Nucleation and Precipitation Strengthening in Dilute Al-Ti and Al-Zr Alloys

Abstract

Two conventionally solidified Al-0.2Ti alloys (with 0.18 and 0.22 at. pct Ti) exhibit no hardening after aging up to 3200 hours at 375°C or 425°C. This is due to the absence of Al 3 Ti precipitation, as confirmed by electron microscopy and electrical conductivity measurements. By contrast, an Al-0.2Zr alloy (with 0.19 at. pct Zr) displays strong age hardening at both temperatures due to precipitation of Al 3 Zr (L1 2 ) within Zr-enriched dendritic regions. This discrepancy between the two alloys is explained within the context of the equilibrium phase diagrams: (1) the disparity in solid and liquid solubilities of Ti in a-Al is much greater than that of Zr in a-Al; and (2) the relatively small liquid solubility of Ti in a-Al limits the amount of solute retained in solid solution during solidification, while the comparatively high solid solubility reduces the supersaturation effecting precipitation during post-solidification aging. The lattice parameter mismatch of Al 3 Ti (L1 2 ) with a-Al is also larger than that of Al 3 Zr (L1 2 ), further hindering nucleation of Al 3 Ti. Classical nucleation theory indicates that the minimum solute supersaturation required to overcome the elastic strain energy of Al 3 Ti nuclei cannot be obtained during conventional solidification of Al-Ti alloys (unlike for Al-Zr alloys), thus explaining the absence of Al 3 Ti precipitation and the presence of Al 3 Zr precipitation.

Grain Refinement during Rapid Solidification of Aluminum–Zirconium Alloys Using Electrospark Deposition

“…Despite the fact that the solute concentration is above the maximum solvus composition of 0.24 at% reported for the metastable AlZr binary system, 28) the present combination of solute concentration and cooling rate suppresses the formation of the ¢A phase, known to act as grain refiner, and a supersaturated Al solid solution possessing a typical columnar solidification structure forms. This is in fact the ideal situation for an aged hardening alloy, 28) but is the worst condition in the situation to develop an alloy for nanostructured solidification grains. Figure 7(a) presents a back scattered micrograph of the Al¹1 at%Zr, showing a band-structure containing precipitates and a zone with a significantly lower precipitate concentration above.…”

Precipitation Evolution and Modeling of Growth Kinetics of L12-structured Al3Zr Particles in Al-0.22Zr and Al-0.32Zr (wt.%) Alloys Isothermally Aged

“…The temperatures employed were 650 and 700 K, ±1K at the times of 4, 12, 24, 100 and 400 h. The aging temperatures and times were adopted because a study in the literature demonstrated that these parameters promote precipitation hardening in hiperperitectic Al-Zr alloys 4 . After the treatments, samples were water-quenched to room temperature.…”

“…In the latter as pylons and missile tail cones 2 . Al-Zr alloys are precipitation hardening and present elevated thermal stability due to the precipitation of nanometer-scale metastable Al 3 Zr L1 2 precipitates 1,3,4 . These particles are coherent, thermodynamically stable 5 and provide hardening even after long times of heat-treatment (400 h) at temperatures of 698 K 6 and nucleate homogeneously in a high number density at the center of the dendrites, where supersaturation is greater 4,6 .…”

“…Al-Zr alloys are precipitation hardening and present elevated thermal stability due to the precipitation of nanometer-scale metastable Al 3 Zr L1 2 precipitates 1,3,4 . These particles are coherent, thermodynamically stable 5 and provide hardening even after long times of heat-treatment (400 h) at temperatures of 698 K 6 and nucleate homogeneously in a high number density at the center of the dendrites, where supersaturation is greater 4,6 . Al 3 Zr precipitates present a series of different morphologies, depending on processing conditions these may be spheroidal [6][7][8] , rod-like 6,8 , petal-like 6,9,10 , cauliflower 6 , cellular 11,12 and disk-like 13,14 .…”

“…The values for r * are calculated in Eq. (4) 26 , where γ = 0.1033 J/m 2 27 is the interfacial energy of the Al 3 Zr L1 2 precipitate with the Al matrix at 650 K, V a is the atomic volume, k = 1.381E-23 J/K is the Boltzman constant 28 , R = 8,314 kJ/mol the universal gas constant 28 , T the absolute temperature and C L12 a (Eq.5) 4 is the Zr concentration in the matrix in equilibrium with the Al 3 This calculation step considers that the metastable L1 2 -structured Al 3 Zr particle has its stoichiometric composition and no other intermetallic compounds consume Zr from the matrix. Also, during nucleation, the Zr concentration at the precipitate/matrix interface is equal to the solute concentration in the matrix at the center of dendrite branches 26 .…”