Which of atomic bonds are the weakest ones?
There are two major bond types: weak bonds between molecules, and strong bonds between atoms.
Weak bonds - van der Waals bonds
All atoms and molecules have a weak attraction for each other. This is called the van der Waals attraction
All liquids and some solids are formed through this kind of weak bond.
Hydrogen bonds which are commonly referred to in biology, which hold our two strands of DNA together, and help shape proteins into their proper conformation, fall into this category.
Strong bonds - ionic and covalent
When two atoms bond together very strongly they do so via either a covalent bond or an ionic bond.
These bonds are MUCH stronger than the weak van der Waals bonds that help molecules to stick to each other.
Atomic bonds overwiew: ionic: high energy, nondirectional (ceramics)
covalent: variable energy (high in diamond, low in bismuth), directional (semiconductors, ceramics polymer chains)
metallic: variable energy (high in tungsten, low in mercury), nondirectional (metals)
secondary: the smallest energy, directional
Generally, secondary bond is the weakest one.
The strongest bonds to the weakest: ionic > polar covalent > non-polar covalent > hydrogen
Describe factors that have the biggest influence on Young’s modulus.
Generally, Young’s modulus increases with higher melting temperature Tm
Young's Modulus depends only on the material, not its geometry, thus allowing a revolution in engineering strategies.
Metallic bonds have the biggest influence on Young’s modulus. The higher bond energy, the bigger Young’s modulus is.
Where we use superalloys?
They are used in parts of gas turbines which undergo high temperatures and where resistance to corrosion and deformation is needed.
These materials are also used as a replacement for stainless steel in an acid and salt environments. They are also used in many other branches of industry, such as military industry, nuclear, automotive and chemical industries.
What is the basic mechanism of duraluminum strengthening?
(The main alloying constituents of duraluminum are copper, manganese, and magnesium) After subjecting an alloy to supersaturation and aging (precipitation hardening), it has a high mechanical strength. The best mechanical properties are obtained after natural aging.
Which mechanism of steel strengthening increases yield and decreases temperature of transition to brittle state at the same time?
In general, the yield strength increases with strain rate and decreases with temperature. However, the temperature of transition to brittle state decreases at the same time in case of grain refinement. Steel resistivity to brittle fracture, under certain operating conditions, to a large extent relies on many metallurgical factors and grain size is one of them. Steel resistivity to brittle fracture, under certain operating conditions, to a large extent relies on many metallurgical factors and grain size is one of them.
Why low carbon steel has higher ductility than medium carbon steel?
High carbon content indicates larger percent of hard cementite so the strength (but also higher brittleness) increases. When the content of carbon is low, there can be a lot of ferrite and the material is more ductile. That is due to the natural properties of carbon.
What is the basic element that increases steel resistivity to corrosion?
This element is chromium. When the chromium content is close to 10.5% or higher, a thin layer of chromium and iron oxides forms on the surface of steel (it is so called passive layer). It causes increase in chemical potential ie corrosion resistivity. The layer protects the steel like it was a special coating. It must have the ability to recover in case of mechanical damage of the surface. When the content of chromium is low, the layer of oxides is porous and not so cohesive with the material which let the corrosive factor easily get to the surface and cause the corrosion.
Write down the shortest definition of martensite.
Martensite is a metastable phase formed in steel and other materials by diffusionless athermal transformation.
Describe the process of steel toughening.
This is a heat treatment that involves quenching connected with high tempering. Steel is first heated so that some phases get melted and only the needed phase remains. Then, the object is quickly cooled (quenching) and then tempered. This process leads to:
increase in plastic properties
increase in resistivity to tensile
higher resistivity to fracture toughness i.e. the critical stress intensity factor in plane strain or “Rice integral”
Why we should not quench aluminium alloys?
This decreases mechanical properties of aluminum.
What should steels that need high weldability at big constructions (bridges, buildings, pipelines) contain?
Manganese in small amounts (0.8%) improves the weldability of the steel. The most beneficial is manganese content within 0.30-4-0.75% . Silicon (up to 1%) does not influence weldability. They should not contain a lot of carbon as it lowers weldability (or max. 0.22% C). And they can contain some microalloying: V, Al, Ti, Nb i N.
What metals show temperature of transition to brittle state?
Metals that have spatially centered lattice.
Increase in content of pearlite causes…
…increase in hardness but decrease in plasticity.
Which element should be added to steel to get austenite at room temperature?
Manganese lowers the temperature of austenitic transition and in case of content equal about 15% it stabilize and allows getting austenitic structure at normal temperatures. Also nickel lowers the temperature of austenitic transition and quenching rate.
15. Brasses are alloys made of…
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. It may contain additives of other metals such as lead, aluminum, tin, manganese, iron, chromium and silicon.
Describe mechanical properties of ceramics.
Ceramics have low tensile and bending resistivity. Ceramic objects are susceptible to impacts and brittleness, they have low resistivity to mechanical and thermal shocks. Machining is very hard in their case due to high brittleness. However, porous ceramics exhibits high strength 25-1000 Mpa, on par with steel or stone. Engineering ceramic is one of the hardest materials after diamond.
Microstructure imperfections
point defects: vacancies and interstitials
paired anions and cations
dislocations
boundaries imperfections
Refractive index.
The refractive index of the core is slightly higher than the coat. The light beam is moved continuously in the core, because there is total internal reflection of beam which is reflected from the plane of the passage of the core to the shell.
When we use hard grinding wheels?
We use hard grinding wheels in case of soft materials treatment. It is conversely in case of soft grinding wheels. With hard grinding wheels we can perform inter alia profile grinding or wet grinding with the use of a coolant. On the other hand, soft wheels are indispensable when it comes to the treatment of products in case of which it is unacceptable to produce large quantities of heat and in case of big surfaces grinding.
A by-product is formed in case of condensation polymer.
Indicate which fiber reinforced polyester composite has the highest modulus of elasticity. – carbon fibers.
What is the special feature of electrostrictive polymers?
They shows mechanical deformation under the influence of an applied voltage.
What is the characteristic feature of thermosets?
Their final process of forming is irreversible i.e. they can be formed only once.
24. Indicate which type of reinforcement is used in composite materials in order to obtain an isotropic or nearly isotropic material.
Composites reinforced with particles.
25. What are the aims of bionics?
Bionics (also known as biomimicry, biomimetics, bio-inspiration, biognosis, and close to bionical creativity engineering) is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology.