In-situ tensile deformation behavior of powder metallurgy Ti6Al4V alloys
Introduction
In recent years, powder metallurgy (PM) titanium (Ti) alloys have received significant attention because of the advantages of no segregation, fine grain, low cost and near net shape formability [[1], [2], [3]]. In the traditional PM process, PM Ti parts are fabricated by mixing, pressing and vacuum sintering of blended elemental (BE) powders [[4], [5], [6]]. However, the wide application of PM Ti alloys is limited by their low mechanical properties at room temperature [[7], [8], [9]]. Many attempts have been carried out to improve the properties of PM Ti alloys. It has been reported that equal-channel angular pressing (ECAP) [10,11], hydrogen sintering and phase transformation (HSPT) [12,13], and hot isostatic pressing (HIP) [14,15] are able to increase the relative density of the products, thus achieving better mechanical properties. The mechanical properties of Ti6Al4V alloys prepared by different PM processes are comparable with those of wrought parts. Until now, most of the studies on PM Ti alloys have been focused on the process improvement [16,17], while little attention has been paid on the fracture mechanism of PM Ti alloys, especially in the aspects of crack initiation and propagation. According to previous studies [18], the in-situ tensile deformation process which allows to real time track the crack initiation and propagation is proved to be an effective method to study the fracture mechanism of metallic parts. Therefore, it is necessary to investigate the in-situ tensile deformation behavior of PM Ti alloys for better understanding the fracture mechanism.
In-situ tensile deformation process is based on the tensile test platform of scanning electron microscopy (SEM) observation. The in-situ tensile specimen can be dynamically observed and recorded the initiation and propagation of microstructure and crack during the tensile deformation process [[19], [20], [21]]. In general, traditional SEM analysis can only be used to characterize the longitudinal section of the specimen after deformation. Compared with the traditional SEM, the SEM observation of in-situ tensile deformation process can be applied to accurately track the deformation behavior and fracture characteristic of the material in real time [[22], [23], [24]]. In terms of the in-situ deformation behavior analysis, it is conducive to understanding the fracture mechanisms of PM Ti alloys prepared by different processes.
Therefore, it is of interest to systematically investigate the in-situ tensile deformation behavior of PM Ti alloys. In this work, Ti6Al4V alloys were prepared by PM process. In order to improve their properties, parts of the as-sintered samples were forged followed by heat treating. The in-situ tensile deformation process was carried out on the as-sintered and forged PM Ti6Al4V alloys. The crack initiation and propagation of the specimens were observed and recorded dynamically, which provides a scientific theoretical basis for revealing the fracture behavior of PM Ti alloys and improving the PM fabrication process to achieve high mechanical performance.
Section snippets
Material preparation
Commercial Ti6Al4V powders with an average size of 22.5 μm were used as raw materials, which were provided by Tiantailong (Tianjin) Metal Materials Co., Ltd. Micrographs of the raw powders are shown in Fig. 1. The raw powders had irregular shapes. Firstly, the powders were compacted by cold isostatic pressing (CIP, DJY800) at 200 MPa for 60 s. Then, the green compacts were heated to 1300 °C with a heating rate of 2 °C /min and held for 2 h under a vacuum of 10−3 Pa. After that, a furnace
Microstructure and mechanical performance
The chemical composition and density of the as-sintered and forged PM samples are shown in Table 1. It can be seen that the chemical composition of different samples was almost the same, while the densities showed a big difference. The density of the as-sintered sample was about 4.30 g/cm3 with a relative density of 97.0%. After forging, the density increased to 4.42 g/cm3 and the relative density was nearly 100%. The density result was as expected. In general, pores inevitably exist in the
Conclusions
- (1)
In-situ tensile deformation behavior of the as-sintered and forged PM Ti6Al4V alloys was systematically studied.
- (2)
The cracks of the as-sintered samples originated from the pores and propagated rapidly along the phase boundary. The driving force of fracture was normal stress. The connection of micro-cracks caused by cracks and slip leaded to material fracture.
- (3)
The crack initiation of the forged PM sample occurred between fine α lamellae, with a direction of 45° to the loading direction. The driving
Author statement
- 1.
Carried out related experiments: Boxin Lu and Yanru Shao. Beyond that, Boxin Lu performed the data analyses and wrote the manuscript.
- 2.
Helped perform the analysis with constructive discussions: Xinyue Zhang, Cunguang Chen, Haiying Wang and Ce Zhang.
- 3.
Helped to polish, revise the manuscript, provide the conception of this study and conduct the experiment instruction: Fang Yang, Zhimeng Guo.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgment
This study was funded by the State Key Lab of Advanced Metals and Materials (No. 2019-ZD08), the Fundamental Research Funds for the Central Universities (No. FRF-TP-18-025A1), the Guangdong MEPP Fund (Grant No. GDOE[2019]A16), and the National Key R&D Program of China (No. 2016YFB1101201).
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