Enabling the development of ductile powder metallurgy titanium alloys by a unique scavenger of oxygen and chlorine
Graphical abstract
Introduction
The oxygen (O) content has been one of the key issues that affect the powder metallurgy Ti business in terms of the cost of production and the resulting mechanical properties [[1], [2], [3], [4], [5], [6]]. Reducing the detrimental effect of oxygen therefore plays a key role in enabling the development of low-cost high performance PM Ti alloys. The oxygen content of the sintered part strongly depends on the oxygen content of the raw Ti powder used, where the oxygen is mainly present in the form of TiO2, Ti2O3 and TiO from the outermost surface to innermost layer in the surface oxide film on Ti powder [7]. These oxides start to dissolve into the underneath titanium matrix from about 670 °C [7], resulting in the largely reduced ductility.
Currently, in order to reduce the decline of ductility, rare-earth-containing (RE-containing) hydrides have been introduced to scavenge the oxygen from the Ti solid solution during sintering [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17]]. However, the significant scavenging by RE-containing hydrides occurs after the surface titanium oxide films have disappeared. The oxygen-scavenging process is controlled by the diffusion of oxygen [9,13]. Oxygen is therefore difficult to be fully scavenged, evidenced by the resulting oxygen-deficit RE oxides after even 120 min of scavenging at 1350 °C [13,14]. As a result, the improvement of the ductility is still limited. The practical solution is to develop new scavenger that ideally can consume the surface oxide film prior to its significant dissolution into Ti. The recent study showed that LaB6 was able to scavenge oxygen before the disappearance of oxide film [7]. NdB6 is another type of commercially available lower-cost RE-containing compounds (USD $0.35/g) and has similar crystal structure to LaB6. However, it remains unknown if the unique oxygen scavenging feature also happens on NdB6.
This paper presents a detailed study of whether and how NdB6 can scavenge O from Ti powder during sintering, particularly focusing on whether the surface oxide films can be consumed by NdB6 before they dissolve into the titanium matrix. In addition, commercially pure Ti (CP-Ti), Ti-6Al-4V and Ti-10V-2Fe-3Al (weight percent throughout) were selected as the base alloy to investigate the effect of NdB6 addition on the sintering densification, microstructure and mechanical properties. In comparison of the effectiveness of NdB6 in improving the mechanical properties, other RE hydrides were introduced as references. These results are important for capitalizing on the scavenging effect of NdB6 for the fabrication of low cost high performance PM Ti alloys.
Section snippets
Experimental procedure
Hydride-dehydride (HDH) Ti powder (−250 mesh, 99.4% purity, 0.25 wt.% O) supplied by General Research Institute for Nonferrous Metals, China, and two master alloy powder products supplied by Baoji Jia Cheng Rare Metal Materials Co. Ltd., China 66.7V-13.3Fe-20Al (−325 mesh, 99.5% purity) and 40V-60Al (−325 mesh, 99.5% purity) were used. NdB6, LaH2 and YH2 powders (−500 mesh, purity 99.5%) used were supplied by Yaguang New Materials Co. Ltd., China. Elemental B supplied by the same company was
Phase identification
Fig. 1 shows the XRD results of CP-Ti-1.0NdB6, Ti-6Al-4V-1.0NdB6 and Ti-10V-2Fe-3Al-1.0NdB6 after sintering at 1350 °C for 120 min NdB6 disappeared in each case. Apart from the α and β phases, Nd2O3 is detected in each as-sintered sample, indicative of scavenging of oxygen by NdB6.
Fig. 2 shows the as-sintered microstructures of CP-Ti and CP-Ti-0.3NdB6 after sintering at 1350 °C for 120 min. The bright particles observed in the as-sintered CP-Ti-0.3NdB6 were confirmed to be all Nd-containing
Scavenging pathway of O and Cl by NdB6
The scavenging of O by NdB6 began by forming an interfacial NdBO3 layer at 670 °C, see Fig. 5b. Because the dissolution of oxygen into the underneath titanium matrix started with the thermodynamically least stable innermost TiO from about 670 °C, the NdBO3 layer formed at 670 °C was caused by the reaction between NdB6 and the outer TiO2 layer in the surface oxide film. However, the diffusion coefficient of O in α-Ti is sluggish (∼1 × 10−13 m2/s) [19], NdB6 therefore had a long time to react
Conclusion
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NdB6 can scavenge O prior to the active dissolution of the surface oxide film. It begins by forming an interfacial NdBO3 layer from 670 °C due to the reaction between the titanium surface oxide film and NdB6, followed by diffusion of oxygen through the loose NdBO3 layer. The scavenging of oxygen essentially finishes at 1130 °C during heating. NdBO3 decomposes into Nd2O3 above 1130 °C, which enables the subsequent scavenging of chlorine to form NdClxOy. The scavenging of chlorine mainly occurs
Acknowledgements
The work was supported by the Key Research Program of Frontier Sciences (Grant No. QYZDB-SSW-JSC045), Chinese Academy of Sciences and National Youth Thousand Plan Program.
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