Tensile properties improvement by homogenized nitrogen solid solution strengthening of commercially pure titanium through powder metallurgy process

Abstract

In this study, interstitial solid solution behavior of nitrogen atoms dissolved in commercially pure Ti fabricated by spark plasma sintering (SPS) followed by the extrusion process was investigated. Microstructural observations of the extruded Ti-(N) samples before homogenization, ExTi-(N), revealed the nitrogen element's segregated regions as a result of a short time of the SPS process. Because of the inadequate sintering time, nitrogen atoms were unable to dissolve in the α-Ti matrix homogeneously, which caused a material degradation through brittle failure initiating at high nitrogen absorbed regions. Applying homogenization heat treatment at 1123 K for 180 min followed by hot extrusion, improved microstructural homogeneity without the nitrogen segregation. Furthermore, grain morphology was transformed from coarse elongated grains with substructures into refined equiaxed α-Ti grains. The yield strength and ultimate tensile strength of the Ti-(N) sample with 0.69 wt%N reached maximum 957 MPa and 1117 MPa, respectively, which represented 148% and 214% increments over as-extruded pure Ti sample. In addition, a 50% enhancement of elongation of homogenized Ti-(N) alloys compared to the non-homogenizing condition was achieved. The improvement of strength and elongation was primarily attributed to grain refinement and solid solution strengthening. The quantitative evaluation of strengthening and mechanisms was discussed in detail by the Hall-Petch formula and Labusch model.