Fatigue life of Ti alloys is decreased by tensile stress dwell even at room temperature, which is called Cold Dwell Fatigue (CDF). CDF is similar to creep-fatigue of some metals (stainless steel, Ni-base alloy, and so on) in the point of decrease of fatigue life with stress or strain dwell at high temperature. In this study, time exhaustion rule and ductility exhaustion rule based on linear cumulative damage rule were calculated for CDF life assessments. In the case of time exhaustion rule, total damage was calculated as D F +D C < (0.03, 0.015) (D F : fatigue damage, D C : creep damage). In the case of ductility exhaustion rule, total damage was calculated as D F +D C~1 . Additionally, the feature of fracture surfaces after CDF tests changed with increase of dwell time. This trend corresponded to the balance between fatigue damage and creep damage calculated by ductility exhaustion rule.
Low cycle fatigue life of Ti and Ti alloys decreases due to tensile stress dwell even at ambient temperature. This phenomenon is called Cold Dwell Fatigue (CDF). CDF properties are affected by dwell time, test stress, and microstructure. In our previous study, the effects of dwell time and the life assessment by linear cumulative damage rule were reported. The effects of test stress and microstructure for CDF properties and life assessment are discussed in this study. Test stress was from 0.85 to 0.955 0.2 and influenced fatigue life. As for strain accumulation during stress dwell, rupture time was affected by both test stress and microstructure. On the other hand, rupture elongation was affected by only test stress. Considering stress dependence on rupture elongation was important for ductility exhaustion rule in linear cumulative damage rule and lead to calculation of creep-fatigue damage as DF+DC~1 (DF: fatigue damage, DC: creep damage).
The deformation behavior and microstructure evolution of Ti-6Al-4V alloy under room temperature creep was investigated using mechanical test and scanning electron microscope observation with electron back-scatter diffraction method. The alloys were creep deformed and ruptured under initial stresses of 874 MPa, 889 MPa and 904 MPa at room temperature. The rapid stress change test revealed that creep deformation was controlled by the viscous slip motion of dislocations. The stress exponent was estimated as 59. The strain rate of acceleration creep region calculated by the Norton s law with the high stress exponent was inconsistent with that measured by the experimental creep test. Using the slip trace analysis, it was found that single dislocation slip in basal and prism were mainly activated in the early stage of creep, and multiple slips were often observed as the deformation progresses. Especially, the multiple slip including 1st pyramidal slip believed to be effective for suppressing strain rate acceleration in creep. In addition, the work hardening behavior during creep showed a strain rate dependence, indicating that the lower the strain rate is, the more work hardening occurs.
Low cycle fatigue (LCF) life at ambient temperature of Ti alloys is well known to decrease with stress dwell. This phenomenon, called cold dwell fatigue (CDF), is influenced by the peak stress, dwell time, and microstructure. For this study, the CDF life was evaluated by the linear cumulative damage rule.The influence of test conditions and microstructure on the linear cumulative damage rule was also verified. By the linear cumulative damage rule, when creep damage is calculated using the time exhaustion rule, theCDF damage was evaluated by the inequality of D Total = (D F , D C ) ≤ (0.01, 10 À6 ). However, the CDF damage can be evaluated in the range of D Total = 0.6-1.2 when creep damage was calculated using the ductile exhaustion rule. Results indicate that the evaluation was almost independent of the dwell time, peak stress, and microstructure, so it is also a versatile method for evaluating CDF responses.
In this study, we conducted a quantitative evaluation of dislocation density by scanning electron microscopy electron channeling contrast imaging for α grains of a Ti-6Al-4V alloy deformed at room temperature. The depth of visibility of dislocations is experimentally measured as 140-160 nm by a serial sectioning observation. This result is compared to the theoretical value and applied to evaluate dislocation density. These factors confirm that the theoretically calculated value of the depth of visibility, at five to six times the extinction distance, is valid for the HCP-Ti alloy.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.