The far-from-equilibrium solidification during additive manufacturing often creates large residual stresses that induce solid-state cracking. Here we present a strategy to suppress solid-state cracking in an additively manufactured AlCrFe2Ni2high-entropy alloy via engineering phase transformation pathway. We investigate the solidification microstructures formed during laser powder-bed fusion and directed energy deposition, encompassing a broad range of cooling rates. At high cooling rates (104−106 K/s), we observe a single-phase BCC/B2 microstructure that is susceptible to solid-state cracking. At low cooling rates (102−104 K/s), FCC phase precipitates out from the BCC/B2 matrix, resulting in enhanced ductility (~10 %) and resistance to solid-state cracking. Site-specific residual stress/strain analysis reveals that the ductile FCC phase can largely accommodate residual stresses, a feature which helps relieve residual strains within the BCC/B2 phase to prevent cracking. Our work underscores the value of exploiting the toolbox of phase transformation pathway engineering for material design during additive manufacturing.
Test for stress relief cracking susceptibility in creep resistant chromium-molybdenum steels
An externally restrained stress relief cracking test was developed and demonstrated in testing susceptible and resistant to cracking welds in Cr–Mo steels. Compared to other externally restrained tests, it simultaneously applies stress and compensates thermal expansion during heating to post-weld heat treatment temperature and utilises digital image correlation for quantification of key characteristics of the stress relaxation and stress relief cracking phenomena. In contrast with resistant to stress relief cracking materials, susceptible materials experienced lower levels of stress relaxation, strain absorption, and sustained mechanical energy, with accelerated kinetics of strain accumulation and strain localisation leading to failure. The processes of stress relief cracking and stress relaxation were quantified as low strain – slow strain rate – low energy phenomena.
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- Award ID(s):
- 2052747
- NSF-PAR ID:
- 10506620
- Publisher / Repository:
- Taylor and Francis Online
- Date Published:
- Journal Name:
- Science and Technology of Welding and Joining
- Volume:
- 27
- Issue:
- 4
- ISSN:
- 1362-1718
- Page Range / eLocation ID:
- 265 to 281
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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We report on the absence of strain relaxation mechanism in Al 0.6 Ga 0.4 N epilayers grown on (0001) AlN substrates for thickness as large as 3.5 μm, three-orders of magnitude beyond the Matthews–Blakeslee critical thickness for the formation of misfit dislocations (MDs). A steady-state compressive stress of 3–4 GPa was observed throughout the AlGaN growth leading to a large lattice bow (a radius of curvature of 0.5 m −1 ) for the thickest sample. Despite the large lattice mismatch-induced strain energy, the epilayers exhibited a smooth and crack-free surface morphology. These results point to the presence of a large barrier for nucleation of MDs in Al-rich AlGaN epilayers. Compositionally graded AlGaN layers were investigated as potential strain relief layers by the intentional introduction of MDs. While the graded layers abetted MD formation, the inadequate length of these MDs correlated with insignificant strain relaxation. This study emphasizes the importance of developing strain management strategies for the implementation of the single-crystal AlN substrate platform for III-nitride deep-UV optoelectronics and power electronics.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1080/13621718.2022.2050133
