Titanium and titanium alloys. Josef Stráský

Transcription

1 Titanium and titanium alloys Josef Stráský

2 Lecture 4: Production technologies, experimental investigation, modern problems Technology Casting, forming Experimental methods Metalography Scanning and transmission electron microscopy X-ray diffraction Phase transformations investigation Mechanical properties and fatigue Titanium aluminides Shape memory effect Biocompatible alloys Ultra-fine grained Ti

3 Casting Liquid titanium is extremely reactive Casting must be done at vacuum of very clean inert atmosphere (He, Ar) Currently, there is no continuous process The bigger batch (size of primer ingot) the better economic efficiency typical ingot size: tones Other option is electric arc melting (feasible only for research and development) Typical casting defects are segregation of either or stabilizing elements Homogenization treatment: C above -transus temperature; hrs in industrial process; in laboratory usually 2-4 hrs

4 Forming Forging and rolling are commonly use to produce rods and sheets (sheets constitute 40% of production) First forming steps are done usually above -transus temperature due to increased formability The temperature of future forming depends on thy particular alloy (and its type), formability and required properties The temperature control is essential due to undergoing phase transformations that may cause degradation of mechanical properties or even embrittlement during forming Precise time and temperature control allows wider use of metastable β- Ti alloys since material properties depend on complete history of material processing Hot working refer to forming above recrystallization temperature (usually it refers to forming above -transus temperature) Cold working is forming below recrystallization temperature (well-bellow -transus temperature, only rarely at room temperature) Stress-relief treatment is required (sometimes interferes with phase transitions)

5 Other technologies Machining, cutting Machining and cutting of Ti alloys is complicated, time demanding and costly Ti alloys are extremely tough causing extreme heat generation and increased tool wear Processes are costly due to low machining/cutting rate, cooling requirements and frequent tool replacements Generated heat may also cause a) contamination, b) microstructural/phase transtion causing deterioration of mechanical properties (Near)-net shape casting Continuously increasing percentage of casted final products Reduction of costly machining and welding Powder metallurgy Advanced technology for net-shape processing; small-scale production and chemical composition variability However, industrial application is limited due to extreme titanium reactivity (requiring vacuum/inert atmosphere) Continuous improvement in available technologies (e.g. field assisted sintering technique) Superplastic forming Ti alloys can be formed superplastically, but usually at high temperatures (>900 C) and using low strain rate (10-4 s -1 ), which significantly limits industrial use Superplasticity depends on grain-size, therefore advances in grain size control (e.g. severe plastic deformation) can make superplastic forming feasible Welding The main issue is atmospheric contamination of welds that are extremely heated Other issue are undergoing phase/microstructural transformations that significantly affect mechanical properties of the weld

6 Experimental techniques metallography (light microscopy) Sample preparation Standard grinding using series of grinding papers Abrasives of finer papers (starting from 800 mesh) tend to dig into the material This can be avoided by short etching by weak Kroll s etchant after each grinding/polishing step Alumina is preferred to diamond pastes for polishing Polishing should be done down to 0.05 μm fineness Hydrogen peroxide and/or very weak solution of hydrofluoric acid can be added during polishing Grinding and polishing is generally much more complicated for β-ti alloys (especially when in β solution treated condition) Etching Kroll s etchant is used to image different phases (i.e. α and β typically to show microstructure of Ti6Al4V alloy) Oxalic acid shall be used to show grains (e.g. in pure Ti or solution treated β alloys)

7 Light microscopy Ti-35Nb-7Zr-5Ta and Ti-35Nb-7Zr-5Ta-1Si alloys Biocompatible, low-modulus alloy Differential interference contrast (Nomarski contrast) Částice (Ti,Zr) 5 Si 3 a relikty z broušení

8 Scanning electron microscopy The main requirement is flat, smooth and clean surface 1. Similar procedure to light microscopy might be used Usually sufficient for Ti-6Al-4V, but not for -alloys 2. Vibratory polishing Excellent labor-saving polishing method Vibratory polisher is currently produced only by Buehler Three steps polishing: Alumina 0,3 m, Alumina 0,05 m and colloidal silica 8 hours (or more) each step Precise cleaning of samples and holders required between steps (or even short etching by diluted hydrofluoric acid) 3. Electrolytic polishing Electrolyte: 5% H 2 SO 4 ; 1,25% HF, in methanol Room temperature, 30 s Results are mixed, strongly depending on alloy

9 Scanning electron microscopy alloy (Ti-6Al-4V) Metastable alloy (Ti LCB) Scanning electron microscopy Z-contrast and phase can be distinguished by back-scattered electrons thanks to different chemical composition (Z-contrast) -phase contains more heavier elements it appears lighter

10 SEM observations of Ti-6Al-7Nb alloy - Biomedical alternative to Ti-6Al-4V alloy - SEM image of duplex structure Z contrast dark alpha interior Al enriched, Nb depleted edge equilibrium composition bright beta - caused by double step annealing ( C) 10

11 EBSD observations Ti-6Al-7Nb alloy LEFT: EBSD image (orientation) image map) RIGHT: grain boundaries with misorientation UP: distribution of grain boundaries Alpha lamellae are not created randomly but follows Burgers relationship between and some misorientations are preferred

12 Transmission electron microscopy Thin foil preparation is essential 1. Electrolytic polishing Electrolyte: 300 ml methanol, 175 ml butanol, 30 ml perchloric acid As low temperature as possible (-50 C) Different phases are polished with different rate Tricky for β-alloys 2. Ion polishing Material removal by Ar ions under small-angle (PIPS) Uniform removal, but smaller area for TEM observations, alost material independent, but time demanding (24 hrs and more) 3. FIB focused ion beam Usually part of scanning electron microscope Area for manufacturing TEM foil can be selected by SEM observations Multi-step process with decreasing energy of FIB leads to high quality foil

13 Transmission electron microscopy Essential for ω-phase observations Devaraj et al., Acta Mat 2012 Ti-9Mo a,b) water quenched from -field; c)-e) 475 C/30 min.; f)-h) C/48 hod.

14 In-situ observations In-situ methods Diferential scanning calorimetry Electric resistivity measurements Microscopic and diffraction methods in in-situ arrangement Ti LCB electric resistivity measurements and differential scanning calorimetry (DSC) I. Dissolution of athermal fáze diffusion-ess process II. Stabilization and growth of isothermal phase diffusion process III. Dissolution of phase IV. Precipitation of phase V. Dissolution of phase VI. Above -transus temperature pure phase

15 Mechanical properties and fatigue Standard tensile tests Ti alloys undergo necking, moreover many β-alloys show work softening samples should be manufactured according to appropriate standards (e.g A5 standard) Increased work hardening paradoxically increase elongation due to avoiding necking Ti and Ti alloys are extremely notch sensitive (!) Uneven surface (groove, scratch) serve as fatigue crack initiation site with fatal effect on fatigue performance Samples must carefully polished (prefereably in longitudial direction) or electro polished to obtain comparable results Fatigue performance can be significantly improved by surface treatment processes Shot-peeining, sand blasting, laser shock processing, ball burnishing etc. Surface nitridation (hard surface layer TiN),

16 Main areas of current research and development Isolation of Ti Complicated Kroll s process causes high-price of titanium Strong incentives for developing new process Casting and alloying No continuous process available; typical batch size: tones low production flexibility New processes (powder metallurgy, magnetic levitation casting) are being developed, however still more expensive Significant improvement would decrease price of titanium leading to massive use in automobile industry

17 Main areas of current research and development New alloys Rapid development mainly in the field of metastable -alloys Elimination of expensive alloying elements Minimization of segregation problems simplifying casting procedure Tailored composition to intended properties/use Thermo-mechanic treatment optimization Thermo-mechanic treatment determines phase composition and microstructure that significantly affect mechanical properties; however these relationships are still not completely qualitatively and quantitatively understood Complete history matters annealing/forming/ageing temperatures and times, and also all heating and cooling rates during production Requires precise production control The effect of ageing on microhardness of LCB alloy

18 Requirements Biocompatible alloys Using biocompatible elements (Ti, Nb, Zr, Ta, Mo) Elimination of toxic elements (V, Sn) Sufficient strength and formability Lowering the elastic modulus Stiff implant causes stress shielding Elastic modulus of bone 30 GPa Elastic modulus Ti-6Al-4V 120 GPa Achievable elastic modulus in metastable beta alloys: GPa Cost is not the key factor due to specialized application that require low amount of material with outstanding properties

19 Biocompatible alloys alloys The most used is Ti-6Al-4V But: vanadium is toxic (but: it is probably not dissolved from the implant) Biocompatible alternative Ti-6Al-7Nb Equivalent properties to Ti-6Al-4V alloy No toxic vanadium Metastable -alloys Commercial alloys TMZF and TNTZ Comparatively low-strength in beta-solution condition, further ageing increases elastic modulus TMZF (Ti-12Mo-6Zr-2Fe) Yield stress: MPa; Ultimate tensile strength (UTS): MPa Elastic modulus: GPa TiOstalloy - TNZT (Ti-35Nb-7Zr-5Ta) Yield stress MPa; UTS: MPa; Elastic modulus: 55 GPa (after quenching from region) Possible improvements Strengthening by intermetallic particles (Ti,Zr) 5 Si 3, TiC, TiN, Employing. pseudoelasticity (martensitic transformation during deformation SIM stressinduced martensite) decreased elastic modulus Employing of ultra-fine grained material increased strength and biocompatibility

20 Shape memory alloys Shape memory effect Martensite transformation Diffusionless, reversible Martensite is created upon cooling Can be deformed Upon heating transforms back to austenite and the original shape is restored Nitinol (Ti-Ni) Martensitic transformation temperature is around room temperature (or 37 C) Temperature can be fine-tuned by Ni content Applicable in blood vessels stents Stent is cooled and deformed, then it is moved to correct position; after heating exactly to 37 C it restore its shape serve as blood vessel reinforcements

21 Titanium aluminides Intermetallic compounds Ti 3 Al ( 2), TiAl( ) High strength at elevated temperatures Excellent creep resistance up to 750 C Currently not widely applied in industry Promising potential to replace nickel superalloys in some parts of airplane engines Cost and weight saving But: very limited formability Can be partly increased by alloying (so-called gamma alloys) Can be improved by sophisticated manufacturing (powder metallurgy, sintering etc.)

22 Ultra-fine grained materials Employing severe plastic deformation (SPD) methods for manufacturing material with high concentration of defects and grain size below 100 nm Sever plastic deformation Deformation of material that does not reduce size of the product (contrary to forging, extrusion, rolling etc.) Material can be deformed repetitively ECAP - equal channel angular pressing HPT - high pressure torsion ECAP-Conform continuous ECAP ECAP HPT ECAP-Conform

23 Ultra-fine grained materials Advantages Increased strength thanks to reduced grain size and increased defect concentration Small grains allow superplastic forming (!) Increased biocompatibility Disadvantages Limited size of final products Technology is currently developed only for CP-Ti and Ti-6Al-4V Manufacturing is expensive and must be rationalized by cuttingedge applications CP-Ti is currently used for dental implants (stents)

24 Lecture 4: Summary Casting, forming are complicated and expensive Precise production control is required for phase composition and microstructure control Microscopic methods require precise sample preparation Shape memory alloys (SMA) Shape memory effect due to martensitic transformation Ti-Ni (nitinol), used in medicine Titanium aluminides High strength at elevated temperatures, creep resistance (up to 750 C) Low formability, developing field Ultra-fine grained materials Severe plastic deformation grain size < 100 nm Increased strength and biocompatibility; modern, fast developing field

25 Titanium and titanium alloys Josef Stráský Thank you! Project FRVŠ 559/2013 is gratefully acknowledged for providing financial support.