| Ti5Mo5V3Al-xCr(x= 1.0-9.0) alloys are new β titanium alloys with good balance of higher tensile strength and good toughness. In this paper, five different Cr content of Ti5Mo5V3Al-xCr(x=1.0,3.0,5.0,7.0,9.0) alloys were chosen for the research. The calculation method, the metallographic method, differential scanning calorimetry (DSC) and method of in situ thermal expansion were used to study the solid phase transformation in continuous cooling process and the isothermal aging process, respectively. Isothermal compression testing of alloys are performed within a temperature range of 923-1173K and strain rate range of 0.001-1.0s-1. The effects of temperature and strain rate on hot compression behavior of alloys with different Cr content are discussed. The mechanical properties of alloys in the different heat treatment process were studied with the methods of analysis including optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The effects of Cr content on the properties and microstructures of alloys are discussed and the comprehensive performances of alloys are analyzed. The application characteristics of alloys and the potential engineering application types are discussed based on the research results. The main results are summarized as follows:The temperature of a+β/β phase transition decreased linearly with increasing’Cr content. With increasing 1% Cr content, the T(a+β)/β decreased 17.5℃. The microstructures of alloys at the different temperature show the ability to stable β phase improved with increasing Cr content, and the type of alloys are transformed from near β titanium to stable β titanium alloy gradually according to Mo equivalent.The precipitation rates of a phase of 1Cr and 3Cr alloys are high in the investigated temperature range(420℃～620℃).5Cr and 7Cr alloy show the fastest rates of the precipitation of a phase in the temperature range of 520℃-620℃.9Cr alloy shows the slowest phase transition rates in the whole of investigated temperature range.The compression deformation behaviors of a newly developed Ti5Mo5V3Al-xCr (x=1, 3,5,7 and 9) alloys are investigated at temperatures ranging from 923 to 1173K and under constant strain rate loadings ranging from 0.001 to 1.0 s-1. Low stress level flow stress equation of alloys when deformed in β phase region and high stress level flow stress mode of alloys when deformed in a+β phase region are established. Theoretical model predictions are in agreement with the experimental results. Each alloy’s dynamic stress-strain response, temperature sensitivity, strain rate sensitivity and hot deformation activation energy are discussed. The results show the temperature sensitivity of flow stress of Ti5Mo5V3Al-xCr alloys decreases with the increase of Cr content and strain rate. The strain rate sensitivity of flow stress of alloys increases with the increase of deformation temperature and which is insensitive to the Cr element content of alloys over the range investigated (1.0%-9.0%). The hot deformation activation energy of alloys both in (a+β)-phase region and β-phase region are discussed. When the alloys were deformed in (a+β)-phase region, the deformation activation energy of alloys are consistent with the proportion of a-phase in alloys, and which are all decreased with the increase of Cr content. When the alloys were deformed in β-phase region, the deformation activation energies of alloys are associated with the starting grain size prior to deformation, and the activation energy increases with increasing the grain size.The mechanical properties results show the aging strengthening capacity of alloys decreased with increasing Cr content. The aging strengthening mechanism of Ti5Mo5V3Al-xCr alloys is mainly caused by the precipitation of as phase in the process of aging treatment. The aging strengthening capacity of alloys is closely related to the content of as phases and the size of as phase. The precipitation mechanism of as phase is mainly dominated by the content of Cr element in Ti5Mo5V3Al-xCr alloys. Equivalent aging process of alloys (1Cr,3Cr,5Cr and 7Cr) can be written as [575-15(x-1)]℃/[2+1.5(x-1)]h, and 1≤x≤7. Based on the phase stability, strengthening effect, strength-plastic matching and thermal plastic processing performance of alloys,5Cr alloy shows an excellent comprehensive performance.Analysis of engineering applications show 1Cr and 3Cr alloy are suitable for use as high-strength and high hardness structures through thermal deformation forming.5Cr alloy can be formed by the methods of cold or thermal deformation, and it is suitable for use as the structures with high strength and high toughness.7Cr and 9Cr alloy are suitable for forming by cold deformation and used as cold-rolled sheets.9Cr alloy is also suitable for warm drawing and used as wires. Overall, the suitable deformation methods of alloys are transitioned from cold deformation to thermal deformation with increasing Cr content. |
PDF Full Text Request