Abstract
Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface. However, whether the very-high-cycle fatigue (VHCF) property can be improved by GS is questioning due to the different crack initiation mechanisms between low-, high-cycle and VHCF. In this paper, GS of a Ti-6Al-4V alloy is generated by pre-torsion and characterized by electron backscatter diffraction. Then the VHCF behavior of the GS specimen is studied. The fractography and synchrotron radiation X-ray microtomography presented detailed characteristics of the internal crack initiation region in VHCF of the titanium alloy with GS. The results indicated that, in contrast to the low- and high-cycle regimes, the VHCF strength is reduced for the specimens with GS. Thus, the GS induced by pre-torsion cannot enhance the VHCF strength of the titanium alloy. This implies that VHCF test (property) is an important consideration for the microstructural designed materials. The graphical abstract is available in Supplementary information.
Highlights
Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface
very-high-cycle fatigue (VHCF) test is very time consuming, e.g. it takes 4 days to reach 107 cycles and more than 1 year for 109 cycles with conventional frequency of 30 Hz via a servohydraulic system. This limits the VHCF research and the VHCF behavior is still rarely studied for metallic materials with a designed microstructure, such as GS, heterogeneous, hierarchical, etc
As an accelerated testing method, ultrasonic cycling with resonant frequency 20 kHz is employed for the VHCF test, which takes about 8 minutes to reach 107 cycles and 14 hours to 109 cycles
Summary
Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface. The GS induced by pre-torsion cannot enhance the VHCF strength of the titanium alloy This implies that VHCF test (property) is an important consideration for the microstructural designed materials. VHCF test is very time consuming, e.g. it takes 4 days to reach 107 cycles and more than 1 year for 109 cycles with conventional frequency of 30 Hz via a servohydraulic system This limits the VHCF research and the VHCF behavior is still rarely studied for metallic materials with a designed microstructure, such as GS, heterogeneous, hierarchical, etc. Whether the VHCF fatigue property can be improved by the layer of GS is still questioning as the crack initiation mechanism in VHCF differs from that in LCF and HCF regimes[11,13–15]. For titanium alloys[14,15], there is no evident difference between ultrasonic (20 kHz) fatigue data and conventional frequency (30 Hz) tests by servo-hydraulic machines in HCF and VHCF regimes
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