Titanium Flashcards
What are the main ores used in Titatium Production?
- Ilmenite
- Rutile
Phase Transformation
Change in crystallographic arrangement of atoms at higher temperatures
When does phase transformation happen for Pure titanium?
882oC
HCP ► BCC
What is a phase?
Any part of a material with a distinct crystal structure and/or chemical composition
What are the 2 designation off Ti alloys and what do they mean?
- Alpha- alloying elements act to stabilise hexagonal alpha phase
- Beda- alloying elements act to stabilise BCC beta phase
Alpha stabilsing elements are
Al, Sn, Zr and O
Beta stabilising elements are
Mo, V, Cr and Nb
What is the strongest Alpha staibilser?
Al. Dissolves in the Ti matrix and acts as a strengthening agent up to 550oC
What happens when you increase the amount of alpha stabilsers?
Increase in transfromation temperature between alpha and beta phase
What do Beta stabilisers do?
- Lower the beta transus temp.
- Provide solid solution strengthening by preferentially dissolving in the beta phase
What are the 2 categories of beta stabilisers?
- Isomorphous- Alloying element and beta phase mutually soluble (V, Mo)
- Eutecoids- Soild state reaction in which 2 new phases are precipated over a range of compositions (Fe, Mn, Cr)
What are (alpha+beta) systems?
Contain both types of stabilising elements below levels where detrimental phases form
What is the Kroll Process?
- Chlorination of the ore to TiCl4
- Purification of TiCl4
- Reduction of TiCl4 to Ti sponge
- Processing of sponge
How is Ti melted?
Electrode arc melting under vacuum (VAR) - Compacts are formed from ti granules and alloying elements then welded together
Problems with VAR?
- Random occurence of high density- From contamination of raw materials
- Random occurence of hard alpha inculsions- From high concentrations of interstitial alpha stabilsers
How to over come VAR problems?
Cold Hearth process
What furnaces are available for cold hearth processes?
- Electron beam
- Plasma torch
Both reduce and even eliminate defect population
Disadvantage of Cold hearth process?
Less efficent in terms of melting capability.
- EB- loses in high vapour pressure elements
- Plasma- Less effective hydrogen removal
How does a cold hearth process work?
- Raw material enter a water cooled copper hearth at one end where they are melted by electrom beam or plasma
- Heating is maintained and HDI and hard alpha particles get trapped in skull
- Molten metal then moves to a cooled copper mold and become an ingot
Slow Cooling rate (air or furnace cool) in heat treatment leads to
Nucleation and diffusion of atoms- widmanstattan microstructure
Fast cooling rates leads to
hexagonal alpha- needle like form
