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Li, Shilei

Publications and source records attributed to Li, Shilei.

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Unveiling the origins of work-hardening enhancement and mechanical instability in laser shock peened titanium

Laser shock-peening (LSP) produces complex gradients in microstructures and residual stresses, which enables fabrication of metallic engineering components with superior mechanical properties. In this work, we use non-destructive and spatially resolved high-energy X-ray diffraction (HE-XRD) to investigate the influences of LSP on microstructure, surface topography, and residual stress for high purity titanium plates. LSP is found to produce large compressive in-plane residual stresses near the peened surface which monotonically decay to zero about 2.5 mm below the surface. These properties were also tracked during in-situ tensile loading, allowing stress partitioning and work-hardening rates to be measured as a function of depth. The surface region is found to have the highest work-hardening rate and remain mechanically stable up to sample failure. In addition, a crystal rigid rotation along a transversal axis of about 27 & nbsp; was observed at large applied strain near the surface region, which is attributed to the formation of denser dispersed shear bands. Furthermore, a sudden drop in tensile stress was observed at a depth of similar to 160 mu m from the LSP surface, indicating the formation of a localized mechanical instability zone (LMIZ). The origin of this LMIZ could be attributed to the increase in shear stress induced by a load transfer from LSP surface due to the formation of denser dispersed shear bands. These observations indicate a localized transition in deformation behavior has occurred from dispersed shear banding to LMIZ and homogeneous deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

External-Field-Induced Phase Transformation and Associated Properties in a Ni 50 Mn 34 Fe 3 In 13 Metamagnetic Shape Memory Wire

Metamagnetic shape memory alloys exhibit a series of intriguing multifunctional properties and have great potential for applications in magnetic actuation, sensing and magnetic refrigeration. However, the poor mechanical properties of these alloys with hardly any tensile deformability seriously limit their practical application. In the present work, we developed a Ni-Fe-Mn-In microwire that exhibits both a giant, tensile superelasticity and a magnetic-field-induced first-order phase transformation. The recoverable strain of superelasticity is more than 20% in the temperature range of 233–283 K, which is the highest recoverable strain reported heretofore in Ni-Mn-based shape memory alloys (SMAs). Moreover, the present microwire exhibits a large shape memory effect with a recoverable strain of up to 13.9% under the constant tensile stress of 225 MPa. As a result of the magnetic-field-induced first-order phase transformation, a large reversible magnetocaloric effect with an isothermal entropy change ΔS m of 15.1 J kg –1 K –1 for a field change from 0.2 T to 5 T was achieved in this microwire. The realization of both magnetic-field and tensile-stress-induced transformations confers on this microwire great potential for application in miniature multi-functional devices and provides an opportunity for multi-functional property optimization under coupled multiple fields.

36 MATERIALS SCIENCE↗
DOE OSTI2020

In situ investigation of the deformation behaviors of Fe 20 Co 30 Cr 25 Ni 25 and Fe 20 Co 30 Cr 30 Ni 20 high entropy alloys by high-energy X-ray diffraction

In situ synchrotron-based high-energy X-ray diffraction (HE-XRD) technique was employed to investigate the mechanical behaviors and microstructural evolution of face-centered cubic (FCC) Fe 20 Co 30 Cr 25 Ni 25 and Fe 20 Co 30 Cr 30 Ni 20 high entropy alloys (HEAs) during tensile deformation. Fe 20 Co 30 Cr 30 Ni 20 HEA has a good combination of strength (ultimate tensile strength of 864 ± 35 MPa) and ductility (elongation of 0.627 ± 0.021). The HE-XRD investigation reveals that Fe 20 Co 30 Cr 30 Ni 20 HEA has the transformation-induced plasticity (TRIP) effect, which starts at a critical stress of ~555 MPa. Furthermore, transmission electron microscopy confirmed this deformation-induced new phase is hexagonal-close-packed structured ε-martensite, which follows an orientation relationship of {111} γ //(0001) ε , and (110) γ //[11 2 ¯ 0] ε with the FCC γ matrix. The observation of deformation twins in the deformed samples of the two studied HEAs proves that twinning-induced plasticity (TWIP) effect occurs in both HEAs. The combination of TRIP and TWIP effects lead to the high strength, large ductility and improved strain hardening behavior of Fe 20 Co 30 Cr 30 Ni 20 HEA.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Unprecedented non-hysteretic superelasticity of [001]-oriented NiCoFeGa single crystals

Superelasticity associated with the martensitic transformation has found a broad range of engineering applications. However, the intrinsic hysteresis and temperature sensitivity of the first-order phase transformation significantly hinder the usage of smart metallic components in many critical areas. In this paper, we report a large superelasticity up to 15.2% strain in [001]-oriented NiCoFeGa single crystals, exhibiting non-hysteretic mechanical responses, a small temperature dependence and high-energy-storage capability and cyclic stability over a wide temperature and composition range. In situ synchrotron X-ray diffraction measurements show that the superelasticity is correlated with a stress-induced continuous variation of lattice parameter accompanied by structural fluctuation. Neutron diffraction and electron microscopy observations reveal an unprecedented microstructure consisting of atomic-level entanglement of ordered and disordered crystal structures, which can be manipulated to tune the superelasticity. The discovery of the large elasticity related to the entangled structure paves the way for exploiting elastic strain engineering and development of related functional materials. NiCoFeGa single crystals exhibit large non-hysteretic superelasticity over broad temperature and composition ranges. It is attributed to the continuous phase transition with applied stress, which is related to the fluctuation of entangled ordered and disordered crystal structures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Surface Integrity and Oxidation of a Powder Metallurgy Ni-Based Superalloy Treated by Laser Shock Peening

Laser shock peening is a mechanical surface treatment which can induce large compressive residual stresses and microstructural changes in the material by using repetitive shocks from laser pulses. In this work, the surface integrity (surface microstructure, topography, hardness and residual stress) of a laser shock peening treated powder metallurgy Ni-based superalloy was investigated. Laser shock peening treatment introduces large plastic deformation especially at a depth of about 100 μm from the surface, which increases the local hardness. The residual stress from the surface to the interior of the sample was investigated by synchrotron X-ray diffraction. The maximum compressive stress reaches 400 MPa at the surface, while the depth of the compressive stress is about 0.7 mm. Lastly, the oxidation behavior of the treated and untreated samples was evaluated by thermal exposure at 700°C for 312 h. The LSP treatment decreased the thickness of oxide layer, thereby showing improved oxidation resistance.

36 MATERIALS SCIENCE↗