Titanium Alloys Flashcards
What are different ways of extracting titanium?
➢ Sodium reduction process (hunter process)
➢ Direct oxygen reduction process (electrolytic process)
➢ Kroll Process, 2𝑀𝑔 + 𝑇𝑖𝐶𝑙4 → 2𝑀𝑔𝐶𝑙2 + 𝑇𝑖
What are properties of titanium?
➢ High strength to weight ratio
➢ Low density
➢ High corrosion resistance
➢ Biocompatible (non-toxic and it is not rejected by the body)
What are applications of titanium?
➢ Aerospace Applications: such as engines and airframes.
➢ Chemical Processing: Many chemical processing operations
specify titanium to increase equipment lifetime.
➢ Petroleum: In petroleum exploration and production, flexible
titanium pipe’s light weight, makes it an excellent material for
deep sea production risers.
➢ Automotive applications: Particularly in motorcycling racing, This
area is extremely challenging because of its cost sensitivity.
➢ Consumer products: such as spectacle frames; cameras; watches;
jewelleries and various kinds of sporting goods.
➢ Biomedical field: Such as surgical implements and implants.
➢ Architectural applications: Such as exterior walls and roofing
materials.
What structure occurs in titanium alloys?
➢ Pure titanium crystalline structure undergoes a transformation
from hcp (α – at lower temperature) to bcc (β – at higher
temperature) by increasing the temperature up to 882C (β Tanus
temperature) and the mentioned single- phase regions are
separated by two-phase region of α+β.
➢ Alloying elements in titanium are usually classified in two groups
of α and β stabilizing additions depending on whether increase or
decrease α/β transformation temperature of 882C
What are “a” stabilizers and what do they do?
➢ Substitutional elements such as Al, Sn (Tin), Ga, Ge
➢ Interstitial elements such as O, N and C
➢ Thus, unalloyed titanium and titanium alloys with α stabilizers
(either singly or in combination) are called α- alloys which have
hcp crystalline structure at room temp.
➢ Al is the main alloying addition in this kind of alloys and
increases the transformation temperature.
What characteristics do “a” stabilizers add?
➢ Generally non heat treatable
➢ Medium strength
➢ Good notch toughness
➢ Good high Temperature creep resistance
What are the two types of solid solution strengthening?
(a) Interstitial atom: move into empty part of the lattice structure,
makes it hard for dislocations to move.
(b) Substitutional: atoms are replaced or substituted into empty
atom location, they are bigger, distort the crystal lattice, strain,
inhibit movement of dislocations.
What do “β” stabilizers do?
β stabilizers : make the phase stable to lower temperatures,
(usually not alloyed to sufficient extent to produce “β” phase at room temperature). The main elements are vanadium (V) and molybdenum (Mo).
What are α+β alloys and their features?
➢ α-β alloys support a mixture of α and β at room temperature.
➢ α+β alloys are a very widely used class of Ti alloys. They contain
enough stabilizing elements to permit significant amounts of β
phase to be retained in the microstructure at room temperature
(resulting in the α+β structure).
➢ Aluminium (Al) is added to the alloy as α-phase stabilizer and
hardener due its solid solution strengthening effect. Vanadium (V)
stabilizes ductile β-phase, providing hot workability of the alloy.
➢ Titanium α+β Alloys are heat treatable and have high tensile
strength, and fatigue strength, low ductility, good hot formability
and creep resistance up to 425 ° C.
What are features of metastable β alloys?
➢ Further increased content of - stabilizingelements
* The temperature “martenzite start” of phase transition β → α is
decreased below room temperature.
* Phase α is not formed during quenching.
➢ The strength is produced by: Ageing metastable β phase to
produce some α phase. (Ageing causes precipitation of fine α
phase (dispersion strengthening)).
What are features of stable β alloys?
➢ Even more increased content of β - stabilizing elements
* Beta transus temperature can be decreased below RT
* After cooling to room temperature, β phase remain stable
➢ β alloys are exceedingly formable and they are not suitable for low
temperature applications (unlike α-alloys which are suitable for
cryogenic applications.)
What are heat treatments of titanium alloys?
- Cold water quenching (CWQ)
Holding at 1hr 1066˚C produces an all β structure. Quenching then produces an α’ (titanium martensite structure).
➢ α’ consists of small individual platelets structure. It does not develop the high strengths of Fe – C martensites since interstitial elements are more soluble in the low temperature (hexagonal close packed) phase, than they are in the β phase. - Air Cooling from 1066˚C
➢ Intermediate cooling rate produces “acicular” (basket weave) α
phase
➢ Fatigue cracks may nucleate relatively easily in this structure,
however there is a tortuous path for them to follow if they are to
subsequently grow.
➢ Results in generally good fatigue, and fracture toughness,
properties - Furnace Cooling from 1066˚C
➢ This is close to equilibrium conditions and the resulting
microstructure is a coarse plate-like phase and retained,
intergranular β .
➢ Course grain size results in reduction of strength.
