Through the analysis of ordered phase morphology and degree of order, it is found that Fe-6.5wt.%Si alloy ingot has the highest degree of order, Fe-6.5wt.%Si alloy air-cooled cast strip has a lower degree of order, and Fe-6.5wt.%Si alloy air-cooled cast strip has the lowest order. According to the phase diagram of Fe-Si alloy, Fe-6.5wt.%Si alloy has A2 disordered structure above 760°C, and with the decrease of temperature, B2 ordered structure is formed in A2 disordered matrix. Since the transition from A2→B2 is secondary, the transition speed is faster. A further drop in temperature will lead to further ordering between the next-nearest neighbor atoms, and the DO3 ordered phase will nucleate in the existing B2 ordered phase domain. The transition from B2 to DO3 is a first-order transition, the time required for the transition longer than the transition time of A2→B2 . At the same time, due to the relatively simple lattice structure of the B2 ordered phase, only one Fe atom and one Si atom can be formed . During cooling, each pair of Fe atoms and one Si atom is the core of a B2 ordered phase, and this core grows rapidly so that it swallows up the surrounding similarly occupied lattice. Therefore, it is complicated to completely suppress the B2 ordered phase utilizing rapid cooling. The DO3 ordered phase has a higher-order and is a higher-order ordered phase formed based on the B2 ordered phase. The formation of a DO3 ordered phase unit cell requires more Si atoms to participate, and the temperature of this ordered region is lower (<620°C), and the atomic mobility is poor. When the cooling rate is fast, the B2 ordered phase cannot provide a sufficient basis for the nucleation and growth of the DO3 ordered phase. In addition, Si atoms are too late to occupy probabilistically in the way of the DO3 lattice, so DO3 has an ordered phase that can be quickly and completely suppressed. Due to the slow cooling rate of Fe-6.5wt.%Si alloy ingots, smooth and curved B2 ordered phases and granular DO3 ordered phases were formed in the samples. Due to the fast cooling rate of the Fe-6.5wt.%Si alloy air-cooled cast strip and water-cooled cast strip, the nucleation and growth of DO3 ordered phases in the samples can be easily and completely suppressed by rapid cooling. Although the B2 ordered phase could not be completely inhibited, the growth of the B2 ordered phase was inhibited.
Because the existence of the ordered phase will produce an ordered strengthening effect in the matrix, increasing the hardness and decreasing the plasticity of the material . This is because, in ordered alloys, the movement of dislocations disrupts the ordered arrangement of atoms, creating additional antiphase domain boundaries on the slip plane. Yoshimi  investigated the effect of APB on the tensile properties of Fe3Al alloys and found that dislocations tend to aggregate at antiphase boundaries. They suggested that antiphase boundaries have some hindering effect on the movement of dislocations in the alloy. Since Fe-Si alloys have high antiphase boundary energy, dislocations tend to move in pairs to reduce the antiphase boundary energy of the system. Compared with ordinary unit dislocations in disordered alloys, superdislocations have larger slip resistance and insufficient independent slip systems during the movement process, resulting in reduced plasticity and poor processing performance [27, 28]. The antiphase boundary energies decrease with decreasing ordering degrees, and the antiphase boundary energies affect the dislocation configuration in ordered alloys. The lower the antiphase boundary energy, the larger the distance of superdislocations, and the more favorable it is to decompose into single dislocations. Compared with paired superdislocations, single dislocations are easier to move and cross-slip. Thereby increasing the mobility of dislocations . In Fe-6.5wt.%Si alloy, the resistance of dislocation motion in the DO3 ordered phase is much larger than that in the B2 phase. That is to say, the mobility of dislocations in the B2 phase is high, while the mobility of dislocations in the DO3 phase is poor. In the DO3 phase, dislocations move slowly, and it is not easy to cross or bypass obstacles, resulting in superdislocations easily aggregated in grains or grain boundaries. Dislocations with different Burger vectors react to form a dislocation network, which further reduces the mobility of dislocations and becomes a new obstacle for other moving dislocations, thus forming a vicious circle. Therefore, in the present experiments, the variation in ductility and hardness of the Fe-6.5wt.% Si alloy at room temperature can be attributed to the influence of ordered phases. The greater the content and size of the ordered phase, the worse the plasticity of the material. It can be seen from Fig. 9 that the ordered phase size is positively correlated with the hardness. In the Fe-6.5wt.%Si alloy ingot sample, the B2 ordered phase has the largest size, and a larger DO3 ordered phase is observed. The Fe-6.5wt.%Si alloy ingot has the largest hardness and the worst room temperature ductility. On the contrary, the hardness of the Fe-6.5wt.%Si alloy water-cooled cast strip is the smallest, and the room temperature plasticity is relatively the best.
In Fe-6.5wt.%Si alloys, due to ordered phases in the matrix, the Burger vector of the total dislocations will become longer, resulting in the total dislocations becoming extremely unstable. Typically, a single global dislocation resolves into two, and four quantile dislocations in the B2 and DO3 ordered phases, respectively . Figure 10 is a schematic diagram of the dislocation structure on the (110) close-packed plane in the ordered structure. In the B2 ordered structure, one complete dislocation decomposes into two dislocations, b1 and b2, connected by band-like APBs. Similarly, the DO3 ordered structure decomposes into four dislocations, b1, b2, b3, and b4, which are connected by two kinds of band-like APBs, namely APB1 and APB2. Ribbon APBs are energetic and proportional to the order of the matrix . This means that the energy of banded APBs between paired superdislocations in Fe-6.5wt.%Si alloy ingots is much higher than that of Fe-6.5wt.%Si alloy air-cooled cast strip and water-cooled cast strip. The "ordered strengthening" effect occurs when the Fe-6.5wt.%Si alloy undergoes plastic deformation. The b1 dislocation in the B2 structure will slip along the direction of the arrow in Fig. 10, and the ribbon APB will also be elongated, increasing the area of the APB, so additional energy is required to facilitate this process. Moreover, the higher the matrix order is, the higher the additional energy required. Similar processes will also occur in the DO3 ordered structure, but the DO3 ordered structure has a higher-order degree and requires more energy [23, 28]. To sum up, this is also why the plasticity of Fe-6.5wt.%Si alloy air-cooled cast strip and water-cooled cast strip is much higher than that of Fe-6.5wt.%Si alloy ingot.
According to the above discussion, it can be known that the Fe-6.5wt.%Si alloy ingot needs larger external stress to start slipping. However, before the superdislocations slip, the gradually increasing external stress will generate stress concentrations inside the material. When the strength of these stress concentrations exceeds the maximum strength of the matrix, macroscopic cracks will follow, Fe-6.5wt in Fig. 7. The stress-strain curve of the alloy ingot, which fractured in the form of cleavage fracture in the elastic stage, also confirmed the above inference. On the other hand, under the influence of a high degree of order, the intragranular stress concentration due to the difficulty of superdislocation slippage also makes Fe-6.5wt. The area of transgranular fracture  is consistent with the results of the fracture SEM fracture morphology in Fig. 8a.