Synthesis of nanocrystalline Zn-22 Pct Al using cryomilling

Abstract

In the present investigation, the synthesis of nanocrystalline Zn-22 pct Al by ball milling was studied. The microstructural evolution during cryomilling and subsequent annealing was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Observations made during the cryomilling of the alloy reveal three findings. First, minimum average grain sizes of about 33 nm for the Al phase and 41 nm for the Zn phase are reached as cryomilling time increases to 16 hours. Second, the morphology of the powders changes from spherical (as-sprayed) to flaky (milled 8 hours) and then back to spherical again (milled 16 hours). Third, the microstructure transforms from two-phase coarse structure (0.8 m, as-sprayed) to bimodal structure (milled 8 hours) and then to a uniform fine-grained structure (milled 16 hours). The minimum grain size characterized by the peak broadening of the XRD patterns is much larger than that reported in previous work on Al and Zn but agrees well with those predicted from the approximate linear relationship between the minimum grain size and the critical equilibrium distance between two edge dislocations in a pileup. The mechanism of grain size refinement is discussed at three different levels: macroscopic level (individual powders), mesoscopic level (individual small fragments), and microscopic level (individual grains). The excellent thermal stability of the milled powders during subsequent annealing has been attributed to three factors: the nature of the eutectoid structure, grain-boundary pining by dispersions, and microporosity in the particles.

Mechanical Behavior of Cryomilled CP-Ti Consolidated via Quasi-Isostatic Forging

“…Microstructure evolution and thermal stability Grain-size analysis via XRD and TEM indicated that the final average grain size obtained after 8 hours of cryomilling is~20 nm, for both liquid-nitrogen and liquid-argon-cryomilling experiments. In related previous studies, [17,26,27] the minimum grain size (d min ) obtainable during mechanical milling was explained and modeled; the dislocation pile-up formation and recovery rates were compared. These suggest that there is an approximately linear relationship between d min and the critical equilibrium distance between two edge dislocations in a pileup, L c ; this was calculated from the expression [28] …”

“…[14] Studies of cryomilling show that the milling time is significantly shorter for cryomilling than it is for mechanical milling, at room temperature. [15][16][17] This behavior is attributed to the suppression of the recovery effect at cryogenic temperatures. [16] Other studies have shown that cryomilled and consolidated nanostructured metals are thermally stable; this stability permits the consolidation of large-scale components such as forgings and rolled plate.…”

Role of Different Fractions of Nano-size SiC and Milling Time on the Microstructure and Mechanical Properties of Al–SiC Nanocomposites

“…Significant peak broadening in both constituents, Al and SiC, occurs during milling. In the case of SiC, this may be related to particle size refinement, while for Aluminum it can be related to build-up of the induced heterogonous strains by an increase in structural defects such as dislocations [18] and formation of secondary grain boundaries causing formation of a nanostructure composite [19]. Figure 7a gives information on the crystallite size of Al in different samples, with variant amounts of reinforcement, as a function of milling time.…”

“…Since then, deformation in extremely small grains occurs by GBs rotation. This is accompanied by a reduction in overall Trans Indian Inst Met dislocation density by annihilation at grain boundaries [18] (dynamic recovery), which leads to a rigorous decline in increasing rate of microstrain. Also, temperature rise effect brings about static recovery and strain relaxation [12,20,25].…”

Processing and microstructural evolution of powder metallurgy Zn-22 Pct Al eutectoid alloy containing nanoscale dispersion particles

“…[3] More recently, this process has attracted considerable interest, primarily because of its potential to generate nanocrystalline and other nonequilibrium structures in large quantities. [4,5,6] In addition to structural refinement, another inherent characteristic associated with milling is the formation of nanoscale dispersions by the promotion of in-situ reactions between the powder matrices and environmental gases such as oxygen, or milling media such as nitrogen. For example, Luton et al [7] demonstrated that a dispersion of nanoscale aluminum oxy-nitride particles formed in Al powders during ball milling in liquid nitrogen (cryomilling).…”