## 3 Results
### Flow Behavior and Hardness  
The flow stress curves in Figure 5 were obtained from hot compression tests on the Gleeble using deformation temperatures (800, 900, and 1000 \({}^{\circ}\)C) and stain rates (0.1 and 5 s\({}^{-1}\)). The flow stress was sensitive to both the deformation temperature and strain rate. As expected in materials with positive strain rate sensitivity, the flow stress increased with increasing strain rate and decreasing deformation temperature [13; 18]. Generally, it is reported that when low-density steels are subjected to hot working, the flow stress is higher at low temperatures and high strain rates, while it is lower at high temperatures and low strain rates [2]. The curves show that the flow stress increased drastically at the beginning of deformation until a peak stress, after which the softening effects on flow stress became obvious. The initial increase in flow stress depicts the dominance of work hardening [13; 14; 15]. This metallurgical phenomenon is controlled by the relationship between dislocation mean free path and dislocation length [19]. The work hardening rate is increased by decreasing the mean free path of dislocations [20]. The decrease in the mean free path depends on dislocation density and grain size; therefore, higher strain rates increase dislocation density while smaller grain size increases grain boundary area. Therefore, the mean free path of dislocation is decreased by high dislocation density and small grain size, causing dislocation motion impediment [19]. The flow stress curves show varying softening trends for the different testing conditions, suggesting that multiple softening mechanisms may have influenced the deformation process. It has been reported that the flow behavior of low-density steels is influenced by softening mechanisms such as continuous dynamic recrystallization, discontinuous dynamic recrystallization (DDRX), dynamic precipitation of intragranular K-carbides, and dynamic recovery [13]. Dynamic recrystallization (DRX) causes grain refinement, and the DRX grains' nucleation generally begins at a critical strain just before the peak stress is reached [21]. As seen in Figure 5a,b, the curves at 0.1 s\({}^{-1}\) strain rate show a rough flow oscillation depicting the possible occurrence of DDRX [22]. [PERSON] and [PERSON] [23] focused on the mechanical aspect of DRX and reported that oscillating flow stress or cyclic flow stress are signatures of DDRX, but depending on the deformation temperature and strain rate, the multiple peaks for oscillating flow stress may transform to a single peak. Similar to the observation in Figure 5a,b, they emphasized that flow oscillations are expected at low strain rates and high deformation temperatures. [PERSON] and [PERSON] [24] later advanced this by looking at the microstructural aspect of DRX, establishing that flow oscillations with multiple peaks are related to the ratio of the initial grain size and the size of a stable dynamically recrystallized grain. In the case of flow oscillations, they showed that the initial grain size is less than the stable dynamically recrystallized grain and, for that reason, grain  
Figure 4: X-ray diffraction pattern of Fe-30.9 Mn-4.9 Al-4.5 Cr-0.4C and Fe-21.3 Mn-7.6 Al-4.3 Cr-1C low-density steels. coarsening occurs during deformation until the size of a stable dynamically recrystallized grain is reached. DRX appeared to be the dominating softening mechanism at a low strain rate because there is more time for the nucleation of new grains and grain growth. The flow stress at 800 and 900 \({}^{\circ}\)C declined continuously after peak stress and the strain exceeded 0.2 (Figure 5b), indicating continuous flow softening. This observed trend portrays a possible occurrence of deformation-induced defects, such as cracking or, in other instances, resulting from geometric dynamic recrystallization (GDRX), otherwise known as dynamic globularization, where elongated grains are transformed to a globular morphology ([PERSON] and [PERSON], 2015; [PERSON], 2015).