MECH 473 Ferrous and Non-Ferrous Materials

Student Name ………………………… Student Number …………………

Assignment #2

Marks shown in brackets ( ) at the end of each question.

Due Friday March30, 2012

  1. What are the principle means of strengthening Copper and its alloys and give an example of a material using these means? (5)

1) Solid solution strengthening, eg. Commercially pure Cu, Cu-Ni alloys have complete solid solubility, Cu-35%Zn, Cu-10%Zn

2) Grain size strengthening – pure Cu

3) Strain hardening – pure Cu, Cu-35%Zn

4) Age hardening – Cu-2%Be

5) Phase change hardening – martensite (Cu-Al) and eutectoid reaction (Cu-Mn)

  1. When copper and its alloys strengthen by cold working how are the crystal and its dislocation process(es) involved in the strengthening? (2)

(Describe the dislocation mechanism involved in cold working Cu and its alloys)

Copper and its alloys have the FCC structure. It work hardens by the dislocations having B = ao/3<110>{111} being able to interact with each other to form a dense, tangled network where their strain fields resist their movement thereby providing strengthening.

  1. What copper alloy is used for thermocouples and why? (1)

Cu 45%Ni, constantan is used for precision standard resistors and rheostats to measure temperature as thermocouples because it has the highest electrical resistivity combined with a very low temperature dependence of resistivity.

  1. What Copper alloy is able to make stiff wires and springs? What are thecurrent and a modern application of it at a very small scale? (1)

An alloy containing 1.8 % Be- 0.25% Co has high hardness and excellent stiffness – so it is used for miniature springs and for current and future MEMS applications.

  1. What are two Cu-based shaped memory alloys and how do they work? (1)

The most effective and widely used Cu alloys include CuAlNi and CuZnAl.

The shape change involves a solid state phase change involving a molecular rearrangement between Martensite and Austenite upona temperature change of only about 10 oC.

  1. What is coring and why does it occur in Cu-Ni alloys? (1)

Coring is segregation of the Ni concentration from the center to edge of the grains produced during solidification due to the low diffusivity of Cu and Ni.

  1. What is the most widely used cast Cu alloy and why? (1)

The three-fives bronze – 5%Sn 5%Zn and 5%Pb – is the most widely used casting alloy – it is a combined brass and Sn bronze – with good mechanical casting and corrosion properties and excellent machinability (due to Pb).

  1. Describe the three steps that would be required to age harden the Cu-2%Be alloys. (3)
  1. Solution Treatment – the alloy is heated above the solvus temperature (above 576 C and usually up to 800 C) into a single phase region of the phase diagram to dissolve any secondary phases such as precipitates. The material is held at this temperature until a homogeneous solid solution is produced.
  2. Quench – the alloy is rapidly cooled so the atoms do not have enough time to diffuse to potential nucleation sites. The alloy remains as a single phase material that is supersaturated with Be. If the material is work hardened, the increase in dislocations density can be used as nucleation sites during aging.
  3. Aged – The alloy is heated to a temperature below the solvus (below 576 C and usually at 400 C) so the atoms can diffuse to numerous nucleation sites to produce precipitates. Ideally, uniform highly dispersed, ultrafine precipitates form to give the best effect in age hardening or precipitate strengthening.
  1. What four conditions must be meet to age harden the Cu-2%Be alloys? (2)

The four conditions that must be satisfied for an alloy to have an age-hardened response during heat treatment.

1) The Cu-2%Be alloy system must display a decreasing solid solubility with decreasing temperature. In other words, the alloy must form a single phase on heating above the solvus line, then enter a two-phase region on cooling.

2) The matrix should be relatively soft and ductile, and the  precipitate must be hard and brittle.

3) The alloy must be quenchable.

4) The  precipitate must have coherence with the Cu matrix.

  1. Other than Cu alloys, what other alloys are age hardenable? (1)

Many important alloys are age-hardenable including stainless steels and alloys based on aluminum, magnesium, titanium, nickel, chromium, and iron.

  1. What alloying addition to Al has very low solubility and how is it used by the electronics industry. (1)

Si has very low solubility in Al. There is no Si-Al phase at 100%Si on the phase diagram. This makes it good for an interconnect of components in devises for the electronics industry.

  1. Why must Mg be added to Al-Si alloys to make them age hardenable? What can be added to Al-Mg alloys to make them age hardenable? (1)

Mg needs to be added to Al-Si alloys to make them age hardenable because an Al-Si precipitate is not formed whereas a Mg-Si precipitate will form enabling aging. Zn can be added to Al-Mg alloys to make them age hardenable.

  1. What is added to Al-Si alloys and how are they heat treated to enhance their strength and toughness. (1)

The addition of Na just before casting allows a hypereutectic alloy to become supercooled, which shifts the eutectic from 11 to 13% Si and causes the Si to form as very fine particles giving high strength and ductility. Rapid cooling gives smaller, more rounded precipitates with greater ductility.

  1. What is the strengthening phase in Al-Cu alloys? Why is it so effective in strengthening these alloys? On what close-packed atomic planes does this phase block the motion of dislocations? Why do the strength of these alloys fall off if they are aged too long? (2)

The  phase is the strengthening phase of the precipitates. They are effective because they are coherent with the matrix. These precipitates block the motion of dislocations on the close-packed {111} planes. Their strength falls off if they are aged too long because of ripening (precipitates get round and large).

  1. Why do Al alloys initially soften during aging and why does prior stretching of an Al alloy enhance its aging response? (1)

Al alloys initial soften because the dislocations are annealed out at the aging temperature. Prior stretching enhances the aging response because the dislocations are nucleation sites for precipitate formation.

  1. What Al alloy is known for superplasticity and why? What can be added to enhance its precipitation hardening (1)

Al-Zn alloys are good for superplasicity because Al can dissolve a high percentage of Zn. Mg, Cu and Cr can be added for precipitate strengthening in Al-Zn alloys.

  1. Why is pure Al used to clad Al alloys? Why is it used to clad steel cables used by the power industry? (1)

Pure Al helps prevent corrosion of Al alloys. It’s used to clad steel cables used for the transmission of electricity.

  1. If an Al alloy corrodes (other than forming its protective oxide), what is/are the mechanism(s) by which it corrodes? (1)

(Describe the dislocation mechanism involved in corroding Al and its alloys.)

Thermal fluctuations or mechanical deformation cause precipitates existing on the surface of an alloy to crack the protective oxide enabling corrosion products to penetrate and corrode the Al alloy.

  1. What is the structure of the corrosion product on the surface of the Al material and why. (1)

The corrosion product is usually a white pit on the surface caused by the dislocations of FCC Al to be able to interact (join each other), which enables the corrosion product to be able to penetrate deep within the material rather than forming a scale like rust similar to a plain carbon steel.

  1. Explain why aluminum alloys containing more than about 17% Mg are not used. Assume that the  phase is an intermetallic compound and a eutectic structure is produced. (1)

When more than 17% Mg is added to Al, a eutectic microconstituent consisting of the  phase and a brittle  phase is produced during solidification as seen in its phase diagram. This eutectic contains

Most of the eutectic is thus the brittle  intermetallic compound and it will likely embrittle the eutectic. The brittle eutectic, which is the continuous microconstituent, will make the entire alloybrittle.

  1. What is a common problem of cast aluminum alloys occurring during solidification and what are two methods by which it can be reduced. (1)

Shrinkage occurs between the dendrite that create pores, which reduces Al’s density and act as defects for failure. They can be reduced by controlling the solidification process to produce smaller dendrites and by compressing the Al part that will squish the pores to reduce their size and increase the density.

  1. The company that you are working for welds aluminum parts together using the GMAW (MIG) process. Currently, they are using CO2 gas to protect the aluminum during the welding process. However, the CO2 gas is too cold and leaves a rough finish on the welds. What gas would you choose to improve the finish on the welds? Why would you choose this particular gas? (1)

Answer: Choose argon gas because it does not cool the weld as quickly. This allows the weld puddle to smooth out more before freezing.

  1. Stainless steels can suffer from “weld decay”. What is it? (1)

Weld decay occurs in the stainless steels and is the precipitation of (CrFe)4C, which contains 70 % Cr, at grain boundaries causing the concentration of Cr in the adjacent grains to fall below 12%, which degrades the corrosion resistance properties of the stainless steels. The optimum temperature for precipitation of (CrFe)4C is around 650 oC, which is attained in the heat affected zone adjacent to a fusion weld.

  1. What are three methods that can be used to eliminate weld decay and why? (1)

The thee methods to eliminate weld decay are to use a low carbon steel, use a higher Cr steel or add W and Nb as additives. The first two methods reduce the C in the steel to help prevent the formation of (CrFe)4C. The second method has sufficient Cr to protect the steel even if (CrFe)4C forms.

  1. Submerged-arc welding is used for the high-output welding of large steel panels (such as ship decks) in the flat position. Describe the Submerged-Arc Welding process. (1)

Submerged-arc welding is both fast and automatic. Heat is supplied by an arc between a bare metal electrode and the work piece while the shielding is provided by a blanket of granular flux.The electrode and the flux are both fed continuously as the arc is traversed along a pre-machined vee interface. The flux melts in the vicinity of the arc and then solidifies to form a protective cover over the cooling weld.The flux is an insulator in the solid state but become highly conductive on melting and thus maintains the submerged arc. High quality welds can be produced at high rates of weld metal deposition. The high deposition rates are achieved with narrow electrodes passing high currents, which results in a relatively coarse columnar structure and an enlarged heat affected zone causing poorer low temperature ductility.

  1. How should the welding rod match the base steel metal that is heat treatable and non-heat treatable in terms of Ni, Mn, Cr and Mo? (1)

In non-heat treatable steels the filler usually has slightly more Mn and Ni.

In heat treatable steels, the Ni, Cr, and Mo content of the filler is usually increased to counteract evaporation losses when passing through the arc.

  1. Draw the microstructure of steel in the fusion zone and heat-affected zone in a weld showing in a) the initial structure at the maximum temperature, b) the structure after cooling of a low-carbon steel and c) the structure after cooling of a high hardenability steel. (3)

  1. By what means does the FIB (Focus Ion Beam) weld materials at the nanoscale? (1)

The metal-organic gas that is introduced into the vacuum chamber is broken down by the focused ion beam, which deposits the metal atoms onto the surfaces being welded.

  1. What is the crystal structure of Mg? What are its close-packed planes and directions? (not required for the assignment but may be required for the 2nd mid-term)

Mg is HCP up to its melting point. Its close packed planes are the (0001) and its close-packed directions are the <1120>.

  1. Mg being a highly reactive metal, how is it protected for open and closed crucibles during melting? (not required for the assignment but may be required for the 2nd mid-term)

It must be covered with a flux during melting.

For covered crucibles the flux is 20% KCl, 50% Mg2Cl, and 15% CaF2.

For open pots the flux is 55% KCl, 34% Mg2Cl, 9% BaCl2 and 2% CaF2.

  1. At what temperatures are Mg alloys solution treated and what happens if the temperature is too high? (not required for the assignment but may be required for the 2nd mid-term)

To dissolve magnesium alloy precipitates, it is solution treated at 390 – 410 C.

If the solution temperature is too high,it will “burn” where low melting grain boundary phases are exuded at the surface.

A grey-black powder appears on the surface

Internal voids form due to evolution of gaseous phases

  1. On a stiffness basis, how does Mg match up with Al and steel? On a mass basis, how does it match?(not required for the assignment but may be required for the 2nd mid-term)

Mg has a modulus of elasticity of 45 GPa – compared to 71 GPa for Al and 200 GPa for steel.

Mg density is 1.8 g/cm – compared to 2.8 g/cm for Al and 7.9 g/cm for steel – on a mass basis, Mg has the greatest stiffness – and steel the least.

  1. By what means can Mg alloys be welded and what’s possible for temporary repairs in the field? (not required for the assignment but may be required for the 2nd mid-term)

Mg must be protected from the atmosphere by an inert gas using a tungsten arc or consumable Mg

It can be welded using a gas torch with suitable flux for temporary repairs in the field.

  1. In Mg-Zn alloys show how much alpha phase exists in the eutectic assuming the maximum solubility of Al in the delta phase is 73%. Is the eutectic alloy ductile? (not required for the assignment but may be required for the 2nd mid-term)

The Mg-Zn alloy is not ductile but brittle because of the delta phase is brittle.

  1. What are the two means to strengthen Mg-Al alloys and at which compositions do they apply? (not required for the assignment but may be required for the 2nd mid-term)

Alloys containing up to 3 wt% Al are solution strengthened

Alloys with 6-9 wt% Al can be precipitation hardened

  1. At what temperatures can wrought Mg alloys be forged into shapes? By what means and temperature are they strengthened and why? (not required for the assignment but may be required for the 2nd mid-term)

All solid solution Mg alloys can be hot forged at 300 – 400 C. Precipitation hardening during subsequent aging at room temperature is required as they are not heat treatable.

  1. In sand cast Mg alloys, what problems can arise due to the reactive nature of Mg and how is it mitigated in a similar way as in Al alloys? (not required for the assignment but may be required for the 2nd mid-term)

The reactive nature of Mg also means that sand cast alloys are subject to microporosity caused by evolution of hydrogen with a consequent deterioration of its mechanical properties. Insoluble gases such as He and Cl are bubbled through the melt before casting to remove reactive gases such as H.

  1. Why are die cast Mg alloys significantly stronger than sand cast alloys and give three uses for them? (not required for the assignment but may be required for the 2nd mid-term)

Die cast alloys are significantly stronger than sand cast alloys as they are not susceptible to microporosity.

They are used for auto wheels, for crankcases for air cooled engines like VWs and for dash boards in GM trucks.

  1. What are the low-temperature and high-temperature phases of Titanium and how can the high temperature phase be partially and fully stabilized to low temperatures? (2)

The low-temperature phase is hcp. The high temperature phase is bcc, which can be stabilized by alloying with alloying additions of Mn Fe Cr Cu Ni H for partially stabilized and Mo V Ta Nb for fully stabilized bcc phase.

  1. What are the two mechanisms of deformation in Ti and what are the slip plane and directionsactive in the hcp material? (2)

Twinning and dislocations are the deformation mechanisms. The slip plane and direction in the hcp material are the (0001) and .

  1. What impurities easily dissolve in Ti and how are they expressed? (2)

O N and C easily dissolve in both hcp and bcc Ti as interstitials and are conveniently expressed in terms of its yield strength at 0.2% offset and in terms of the percent oxygen equivalent, i.e., % O equivalent = (%O) + 2(%N) + 0.67(%C).

  1. What alloying addition to the +Ti alloys is added for high temperature creep resistance and why is it effective? (2)

Small amounts of Si are added to the + alloys to increase high temperature creep resistance due to insoluble silicide phases.

  1. What similar alloys to the Ti alloys are used in the nuclear industry and why? (2)

Zr and its alloys, i.e., Zircaloys (Zircaloy-1. Zircaloy-2 and Zircaloy-4) are similar to Ti and are used as cladding of fuel rods in nuclear reactors because they have very low absorption cross-section of thermal neutrons.

  1. Why is Ta a special material and what new method of processing enables it to be used commercial purposes. Give an example of an application. (2)

Pure Ta is one of the most corrosion resistant materials. It is now being deposited as Ta gas onto material surfaces that require extra-strong protection to form a thin, protective film of Ta.

Its applications include the production of H, biomass to ethanol, oil & gas, etc that involve a highly corrosive environment.

  1. What is the structure of Nb over its entire temperature range and give two methods of how it is strengthened for high-temperatures applications? (2)

Nb is bcc. Nb is solid solution strengthened using the additions of Zr, Mo W or Ti and dispersioned strengthened using ZrO2 ThO2 ZrC and HfC, which strengthen by preventing recrystallization and grain boundary sliding.