Oxygen scavenging, grain refinement and mechanical properties improvement in powder metallurgy titanium and titanium alloys with CaB6
Graphical abstract
Introduction
Due to its near-net-shape capabilities, homogenous microstructures, and isotropic properties, powder metallurgy (PM) technology has long been regarded as a promising method to reduce the cost of Ti and Ti alloys [1]. But, one inevitable issue with the PM Ti alloys is that interstitial oxygen exists, which significantly affects not only ductility but also the stress-corrosion cracking resistance and fracture toughness [[2], [3], [4]]. For example, the tensile elongation of Ti-6Al-4 V drops from over 10% to below 5% once the O content exceeds 0.33 wt%, and further to <2% when the O content is obove 0.45 wt% [5]. For PM Ti alloys, they usually contain a much higher level of O than conventional ingot metallurgy (IM) Ti alloys due to the inevitable contact with air of Ti powder. Now, most low-cost hydride–dehydride (HDH) Ti powder products contain over 0.25 wt% O. The subsequent powder treatment and sintering process will easily increase 0.1 wt% O, exceeding the critical value of 0.3 wt% (the ASTM Standard B988–13 for PM Ti) for most Ti alloys. Therefore, mitigating the detrimental effect of O in PM Ti plays a key role in preparation of low-cost and high-performance Ti alloys.
Rare earth (RE) elements have been long known as the efficient scavengers of O for both IM and PM Ti alloys due to their higher chemical affinity with O [[6], [7], [8]]. RE elements are usually introduced into PM Ti alloys as the form of RE hydride [9] (YH2, LaH2), RE-containing compounds like borides [10,11] (LaB6) or silicides [12] (CeSi2) and some kinds of RE master alloy [13] (Al-Nd). But, due to the cost and scarcity of RE, the most pressing task is to seek a new compound as the O scavenger to replace the RE for the preparation of low-cost and high-performance Ti alloys.
The calcium (Ca) and its compounds were used in the deoxidation of Ti. For instance, the O content contained in the powder was decreased from 2500 ppm to 920 ppm with the use of a non-contact deoxidation pot and Ca vapor [14]. But, elemental Ca is unstable and low-melting-point. It is hard to be introduced in Ti alloys. Among the Ca compounds, calcium hexa boride (CaB6) appears to be one such promising candidate. Firstly, it is commercially available and very stable at room temperature. Then, it is cheaper than all forms of RE-containing compounds, while O has a higher chemical affinity with Ca than that with RE. Finally, B would enhance the sintered density and refine the grain, and the in-situ TiB is an effective reinforcement phase for Ti alloys [15].
To date, no related reports about the introduction of CaB6 in PM Ti alloys. It is unknown if CaB6 will be able to scavenge O from the Ti powder and how the mechanical properties will respond accordingly. This paper presents a systematic study of the effects of CaB6 in CP-Ti and Ti-6Al-4 V on the sintering density, O scavenging, microstructural evolution and mechanical properties.
Section snippets
Experimental procedure
The raw materials were HDH Ti powder (99.5% purity, 0.35 wt% O, −325 mesh), Ti-6Al-4 V powder (99.5% purity, 0.4 wt% O, −325 mesh), CaB6 powder (99.9% purity, D10 = 0.64 μm, D50 = 5 μm, D90 = 18.6 μm), B powder (99.9% purity, D50 = 5 μm). The SEM micrographs of the powders are shown in Fig. 1, in which both HDH Ti and Ti-6Al-4 V powder have the irregular morphology with rough and uneven surface, while CaB6 powder has a regular cubic shape. HDH Ti and Ti-6Al-4 V powder were separately mixed with
Relative density
The relative densities of the compacts after CIP are approximately 84.6 ± 1.3%. The relative densities of the sintered samples are shown in Fig. 2. The relative density of Ti-xCaB6 increased first with increasing CaB6 content from 0 to 0.2 wt%, reaching its maximum value 99.3%, then decreased afterwards. The SEM images of Ti and Ti-0.2CaB6 are shown in Fig. 3(i) and Fig. 3(j), respectively. There are a few pores about 2–12 μm distributing in the Ti matrix, but no visible pores are shown in the
Conclusions
The density, microstructure, and mechanical properties of as-sintered Ti-xCaB6 and Ti-6Al-4 V-yCaB6 samples were investigated, and the following results were obtained:
- (1)
CaB6 appears to be one promising addition to Ti and Ti alloys by forming Ca-Ti-O particles to scavenge the solid solution O, resulting in the improvement of sinter-ability. The relatively density increases from 97.1% to 99.3% in Ti-0.2CaB6 and from 97.2% to 98.8% in Ti-6Al-4 V-0.1CaB6. The improvement is the function of both Ca
Acknowledgements
This work was supported by the State Key Lab of Advanced Metals and Materials (No. 2016-ZD02).
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