Changes in microstructure, solidification path and hydrogen permeability of Nb–Hf–Co alloy by adjusting Hf/Co ratio

Abstract

The effect of Hf/Co ratio on microstructure, solidification path and hydrogen permeability of as-cast Nb 40 Hf 30+x Co 30-x and Nb 35 Hf 32.5+x Co 32.5-x (x=-2, 0, 2, 4) alloys is investigated. The above two series of alloys mainly consist of the primary BCC-(Nb, Hf) and the eutectic {BCC-(Nb, Hf) + B f-HfCo} phases, while Nb 40 Hf 34 Co 26 alloy additionally contains small fractions of the Hf 2 Co phase that results from a ternary eutectic reaction. Increase of Hf/Co ratio induces higher fraction of primary BCC-(Nb, Hf) and Hf 2 Co phases. Two kinds of solidification paths are found in this two series of alloys, particularly [L→BCC-(Nb,Hf)]→[L→BCC-(Nb,Hf) + B f-HfCo] and [L→BCC-(Nb,Hf)]→[L→BCC-(Nb, Hf)+B f-HfCo]→[L→BCC-(Nb, Hf) + B f-HfCo+ Hf 2 Co]. The permeability increases with increasing Hf/Co ratio and increasing volume fraction of the primary BCC-(Nb, Hf) phase. Nb 40 Hf 32 Co 28 (x = 2) exhibits a high hydrogen permeability of 5.21×10 −8 mol H 2 m-1 s-1 Pa-1/2 at 673 K and excellent durability in hydrogen permeation (~72 h), which becomes superior to that of the Nb 40 Hf 30 Co 30 previously reported to be the best in the Nb-Hf-Co alloy series. The present work demonstrates that the hydrogen permeability of this alloy series can be optimized by appropriately adjusting the Hf/Co ratio for a given niobium content.

Phase Equilibria of the V-Ti-Fe System and Its Applications in the Design of Novel Hydrogen Permeable Alloys

“…The testing temperature and pressure were 523-673 K and 0.1-0.5 MPa, respectively. Detailed steps for hydrogen permeation measurement are described in our previous work [20,21]. To obtain the hydrogen solubility, the hydrogen absorption test was also conducted for all the samples using a standard Sieverts-type device.…”

Microstructural Changes and Hydrogen Permeability of Rolled and Annealed Nb₃₀Ti₃₅Co₃₅ Alloys

“…．この方法では装置全体を小型 化できることや，水素生成に必要なエネルギー量を抑えられ るという利点がある 10) ．水素分離膜として Pd 合金が用いられ ており，今 日では様々な Pd 系 合金 について研 究されてい る [11][12][13] ．しかし，Pd は高価で環境負荷が大きいため，より安 価で耐水素脆化性に優れた水素透過膜の開発が必要である． V, Ta, Nb といった V 族元素は Pd より大きな水素透過性を 示すことが知られている 14) ．しかし，これらの元素は水素化 により水素化物を形成し著しく脆化することが知られてい る 15) ．Pd 代替水素透過合金では，優れた水素透過性と良好な 耐水素脆化性の両立が不可欠である．V に水素吸蔵を抑制す る Ni を固溶させた V-Ni 合金 [16][17][18][19] [21][22][23][24] や Nb-TiCo [25][26][27][28] 合金を開発した．これらの合金では，bcc-(Nb, Ti)相 が水素透過性を担い{bcc-(Nb, Ti) + TiNi (TiCo)}共晶組織が耐 29) ， Nb-ZrNi 30) ， Nb-ZrCo 31) ，V-TiNi 32) ，Ta-TiNi 33) といった同様のコンセプトを (c)…”

Microstructural Changes and Hydrogen Permeability of Rolled and Annealed Nb₃₀Ti₃₅Co₃₅ Alloys

“…These alloys are expected to be commercially viable. Other dual-phase alloys, such as Nb HfCo, 29) NbZrNi, 30) NbZrCo, 31) VTiNi, 32) and TaTiNi, 33) also have both hydrogen permeability and resistance to hydrogen embrittlement.…”