Quantitative and Qualitative Relationship between Microstructural Factors and Fatigue Lives under Load- and Strain-Controlled Conditions of Ti–5Al–2Sn–2Zr–4Cr–4Mo (Ti-17) Fabricated Using a 1500-ton Forging Simulator

Abstract

The fatigue lives of forged Ti-17 using a 1500-ton forging simulator subjected to different solution treatments and a common aging treatment were evaluated under both load-and strain-controlled conditions: high and low cycle fatigue lives, respectively. Then, the tensile properties and microstructures were also examined. Finally, the relationships among fatigue lives and the microstructural factors and tensile properties were examined.The microstructure after solution treatment at 1203 K, which is more than the ¢ transus temperature, and aging treatment exhibits equiaxed prior ¢ grains composed of fine acicular ¡. On the other hand, the microstructures after solution treatment at temperatures of 1063, 1123, and 1143 K, which are less than the ¢ transus temperature, and aging treatment exhibit elongated prior ¢ grains composed of two different microstructural feature regions, which are acicular ¡ and fine spheroidal ¡ phase regions. The 0.2% proof stress, · 0.2 , and tensile strength, · B , increase with increasing solution treatment temperature up to 1143 K within the (¡ + ¢) region, but decrease with further increasing solution treatment temperature to 1203 K within the ¢ region. The elongation (EL) and reduction of area (RA) decrease with increasing solution treatment temperature, and it becomes nearly 0% corresponding to a solution treatment temperature of 1203 K. The high cycle fatigue limit increases with increasing solution treatment temperature up to 1143 K, corresponding to the (¡ + ¢) region. However, it decreases with further increase in the solution treatment temperature to 1203 K in the ¢ region. The fatigue ratio in high cycle fatigue life region is increasing with decreasing solution treatment temperature, namely increasing the volume fraction of the primary ¡ phase, and it relates well qualitatively with the volume fraction of the primary ¡ phase when the solution treatment temperature is less than the ¢ transus temperature. The low cycle fatigue life increases with decreasing solution treatment temperature, namely increasing the volume fraction of the primary ¡ phase. The low cycle fatigue life relates well quantitatively with the tensile true strain at breaking of the specimen and the volume fraction of the primary ¡ phase for each total strain range of low cycle fatigue testing. [

Quantitative relationship between microstructural factors and fatigue life of Ti-5Al-2Sn-2Zr-4Cr-4 Mo (Ti-17) fabricated using a 1500-ton forging simulator

“…The heat treatment processes are schematically shown in Fig. 1 [6]. The samples subjected to aging after the different STs were denoted as ST/1063 K, ST/1123 K, ST/1143 K, and ST/1203 K, according to the ST temperatures.…”

“…5 [6]. Therefore, the relationship between the R value and V(%) can be expressed by the following equation [6]. R = 0.80・e 0.0029V (%) (1) N fs , was found to be well related to the tensile true strain at the breaking of the specimen (ε tt ) as shown in Fig.…”

“…7 [6]. Therefore, the relationship between N fs and ε tt can be expressed by the following equation for each Dε t [6].…”

Recent activities of titanium research and development in Japan

“…Next, the forged specimens were solution treated and aged (STAed), their microstructure evolution was investigated, and a prediction model for STA was established [6]. The tensile strength and fatigue life of the specimens forged using the 1500 t press and STAed were tested to understand the microstructural factors that control the mechanical properties, and a prediction model for tensile strength was established [7,8]. To apply the prediction model to the 1500 t forging press, coefficients of friction and thermal transfer were determined by comparing experiments conducted using the hot forging emulating system and Finite Element Method (FEM) [9].…”

Effect of microstructure on tensile properties of Ti-17 alloys forged using a 1500-ton forging simulator

“…The microstructures of as-forged samples and the solution treated and aged samples using the 1500 ton forging press were observed and the prediction models were improved to describe them precisely. The effect of the microstructure factors such as the volume fraction of the primary and secondary alpha phase and the grain size and the aspect ratio of the beta phase on tensile properties and fatigue was also investigated for samples forged using 1500 ton forging press, solution treated and aged materials [11,12]. These models were connected each other as a subroutine and inserted to FEM software to simulate microstructure and tensile strength.…”