L Journal of Alloys and Compounds 293–295 (1999) 872–876 Ga site occupancy in HDDR-treated Nd Fe B-based alloy by XAFS 2 14 a, a ,1 b c c * Makoto Matsuura , Ahmad Ashfaq , Masaki Sakurai , Tosiro Tomida , Naoyuki Sano , d Satoshi Hirosawa a Miyagi National College of Technology, Nodayama Natori Miyagi 981-1239, Japan b Institute for Materials Research, Katahira-cho Sendai 980, Japan c Sumitomo Metal Industries Ltd., 1 –8 Fuso-cho, Amagasaki 660, Japan d Sumitomo Special Metals Co., Ltd., 2-15-17, Mishima-gun, Osaka, 618, Japan Abstract The changes of local structure for Ga in Nd Fe Co Ga Zr B during hydrogenation, disproportionation, desorption and 13.0 68.2 10.8 1.0 0.1 6.9 recombination (HDDR) process are studied by fluorescence XAFS in order to understand the evolution of magnetic anisotropy by HDDR process. Comparing the observed Ga XAFS data with the calculated ones, using ab initio calculation (FEFF), Ga is proved to occupy preferentially an Fe(c) site in the Nd Fe B structure before disproportionation and dissolves in a-Fe after the disproportionation. In the 2 14 recombined state Ga redistributes into two phases, i.e. Nd Fe B and a Nd-rich phase. The behavior of Ga during the HDDR process 2 14 differs from Zr which occupies Fe(j ) sites before disproportionation and after the disproportionation.  1999 Elsevier Science S.A. All 2 rights reserved. Keywords: Nd Fe B; HDDR; XAFS; Site occupancy; Ga-additive 2 14 1. Introduction that these fine Nd Fe B particles have the same crys- 2 14 talline axis as that of the original coarse grains [7]. From By utilization of hydrogen, high coercivity has been these observations they concluded that these fine achieved in Nd Fe B-based magnets [1,2]. The HDDR Nd Fe B grains are likely to act as recombination centers 2 14 2 14 (Hydrogenation Disproportionation, Desorption and Re- during dehydrogenation process. Uncertainties remain, combination) process refines originally coarse grains of however, about the details of the growing process of Nd Fe B to about 0.3 mm which significantly increases recombined Nd Fe B from the decomposed phases. 2 14 2 14 its coercivity. Moreover, when a small amount of additives It is important that this crystallographic memory effect like Ga, Zr, Nb and Hf are included together with Co in a in the HDDR process occurs when small amounts of such Nd Fe B-based alloy, the crystalline axes of recombined elements like Ga, Zr, Nb and Hf are added to the 2 14 Nd Fe B grains are aligned parallel to those of the Nd Fe B-based alloys. The roles of such additives on the 2 14 2 14 original coarse grains resulting in higher magnetic aniso- magnetic anisotropy in the HDDR process have not yet tropy [3,4]. Recently, only a single additive (Zr) was been clarified. In the present work we focus our attention reported to be able to produce magnetic anisotropy in on the behavior of Ga in Nd Fe B-based alloy by HDDR process [5]. One of the Nd Fe Co Ga Zr B (hereafter abbreviated as 2 14 13.0 68.2 10.8 1.0 0.1 6.9 controversial points about this HDDR process is why and NdFeCoGaZr) during the HDDR process by means of how the original crystalline axis of coarse grains is fluorescence XAFS. Information on the local structures for inherited by the recombined Nd Fe B fine grains after such additives in NdFeCoGaZrB before and after hydro- 2 14 addition of such elements. From microscopic studies, genation is of great importance for revealing the roles of Uehara et al. have proved that very fine Nd Fe B the additives on the crystallographic memory effects. 2 14 particles less than 50 nm in diameter are embedded in the decomposed phases of a-Fe and Fe B [6] in the dis- 2 proportionated state. Tomida et al. observed by HRTEM 2. Experimental procedure An ingot of NdFeCoGaZrB alloy was prepared in an *Corresponding author. 1 JSPS postdoctoral fellowship for foreign researchers. induction furnace. The ingot was pulverized (about 100 0925-8388 / 99 / $ – see front matter  1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388(99)00381-3