Microstructure evolution and mechanical properties of P/M Ti-22Al-25Nb alloy during hot extrusion
Introduction
In recent years, Ti2AlNb based alloy, as a promising structure material, has been received more and more attention due to its high strength-to-weight ratio, great fracture toughness and good workability since it was discovered by Banerjee in 1980s [1], [2], [3], [4]. Compared with the conventional titanium alloys, Ti2AlNb alloys include three different phases: O phase (Cmcm symmetry based on Ti2AlNb), hcp α2 phase (Ti3Al, DO19 structure) and bcc β phase (disordered structure) or B2 phase (ordered structure). In addition, their phase equilibria and microstructural evolution are complicated. The volume fraction, size, and morphology of these constituent phases are sensitive to their thermal processing and heat treatment. C.J. Boehlert [5] discovered that the O phase was precipitated as lamellar form within the B2 matrix when the alloy was heated in O+B2 phase region. C. Xue [6], [7] systematically studied morphology and size of O phase and found that yield strength and elongation agreed well with Hall-Petch relationship. In addition, it has been reported that materials cooled from the high temperature B2 single-phase region exhibited an (O+B2) two phase lamellar microstructure with superior mechanical properties [8]. As a second generation of orthorhombic alloys, the Ti-22Al-25Nb alloy, which offering good mechanical properties both at room and elevated temperatures [9], [10], [11], has excellent potential to be applied in engineering practice [12] and a number of beneficial contributions regarding this kind alloy have been conducted by many scientists [13], [14], [15].
Powder metallurgy (P/M), as a widely used material fabrication approach, shows several advantages, involving near net shape forming, low-cost raw material and preparation process [16], [17]. Compared with traditional cast forming, high degrees of chemical homogeneities can be obtained and large scale segregations are avoided in P/M process [18], [19]. Additionally, no matter which kind of the fabrication techniques is adopted, subsequent thermo-mechanical processing is always involved to optimize the microstructure and improve the mechanical properties. Hot extrusion is often used to break down coarse microstructures and obtain preforms with high performance [20], [21]. However, there are few studies describing the effects of hot extrusion on microstructural evolution and mechanical properties of Ti-22Al-25Nb alloys, especially for the P/M Ti-22Al-25Nb alloy.
In this paper, Ti-22Al-25Nb alloy bars were prepared by elemental powder metallurgy and hot extrusion. The effect of hot extrusion was investigated on microstructure, texture and mechanical properties of P/M Ti-22Al-25Nb alloy. Furthermore, microstructure-property relationship of the as-extruded Ti-22Al-25Nb alloy was discussed in detail.
Section snippets
Experiments
In this work, Ti-22Al-25Nb alloy billets were prepared by elemental powder metallurgy (EPM) [22]. Firstly, Ti (99.99%, <325 mesh), Al (99.99%, <200 mesh) and Nb (99.95%, <325 mesh) powders were low-energy milled using a planetary ball-milling machine for 190r/min for a period of 4.0 h under the argon protection and then step-sintered at 630 °C/1.0 h/25 MPa and 1250 °C/2.0 h/35 MPa under a vacuum of 10−2 Pa. The initial microstructure comprising of α2, B2 and O phases was shown in Fig. 1. It can be
Microstructural analysis
According to the phase transformation analysis of Ti-22Al-25Nb alloy conducted by Wang et al. [10] using differential scanning calorimetry, the estimated temperature ranges of the phase fields are TB2>1060 °C,1000 °C<TB2+α2<1060 °C, 975 °C<TB2+α2+O<1000 °C and TB2+O<975 °C, which are confirmed with the typical phase diagram of Ti-22Al-xNb alloy depicted in Fig. 2 [5], [23]. In this work, the extrusion deformation is processed in B2 phase region, and the subsequent cooling process goes quickly through
Conclusions
The present work has investigated the microstructure evolution and mechanical property of Ti-22Al-25Nb alloy prepared by elemental powder metallurgy and subsequent hot extrusion process. Main conclusions are drawn as follows:
- 1.
The Ti-22Al-25Nb alloy bars with excellent performance can be prepared by hot pressed sintering and subsequent hot extrusion from element powders.
- 2.
After hot extrusion at B2 phase region, bulks of α2 phase were eliminated and the B2 grain size was elongated or significantly
Acknowledgments
This work was supported by the National Natural Science Foundation of China (51275129), the National Science and Technology Major Project of China (2014ZX04001-141), and the Sci-tech Major Project in Shandong Province (2016ZDJS03A01).
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