GKSS FORSCHUNGSZE ZENTRUM DEVELOPMENT OF RECYCLABLE Mg-BASED ALLOYS: AZ91D AND AZC1231 PHASE INFORMATION DERIVED FROM COOLING CURVE ANALYSIS A. J. Gesing N. D. Reade J. H. Sokolowski C. Blawert D. Fechner N. Hort Mg Technology Symposium, TMS 2010, Seattle University ESING of Windsor ONSULTANTS INC 1
GKSS Motivation FORSCHUNGSZE ZENTRUM There is a wealth of information to be extracted from a simple cooling rate curve, when it is combined with equally simple bulk alloy composition along with micro-structural and micro-analytical data. Such non-equilibrium thermo-analytical data – acquired from a representative sized UMSA test sample under solidification rates matching those typical of commercial casting processes – are useful for both alloy development and for the development of melt treatment, refining and casting processes. We report on the work performed at the University of Windsor, Ontario and GKSS research center, Germany that significantly expanded the capabilities of the UMSA thermo-analytical system, and applied it to re-interpretation of data for two potentially recyclable Mg alloys, AZ91D and AZC1231. University ESING of Windsor ONSULTANTS INC 2
GKSS Contents FORSCHUNGSZE ZENTRUM • Determination of alloy phase composition and weight distribution • UMSA system calibration by pure Mg solidification measurement • Thermal analysis baseline determination • Thermal peak deconvolution • Determination of enthalpy of formation of solid phases • Quantitative evolution of weight fractions of solid phases during solidification • Estimate of the evolution of composition of residual liquid during solidification University ESING of Windsor ONSULTANTS INC 3
GKSS Phases in AZC1231 FORSCHUNGSZE ZENTRUM a phase : HCP-Mg + Al, Zn in solid solution ● Susceptible to corrosion b phase : Mg 17 Al 12 + Al, Zn, Cu in solid solution ● Forms stable passive layer of Al 2 O 3 •MgO ● Partially embeds t phase ● Continuous b phase embedding a and t phases is the most effective topology for corrosion protection. t phase : AlMgZnCu solid solution ● Prevents formation of noble Mg-Cu binary intermetallics ● Reduces electrolytic corrosive activity of Cu Al 8 Mn 5 phase : + Fe and Ni in solid solution ● Prevents formation of noble Al-Fe and Al-Ni binary intermetallics ● Reduces electrolytic corrosive activity of Fe and Ni ● Can be selectively precipitated and settled out of the melt University ESING of Windsor ONSULTANTS INC 4
GKSS FORSCHUNGSZE ZENTRUM Calculation of phase distribution from the average alloy composition and phase micro-composition Phase Alloy Mg Al Zn Mn Si Cu Fe Ni Calculated Average OES alloy elemental composition (wt%) Phase AZ91D 90.32 8.75 0.67 0.20 0.054 0.006 0.002 0.0006 Distribution AZC1231 83.91 11.70 3.04 0.48 0.39 0.47 0.009 0.003 (wt%) Average EDX elemental phase micro-composition (wt%) 86.6 AZ91D 97.22 2.39 0.25 0.07 0.02 0.01 0.01 0.02 a Mg AZC1231 96.16 2.9 0.94 74.6 AZ91D 1.33 41.94 1.07 51.86 1.00 0.41 2.00 0.38 0.1 Al 8 Mn 5 AZC1231 1.44 40.43 0.98 51.68 2.14 0.49 2.83 0.8 AZ91D 13.9 53.19 41.68 4.73 0.12 0.07 0.05 0.13 0.04 b Mg 17 Al 12 AZC1231 51.07 40.02 8.07 0.02 0.02 0.76 0.02 21.8 AZ91D t MgAlZnCu AZC1231 31.22 33.26 11.53 0.68 0.03 22.54 0.05 0.69 1.7 AZ91D 63.38 36.62 0.06 Mg 2 Si AZC1231 63.38 36.62 1.0 University ESING of Windsor ONSULTANTS INC 5
GKSS FORSCHUNGSZE ZENTRUM Comparison of alloy phase distribution Mg 2 Si t Al 8 Mn 5 Mg 2 Si Al 8 Mn 5 0.06% 1.00% 0.11% 0.80% 1.71% b 13.9% b 21.84% a a 74.64% 86.6% AZ91D AZC1231 University ESING of Windsor ONSULTANTS INC 6
SEM-SE 15 kV GKSS FORSCHUNGSZE ZENTRUM Microstructure of AZ91D (1) α Mg, (2) β Mg 17 Al 12 , (3) Al 8 Mn 5 , (5) MgO oxide skins and pores. University ESING of Windsor ONSULTANTS INC 7
SEM-SE 15 kV GKSS FORSCHUNGSZE ZENTRUM Microstructure of AZC1231 (1) α Mg (2) β Mg 17 Al 1 (3) Al 8 Mn 5 (4) τ MgAlZnCu (5) MgO, oxide skins and pores (6) Mg 2 Si University ESING of Windsor ONSULTANTS INC 8
GKSS FORSCHUNGSZE ZENTRUM Commercial purity Mg solidification 3 660 3 100% B mix dT/dt - B mix 2 650 2 B liq 67% B solid F solid F solid [J %] 1 640 1 33% Temperature ( C ) dT/dt (K/s) dT/dt ( K/s ) 0 630 0 0% dT/dt -1 620 -1 -33% B mix dT/dt - B mix -2 610 T -2 -67% -3 600 -3 -100% -10 10 30 50 70 90 110 600 620 640 660 680 time (s) Temperature ( C ) The offset between two baseline curves on a cooling rate vs temperature plot at 660 is 0.3 K/s for pure Mg. We subsequently used this observation in extrapolating the quadratic fit to the solid baseline for the alloy cooling rate curves. University ESING of Windsor ONSULTANTS INC 9
ZENTRUM Pure Mg Thermal Property Data GKSS FORSCHUNGSZE 35 45 Mg H 0 L = 3.429781E-02 T - 5.42735 34 40 C p liquid = 34.309 Cp solid Mg 33 Cp liquid 35 H° solid 32 H° liq 30 31 Cp (J/mole) H 0 (kJ/mole) 25 30 20 C p solid = 1.635961E-08 T 3 29 -2.7174556E-05 T 2 + 0.0251579 T 15 + 19.350967 28 10 27 5 26 s = 5.47017E-06 T 2 + 2.16869E-02 T - 6.94747 H 0 25 0 300 500 700 900 1100 1300 300 500 700 900 1100 1300 Temperature (K) Temperature (K) Radically different C p – temperature trend C p is the slope of H 0 – temperature plot. for solid and liquid Mg translates to D H 0 individual thermal analysis baselines. solidification decreases on super-cooling Step in C p at melting translates to offset in pure Mg. baselines. University ESING of Windsor ONSULTANTS INC 10
GKSS UMSA calibration by enthalpy of FORSCHUNGSZE ZENTRUM commercially pure Mg solidification UMSA setup was calibrated by solidification of a commercially pure Mg sample with known enthalpy of melting. The enthalpy of melting is proportional to the area under the baseline corrected peak on a cooling rate vs time plot. -8.49 kJ/mol D H solidifcation at 924K -349.33 J/g Sample weight 10 g Peak area: ( dT/dt – B mix ) vs t 224.26 K Proportionality factor 15.58 J/K For our UMSA setup, this proportionality (calibration) factor for cooling rate curve is 15.58 J/K at 924 K. We assume that this value is applicable to solidification at lower temperatures corresponding to the solidification range of AZ91D and AZC1231 alloys. University ESING of Windsor ONSULTANTS INC 11
GKSS FORSCHUNGSZE ZENTRUM Calculation of baseline value for a semi-solid mix We assume that the baseline for the semi-solid mix (B mix ) is a linear combination of the baseline values for the solid and for the liquid, weighted respectively by the fractions solid (F S ) and liquid (F L ). An iterative solution is required as calculation of F S and F L depends on the position of the baseline. University ESING of Windsor ONSULTANTS INC 12
GKSS FORSCHUNGSZE ZENTRUM Comparison of Baseline Curves AZ91D and AZC1231 0.5 0.5 AZ91D solidification AZC1231 solidification dT/dt dT/dt 0 B liquid 0 B mix B mix B solid B solid B liq -0.5 -0.5 dT/dt (K/s) -1 -1 dT/dt [K/s] -1.5 -1.5 -2 -2 B solid = -5.480934E-06 (T-660) 2 B solid = -5.411848E-06 (T-660) 2 -2.5 -2.5 - 7.599266E-03 (T-660) - 2.70 - 7.557010E-03 (T-660) - 2.71 B liquid = -6.952019E-03 T + 2.166985 B liq = -7.799606E-03 T + 2.764391E+00 -3 -3 300 350 400 450 500 550 600 650 700 300 350 400 450 500 550 600 650 700 Temperature (c) Temperature [C] University ESING of Windsor ONSULTANTS INC 13
Peak deconvolution GKSS for AZ1231 solidification FORSCHUNGSZE ZENTRUM 2 1.5 dT/dt Deconvolution of dT/dt Deconvolution of B mix Mg 2 Si and a Mg peaks 1.5 B mix t, b and a peaks dT/dt - B mix 1 dT/dt - B mix B Mg2Si 1 B thau B beta 0.5 t 0.5 dT/dt [K/s] dT/dt [K/s] a Mg b a Mg 0 0 -0.5 -0.5 -1 -1 -1.5 -1.5 -2 300 350 400 450 500 400 450 500 550 Temperature [C] Temperature [C] On baseline corrected cooling rate vs temperature plot, peak deconvolution involves determining the secondary peak baseline by regression fitting a single polynomial to the primary peak profile both above and below the region of the secondary peak. University ESING of Windsor ONSULTANTS INC 14
GKSS FORSCHUNGSZE ZENTRUM Enthalpy of formation of solid phases Enthalpy Solidification D H f distribution Phase distribution temperature range Alloy J% wt% [J/g] [C] Phase AZ91D AZC1231 AZ91D AZC1231 AZ91D AZC1231 AZ91D AZC1231 Alloy average 100.0% 100.0% 100.0% 99.76% -298.71 -262.52 650-400 660-345 a Mg 90.15% 77.55% 86.56% 74.46% -311.09 - 272.74 595-400 578-360 b Mg 17 Al 12 8.88% 13.81% 13.93% 21.79% -190.52 -165.98 430-400 410-345 t MgAlZnCu 2.05% 1.71% - 313.76 405-380 Mg 2 Si 5.07% 0.059% 1.00% -1,330 560-500 Al 8 Mn 5 0.97% 1.52% 0.108% 0.80% -2,672 -499.20 650-600 650-600 Enthalpy of formation depends on the composition of the liquid and that of the solid product. Both liquid and solid compositions are different for AZ91D and for AZC1231 resulting in lower absolute D H f values for AZC1231 than for AZ91D. D H f values for t MgAlZnCu, Mg 2 Si and Al 8 Mn 5 are highly negative, suggesting low electrochemical activity for noble components trapped in these phases. University ESING of Windsor ONSULTANTS INC 15
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