## 3 Results and Discussion  
HiperFer steel owes its strength to a combination of solid solution strengthening and precipitation hardening using thermodynamically stable (Fe, Cr, Si)\({}_{2}\)(Nb, W) Laves phase particles [25]. Nb and Si are key elements in Laves phase precipitation: Nb is used because when combined in an alloy with tungsten, it improves solid solution strengthening to some extent [45]. Furthermore, it is a strong Laves phase former [46], and because of its comparatively low solubility and high diffusion rate, it facilitates precipitation in ferrite. Si further accelerates nucleation [46; 47; 48; 49; 50] of the Laves phase [25; 35; 46; 51] precipitates. Figure 2 depicts the solid solution strengthening effects of W and Nb in HiperFer steel. The figure indicates a more pronounced impact of W on UTS and a greater impact of Nb on YS\({}_{0.2}\). Figure 2: Impact of W and Nb in solid solution on ambient temperature tensile strength values with a virtual 2.4W1 Nb alloy, equation: YS0.2 = a \(\times\) (1 \(-\) cW \(-\) b); cW: W-content in wt.%, a: 300.05, b: 2.38; UTS = a \(\times\) (1 \(-\) cW \(-\) b); cW: W-content in wt.%, a: 396.53, b: 1.71 [31]. In the case of a 17 Cr2 base alloy (2.4W0.6 Nb), Y50,2/UTS values of 400/660 MPa were achieved in the precipitation heat-treated state [33]. Increasing the W and Nb contents improved the solid solution strengthening effect, along with increasing the volume fraction and reducing the average particle size of the Laves phase precipitates, resulting in enhanced mechanical properties, in particular creep strength [31]. Figure 3 displays the effects of varying the W and Nb contents on the calculated limit stress for dislocation creep \(\sigma_{\text{th}}\) (for details cf. [31; 52]): using W and Nb alloying, an increase in the limit stress of approximately 61% was achievable. The addition of Si mainly resulted in a reduction in the Laves particle size, increased the number of particles and accelerated the precipitation kinetics [53]. From the evaluated increase in limit stress for dislocation creep \(\sigma_{\text{th}}\) by Si-alloying (Figure 4), it could be derived that by Si alloying, an approximate increase of 43% was obtainable. Figure 4: Effect of Si on \(\sigma_{\text{th}}\)[54]. Figure 3: Impact of W and Nb on the threshold stress \(\sigma_{\text{th}}\)[31]. Based on these results, the Nb/Si contents were adjusted to 1.5/1 wt.% in order to maximize the (i) precipitation kinetics and (ii) volume fraction and to decrease the particle size in the HiperFer\({}^{\text{AM}}\) (Table 1) trial alloy. Because of the high melting point discrepancy between W and the other constituents, a moderate content of W (2 wt.%) was chosen to alleviate the expected difficulties in the additive manufacturing process. The increased Nb and Si contents (compared with conventionally casted, rolled and heat-treated HiperFer 17 Cr2 (Table 1)), in combination with the high cooling rates of the AM process, resulted in a highly supersaturated solid solution alloy matrix in the as-built state, which in turn led to rapid precipitation kinetics as a result of the high precipitation pressure toward equilibrium. Despite comparatively short high-temperature periods, the in-build (i.e., temper bead) heat treatment (Figure 5) successfully produced a large amount of finely dispersed intragranular Laves phase particles. In addition, the high angle grain boundaries (HAGb) were covered by Laves phase precipitates, which stabilized the grain structure without embrittling the material. The stabilization of the grain structure is the key factor that allows for dynamic strengthening [27; 55] under fatigue loading. Dynamic hardening/precipitation was successfully exploited in aluminum alloys AA2021, AA6061 and AA7050 to enhance high-cycle fatigue performance [56]. For this purpose, additional fine particles were generated in the particle free zones (PFZs) using suitable cyclic training.