# Synthesis and Properties of Octet NiCr Alloy Lattices Obtained by the Pack Cementation Process
## 3 Results and Discussion
### Compression Curve and Mechanical Properties  
Figure 7 shows the stress-strain curve corresponding to the quasi-static compression of the Ni and NiCr alloy lattices at room temperature. It can be seen in the Figure that during the quasi-static compression process, the lattices presented a linear elasticity region, a plateau region and densification stages, showing the deformation characteristics of typical elastoplastic porous materials. In the linear deformation region, the compressive stress increased linearly with the increase of the strain [33]. The slope of the initial linear phase of the stress-strain curve provided the compression modulus E of the NiCr lattice. At a strain of about 0.05, the stress reached a non-proportional stress Rp1.0, corresponding to the compressive strength. This was followed by a long stress plateau region, where the strain increased, but the stress remained largely unchanged. With the bending, buckling and plastic deformation of the truss rod, the middle position of the lattice was compacted firstly, and the stress was released. The compressive stress was transmitted to the undeformed skeleton part through the deformed skeleton portion, and the above process was repeated, resulting in a slow rising of the stress plateau area. All curves of the nickel-based lattice materials with different compositions showed long stress plateau regions, without significant stress peaks and stress drop stages. The long stress plateau region showed good toughness, indicating that no brittle fractures occurred at the rod and node sites during the lattice deformation. In the densification stage, the stress increased rapidly with the strain due to the compression of the pores of the lattice, and the lattice at this time was equivalent to a dense metal.  
Figure 6: Microhardness of Ni and NiCr alloys.  
The mechanical properties of the lattices with different alloys are shown in Figure 8. With the increase of Cr content and density, the compressive strength and modulus of the lattice NiCr alloys gradually increased. In order to compare mechanical properties of the lattice of different alloy components together in Figure 8, the actual modulus and specific modulus needed to be multiplied by 100 on the values. (Similarly, the value of energy absorption efficiency in Figure 9 needed to be multiplied by 0.1). The compressive strength of the lattices of pure Ni, Ni-16 Cr, Ni-23 Cr, Ni-34 Cr and Ni-45 Cr alloys were 0.7 MPa, 3.1 MPa, 5.3 MPa, 6.8 MPa and 7.3 MPa, respectively. The elastic moduli were 65.2 MPa, 286 MPa, 557.7 MPa, 666 MPa and 771.6 MPa, respectively. The lattice strength and modulus of the Ni-45 Cr alloy were about 10 times and 12 times higher than those of the pure nickel lattice.  
Figure 8: Mechanical properties of different NiCr alloys.  
Figure 7: Stress–strain curves of NiCr alloy lattices with different compositions.  
The specific strength \(\sigma/\rho\) and specific modulus E/\(\rho\) of a lattice can be obtained by density normalization [6; 9; 25]. The specific strengths of pure Ni, Ni-16 Cr, Ni-23 Cr, Ni-34 Cr and Ni-45 Cr lattices were 1.8 MPa/(g/cm\({}^{3}\)), 6.7 MPa/(g/cm\({}^{3}\)), 9.9 MPa/(g/cm\({}^{3}\)), 11.8 MPa/(g/cm\({}^{3}\)) and 11.1 MPa/(g/cm\({}^{3}\)), respectively. The specific moduli were 171.6 MPa/(g/cm\({}^{3}\)), 628.6 MPa/(g/cm\({}^{3}\)), 1048.2 MPa/(g/cm\({}^{3}\)), 1156.3 MPa/(g/cm\({}^{3}\)) and 1169.1 MPa/(g/cm\({}^{3}\)), respectively. The specific strength and specific modulus of the Ni-45 Cr alloy were about six times and seven times higher than those of the pure nickel lattice.  
The mechanical performance improvement was attributed to the increased density and the solution strengthening effect provided by the alloy elements. When the wall thickened and the bearing area became larger, the lattices were more resistant to rod buckling and lattice plastic deformation. These phenomena have been described in the study of NiCr, NiCrAl and NiCrFe alloy foams [10; 20; 21].