Early stages of Al-nanocrystal formation in Al₉₂Sm₈

Abstract

Several proposals involving solute effects, phase separation or quenched-in nuclei and heterogeneous nucleation have been advanced to account for the high nanocrystal density that evolves during primary crystallization in marginal glass-forming alloys, but recent crystallization measurements and kinetics analyses provide new evidence for the role of the as-quenched structure on nanocrystal synthesis. Here, isothermal microcalorimetry investigations and quantitative electron microscopy measurements including high-resolution imaging and electron spectroscopy analyses were performed on a model system at different temperatures well below the glass transition to monitor the nanocrystallization isothermally as a function of time. From the combined measurements, the size distribution and the transformed fraction can be determined with a high accuracy for extended ranges of temperatures and times. In addition, calorimetric measurements in the glassy, liquid and crystalline states of the model alloy serves to analyze, for the first time, the fragility characteristics of a marginal glass-former that presents an important parameter in the context of the kinetic stability of the material against premature crystallization.

Aluminum nanoscale order in amorphous Al92Sm8 measured by fluctuation electron microscopy

“…2,3,15 At a given annealing temperature, some of those clusters are supercritical and grow into stable crystals, which predicts the observed decrease in nucleation rate under isothermal annealing. 4 Figure 2 shows that the degree of MRO, indicated by the height of the peaks, 16 is reduced by the same isothermal anneal. If the sample is crystallizing, why do we measure less order?…”

“…2,3 Devitrification of Al 92 Sm 8 amorphized by deformation does not proceed by primary Al crystallization. 3 When annealed below T g , the melt-spun material exhibits a steadily decreasing nucleation rate, 4 which suggests heterogeneous nucleation. 5 The nucleation site has so far eluded structural or chemical detection, ruling out common sites like second phase interfaces or large impurity clusters.…”

“…The second peak also covers the Al ͗311͘ at 0.82 Å −1 . Figure 3 shows V͑k͒ simulated for both a 30 Å diameter crystalline Al sphere and icosehedron of 12 Al atoms surrounding a Sm atom 12 using an extension of the Dash et al 13 4 Sm sphere simulations ͑not shown͒ fail to reproduce even the experimental peak positions.…”

Fragility, kinetic stability and phase separations in the undercooled state of bulk glass formers – a case study on metallic model systems

“…shows a small DT K . The correlation of this value to fragility is independent of the type of material and can also be evaluated for marginal glass formers such as Al 92 Sm 8 , since T g was determined by modulation calorimetry and since the respective thermodynamic excess functions were also determined either directly or by applying a complete Kirchhoff cycle [33]. Thus, the following sequence is obtained: DT K (PdCuNiP) = 0.134 < DT K (PdNiP) = 0.138 < DT K (PdCuSi) = 0.150 5 DT K (AlSm) = 0.348.…”

Electron beam induced crystallization of amorphous Al-based alloys in the TEM

“…Upon annealing these alloys develop a high density (O10 21 m K3 ) of pure Al nanocrystals [3,4] that form from material with a higher solute concentration than the eutectic alloy of the system [5] when annealed prior to primary crystallization. These alloys show a steadily decreasing heat flow signal in isothermal calorimetry [3,4,6] and a steadily decreasing nucleation rate [7], suggesting heterogeneous nucleation [8]. Heterogeneous nucleation requires a high density of nanoscale structural precursor sites to account for the large number of nucleation events.…”