Our team has been reporting the interpretation of the high-temperature creep mechanism for dual ductile-phase alloy by theoretical analyses, indentation creep tests, and finite-element simulations. The experimental results suggested that the creep strength of the dual ductile-phase alloy closely followed the rule of mixtures and the isostrain rate conditions. The stress exponent *n* for creep was expressed by the harmonic mean weighted by the effective volume fractions of the constituent phases, which strongly depended on the deformation rate. In addition, *n* consistently fell between the corresponding values for the two constituent phases in the power-law creep region. A similar trend was observed for the deformation rate dependence of the creep activation energy *Q*, which was expressed by the weighted arithmetic mean value. Thus, the values of *n* and *Q* quantitatively captured the mechanical contribution of the reinforcing phase to the creep strength of the overall dual ductile-phase alloy. As following, the microstructure of the tested alloy and experimental results of the stress exponent are shown.

**Figure 1** shows a representative SEM image of the three-dimensional microstructure of the dual-phase alloy. In the longitudinal section parallel to the extrusion direction (ED), the discontinuous fibrous LPSO phase (light gray) is aligned with the ED in the *α*-Mg matrix (dark gray). Meanwhile, in the cross-section perpendicular to the ED, the lamellar LPSO phase is uniformly observed in the *α*-Mg matrix. The area fraction of the *α*-Mg matrix is ~75% for all cross-sections (i.e., a volume fraction of *V*_{1}0.75 for the matrix and *V*_{2}0.25 for the remaining LPSO phase). The values of *V*_{1} and *V*_{2} are unchanged after heat treatment.

**Figure 2** shows the IPF map of the longitudinal section immediately under the indentation mark. In this case, indentation was performed with a conical indenter perpendicular to the cross-section at the highest test temperature of 673 K for a loading time of 5.4 ks. During this loading period, neither recrystallization nor microscopic cracking occurs in the region. These findings show that the indentation creep test is performed on a thermally stable microstructure. Although a small amount of as-worked *α*-Mg grains exist in the microstructure, it is thought that they have little influence on the cross-sectional indentation test results.

**Figure 3** shows double logarithmic plots of *ε* ’_{in(si)} versus *p*_{si}/*E* for the *α*-Mg alloy, the LPSO alloy, and the dual-phase alloy. Here, *p*_{s1} of the *α*-Mg alloy is the lowest, *p*_{s2} of the LPSO alloy is the highest, and *p*_{s} of the dual-phase alloy is consistently located between them. In the case of single phase alloy of *α*-Mg or LPSO alloy, the data points follow different straight lines. The gradient line for *α*-Mg alloy corresponds to *n*_{1} = 2.4, and it for LPSO alloy is *n*_{2} = 5.5. In contrast, The data points for the dual-phase alloy follow a continuous curve, when *ε* ’_{in(si)} decreases from 4.5 10^{-3} s^{-1} to 1.0 10^{-4} s^{-1}, *n* increases from 3.0 to 4.5. The *n* value of dual-phase alloy gradually approach the corresponding value of the LPSO phase (*n*_{2} = 5.5) as the deformation rate decreases. These findings indicate that the characteristic creep parameters of dual-phase alloy cannot be explained using the traditional creep theory for single-phase alloys.

Additional results in this paper are as follows:

• |
During the creep of the LPSO phase, the basal <a> slip system was dominant. However, the steady-state creep behaviors of the LPSO phase could not be explained using the conventional creep theory for single-phase alloys. Meanwhile, GBS occurred in the fine-grained |

• |
The creep characteristic values |

• |
The values of |

• |
In the power-law creep region, the LPSO phase effectively suppressed deformation of the |

• |
To further improve the creep strength of the dual-phase alloy, it is effective to increase the toughness and reinforcement efficiency of the LPSO phase. It is important to develop a method for elucidating the high-temperature creep mechanism of such an alloy, thereby reducing the development times of new materials with complicated structures. |