Ti alloys Flashcards

1
Q

general characteristics and properties of titanium

A
  • light (not as much as Al and Mg)
  • very strong with half the weight of steel
  • pretty high melting point (similar to Fe)
  • lot thermal conductivity
  • low CTE
  • elastic modulus half of steel but more that Al, Mg
  • excellent corrosion resistance (tendency to passivate as Al)
  • chatodic behaviour in contact with most of metals
  • High static and fatigue strength (especially when metastable b phase is heat treated to produce strengthening precipitates)
  • high reactivity with oxigen at high temperatures
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2
Q

talk about the 2 different crystal structures of titanium

A
  • T < 888°C: alpha - phase featuring an HCP structure
    -HCP structure
    -less ductile
    -not high formability
  • T > 888°C up to melting temperature of 1670°C: beta - phase featuring a
    -BCC
    -a lot of crystal plane allowed for dislocation sliding
    -more ductile
    -negative effect: ability to absorb hydrogen
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3
Q

how alloying elements on Ti are splitted

A

elements are spilt according to their ability to stabilize a (Al equivalent) or b (Mo equivalent) phases

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4
Q

Titanium possible alloys based on stability of different phases

A

✓ Commercially Pure (CP) Ti
✓ a or quasi-a Ti alloys
✓ a+b Ti alloys
✓ metastable b Ti alloys

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5
Q

tell some alpha stibilizer and beta stabilizer elements

A
  • alpha stabilizer:
    Al, O, N, C
  • beta stabilizer:
    Fe, Mn, Cr, Co, Cu, Si, H
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6
Q

possible thermal treatements on Ti alloys

A
  • a+b and b metastable alloys possible to do: solution annealing followed by artificial aging
  • Solutionising in the region where only b phase is stable to produce a
    homogeneous solid solution
  • solutionising just below this region to produce a two-phase structure
  • Rapid cooling to keep a b metastable
    structure
  • Controlled decomposition of the metastable phase by artificial aging to achieve the precipitation of the a phase into fine strengthening particles
  • Fast cooling also promotes the transformation of metastable b into a form
    of soft martensite called a“. On aging a“ transforms as well
  • For CP titanium, only annealing is feasible as a possible thermal
    treatment. Stress relieving, recrystallization annealing are carried out after forming operations.
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7
Q

effect of alpha and beta stabilizer elements

A
  • Main effects of b-stabilizer elements:
    -higher density
    -higher response to thermal treatments
    -higher achievable strength
    -improved formability
  • Main effects of a-stabilizer elements:
    -increased b-transus temperature
    -higher creep strength
    -improved weldability
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8
Q

mechanical properties of commercialy pure Ti alloys

A
  • increasing the concentration of interstitials, the strength increases but ductility and corrosion resistance are depleted
  • For CP titanium, only annealing is feasible as a possible thermal treatment. Stress relieving, recrystallization annealing are carried out after forming operations.
  • Forms of nitriding and carburizing are possible in order to modify only the surface layers of parts
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9
Q

effects of O2 pickup in alpha phase

A

pickup of O2 induces
- embrittlement (loss in ductility)
- stabilizes the a phase
- On the other hand, the b phase also tends to pick up hydrogen. To avoid this, a slightly oxidising environment (to “block” hydrogen on surface) is created

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10
Q

in which field do we carry out hot plastic deformation in Ti alloys

A

beta or alpha+beta

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11
Q

problem of homogeneous beta structure at high temperature

A

homogeneous b structure is prone to marked grain coarsening at high temperature. When possible a mixed a+b structure is preferred to better control the grain size

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12
Q

talk about the oxide formed over 600 degrees in Ti alloys

A

✓ The oxide formed above 600°C is thick and resistant, requiring mechanical removal (machining) or strong pickling
✓ Most importantly, the oxide formed above 600°C is not passivating anymore the underlying metal. This lead to a continuous pickup of O2 from the environment

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13
Q

talk about Ti alloys welding

A
  • problematic because of titanium easy oxidation at high temperature
  • Low thermal conductivity of Ti makes the localized heat input phase quite easy for welding
  • Care has to be paid to avoid interstitial pickup such as O, H, N, C
  • Welding edges have to be accurately prepared to avoid contamination
  • Inert gas shielding not only in the molten region but also in the HAZ until it cools down below 300°C
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14
Q

machining of Ti alloys

A
  • low thermal conductivity
  • high friction coefficient
  • strenght of Ti
  • chip sticks to tool suface leading to tool rapid wear
  • abundand cooling necessary to avoid overheating
  • lower machining efficiency that steels
  • risk of chips burning (as Mg)
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15
Q

applications of Ti alloys

A
  • aerospace and military (e.g. submarines has to be light, corrosion resistant, strong)
  • process industries, oil sector, power production (e.g. heat exchanger despite low thermal conductivity because sometimes we work with corrosive fluids as sea water)
  • boad induustry, marine industry
  • motorsport and automotive (e.g springs because we want them to be strong, light and because Ti has a bigger elastic area in the stress strain curve)
  • biomedical sector (e.g. protheses)
  • sport equipment, architecture, others (e.g. bicycle frames panels over architectural structures)
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