Ordering of the bcc Phase in a Mg-Sc Binary Alloy by Aging Treatment
- PDF / 1,058,645 Bytes
- 4 Pages / 593.972 x 792 pts Page_size
- 10 Downloads / 186 Views
ody centered cubic (bcc)-type Mg-Sc alloys have offered new insights in the magnesium alloy field because of their strength, ductility, and shape-memory functionality.[1–9] In a Mg-Sc binary system, a bcc (disorder) phase (b) exists even in the Mg-rich region at high temperature, and the b phase can be quenched to room temperature.[3] The metastable b phase shows drastic age-hardening in the temperature range of 473 K to 673 K.[1,2,8] This drastic hardening is due to the formation of a Widmansta¨tten-like microstructure with needle- or plate-shaped hexagonal close packed (hcp) precipitates in the b phase. Moreover, the metastable disordered bcc phase in the Mg-Sc alloy shows a shape-memory effect and superelasticity in association with a thermoelastic martensitic transformation.[3,6] According to reported Mg-Sc binary phase diagrams, a B2 phase (ordered bcc phase) with a composition
YUKIKO OGAWA and HIDETOSHI SOMEKAWA are with the Research Center for Structural Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] YUJI SUTOU, DAISUKE ANDO, and JUNICHI KOIKE are with the Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted January 9, 2019. Article published online May 10, 2019 3044—VOLUME 50A, JULY 2019
range of around 35 to 50 at. pct Sc exists in a low-temperature region, below around 723 K.[10–12] It is reported that Mg-5 wt pct Sc with a hcp (a) phase shows age-hardening due to the nanosized spherical-shaped Mg-Sc precipitates when it was aged under the stress.[13] Meanwhile, it has not yet been clarified how the B2 phase forms during cooling from the high-temperature b phase region or during aging in the quenched b phase. The purpose of this paper is to report our investigation of the ordering behavior via aging in the b phase of a Mg-Sc alloy. A Mg-Sc ingot was prepared by induction melting. The nominal composition was Mg-20 at. pct Sc. The alloy composition was measured with an electron probe micro analyzer (EPMA) and was 19.7–20.0 at. pct Sc 20 at. pct. The ingot was hot- and/or cold-rolled into a sheet and then annealed at 873 K for 3.6 ks, followed by water quenching. The annealed sample possesses an hcp (a) phase in addition to a b phase, where the volume fraction of the a phase was about 37 pct. The b + a sample was aged at 573 K. Microstructures of the aged samples were observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples were etched with acid solution after mechanical polishing for SEM observation. For TEM sample preparation, the samples were polished mechanically to under 100 lm in thickness, then thinned using a dimple grinder and Ar-ion milling with a beam intensity of 3 to 5 keV. Figure 1(a) shows a bright field image of the b phase in the Mg-Sc alloy obtained by annealing at 873 K for 3.6 ks, followed by water quenching. The selected area electron diffraction (SAED) pattern obtained from the region in (a) is shown
Data Loading...
