Ti alloys Flashcards
general characteristics and properties of titanium
- 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
talk about the 2 different crystal structures of titanium
- 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
how alloying elements on Ti are splitted
elements are spilt according to their ability to stabilize a (Al equivalent) or b (Mo equivalent) phases
Titanium possible alloys based on stability of different phases
✓ Commercially Pure (CP) Ti
✓ a or quasi-a Ti alloys
✓ a+b Ti alloys
✓ metastable b Ti alloys
tell some alpha stibilizer and beta stabilizer elements
- alpha stabilizer:
Al, O, N, C - beta stabilizer:
Fe, Mn, Cr, Co, Cu, Si, H
possible thermal treatements on Ti alloys
- 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.
effect of alpha and beta stabilizer elements
- 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
mechanical properties of commercialy pure Ti alloys
- 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
effects of O2 pickup in alpha phase
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
in which field do we carry out hot plastic deformation in Ti alloys
beta or alpha+beta
problem of homogeneous beta structure at high temperature
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
talk about the oxide formed over 600 degrees in Ti alloys
✓ 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
talk about Ti alloys welding
- 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
machining of Ti alloys
- 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)
applications of Ti alloys
- 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)