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论文题目：Effect of trace carbon on the dynamic compressive properties in the as-cast Ti13V11Cr3Al alloy

发表时间：Available online 7 May 2020

刊源：Intermetallics 123 (2020) 106818 [ 点击下载PDF ]

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Fig. 1 shows the microstructural morphology of the Ti13V11Cr3Al titanium alloy. The average grain sizes of the as-cast and heat-treated alloys remain essentially the same (i.e., ~700–800 μm), as shown in Fig. 1(a) and (d). Trace in-situ spherical precipitates (size: 3–4 μm) are uniformly distributed inside the grains in the as-cast alloy, whereas inhomogeneous distribution occurs for the heat-treated alloy (see Fig. 1 (b) and (e), respectively). Furthermore, the grains of the heat-treated alloy are divided into smaller structures by relatively more precipitates than the as-cast alloy. The precipitates accumulate along the grain boundaries before and after the heat treatment, exhibiting a necklace shape. Fig. 1(c) shows a magnified view of the region enclosed in the red circle (see Fig. 1(b)). The dispersive distribution of relatively fine nanoscale precipitates is observed.

Fig. 1. Microstructural morphology of the Ti13V11Cr3Al titanium alloy: (a) OM image at a magnification of 25 × , as-cast, (b) SEM image at a magnification of 500 ×, as-cast, (c) SEM image at a high-magnification (50k × ) of region enclosed in the red circle shown in (b), (d) OM image at a magnification of 25 × , heat-treated, (e) SEM image at a magnification of 500 × , heat-treated.

Fig. 3 shows the TEM-BF images and TEM-EDX results of the as-cast Ti13V11Cr3Al titanium alloy. The TEM-BF images of the β-Ti matrix and TiC precipitate are shown in Fig. 3(a) and (b), respectively. The selected area electron diffraction (SAED) pattern (Fig. 3(a) inset) confirms the body-centered-cubic structure of the β-Ti matrix. Compared with the SEM image in Fig. 1, Fig. 3(b) reveals more detailed morphology, i.e., needle-like precipitates with aspect ratio »1. The chemical composition of the precipitates is determined via TEM-EDX analysis of two typical positions (Fig. 3(b)) shown in Fig. 3 (c) and (d), respectively. Comparative analysis indicates that the C/Ti atom ratio at position 1 (denoted as the precipitate) is ~1, whereas a rather low C/Ti atom ratio occurs at position 2 (denoted as the β-Ti matrix). Therefore, based on the effect of inevitable deviation, the precipitate is determined to be TiC produced by trace carbon. The electron diffraction spots shown in the inset of Fig. 3(b) can further confirm the existence of the face-centered-cubic TiC phase (JCPDS 659622). Furthermore, a small amount of twin structure (indicated by blue lines in Fig. 3(b) inset) forms during the nucleation and growth of TiC, as reported in previous studies.

Fig.3. TEM-BF images and TEM-EDX results of the as-cast Ti13V11Cr3Al titanium alloy: (a) and (b) are TEM-BF images of the β-Ti matrix and TiC precipitate, the corresponding SAED patterns are shown in the insets, (c) and (d) show the TEM-EDX results obtained from two typical positions shown in (b).

ASB is considered a precursor for titanium alloy failure under high strain rates. To validate the deduction that in-situ trace TiC is beneficial to improve dynamic plasticity, the stop rings are used to control strains for the as-cast Ti13V11Cr3Al titanium alloy under dynamic compression loading. An ASB-containing sample (representing the onset of failure) at a strain of ε = 31% is obtained. Fig. 8(a) shows that the main crack is initiated and then propagates along a 45° shear direction. ASBs are observed in the areas where the crack coalescence is insufficient, showing a white-etching band. Fig. 8(b) shows a high-magnification image of the region enclosed in the yellow ellipse (see Fig. 8(a)). A width of ~10 μm is determined for the ASB. Fig. 8(c) shows a highmagnification image of the region enclosed in the red ellipse (see Fig. 8(b)). The interface debonding behavior occurs between nanoprecipitates and matrix, thereby resulting in microscopic grooves in the ASB. This indicates that the TiC precipitates will be pulled out from the matrix by overcoming the friction resistance of the interface. Therefore, considerable energy dissipation is required before the rapid propagation of the main crack, thereby suppressing premature failure and improving the dynamic plasticity of the as-cast Ti13V11Cr3Al titanium alloy. Furthermore, the microstructure comprising the crack tip is revealed by a magnified view (see Fig. 8(d)) of the region enclosed in the blue ellipse (see Fig. 8(b)).

Fig. 8. Microstructural evolution of the ASB: SEM image obtained at a magnification of (a) 100 × , (b) 1500 × , presenting a magnified view of the region enclosed in the yellow ellipse shown in (a), (c) and (d) 20k × , presenting a magnified view of the regions enclosed in the red and blue ellipses, respectively, shown in (b).