (associated with its fcc crystal structure).
Magnesium: is even lower in density (1.7 g/cm3) than aluminium and also has a low melting temperature (650°C), but it suffers from its relatively low ductility (consequence of the hcp crystal structure and the fewer slip systems).
Titanium: has a relatively high density (4.5 g/cm3) and is relatively brittle (due to its hcp structure), but it is still attractive due to its high temperature capabilities (melting temperature: 1668°C).
Titanium is very reactive, what is important to do?
It is important to lead away high temperatures, since the material is more reactive at high temperatures.
Magnesium has HCP crystal structure and fewer slip systems, what does that mean?
It is hard shape
What is thermal conductivity depending on and what does a low value mean?
It is depending on electrons and photons. If the value is low it means that there is a lot of obstacles ( tex in an alloy with different atoms or impurities.)
How much reinforcement is it typically in MMCs?
10-60 vol.%
what is typical for a metallic matrix?
soft and flexible
What is important for the reinforcement in a MMC?
It must have high strength and stiffness
what is needed when it comes to the bond between reinforcement-matrix in MMCs?
The bond must be strong so the load can be transfered from the matrix to the reinforcement
What kind of reinforcement is used in MMCs?
• Two types of particulates
– dispersion strengthened alloys
– large particulate composites (e.g. cermets)
• Fiber reinforcements
– continuous/discontinuous fibers of different materials
Howw do we classify composites?
• Composites – Particle reinforced ~ Large particle ~ Dispersion strengthening – Fiber reinforced ~ Continuous (aligned) ~ Discontinuous (short) = Aligned = Randomly oriented – Structural ~ Laminates ~ Sandwich panels
what is important to remember about the structure when it comes to discontinuous reinforcements?
It will be an anisotropic material due to the alignment of the fibers and this affects the strength
what is it called when the particulate is smaller than 1μm in diameter and what could it be capable of ?
It is called dispersoid and can provide Orowan strengthening.
What is a precipitation?
a solute [löst ämne] dissolved in a metal while both are molten, precipitates as small particles when cooled.
What is a dispersion?
Disperse small, strong particles into a liquid metal, trapping the particles when it is cast in to shape.
What happens if you try to shear a precipitate?
- Large shear stress needed to move dislocation towards precipitate and shear it (works for small precipitates!).
- Dislocation “advances” but precipitates act as “pinning” sites with spacing S.
Is Dispersion strengthened alloys a composite?
- Dispersion strengthened alloys can be considered as composites because there is little or no interaction between the two components and the reinforcement is not soluble in the metal matrix.
- The dispersoids are usually 10-250 nm diameter oxide particles and are introduced by physical means rather than chemical precipitation. They are located within the grains and at grain boundaries but are not coherent with the matrix as in precipitation hardening
- The dispersed particles are sufficiently small in size to hinder dislocation movement and thus improve yield strength as well as stiffness.
- Dispersion strengthened alloys are somewhat weaker than precipitation hardened alloys at room temperature but since overaging, tempering, grain growth or particle coarsening do not occur upon heating, they are stronger and more creep resistant at high temperatures.
What is Cermets?
(actually cemented carbides) are composite materials made of ceramic (cer) and metallic (met) materials that combine the properties of both a ceramic(high temperature resistance and hardness) and those of a metal(such as the ability to undergo plastic deformation.)
One or more carbide compounds bonded in a metallic matrix.
What is the metal used for in cermets?
The metal (mostly Ni, Mo and Co) is used as a binder for an oxide, boride, carbide, or alumina.
Is cermets a MMC or a CMC?
Depending on the physical structure of the material, cermets are considered as metal matrix composites. But cermets are usually less than 20% metal by volume.
What are Cermets used for?
- Tungsten carbide cermets (Co binder) - cutting tools are most common; other: wire drawing dies, rock drilling bits and other mining tools, dies for powder metallurgy, indenters for hardness testers
- Titanium carbide cermets (Ni binder) - high temperature applications such as gas-turbine nozzle vanes, valve seats, thermocouple protection tubes, torch tips, cutting tools for steels
- Chromium carbides cermets (Ni binder) - gage blocks, valve liners, spray nozzles, bearing seal rings
WC-Co,
What do you use them for?
How do you produce it?
What properties does WC and Co have separately?
- Carbides such as WC are used for cutting tool inserts. However, WC is very brittle so it cracks or chips under impact loads. Hence, Co is used as a matrix.
- WC-Co cermets are produced by pressing Co and WC powders into compacts, which are heated above the melting point of Co (liquid phase sintering).
- Upon cooling, the carbide particles become embedded in the solidified Co, which acts as a tough matrix for the WC particles.
- In addition to its strength and toughness, Co is also selected because it wets the carbide particles to give a strong bond.
- Bonding can be enhanced by slight solubility between phases at elevated temperatures used in processing.
What are the 3 different processing techniques for MMCs?
- Liquid state processes
- Solid-state processes
- Deposition processes
What is the DuralcanTM Process and what could be possible problems?
(Liquid state processing)
(Low cost process!)
Al ingot and ceramic particles (usually SiC or Al2O3 of 8−12 μm in size) are mixed and melted. The melt is stirred at a temperature slightly above the liquidus temperature of the alloy to distribute the ceramic particles evenly. After casting, the solidified ingot may also undergo secondary processing by extrusion, forging or rolling.
Possible problems: Reaction of Al and SiC and the formation of brittle compound Al4C3 in the interfacial reaction layer. Al4C3 can be extremely detrimental to the mechanical properties of the MMC.
What is Pressureless) Preform Infiltration ?
Liquid state processing
Infiltration of a preform of fibers or particles with a liquid metal.
Difficult process because of wetting of the ceramic reinforcement with molten metal. When the infiltration of a fiber preform occurs easily (such as with metallic fibers), reactions between the fiber and the molten metal may take place which significantly degrade the properties of the fiber.
