Fig. 1 Al-Hf phase diagram [16]

Fig. 2 Al-rich portion of the Al-Hf phase diagram based on Rokhlin’s results (points, solid lines) and the data in [11,30] (dashed and dash- and dot-lines, respectively)

Fig. 3 Al-Hf-Sc isothermal section at 600 °C near the Al corner redrawn by Raghavan [35] based on the results from Rokhlin et al. [34]

Fig. 4 Al-Hf-Zr isothermal section at 600 °C near the Al corner redrawn by Raghavan [37] based on the results from Rokhlin et al. [36]

Fig. 5 Unit cells of the three structures: a L12 structure, b D022 structure, ideal c/a = 2, c D023 structure, idealc/a = 4

Fig. 6 Angular structure of Al-Hf alloy: a solid illustration, b-d cross sections of (100), (110) and (111), respectively

Fig. 7 Schematic representation of interaction of angular structure and grain boundary reaction

Fig. 8 Continuous and discontinuous precipitation of the L12-A13Hf phase in Al-1.6 wt% Hf alloy aged at 300 °C for 10 h (centered dark field TEM image taken along [100]) [43]

Fig. 9 Continuously formed spherical precipitates in chill cast Al-3.5 wt% Hf alloy aged at 450 °C for 40 h

Fig. 10 Temperature-time-precipitation diagram of solution-treated Al-1 wt% Hf alloy and chill cast Al-3.5 wt% Hf alloy (schematic)

 Fig. 11 Observed sequence of the transformation in the Al-3 wt% Hf-0.5 wt% Si alloy [49] A possible mechanism of phase transformation of Al3Hf from L12 to D022 during aging in a rapidly solidified Al-3 wt% Hf-0.3 wt% Si alloy was investigated by Furushiro and Hori [59]. The H phase was found to have a space group Pmmm (Fig. 12a). The transformation from one phase to the other could be explained by a periodic shear. Thus, a shear of 1/2[110](110)L12 on every second plane led to the H phase which transforms to D022 by a shear of 1/2[010](101) on every second plane (Fig. 12b).

Fig. 12 a Apparent reciprocal lattice of the H phase, b the larger symbol represents the stronger reflection, thesmallest circles of the characteristic spots are placed at ½ <110> in the reciprocal lattice of the L12structure

Fig. 13 a Low-magnification bright-field TEM image of Al-7 wt% Si-0.3 wt% Mg-0.5 wt% Hf-0.2 wt% Y alloy. bHAADF-STEM image from the same alloy sample but different areas. Long nanobelt precipitates together with rectangular-shaped precipitates can be seen in both images [61]

Fig. 14 Relation between grain size and Hf content in the cast alloy

Fig. 15 Mean grain size in castings obtained at various cooling rates versus initial melt overheating above liquidus.Filled square V = 2 × 102 K/s, open square V = 4×103 K/s, filled circle V = 104 K/s, open circleV = 2 × 104 K/s

Fig. 16 Plot of composition versus lattice parameter [43]. Reference here is come from Hori et al. [82]

Fig. 17 Relation between Vickers hardness and Hafnium content in the cast alloy [49]