Iranian Journal of Materials Science and Engineering, Vol. 18, Number 1, March 2021 RESEARCH PAPER 110 Effect of Post-Annealing Process on Nb-Hf Alloy Produced by Spark Plasma Sintering Sasan Ranjbar Motlagh, Hosein Momeni * and Naser Ehsani * h_momeni@mut.ac.ir Received: October 2020 Revised: December 2020 Accepted: January 2021 Faculty of Material & Manufacturing Technologies, Malek Ashtar University of Technology, Iran DOI: 10.22068/ijmse.18.1.11 Abstract: In this study, the effect of annealing treatment on microstructure and mechanical properties of the Nb- 10Hf-1Ti wt.% produced by Spark Plasma Sintering (SPS) was investigated. Scanning electron microscope (SEM), optical microscopy, X-ray diffraction analysis, hardness, and uniaxial tension test were used to characterize the samples. Annealing treatment was carried out in a vacuum of 10 -3 Pa at 1150 °C for 1, 3, 5, and 7 hours and in an argon atmosphere at 1350 °C for 5 hours. Internal oxidation and subsequent hafnium oxide formation causes the hardening of the C103 alloy and drastically increases the hardness and tensile strength. However, HfO 2 particles formed at the grain boundary cause brittleness and cleavage fracture of samples. Volume fraction, particle size, and mean inter-particle spacing of oxides significantly change by annealing which affects the mechanical properties. The SPSed sample at 1500 ˚C is softened by annealing at 1150 ˚C for 5 hours and its hardness and yield strength are reduced from 303 to 230 Hv and 538 to 490 MPa respectively. While annealing at 1350 ˚C for 5 hours increases hardness and yield strength increases to 343 Hv and 581 MPa respectively. Keywords: Annealing treatment, Nb-Hf alloy, Spark plasma sintering, Internal oxidation. 1. INTRODUCTION Niobium alloys with their high melting temperatures and low density are a good substitute for nickel-based superalloys. On the other hand, niobium has poor oxidation resistance and exhibits moderate strength at high temperatures. The addition of elements such as Ti, Hf, W, and Zr increases the strength and corrosion resistance of niobium [1]. C103 by compound Nb-10Hf-1Ti wt. % is one of the important niobium base alloys that has a good production capability. The common method of producing niobium alloys is the use of casting methods such as vacuum arc remelting [2]. The economic aspects of producing niobium alloy castings were also severely hindered by the loss of metal in the form of gates, skulls, and risers. Unlike titanium and nickel alloys, the gates and other casting scrap are not remeltable for niobium alloys due to the low tolerance for interstitial impurities [3]. Powder metallurgy can help in the production of homogeneous chemical composition and microstructures. And also provide the production of dense and close to the final piece shape [4]. Spark plasma sintering (SPS) is a new sintering method used to sinter powders under the effect of current and pressure. SPS has advantages such as application for different materials, high relative density, short sintering time, and production of components with fine grain structures [5]. Several studies have been done on the SPS of niobium [6] and niobium alloys [7,8], and particularly on Nb- Si alloys [9-11]. The effect of SPS process parameters on the microstructure and mechanical properties of Nb-Zr alloy are investigated. The results show that by increasing sintering temperature, the densification of samples is partially increased but their hardness increased significantly due to the reduction of pores [12]. Effect of annealing heat treatment on microstructure and mechanical properties in oxide dispersion strengthened (ODS) alloys fabricated by mechanical alloying and SPS was investigated. The results showed that the microstructure evolution was hardly noticed by annealing at low temperature. Although the grain structure and the nano-oxide of the SPSed sample occur coarse after high temperature heat treatment. it still maintained a high tensile strength [13]. In another case with the same production condition, the hardness of samples decreased and the average grain size increased by increasing annealing temperature [14]. Compared to other more common materials like steel or titanium alloys, niobium alloys have high flow stresses at hot working temperatures. This can lead to very high pressures on metalworking equipment tooling.