Thermomechanical fatigue behavior of the high-temperature titanium alloy IMI 834

Abstract

The isothermal and thermomechanical fatigue (TMF) behavior of the titanium alloy IMI 834 was studied between 350 ЊC and 650 ЊC in air and vacuum, respectively. Transmission electron microscopy (TEM) observations revealed that the microstructure established in the TMF tests was governed by the maximum temperature within the cycle. However, if the maximum temperature does not exceed 600 ЊC, planar dislocation slip prevails and similar microstructures are formed regardless of the test temperature and the testing mode (TMF and isothermal, respectively). As a result, the stressstrain response in TMF tests can be assessed from the corresponding isothermal data. Wavy dislocation slip was found to determine the stress-strain behavior if the maximum test temperature exceeded 600 ЊC. Moreover, in TMF tests with a maximum test temperature of 650 ЊC, the dislocation arrangement formed in the high-temperature part of the hysteresis loop was found to be stable throughout the cycle and to affect significantly the stress-strain response at the low temperatures. Although in-phase (IP) and out-of-phase (OP) TMF tests led to an almost identical microstructure, OP loading was always found to be most detrimental. The interaction between the embrittled subsurface layer, caused by oxygen uptake, and the high tensile stresses developing in the low-temperature part of the hysteresis loop in OP tests eases crack initiation and initial crack propagation and results in reduced fatigue life.

Fatigue of the Near-Alpha Ti-Alloy Ti6242

“…Comparing the LCF and TMF behaviour of Ti6242 with IMI834 [4,10], it can be stated that Ti6242 exhibits a slightly inferior performance. Due to a different chemical composition, the recrystallisation heat treatment can take place closer to the β-transus temperature in the case of IMI834.…”

“…In near-α titanium alloys it has always been observed [4,8] that OP leads to shorter lifetimes when compared to IP. This indicates that environmental attack is more harmful than creep.…”

“…It was found that the maximum cyclic stress of the isothermal test at 650°C matches the maximum stress of the IP test, and also lifetimes were of similar magnitude. Taking into account further results [4], it can be assumed that a prediction of IP behaviour is possible on the basis of isothermal tests at the maximum temperature of the TMF cycle. However, this might be not true for OP conditions.…”

“…However, as the aluminium content increases, the transition temperature for dislocation arrangement increases, too [18]. Pototzky [4], estimated this transition to occur at 600°C for IMI834. Since Ti6242 has the same aluminium content as IMI834 (i.e., 6 mass%), the change in dislocation arrangement should take place at similar temperatures.…”

“…Another important factor to be considered is that if the component is constrained, thermal gradients may cause additional cyclic strains in the material, resulting in thermomechanical fatigue. Almost all high-temperature fatigue investigations have been conducted on the advanced near-α alloy TIMETAL 834 [3][4][5][6][7][8][9][10]. Therefore this paper aims at understanding the (thermal) fatigue mechanisms in Ti6242 and establishing a simple model for fatigue lifetime prediction.…”