# Developing Tungsten-Filled Metal Matrix Composite Materials Using Laser Powder Bed Fusion
## 3 Results
### Heat Treatment  
The pure matrix material MS1 is supposed to be heat treated at 490 \({}^{\circ}\)C for 6 h [12] to reach higher UTS and hardness. However, this heat treatment was not considered to be expedient, as it would possibly promote the formation of intermetallic phases. Because of the nickel-rich MS1, the growth of Ni-W precipitates at the interface between the tungsten particles and steel matrix would be most likely. Such intermetallic precipitates are known to embrittle the interface and reduce the toughness of the material [13]. In contrast to this, a dissolution of potential Ni-W precipitates is desirable. According to [PERSON] [14] and [PERSON] [15], the dissolution of N-W phases takes place at temperatures above 1093 \({}^{\circ}\)C. Therefore, heat treatment at a sufficiently high temperature of 1200 \({}^{\circ}\)C and a shorter duration of 1 h, followed by quenching in water, was attempted. The SEM analysis of a metallographic specimen (60 vol% tungsten) after this heat treatment is presented in Figure 6.  
After a look at the backscatter image of the metallographic specimen, it can be stated that the heat treatment led to the formation of a pronounced interphase (medium grey). The tungsten particles (bright grey) dissolved into the steel matrix (black). The fact that the medium grey areas exhibited no transition from a brighter to a darker grey underlines the formation of an interphase. Moreover, cracks in the interphase were visible. Thermal stress due to heat treatment might be the reason for this phenomenon. This also indicates a brittle behavior of the inter  
Figure 5: SEM images of a failure surface of a tensile test specimen. **(a)** Backscatter image showing tungsten particles (bright) and matrix phase (dark); **(b)** Magnification of the left image. Cleavage of the tungsten particles thoroughly embedded in the steel matrix can be observed.  
Figure 6: Backscatter images of a metallographic specimen after heat treatment (1200 \({}^{\circ}\)C, 1 h). **(a)** Overview at low magnification; **(b)** Close-up revealing the formation of an interphase exhibiting cracks probably caused by thermal stress relief.  
(see Figure 7) revealed that the interphase contained all measurable elements of the matrix and the reinforcement particles.  
Additionally, a fracture surface of a heat-treated sample was analyzed in the SEM. From Figure 8a, it can be concluded that the interface between tungsten particles and steel matrix was weakened due to heat treatment. A ripping-off of the particles at the fracture surface can be stated. Moreover, a radial pattern around the particles can be observed, indicating diffusion processes taking place during the heat treatment. In the backscatter image (b), a cracked tungsten particle can be seen. A core-shell-like structure was formed because of the diffusion processes started by the heat treatment.  
This attempt effectively shows that MMCs can be modified by applying heat treatment steps. This might be used for the design of specific material properties. However, the combination of more than one material enhances the complexity of heat treatments themselves. Therefore, for shorter developing times it might be more expedient to generate MMCs by employing starting materials not necessary to be heat-treated after the building process.  
Figure 8: Dissolution of tungsten particles at a fracture surface of a tensile test specimen due to heat treatment (1200 \({}^{\circ}\)C, 1 h, water quenched). (**a**) Ripping-off of particles can be observed; (**b**) Core-shell-like structures can be found. This phenomenon can be explained by radial diffusion processes inside the particles during the heat treatment.  
Figure 7: EDS measurement of the interphase. The spectrum contains all measurable elements of the MS1 matrix and tungsten.