不同Mo当量钛合金的显微组织演变与低温变形机理

Microstructure Evolution and Cryogenic Deformation Mechanism of Titanium Alloy with Different Mo Equivalent

摘要:

通过调整Mo当量，研究了低温钛合金棒材的显微组织演变及室温和低温下的变形行为与变形机理。研究结果显示，提高Mo当量，β相稳定性提高，α相析出受限。高Mo当量合金形成单相β晶粒，无α相析出；低Mo当量合金形成网篮组织。室温下，稳定β钛合金Ti-38V具有较高塑性，是由于室温下bcc-β相比hcp-α相具有更多滑移系，滑移能力强，塑性变形能力较好，断口呈现大量韧窝特征；(α+β)型钛合金Ti-3Al-6Mo具有较高强度，主要归因于相互交错α层片的强化作用。(α+β)型钛合金具有较高的Schmid因子值，位错滑移容易，在室温/低温以位错滑移为主，低温变形过程中伴随有孪晶变形；稳定β钛合金在晶界处出现位错塞积，存在大量应力集中，且V含量较高，使β稳定性较高，低温变形过程中没有来得及发生应力诱导机械孪晶就失效断裂，变形能力较差，低温下呈现脆性断裂特征。

Abstract:

The microstructure evolution and deformation mechanism at room temperature and cryogenic temperature of cryogenic titanium alloy bars were investigated through controlling Mo equivalent. The results show that with the increase in Mo equivalent, β phase stability is improved, which limits the precipitation of α phase. High-Mo-equivalent alloy forms single-phase β grains without α phase precipitation. As for the low-Mo- equivalent alloy, the basketweave microstructure occurs. At room temperature, the plasticity of stable β alloy Ti-38V is higher, which is attributed to bcc-β phase possessing more slip systems than hcp-α phase. The slip ability of β phase is better than α phase, leading to the excellent plastic deformation ability of β phase. As a result, a large number of dimples occur in the tensile fracture. The strength of (α+β) alloy Ti-3Al-6Mo is higher, which is mainly caused by the strengthening effect of interweaved α lamella. The higher Schmid factor of the (α+β) titanium alloy makes the dislocation slip easier, which makes the dislocation slip act as the primary deformation mechanism at both room and cryogenic temperatures, while deformation twins can also be activated at cryogenic temperature. For the stable β titanium alloy, stress concentration occurs at the grain boundary because of dislocation pile-up. The higher V content increases β stability, which makes the fracture sample at cryogenic temperature without stress-induced mechanical twins. Consequently, the deformation ability is poor and it exhibits brittle fracture characteristics at cryogenic temperature.