Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP® Nb3Sn wires
Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP® Nb3Sn wires
复制标题
控制 Cu-Sn 混合，以便在 RRP® Nb3Sn 焊丝中实现更高的临界电流密度
DOI：
10.1088/1361-6668/aab8dd
复制
发表时间：
2018
影响因子：
3.6
通讯作者：
D. Larbalestier
中科院分区：
文献类型：
--
作者：
C. Sanabria;M. Field;P. Lee;H. Miao;J. Parrell;D. Larbalestier
用于未来圆形对撞机（FCC）的偶极磁体要求的规格远远超过所有现有Nb 3Sn导线的极限，特别是在16 T和4.2 K下的临界电流密度（Jc）超过1500 A mm−2，有效灯丝直径（Deff）小于20 μm。重叠棒工艺（RRP®）是最接近满足这些要求的技术，其Jc（16 T）高达1400 A mm−2，剩余电阻率比> 100，子元件尺寸Ds为58 μm（RRP®导线中的Ds与Deff基本相同）。RRP®目前的一个重要限制是，减小子元件尺寸会使Jc在16 T（Ds = 35 μm）下降低至900 A mm−2。为了了解这种Jc退化的来源，我们已经详细研究了Nb 3Sn形成之前发生的线热处理的Cu-Sn“混合”阶段期间的相演变。使用广泛的微观结构量化，我们已经确定了关键的作用，Sn-Nb-Cu三元相（Nausite）可以发挥。Nausite在Sn源和Cu/Nb细丝组之间形成明确的环，并在300 °C-400 °C范围内充当渗透膜-极大地抑制Sn扩散到Cu/Nb细丝组中，同时支持Cu从细丝组向Sn芯的强烈反扩散。这将Sn核转化为低熔点（408 °C）η相（Cu 6Sn 5）和更理想的ε相（Cu 3Sn）的混合物，其在676 °C下分解。在混合阶段之后，当加热至高于408 °C朝向Nb 3Sn反应时，任何残余的Nb 3Sn分解以在膜的内侧上形成另外的不规则钠锰矿。所有的钠锰矿在进一步加热时分解成NbSn 2，并最终转化成粗晶粒（并且通常是断开的）Nb 3Sn，其对电流传输的贡献很小。了解这一关键的钠锰矿反应途径使我们能够将混合热处理简化为仅在350 °C下进行400 h的一个阶段，这最大限度地减少了钠锰矿的形成，同时通过更好的Cu-Sn混合促进了更高熔点ε相的形成。在Ds为41 μm时，Nausite控制热处理使16 T下的Jc增加了36%，达到1300 A mm−2（即12 T下的2980 A mm−2），并使RRP®更接近FCC目标。
Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb3Sn wires, in particular a critical current density (Jc) of more than 1500 A mm−2 at 16 T and 4.2 K with an effective filament diameter (Deff) of less than 20 μm. The restacked-rod-process (RRP®) is the technology closest to meeting these demands, with a Jc (16 T) of up to 1400 A mm−2, residual resistivity ratio > 100, for a sub-element size Ds of 58 μm (which in RRP® wires is essentially the same as Deff). An important present limitation of RRP® is that reducing the sub-element size degrades Jc to as low as 900 A mm−2 at 16 T for Ds = 35 μm. To gain an understanding of the sources of this Jc degradation, we have made a detailed study of the phase evolution during the Cu–Sn ‘mixing’ stages of the wire heat treatment that occur prior to Nb3Sn formation. Using extensive microstructural quantification, we have identified the critical role that the Sn–Nb–Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C–400 °C range—greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu6Sn5) and the more desirable ε phase (Cu3Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb3Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn2 on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb3Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ε phase through better Cu–Sn mixing. At a Ds of 41 μm, the Nausite control heat treatment increases the Jc at 16 T by 36%, reaching 1300 A mm−2 (i.e. 2980 A mm−2 at 12 T), and moving RRP® closer to the FCC targets.