Yakai Zhao

Yakai Zhao
Nanyang Technological University | ntu · School of Mechanical and Aerospace Engineering (MAE)

Ph.D.

About

50
Publications
6,803
Reads
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963
Citations
Introduction
- Nanomechanical behavior of metallic glasses, high-entropy alloys, and nanostructured materials - Design of novel high-entropy alloys with prominent properties - Hydrogen effects on the mechanical behavior of metallic glasses and high-entropy alloys - Experimental analyis of crystal plasticity with nanomcehanical testing
Additional affiliations
July 2021 - present
Nanyang Technological University
Position
  • Senior Researcher
June 2019 - June 2021
Nanyang Technological University
Position
  • Research Associate
April 2017 - April 2019
Beijing Institute of Technology
Position
  • PostDoc Position
Education
September 2011 - February 2017
Hanyang University
Field of study
  • Materials Science and Engineering
September 2007 - June 2011
Harbin Institute of Technology
Field of study
  • Materials Science and Engineering

Publications

Publications (50)
Article
Full-text available
The effect of hydrogen (H) charging on the shear yield strength (τmax) and shear transformation zone volume (Ω) of Ni–Nb–Zr metallic glass ribbons, with varying Zr content, were studied through the first pop-in loads during nanoindentation. Weight gain measurements after H charging and desorption studies were utilized to identify how the total H ab...
Article
Full-text available
The influence of hydrogen on the mechanical behavior of the CoCrFeMnNi high-entropy alloy (HEA) was examined through tensile and nanoindentation experiments on specimens hydrogenated via gaseous and electrochemical methods. Results show that the HEA's resistance to gaseous hydrogen embrittlement is better than that of two representative austenitic...
Article
Spherical tip nanoindentation experiments on two typical face-centered cubic high-entropy alloys (HEAs), CoCrFeNi and CoCrFeMnNi in as-solutionized and aged conditions were performed using tips of two different radii. Large datasets of the strength at the first pop-in were obtained and their statistical nature were analyzed to gain insights into th...
Article
Full-text available
While considerable effort is made to understand the solid solution strengthening on the deformation behavior of high-entropy alloys (HEAs), relatively little attention is paid to the role of microstructural interfaces, especially twin boundaries (TBs), on the strain-rate sensitivity (SRS) of them. To address this, we have conducted micropillar comp...
Article
The effect of marked change in grain size from coarse-grained to nanocrystalline can affect the hydrogen absorption and plastic deformation behavior in two face-centered cubic high-entropy alloys (HEAs), viz. equiatomic CoCrFeNi and CoCrFeMnNi. Thermal desorption analysis of the hydrogen-charged samples proved that grain boundaries act as hydrogen...
Article
Solidification cells and a high density of dislocations are two common features of additively manufactured (AM) alloys that are processed using techniques such as laser powder bed fusion (L-PBF) and directed energy deposition (DED). A critical assessment of their role in determining the plastic properties (yield strength, σy, and work hardening beh...
Article
The relationship between the microstructural features (such as the solidification cells and initial dislocation densities) and the tensile properties of alloys additively manufactured (AM) using techniques such as laser powder bed fusion (L-PBF) and directed energy deposition (DED) is yet to be firmly established. In this work, a detailed investiga...
Article
The significant strengthening of selectively laser melted (SLM) 304L stainless steel that occurs due to high-pressure torsion (HPT) is examined by recourse to detailed nanomechanical and microstructural characterization. In the as-built alloy, dislocation hardening is the main strengthening mechanism. After HPT, however, the synergistic combination...
Article
Nanocrystalline (NC) dual-phase Al0.7CoCrFeNi HEAs containing face-centered cubic (FCC) and body-centered cubic (BCC) microstructural phases were fabricated by high-pressure torsion (HPT). The influences of hydrogen on the thermal desorption and nanoindentation responses of NC HEA were compared with the coarse-grained alloy. The plastic zone size a...
Article
Full-text available
We developed a new strategy to obtain long whisker (LW) TiB phase (aspect ratio as high as ∼150) using micron-sized AlB2 in a series of TiB/Ti(Al) composites. While three types of TiB phases were present in the composites, microhardness and compression results show that LW-type TiB enhances yield strength dramatically.
Article
The influence of hydrogen on the mechanical behavior of Inconel 718 fabricated by laser powder-bed-fusion was investigated through a series of tensile experiments. Samples subjected to two different post-fabrication heat treatments, viz. direct aging (DA) and homogenization plus aging (HA), were tested. Detailed microstructural characterization sho...
Article
Full-text available
Porosity—and hence poor fatigue performance—is a major concern for additively manufactured (AM) alloys using powders as the raw materials. Since the microstructures in these alloys are also often unique, a detailed understanding of the interactions between the fatigue cracks emanating from the pores and the microstructural features is necessary for...
Article
Full-text available
A modified laser powder bed fusion (LPBF) technique was utilized to fabricate CoCrFeNi high-entropy alloy (HEA) coupon with stepwise gradation (from 0 to ~12 at.%) in Ti, to ascertain the maximum Ti content that can be added to the HEA for enhancing its strength. The microstructure and mechanical behavior of the fabricated coupon were investigated....
Article
The effect of hydrogen (H) charging on the nanoindentation response of a selective laser melted (SLM) 316L austenitic stainless steel was investigated and compared with its conventionally manufactured (CM) counterpart. Results show that the hardness increment in the SLM samples due to H charging is relatively smaller. Thermal desorption spectroscop...
Article
Full-text available
Strain rate sensitivity (SRS) of a face-centered cubic (fcc) CoCrFeNi high-entropy alloy (HEA) with grain sizes ranging from 57 μm to 45 nm was investigated using nanoindentation, and was compared with those reported for conventional fcc metals. Experimental results show pronounced grain boundary strengthening in the HEA. Estimated values of the SR...
Article
Full-text available
The influence of charging method on hydrogen (H) distribution and the resultant nanomechanical behavior of CoCrFeMnNi high-entropy alloy was examined and compared with another face-centered cubic structured alloy, an austenitic stainless steel. Through thermal desorption spectroscopy measurement and theoretical analysis, it was revealed that electr...
Article
Full-text available
The evolution of the microstructure and hardness in a CoCrFeNi high-entropy alloy (HEA) processed by severe plastic deformation (SPD) was studied. SPD-processing was carried out using a high-pressure torsion (HPT) technique up to twenty turns at room temperature that introduces a highest nominal shear strain of about 800. It was found that most of...
Article
Full-text available
AlCoCrFeNi high-entropy alloys (HEAs) were prepared by Bridgman solidification with different solidification rates, and the mechanical behavior of the HEAs was investigated over a wide strain rate range (∼10⁻³∼10³ s⁻¹). Microstructure observations suggest that, with increasing solidification rate, the microstructure evolves from coarse columnar gra...
Article
Full-text available
The influence of hydrogen on the onset of plastic deformation in a CoCrFeMnNi high-entropy alloy (HEA) was examined through the analysis of the load at which first pop-in during spherical nanoindentation experiments occurs on hydrogen-charged and subsequently aged specimens. Results reveal that the dissolved hydrogen lowers the plastic flow resista...
Article
Full-text available
A four-component equimolar high-entropy alloy (HEA) with the composition of HfNbTiZr and body-centered cubic (bcc) structure was processed by HPT at RT. The evolution of the dislocation density, the grain size and the hardness was monitored along the HPT-processed disk radius for different numbers of turns between ¼ and 20. It was found that most o...
Article
Full-text available
Atomic-structure evolution is significant in understanding the deformation mechanism of metallic glasses. Here, we firstly find a dual-step atomic strain variation in laser-shock-peened (LSPed) metallic glasses during compression tests by using in-situ synchrotron X-ray diffraction. Under low compressive load, LSP-deformed zone's atomic-structure s...
Article
Full-text available
The influence of annealing on the constitutive stress-strain response of nanocrystalline (nc) CoCrFeMnNi high-entropy alloy (HEA) was investigated through a series of nanoindentation experiments using five different three-sided pyramidal indenters. The nc HEA, produced by high-pressure torsion (HPT), was subjected to annealing at 450 oC for 1 and 1...
Article
While nanoindentation is a very useful tool to examine the mechanical properties of ion irradiated materials, there are some issues that should be considered in evaluating the properties of irradiated layer. In this study, in order to properly extract the hardness of only-irradiated layer from nanoindentation data, a new procedure is suggested in c...
Article
Full-text available
The influence of electrochemically charged hydrogen (H) on the hardness (HN) of a CoCrFeMnNi high-entropy alloy (HEA) was investigated with nanoindentation. Upon charging, HN of HEA increases by ∼60%, which decreases gradually during subsequent aging at room temperature, and on prolonged aging, the alloy softens to an extent that HN falls below tha...
Article
The effects of specimen size and strain rate on the plastic deformation response of sub-μm-sized nanocrystalline Cu pillars were examined through a series of micro-compression experiments, with particular emphasis on the stochastic nature of the measured responses. A large number of micropillars two different diameters, both with an average grain s...
Article
Time-dependent plastic deformation behavior of nanocrystalline (nc) and coarse-grained (cg) CoCrFeMnNi high-entropy alloys (HEAs) was systematically explored through a series of spherical nanoindentation creep experiments. High-pressure torsion (HPT) processing was performed for achieving nc microstructure in the HEA, leading to a reduction in grai...
Article
Full-text available
The influence of hydrogen on the plastic deformation of nanocrystalline nickel was analysed by recourse to nanoindentation on the uncharged and hydrogen-charged samples. It was revealed that, in nanocrystalline Ni, hydrogen significantly decreases hardness but does not alter the strain rate sensitivity. Through thermal desorption spectroscopy measu...
Article
Time-dependent nanoscale plasticity of nanocrystalline nickel at room temperature was critically explored through a series of micropillar creep and quasi-static compression experiments on rod and tube specimens fabricated by electron beam lithography and electroplating. Enhanced creep rates in tubes as compared to rods, establishes the facilitating...
Article
Full-text available
Thermal desorption spectroscopy and nanoindentation techniques were employed to elucidate the key differences in the hydrogen (H) charging methods (electrochemical versus gaseous) and their consequences on the mechanical response of a low carbon steel. While electrochemical charging enhances the hardness, gaseous charging reduces it. This contrasti...
Article
Full-text available
The influence of absorbed hydrogen on the mechanical behavior of a series of Ni–Nb–Zr amorphous metallic ribbons was investigated through nanoindentation experiments. It was revealed that the influence is significantly dependent on Zr content, that is, hydrogen induced softening in relatively low-Zr alloys, whereas hydrogen induced hardening in hig...
Article
How the applied strain can affect the plasticity evolution of window glass was systematically explored through a series of nanoindentations with various sharp indenters. It was revealed that, as the strain increases, the contribution of shear flow to total plasticity becomes larger, whereas that of densification gets smaller. The results are discus...
Article
Although extensive research has been made for evaluating the activation energy Q of hot deformation in microalloyed steels, almost no attempt has been made for directly comparing the influence of each element. In this study, through a series of hot compression tests of four different microalloyed steels, we have systematically explored the influenc...
Article
Full-text available
Alloy 690 has been widely used as steam generator tubing material in nuclear power plants owing to its excellent properties such as stress corrosion cracking resistance. In the present study, a series of nanoindentation experiments were conducted to explore the nanomechanical characteristics of the grain boundaries (GBs) and grain interior (GI) in...
Article
The existence of an indentation size effect (ISE) in the onset of yield in a Zr-based bulk metallic glass (BMG) is investigated by employing spherical-tip nanoindentation experiments. Statistically significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different struc...
Article
The size of the shear transformation zone (STZ) that initiates the elastic to plastic transition in a Zr-based bulk metallic glass was estimated by conducting a statistical analysis of the first pop-in event during spherical nanoindentation. A series of experiments led us to a successful description of the distribution of shear strength for the tra...

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Project (1)
Project
The proposed research framework in hydrogen embrittlement refers to the challenges and most obvious problems of how to link models, phenomenology, and morphology of hydrogen-related failures of industrial components at different scales and how to successfully translate the insights gained into outcomes of practical value to the engineering community. We are looking for partnerships and the formation of an excellent consortium consisting of complementary partners that will help us to retain our competitive edge over the long term.