Engineering Alloys 307 Lecture 10 Hexagonal Materials David

Engineering Alloys (307) Lecture 10 Hexagonal Materials David Dye Department of Materials, Imperial College Royal School of Mines, Prince Consort Road, London SW 7 2 BP, UK +44 (207) 594 -6811, david. dye@imperial. ac. uk © Imperial College London

Synopsis 2 © Imperial College London • Hexagonal Materials: axial ratios: deformation systems • Effect of Al on twin/slip secondary deformation modes in α-Ti. • Zr: Zircaloy and Zr-2. 5 Nb. Nuclear Applications. Similarity with Ti. ω Phase in Zr. • Be: Properties and uses. Health hazards. Processing • Mg: die casting alloy. Mg sheet. Expansion of use of Mg in the auto industry. Extraction and supply issues. Alloys.

Deformation Systems in Hexagonal 3 © Imperial College London • Predominantly basal <1120>, {0002} slip • But: <5 independent systems for a-slip: no way to accommodate caxis deformation. • Thus: Alternate slip and twin systems System SLIP Plane Direction basal {0001} <1120> TWINS prism 1 {1010} <1120> {1121} pyramidal 1 {1011} <1123> {1012} pyramidal 2 {1011} <1123> {1010} pyramidal 3 {1121} <1123> pyramidal 3 {1122} <1123> CRSS rises

Deformation Systems in Hexagonal 4 © Imperial College London

Twin Systems 5 © Imperial College London

Axial Ratios in Hexagonal Materials 6 © Imperial College London

Axial Ratios Consequence for Modes 7 © Imperial College London • Hexagonal Materials: axial ratios: deformation systems • Ti has a low axial ratio: <c> interplanar spacing is < than <a> interplanar spacing, so <c> slip is relatively easy • Conclusion: as c/a decreases, hexagonal materials become more ductile (? ).

Effect of Alloying: example in Ti 8 © Imperial College London • Ti: usually deforms by prism <a> slip • Effect of Al is to increase favourability of slip of twinning • Also increases strength by Solid Solution strengthening • Increasing Al content also promotes basal slip over prism slip

Effect of changing deformation systems 9 © Imperial College London • Twinning inhibits further slip: lots of cross-hardening between twin and slip systems – Textures develop slower in slipping alloys than twinning alloys – Work hardening is less rapid in slipping alloys

Zirconium 10 © Imperial College London • Two main alloys: Zircaloy and Zr-2. 5 Nb • Poor neutron adsorber (good elastic scatterer): used as a fuel can material and for CANDU pressure tubes • Creep resistance of Zr-2. 5 Nb is good up to ~330 C when extruded • Creep ductility of Zircaloy is high: use as fuel can

Zirconium 11 © Imperial College London

Microstructure of Zr Alloys 12 © Imperial College London

Microstructure of Zr Alloys 13 © Imperial College London Zr-2. 5 Nb As-Extruded + Cold Worked

ω Phase in Zr 14 © Imperial College London • Just as with Ti, there is an ω Phase in Zr, with similar effects. Appears to be common to Zr/Ti – type phase diagrams.

Be 15 © Imperial College London • Almost 0 adsorbtion cross section, almost 0 inelastic scattering, high elastic scattering cross section: used as a neutron reflector • Problem of Berylyosis: asbestosis-like disease affecting ~1% of the population when exposed to Be dust • Very O-affinitive: use attriding rather than plasma spraying to produce powder (avoidance of ~100 nm oxide layer)

Be-Al alloys 16 © Imperial College London • Be also a very lightweight refractory metal: Tm= • A 2 nd phase exists in the Be-Al system, XX • Use Be-Al alloys in rocket engine nozzles

Mg Alloys 17 © Imperial College London • Mg: very low density, but specific stiffness and strength are relatively normal • Problem of ignition with sparks, dust etc.

Mg Alloys 18 © Imperial College London

Mg Alloys 19 © Imperial College London • Major benefit of Mg: low melting point (350 -650 C) and low viscosity in Mg-Al-Zn alloys: good at shape filling and good surface finish • Recyclabe and almost as low-weight as plastics • Good for high-touch cast components such as dashboards in auto, PC casings

World Mg supplies 20 © Imperial College London • Currently rapidly increasing demand for Mg • … but if new mines open, they cannot recoup their cost of capital (current prices too low to justify investment) • … if prices rise, demand will not come through (auto very costsensitive) • … and Chinese production is ramping up, but does not meet quality / trace element requirements • … dilemma! Uncertain as to what will occur

Synopsis 21 © Imperial College London • Hexagonal Materials: axial ratios: deformation systems • Effect of Al, O on twin/slip secondary deformation modes in α-Ti. • Zr: Zircaloy and Zr-2. 5 Nb. Nuclear Applications. Similarity with Ti. ω Phase in Zr. • Be: Properties and uses. Health hazards. Processing • Mg: die casting alloy. Mg sheet. Expansion of use of Mg in the auto industry. Extraction and supply issues. Alloys.