# Synthesis and Properties of Octet NiCr Alloy Lattices Obtained by the Pack Cementation Process
## 2 Materials and Methods  
By combining the preparation processes of a NiCr foam and a NiCrAlTi microlattice, a three-dimensional model of the octet lattice was established firstly, to obtain a rod with a diameter of 0.5 mm and a length of 2.5 mm (Figure 1). The model was imported into a 3D printer, and the polymer template was prepared by stereolithography. Hydroxides, potassium permanganate and chromium trioxide solutions were used for surface coarseization. Then, by immersing the polymer lattice in a solution containing stannous chloride and palladium chloride, a layer of catalytic metal particle palladium was adsorbed on the surface of the resin, forming the nucleation centers. The sample was first electroless-Ni plated and electrodeposited, and then the surface coating was removed to expose the polymer skeleton to the open air [6; 7; 8; 9; 28]. Then, the lattice with the polymer was sintered at 600 \({}^{\circ}\)C to remove the polymer skeleton, and the hollow Ni lattice was reduced under a hydrogen atmosphere at 900 \({}^{\circ}\)C.  
The complete pack cementing process mainly included two steps: packaging chromation and homogenization heat treatment. The packaging chromation process allowed mixing the aluminum, Cr powder (source) and ammonium chloride (activator) as fillers according to the mass ratio of 65:30:5. After about 20 min of mechanical mixing, the filler was poured into a high-chromium stainless steel shell, and the nickel lattice was inserted. The housing was placed at the water-cooled end of a tube furnace and pushed into the hot zone of the furnace at a processing temperature of 1050 \({}^{\circ}\)C (RYJ-2000C, Gaona Aero Material Co., Ltd., Beijing, China) for different times, depending on the desired composition. The packet was then quickly pushed into the other water end of the furnace and cooled to ambient temperature. After this first heat treatment step, the alloyed lattice was homogenized at 1200 \({}^{\circ}\)C for 48 h under flowing Ar, and the Cr-rich coating on the inner and outer surfaces of the Ni hollow truss was diffused into the walls [10; 11; 18; 19; 20; 21; 22; 23; 24; 25].  
To compare the mechanical properties of NiCr and NiCrFe alloy foams, we used a Cr content of the alloy lattice basically equivalent to that of the NiCr foam prepared by [PERSON] and [PERSON] et al. [10; 11; 20]. We then increased the Cr content as much as possible (up to 45%) to obtain higher strength and modulus. Scanning electron microscopy (SEM) was used to observe the surface topography of the NiCr lattice, and EDS was used to measure the elemental distribution of the hollow rod cross section. The metallographic structure of the Ni and NiCr lattices was observed by optical microscopy, and the Feret diameter distribution of 100 grains was extracted from the image by using ImageJ J software. X-ray diffraction (XRD) was used to analyze the phase of the lattice specimens. Their microstructure was observed by using transmission electron microscopy (TEM). A QNESS Q30A+ microhardness tester was used to measure the microhardness of lattice specimens of different compositions. The NiCr lattice material was embedded in epoxy resin, its cross-section was ground and polished. A pressing load of 100 g (quality) was applied to the rods section with a loading time of 10 s. The average measurement of 10 arbitrary points was specified as the microhardness of the sample. Finally, the quasi-static compression performance of the NiCr hollow nickel lattice materials with different compositions were tested by a universal material testing machine. All the compressed samples contained 4 \(\times\) 4 \(\times\) 4 cells, and the indenter displacement speed was set to 2 mm/min.