Fabrication and corrosion resistance of Mg-Zn-Y-based nano-quasicrystalline alloys   Presented by Mainak Saha Department of Metallurgical and Materials Engineering NIT Durgapur

What are quasicrystals? Display a rotational symmetry with no translational symmetry Typically five- or ten-fold symmetry forbidden within conventional crystallography. First obtained as a metastable icosahedral configuration in a rapidly solidified Al-14wt.%Mn alloy, by Dr Dan Shechtman, at USA on 8th August 1984.

What are quasicrystals? Ordered Periodic crystalline  Quasi-crystalline  amorphous QC can have quasiperiodicity along 1,2 or 3 dimensions (at least one dimension should be quasiperiodic) QP QP/P

THE FIBONACCI SEQUENCE The Fibonacci sequence has a curious connection with quasicrystals* via the GOLDEN MEAN () THE FIBONACCI SEQUENCE Fibonacci  1 2 3 5 8 13 21 34 ...  Ratio 1/1 2/1 3/2 5/3 8/5 13/8 21/13 34/21 ...  = ( 1+5)/2 Where  is the root of the quadratic equation: x2 – x – 1 = 0 The ratio of successive terms of the Fibonacci sequence converges to the Golden Mean * There are many phases of quasicrystals and some are associated with other sequences and other irrational numbers

Construction of a 1D Quasilattice B B A a B A B b Rational Approximants B A B B A ba B A B B A B A B bab Each one of these units (before we obtain the 1D quasilattice in the limit) can be used to get a crystal (by repetition: e.g. AB AB AB…or BAB BAB BAB…) B A B B A B A B B A B B A babba 1-D QC In the limit we obtain the 1D quasilattice 2D analogue of the 1D quasilattice Penrose tiling Schematic diagram showing the structural analogue of the Fibonacci sequence leading to a 1-D QC

Inoue’s three empirical rules of QC formation At least three-component system. High negative molar enthalpy of mixing(based on Miedema Model) Atomic radii difference between 2 elements- at least 12% Mg Zn Y Van-der Waals Radius(pm) 173 139 240

Nano-QCs Some common evidences at present. - Known to form in annealed Zr-based and Al-based metallic glasses. - Fabricated in extruded or wrought Mg-based alloys at high temperature. Embedded QCs in a matrix causes significant improvement in mechanical and corrosion properties.

Basic Advantage of quasicrystalline Mg alloy In general, Mg alloys possess poor corrosion resistance. Easily eroded either in acid, neutral or alkali solutions, or even in pure water. Rapidly solidified Mg-Zn-Y alloy bars and ribbons exhibited excellent corrosion resistance.

Experiment The experimental alloys  - produced by a reformed crucible electric resistance furnace Melted under the mixture of SF6/CO2 protective atmosphere, using Mg(99.95%), Zn(99.90%) and Y(99.99%) ingots. Cu(99.99%) and Ni(99.99%) added as powders. Stirring for 2 minutes by impellor at 1073 K and holding for 5 minutes above 1053 K Melt poured and cooled in a wedge-shaped water-cooled copper mold.

Experiment Micro-hardness examined by micro-hardness tester. Electrochemical properties of specimens tested in simulated seawater (2.73% NaCl, 0.24% MgCl2, 0.34% MgSO4, 0.11% CaCl2, 0.08% KCl, 96.5% deionized water, vol. (%)) by electrochemical workstation with a sweep rate of 10 mV.s-1(reference electrode: saturated calomel electrode (SCE)).

Alloy composition(at.%) Middle Tip Mg Zn Y Cu Ni 1# 4# 72.0 26.0 10.0 - 2# 5# 71.0 1.0 3# 6# 0.5

Schematic of wedge-shaped ingot to prepare QCs

Discussion Typical morphology and size of a QC solidified in a cast iron mold -petal-like ,about 12 µm, respectively. Cu and Ni addition to the Mg-Zn-Y – more spherical NQCs with average diameter< 85nm, -probably, due to more grain boundary pinning

TEM image of 4# specimen and SAED pattern of circled region showing a 10-fold symmetry about the central bright spot.

Mg-Zn-Y [1 1 1] [1  0] [ 1 3+ ] [0 0 1] SAD patterns from as-cast Mg23Zn68Y9 showing the formation of Face Centred Icosahedral QC

Successive spots are at a distance inflated by  DIFFRACTION PATTERN 5-fold SAD pattern from as-cast Mg23Zn68Y9 alloy Note the 10-fold pattern  2 3 4 1 Successive spots are at a distance inflated by  Inflationary symmetry

Microhardness of Mg-Zn-Y Nano-QCs

Idea of Mg-Zn-Y NQCs Large negative enthalpy of mixing and/or existence of Frank-Kasper-type-phases - crucial criteria for the formation of NQC phase in any system. Mg-Zn-Y-based QCs just have to Frank-Kasper-type phases Mg-Zn-Y has a certain negative enthalpy of mixing. So theoretically, Mg-Zn-Y-based nano-QCs can be formed in a proper cooling conditions

Idea of Mg-Zn-Y NQCs Additions of Cu and Ni - improves degree of constitutional supercooling of Mg-Zn-Y melts - reduces the crucial criteria radius for forming spherical QCs. With increasing undercooling, Icosahedral clusters found to be nucleating, with very short time for growth.

Electrochemical properties of nano-QC alloys Mg72Zn26Y2 nano-QC alloy shows higher corrosion resistance than industrial pure magnesium. Moreover, Mg71Zn26Y2Cu1 nano-QC has high corrosion resistance in simulated seawater and its corrosion resistance > those of the Mg72Zn26Y2 and Mg71Zn26Y2Cu0.5 Ni0.5 nano-QC

Potentiodynamic plots in aerated seawater at room temperature

Data from Potentiodynamic(Tafel) plot specimen Icorr (microamp/sq. cm) E(mV) Rp(k ohm) Corrosion rate(mpy) Mg 76.16 -1675 6.073 34.8 4# 11.09 -1311 6.925 19.298 5# 2.035 -186.3 14.76 1.522 6# 3.762 -655.4 8.105 3.084

Conclusions The maximum microhardness of NQCs has been dramatically improved to about HV440 which increased by about 280% compared with that of the petal-like QCs fabricated under common cast iron mold cooling conditions. Mg71Zn26Y2Cu1 nano-QC alloy - high corrosion resistance in simulated seawater and its corrosion resistance is much better than those of the Mg72Zn26Y2 and Mg71Zn26Y2Cu0.5 Ni0.5 nano-QC alloys. The key factor for the improved corrosion resistance of magnesium alloy can be ascribed to the formation of nano-QCs and Mg-Y intermetallics. The corrosion resistance of Mg71Zn26Y2Cu0.5 Ni0.5 nano-QC alloy < Mg71Zn26Y2Cu1 nano-QC alloy.

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