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论文题目：Influence of stress-induced α'' phase transformation on mechanical properties in metastable β type Ti-5Al-2.5Cr-5Mo-1Sn alloy

发表时间：Available online 23 May 2022

发表期刊： Journal of Alloys and Compounds [ 点击下载PDF ]

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The materials are heat treated at 930℃ for 30 min, and then cooled to room temperature with air and water, respectively. After heat treatment, the air-cooled alloy consists of β and α phases (Fig. 2a and c). Under the condition of slow cooling, the α stabilizing elements Al and Sn and the β stabilizing elements Mo and Cr have enough time to diffuse, so that the α phase precipitates from the β phase to form a two-phase structure. In contrast, the as-quenched microstructure is composed of the single β phase (Fig. 2b and d). The metastable β phase is completely retained to room temperature under quenched conditions. Statistics on the measured β grain sizes and distribution curves fitted by Gaussian function are shown in Fig. 2e-f. The average β grain sizes of air-cooled and as-quenched alloy are 103.4 μm (standard deviation of 38.3) and 118.8 μm (standard deviation of 50.7) respectively, which are approximately equal. Previous studies showed that the size of β grains depends on the holding temperature and holding time above the β transus temperature. The heat treatment processes in this work keep these two parameters constant. Therefore, the influence of the initial β grain size on the mechanical properties of the two heat treatments states can be excluded.

Fig. 2. EBSD analysis of the air-cooled and as-quenched alloys. (a),(b) IPF maps of the air-cooled and as-quenched alloys, respectively. (c),(d) Phase maps of the air-cooled and asquenched alloys, respectively. (e),(f) Statistical histograms and distribution curves for the β grain sizes in (a) and (b), respectively.

By comparison, it is found that the stress-induced α'' phase exhibits the morphology of bands. These bands are identified as an orthorhombic structure (space group: cmcm 63) by the EBSD IPF map (Fig. 7a). The body centered cubic (BCC) β phase and the orthorhombic α'' phase are shown in red and blue respectively in the EBSD phase map (Fig. 7b). The KAM map shows that the localized misorientation angle distribution in the scanned area exhibits non-uniformity (Fig. 7c), reflecting the different degrees of plastic deformation among various microstructures. The ductility of titanium alloy composed of single β phase is usually low, which is due to the intergranular fracture caused by the concentration of strain at β grain boundaries. This phenomenon is also shown in Fig. 7c, the value of average misorientation at the β grain boundary is higher than that in the β grain interior. Be similar with the grain boundaries region, the stress-induced bands region also exhibits a high average misorientation value, which indicates that the degree of plastic deformation in this region is higher than that of β matrix. Therefore, the formation of stress-induced α'' phase during the stretching process plays a role in coordinating the strain. The higher strain accumulation in the bands avoids the strain concentration at the grain boundaries, thus promoting the ductility of the material.

Fig. 7. EBSD analysis of the bands structure after tension. (a) IPF map. (b) Phase map. (c) KAM map.

The as-quenched samples after tension are further characterized by TEM. Bright-field (BF) image reveals the existence of parallel band structures in the β matrix with an average width of ~60 nm (Fig. 8a). The bands are identified as α'' phase by the selected area electron diffraction (SAED) along the [01-1]_β axis (Fig. 8b). Fig. 8c shows the high resolution (HR) TEM image at the interface of β/α''. Combined with HRTEM image and SAED, the orientation relationship between β and α'' phases can be judged as [01-1]_β//[001]_a'', [011]_β//[010]_a''and [100]_β//[100]_a'', as shown in diagram Fig. 8d.

Fig. 8. TEM analysis of the bands structure. (a) TEM BF image. (b) SAED pattern along the [01-1]_β zone axis recorded from the circled area in (a). (c) The HRTEM image of the β/α’’ interface. (d) Schematic diagram of the orientation relationship between the β and α'' phases.