Past Paper Theory MMC's Flashcards
(11 cards)
Describe the advantages and disadvantages of using MMC’s as an alternative to monolithic metals in engineering applications.
Monolithic materials present limitations in achieving a good combination of strength, stiffness, toughness and weight (density).
Composite MMC’s provide significantly improved properties, including high specific strength, specific modulus, damping capacity and wear resistance comped to unreinforced alloys.
Advantages:
Specific strength and stiffness.
Tailorable design.
Fatigue life.
Dimensional stability.
Corrosion resistance (more for CMC and PMC).
Higher operating temperatures (especially against PMC).
Disadvantages: Lack of automation techniques. High cost of fabrication. Complex mechanical characterisation. Complex repair. May not satisfy all required properties. Limited standardisation.
For an Al/Al2O3 system, outline how the relative values of thermal expansion and young’s modulus may change for the monolithic metal compared to the MMC.
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Indicate any difference in performance in the values of Young’s modulus for discontinuous versus continuous reinforcements.
Discontinuous reinforced MMC’s seem to offer a balance between performance and cost. Cost is lower and has good properties (young’s modulus, CTE and wear). Long fibres and other types can be more costly for a slightly better performance but it all depends on the specific application.
Describe how specific modulus, coefficient of thermal expansion and tensile strength may vary for reinforced compared to unreinforced for MMCs.
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For discontinuous as compared to continuous indicate any differences in the specific modulus, thermal expansion and tensile strength.
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Outline how tensile strength and thermal expansion coefficient at elevated temperatures may change for MMC’s compared with the matrix material.
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Indicate the relative advantages and disadvantages of using MMC’s compared to the unreinforced materials in conditions involving impact of solid particles at room temperatures.
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Indicate how additions of ceramic reinforcements to metals may change the mechanical and thermal properties compared to monolithic materials.
Ceramic reinforced metals are embedded with fibres to improve properties, especially in high temperature applications. Relatively hard and brittle, but excellent high temperature properties. This reinforcement also enhances fracture toughness and strength compared to monolithic materials. Poor bonding rather than good bonding is required.
For sliding wear of a Ni-Cr/WC composite in air, describe how frictional heating can cause oxidation.
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Suggest reasons why the oxidation process can increase and reduce wear.
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For Ni-Cr alloys containing 14% and above, describe the possible oxide scales which may form on the surfaces.
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