Novel Heating-Induced Reversion during Crystallization of Al-based Glassy Alloys

Abstract

Thermal stability and crystallization of three multicomponent glassy alloys, Al₈₆Y₇Ni₅Co₁Fe_0.5Pd_0.5, Al₈₅Y₈Ni₅Co₁Fe_0.5Pd_0.5 and Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁, were examined to assess the ability to form the mixture of amorphous (am) and fcc-aluminum (α-Al) phases. On heating, the glass transition into the supercooled liquid is shown by the 85Al and 84Al glasses. The crystallization sequences are [am] → [am + α-Al] → [α-Al + compounds] for the 86Al and 85Al alloys, and [am] → [am + α-Al + cubic Al_xM_y (M = Y, Ni, Co, Fe, Pd)] → [am + α-Al] → [α-Al + Al₃Y + Al₉(Co, Ni)₂ + unknown phase] for the 84Al alloy. The glass transition appears even for the 85Al alloy where the primary phase is α-Al. The heating-induced reversion from [am + α-Al + multicomponent Al_xM_y] to [am + α-Al] for the 84Al alloy is abnormal, not previously observed in crystallization of glassy alloys, and seems to originate from instability of the metastable Al_xM_y compound, in which significant inhomogeneous strain is caused by the mixture of solute elements. This novel reversion phenomenon is encouraging for obtaining the [am + α-Al] mixture over a wide range of high temperature effective for the formation of Al-based high-strength nanostructured bulk alloys by warm working.

Figure 1. Differential scanning calorimetry curves of the melt-spun 86Al, 85Al and 84Al amorphous alloys.

The glass-transition temperatures T_g and peak temperatures of the exotherms are marked.

Figure 2

X–ray diffraction patterns of the (a) 86Al, (b) 85Al and (c) 84Al glasses annealed for 300 s at each peak temperature of the first and the second exothermic peaks shown in Fig. 1, (d) Al₈₅Y₉Ni₄Co₁Fe_0.5Pd_0.5 and (e) Al₈₄Y₉Ni₅Co₁Fe_0.5Pd_0.5 glasses at each peak temperature of their first and second exothermic peaks.

Figure 3

(a) Bright-field TEM image, (b) selected-area electron diffraction pattern of the 85Al alloy annealed for 300 s at a temperature just above the first exothermic peak, and (c) bright-field TEM image, selected-area electron diffraction pattern (d) and enlarged bright-field TEM image (e), of the Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ initially fully glassy alloy annealed for 300 s at 580 K corresponding to the first exothermic peak. Areas A, B and C are identified as α-Al, Al_xM_y compound and residual amorphous phase respectively.

Figure 4

High resolution HREM images (a), (b) and (c) taken from regions A, B and C, respectively, in the enlarged bright-field TEM image shown in Fig. 3(e).

Figure 5

Bright-field TEM image (a), selected-area electron diffraction pattern (b) and HREM image (c) of the initially fully glassy alloy Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ annealed for 300 s at 620 K corresponding to the second exothermic peak (Fig. 1).

Figure 6

Vickers hardness as a function of (a) isochronal annealing temperature for the initially fully glassy 86Al, 85Al and 84Al alloys and (b) time of annealing at the indicated temperatures of the initially fully glassy 84Al alloy.

Figure 7

The relative free energies of the phases relevant for the crystallization of Al₈₄Y₉Ni₄Co_1.5Fe_0.5Pd₁ glass. In this schematic figure, the alloy is treated as a binary (Al + solute) system. The primary (arrow 1) and secondary (arrow 2) crystallization reactions are shown; ultimately (point 3) the stable phases are attained on heating.

Figure 8

Vickers hardness of fully and partially amorphous Al-based alloys as a function of the estimated solute content in the amorphous phase after various annealing treatments.