Engineering topics

Niu, Hongwei

Publications and source records attributed to Niu, Hongwei.

4 records

DOE OSTI2024

Training models using forces computed by stochastic electronic structure methods

Abstract Quantum Monte Carlo (QMC) can play a very important role in generating accurate data needed for constructing potential energy surfaces. We argue that QMC has advantages in terms of a smaller systematic bias and an ability to cover phase space more completely. The stochastic noise can ease the training of the machine learning model. We discuss how stochastic errors affect the generation of effective models by analyzing the errors within a linear least squares procedure, finding that there is an advantage to having many relatively imprecise data points for constructing models. We then analyze the effect of noise on a model of many-body silicon finding that noise in some situations improves the resulting model. We then study the effect of QMC noise on two machine learning models of dense hydrogen used in a recent study of its phase diagram. The noise enables us to estimate the errors in the model. We conclude with a discussion of future research problems.

Ceperley, David M. (ORCID:0000000150826271)↗

DOE OSTI2023

Stable Solid Molecular Hydrogen above 900 K from a Machine-Learned Potential Trained with Diffusion Quantum Monte Carlo

Here, we survey the phase diagram of high-pressure molecular hydrogen with path integral molecular dynamics using a machine-learned interatomic potential trained with quantum Monte Carlo forces and energies. Besides the HCP and C2/c–24 phases, we find two new stable phases both with molecular centers in the Fmmm–4 structure, separated by a molecular orientation transition with temperature. The high temperature isotropic Fmmm–4 phase has a reentrant melting line with a maximum at higher temperature (1450 K at 150 GPa) than previously estimated and crosses the liquid-liquid transition line around 1200 K and 200 GPa.

08 HYDROGEN↗

DOE OSTI2018

Microstructure evolution, grain morphology variation and mechanical property change of Cu-Sn intermetallic joints subjected to high-temperature aging

Highlights: • The δ-Cu{sub 41}Sn{sub 11} could be formed by reaction of 11Cu{sub 3}Sn + 9Cu → Cu{sub 41}Sn{sub 11} during aging above 350 °C. • The temperature effects on the growth rate and grain morphology of Cu{sub 41}Sn{sub 11} phase were found. • The Cu{sub 41}Sn{sub 11} joints with refined grains exhibited higher hardness and shear strength. • The Cu{sub 41}Sn{sub 11} joints exhibited a combination of transgranular and intergranular fracture during shear test. - Abstract: In this work, the aging treatment was carried out on full Cu{sub 3}Sn joints at different temperatures for various durations, the microstructure evolution, grain morphology variation and mechanical property change of joints were investigated. The temperature effects on the growth rate and grain morphology of Cu{sub 41}Sn{sub 11} phase were found: The complete transformation from Cu{sub 3}Sn phase to Cu{sub 41}Sn{sub 11} phase in joints needed 300 h at 400 °C, but 4 h at 500 °C. The Cu{sub 41}Sn{sub 11} phase formed at 400 °C consisted of columnar grains with an average width of 23.5 μm, while the Cu{sub 41}Sn{sub 11} phase formed at 500 °C consisted columnar grains with an average width of 8.5 μm. The average hardness and shear strength values of the Cu{sub 41}Sn{sub 11} joints formed at 400 °C were 432.5 HV and 62 MPa, while the joints formed at 500 °C exhibited higher hardness and shear strength (451.5 HV and 65 MPa). Besides, the Cu{sub 41}Sn{sub 11} joint was found to exhibit a combination of transgranular and intergranular fracture during shear test.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Effect of processing parameters on the formation of C{sub f}/LAS composites/Ag−Cu−Ti/TC4 brazed joint

C{sub f}/LAS composites were successfully jointed to TC4 alloy with Ag−Cu−Ti filler by vacuum brazing. The interfacial microstructure of TC4/C{sub f}/LAS composites joints was characterized by employing scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-diffraction (XRD) and transmission electron microscopy (TEM). The determination of the thin interfacial reaction layer (TiSi{sub 2} + TiC layer) was realized by TEM. The effect of holding time on the interfacial microstructure and shear strength were investigated. With the increasing holding time, the thickness of diffusion layer, Ti{sub 3}Cu{sub 4} layer, and TiSi{sub 2} + TiC layer increased obviously, on the contrary, that of Ti−Cu intermetallic compound layers decreased gradually. Besides, blocky Ti{sub 3}Cu{sub 4} phase was coarsened when the joint was brazed at 890 °C for 20 min, which deteriorated the mechanical properties of the joint dramatically. The interfacial evolution of TC4/C{sub f}/LAS composites joint and the formation of TiSi{sub 2}, TiC, Ti{sub 3}Cu{sub 4}, TiCu and Ti{sub 2}Cu phases were expounded. The maximum shear strength of 26.4 MPa was obtained when brazed at 890 °C for 10 min. - Highlights: •The thin interface reaction layer was determined to be TiSi{sub 2} + TiC layer by TEM. •Holding time had influence on the interfacial microstructure and joint properties. •Microstructural evolution mechanism and reactions of brazed joints were expounded.

36 MATERIALS SCIENCE↗