Microsc. Microanal. 23, 350–359, 2017 doi:10.1017/S1431927616012599
© MICROSCOPY SOCIETY OF AMERICA 2017
Effect of Cu on Nanoscale Precipitation Evolution and Mechanical Properties of a Fe–NiAl Alloy
Qin Shen,1 Hao Chen,2 and Wenqing Liu1,*
1Key Laboratory for Microstructures, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, People’s Republic of China 2Key Laboratory for Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Abstract: The microstructural evolution of precipitation in two model alloys, Fe–NiAl and Fe–NiAl–Cu, was investigated during aging at 500°C for different times using atom probe tomography (APT). The APT results reveal that the addition of Cu effectively increases the number density of NiAl precipitates. This is attributed to Cu promoting the nucleation of NiAl particles by increasing the chemical driving force and decreasing the interfacial energy. The NiAl precipitates of the Fe–NiAl–Cu alloy grow and coarsen at a slower rate than that of the Fe–NiAl alloy, mainly due to the slower diffusion rate of the Cu atoms. The mechanical properties of the two alloys were characterized by Vickers hardness and tension tests. It was found that the addition of Cu results in the formation of core–shell precipitates with a Cu-rich core and a NiAl shell, leading to a dramatic improvement of peak hardness and strength. The effect of Cu on precipitation strengthening is discussed in terms of chemical strength and coherency strength.
Key words: Cu, NiAl nanoparticles, precipitation, atom probe tomography INTRODUCTION
Ultra-high-strength steels show increasing importance in aerospace, power generation, ship building, and automotive industries because of their high strength, high fracture tough- ness, and good ductility. Precipitation hardening (PH) is recognized as one of the most effective strengthening methods for ultra-high-strength steels (Argon, 2008; Hayashi et al., 2008; Yen et al., 2011; Trotter et al., 2014; Huang et al., 2015), which is mainly attributed to the nanoscale precipitates. Previous studies reported that several PH steels containing nickel and aluminum can be hardened remarkably by aging at tempera- tures above 400°C (Seetharaman et al., 1981; Taillard&Pineau, 1982; Hochanadel et al., 1994), and that the strengthening is due to the precipitation of ordered NiAl nanoparticles with a B2 crystal structure (Guo et al., 2003; Ping et al., 2005). In addition to Ni andAl,Cu is also an important alloying element in PH steels leading to the precipitation of Cu-rich nano- particles, as reported in Fe–Cu alloys (Hornbogen & Glenn, 1960; Goodman et al., 1973). Recently, there has been an increasing interest in the
co-precipitation of Cu-rich and NiAl nanoparticles of high- strength steels, which offers a promising way to effectively strengthen steels without a significant reduction in ductility (Wen et al., 2013; Zhang et al., 2013; Kapoor et al., 2014; Wang et al., 2015). The previous research showed that the NiAl nanoparticles are usually found to form concomitantly with Cu-rich nanoparticles in Ni–Al–Mn–Cu-containing steels. The influence of Cu addition on NiAl precipitation was
*Corresponding author.
wqliu@staff.shu.edu.cn Received June 3, 2016; accepted December 1, 2016
also studied. Schnitzer et al. (2010) showed that the addition of Cu accelerates the precipitation of the NiAl nanoparticles in Fe–Cr–Ni–Al–Ti maraging steel, whereas Horing et al. (2009) reported that there is no difference in density and size of pre- cipitates in Fe–Cr–Ni–Al and Fe–Cr–Ni–Al–Cu alloys at the early stages of aging. However, Cu inhibits the growth of the precipitates during further aging. Previous work was focused mainly on the Fe–Cu–Ni–Al-based steelsoralloyswithrela- tively complex compositions. Therefore, it is very challenging to quantify the effect of Cu on the precipitation of NiAl phase due to the synergistic effects of many alloying elements. In this study, two model alloys, Fe–NiAl and Fe–
NiAl–Cu were chosen, and their precipitation behaviors were investigated using atom probe tomography (APT). Particularly, the effects of Cu on the nanoscale NiAl precipitates and the mechanical properties are discussed.
MATERIALS ANDMETHODS
The Fe–NiAl and Fe–NiAl–Cu alloys, whose compositions are listed in Table 1, were melted in a vacuum induction furnace frommetalswith purities above 99.8wt% and cast into awater- cooled copper mold. Then, the casting plates were forged at 1,100°C and hot rolled at 1,200°C to 50% thickness reduction. Small sheets were cut from the rolled plate, and were sub- sequently solution heat treated at 900°C for 2h, followed by water quenching to room temperature. Finally, these small sheets were aged at 500°C for various selected durations (0.5, 1, 2, 4, 8, 16, 32, 64, 128, 256, and 512 h). The microhardness was measured using Vickers microindentation (500 g load and indentation time 10 s).
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284
