## 2 Materials and Methods  
The chemical compositions of the alloys studied are listed in Table 1. The alloys contained \(\sim\)9 wt.% Cr, three levels of Hf contents and low levels of the interstitial elements. The concentrations of the transition metals and carbon were analyzed using the infrared absorption method after combustion in an induction furnace (JIS G1211), and those of nitrogen and oxygen using the inert gas fusion infrared absorption method. These alloys are designated with their Hf contents in this paper.  
Each sample was prepared by vacuum induction melting as a 1-kg ingot. Each ingot was hot rolled to a bar shape with a rectangular cross-section of 12 \(\times\) 8 mm in size. The hot-rolled bars were machined to test pieces with a gauge portion of 30 mm in length and 8 mm in diameter. The following sequential heat treatment was performed for the test pieces of all the alloys using an infrared furnace: heating to 1300 \({}^{\circ}\)C, holding for 5 min, cooling at a rate of 10 \({}^{\circ}\)C/s to 1000 \({}^{\circ}\)C, holding for 20 s, cooling at a rate of 10 \({}^{\circ}\)C/s to 750 \({}^{\circ}\)C, cooling at a rate of 0.1 \({}^{\circ}\)C/s to 600 \({}^{\circ}\)C and air cooling. This heat treatment scheme was designed to achieve interphase precipitation to occur through the 8-mm-sized sample in the 0.33 Hf alloy based on our preliminary experiments [10]. As shown in the next section, fine particles were formed by interphase precipitation (hereafter called IP particles) in the 0.33 Hf alloy, while precipitation did not occur in the heat treatment in the other two alloys. In the 0.09 Hf alloy, fine Fe\({}_{2}\)Hf particles were precipitated in the transformed ferrite matrix during the creep tests.  
The heat-treated test pieces were machined to cylindrical brimmed creep test specimens with a gauge portion of 30 mm in length and 6 mm in diameter. Creep tests were performed using a lever-type single creep testing machine (TOSHIN KOGYO., LTD, Japan) in air at constant load under temperature/stress conditions of 650 \({}^{\circ}\)C/60 MPa, 700 \({}^{\circ}\)C/40 MPa and 700 \({}^{\circ}\)C/30 MPa. The microstructures of the creep test specimens before creep test and after creep interrupt and creep rupture tests were observed with a field emission scanning electron microscope (FESEM, JSM-7000F, JEOL Ltd., Japan) equipped  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample Designation**} & \multicolumn{8}{c}{**Chemical Composition/wt.\%**} \\ \cline{2-7}  & **Cr** & **Hf** & **Zr** & **C** & **N** & **O** & **Fe** \\ \hline
0.03 Hf & 9.0 & 0.03 & \textless{}0.001 & 0.003 & 0.001 & 0.004 & bal. \\
0.09 Hf & 9.2 & 0.09 & \textless{}0.001 & 0.002 & 0.001 & 0.002 & bal. \\
0.33 Hf & 9.0 & 0.33 & 0.003 & 0.002 & 0.001 & 0.002 & bal. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of the alloys used in the present study.  
with a backscattered electron (BSE) detector which allows electron channeling contrast imaging (ECCI) and an electron backscatter diffraction (EBSD) detector. The phases were identified by X-ray diffraction (XRD) with Cu radiation using a thin Ni filter (Mini Flex 600, Rigaku Corp., Japan). The specimens before creep tests were cut 1 mm inside the sample surface and the creep-interrupted and creep-ruptured specimens were cut from their gauge portions. The cut sections were ground, mechanically polished down to 1 um Al\({}_{2}\)O\({}_{3}\) polishing suspension and, finally, chemically polished with SiO\({}_{2}\) polishing suspension. The creep-interrupted specimens were observed with a scanning transmission electron microscope (STEM, JEM-2100F, JEOL Ltd., Japan) with an energy dispersive X-ray spectroscopy (EDS) detector. The samples were obtained from their gauge portions and prepared by twin-jet electro-polishing in perchloric acid solution (10% perchloric acid + 90% ethanol) at \(-\)10 \({}^{\circ}\)C.