Search NASA⌕ Search

Engineering topics

Sun, Tianyi

Publications and source records attributed to Sun, Tianyi.

8 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

High‐Temperature Mechanical Behavior of Single‐Crystal FeCrAl Alloy Under In Situ Micropillar Compression

FeCrAl cladding is one of the candidate materials for the near‐term accident‐tolerant fuel technologies under development. Research on high‐temperature mechanical behaviors of single‐crystal FeCrAl alloy is rather limited. Previous studies have reported the mechanical property of low‐index orientation in single‐crystal FeCrAl alloy at room temperature. However, the critical resolved shear stress to activate slip systems can be orientation and temperature dependent. Here, single‐crystal grains in a coarse‐grained FeCrAl alloy with different crystallographic orientations are selected to preferentially activate {110}<111> slip systems or {112}<111> slip systems. Micropillars are fabricated in the selected single‐crystal grains and tested at elevated temperatures in situ in a scanning electron microscope. The critical resolved shear stresses of {110}<111> slip systems and {112}<111> slip systems are determined at various temperatures. The critical resolved shear stress shows a temperature dependence and orientation independence. This study provides important insight for understanding the deformation mechanisms of FeCrAl alloys at elevated temperatures.

Sun, Tianyi↗
DOE OSTI2023

In Situ Study on Cu-to-Cu Thermal Compression Bonding

Cu-to-Cu thermal compression bonding (TCB) has emerged as a promising solution for ultrafine pitch packaging in 3D integrated circuit technologies. Despite the progress made by conventional Cu-to-Cu TCB methods in achieving good mechanical strength of the Cu bonds, the bonding processes generally require high temperature and high pressure, which may degrade the performance and reliability of the device. Therefore, it is imperative to investigate the processing parameters to understand the bonding mechanism and achieve effective TCB at a low temperature and low pressure. Here, we developed an in situ TCB technique inside a scanning electron microscope. The in situ TCB method enables a real-time observation of bonding development, which provides critical insights into how the texture and microstructure of Cu bumps may influence the creep and surface diffusion during the bonding process. This work features a strategy to advance our understanding of the bonding mechanisms and provides insight into tailoring the microstructure of Cu for bonding at a low temperature and low pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Gradient nanostructured steel with superior tensile plasticity

Nanostructured metallic materials with abundant high-angle grain boundaries exhibit high strength and good radiation resistance. While the nanoscale grains induce high strength, they also degrade tensile ductility. We show that a gradient nanostructured ferritic steel exhibits simultaneous improvement in yield strength by 36% and uniform elongation by 50% compared to the homogenously structured counterpart. In situ tension studies coupled with electron backscattered diffraction analyses reveal intricate coordinated deformation mechanisms in the gradient structures. The outermost nanolaminate grains sustain a substantial plastic strain via a profound deformation mechanism involving prominent grain reorientation. This synergistic plastic co-deformation process alters the rupture mode in the post-necking regime, thus delaying the onset of fracture. The present discovery highlights the intrinsic plasticity of nanolaminate grains and their significance in simultaneous improvement of strength and tensile ductility of structural metallic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Bridging microscale to macroscale mechanical property measurements of FeCrAl alloys by crystal plasticity modeling

FeCrAl alloys are candidates for accident tolerant fuel cladding of light water reactors. In this work, a microstructure- and temperature-dependent crystal plasticity model is employed to bridge microscale to macroscale mechanical property measurements of FeCrAl alloys. With the visco-plastic self-consistent (VPSC) polycrystal plasticity framework, a mechanism-based single crystal plasticity (MSCP) model adopts the Arrhenius type rate equation to describe the dependence of the critical resolved shear stress for dislocation slips on their temperature-dependent intrinsic frictional resistance and the microstructure-dependent irradiation hardening. The intrinsic frictional resistance associated with {110}<111> and {112}<111> slip systems were measured by in-situ micromechanical testing on unirradiated/irradiated samples at 25-500 °C. The irradiation hardening is estimated by the Bacon-Kocks-Scattergood (BKS) model with density and size of radiation-induced defects measured from microstructural characterization. Several features associated with thermo-mechanical behavior of unirradiated/irradiated polycrystalline FeCrAl alloys are captured. High density of deformation-induced dislocations and radiation-induced defects results in obvious hardening at room temperature, which is weakened at high temperature, and facilitates damage evolution during deformation. Moreover, both high temperature and radiation-induced defects, which facilitate dislocation multiplication, trigger large hardening rate. Finally, the proposed method together with application of accelerator-based ion irradiation technique is a surrogate approach to simulate neutron damage, improving the efficiency associated with evaluation of mechanical properties of FeCrAl alloys exposed to temperature, stress and radiation conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

In situ study on heavy ion irradiation induced microstructure evolution in single crystal Cu with nanovoids at elevated temperature

Understanding the response of nanovoids to irradiation damage advances our capability to design radiation tolerant materials. Recent in situ studies reveal the shrinkage of hexagon-shaped nanovoids in (110) textured Cu. However, the evolution of voids with irregular geometries under irradiation is less well understood. Here, in situ Kr ion irradiation was performed at 100 °C on single-crystal Cu (112) that possesses nanovoids with high aspect ratio. In situ studies show these elongated voids fragment into smaller voids and shrink gradually with increasing dose. Phase field simulations confirm that fragmentation of the void is governed by the competing kinetics between atoms diffusing toward and away from the elongated nanovoid. In conclusion, cost irradiation analysis reveals the formation of high-density defect clusters.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2022

Tribological behaviors of nanotwinned Al alloys

Wear-induced damages cause significant materials loss each year. Al alloys are widely used by industry but usually have low wear resistance. Here, we compare the tribological behaviors of ultrafine grained Al, a nanoprecipitate hardened Al 7075 alloy and nanotwinned Al–Ni alloys using the nanoscratch. The nanotwinned Al–Ni alloys exhibit lower coefficient of friction and much greater wear resistance than the Al and Al 7075 alloys. Additionally, the enhanced wear properties of Al–Ni alloys arise from their high strength and the evolution of nanotwinned microstructures into gradient nanograins during wear. These findings on fundamental wear mechanisms in nanotwinned alloys may advance the discovery of wear-resistant metallic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Deformation mechanism in nanolaminate FeCrAl alloys by ,em>in situ micromechanical strain rate jump tests at elevated temperatures

FeCrAl alloys have attracted extensive attention as one of the potential cladding materials for the commercial light water reactors. Although the creep behaviors of coarse-grain FeCrAl alloys have been investigated at elevated temperatures, little is known on the deformation mechanism of fine-grain FeCrAl alloys. We performed surface mechanical grinding treatment to fabricate nanolaminated ferritic FeCrAl alloys. In situ micropillar compression tests were applied to determine the underlying deformation mechanism of nanolaminated FeCrAl alloys at elevated temperatures. The strain rate sensitivity and activation energy for the dominant deformation mechanism were estimated considering the presence of a threshold stress.

36 MATERIALS SCIENCE↗