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Liang Tian

University of Michigan, Materials Science and Engineering, Post-Doc

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Liang does research in Metallurgy, Mechanics and Materials Physics and machine learning and large-scale scalable parallel scientific software development. His research interests are powder metallurgy and severe plastic deformation, lightweight metallic nanocomposites, computational modeling techniques in kinetics and dynamic evolution of a broad range of materials classes, thermodynamics (e.g. DFT, molecular dynamics, mesoscale phase field, cluster expansion, Monte Carlo, Activation and Relaxation Techniques), data-driven materials design by machine learning and scientific computational software development. He is easy to work with, independent, responsive and curious about interesting problems and always glad to collaborate. He can be reached at email: liangtianisu@gmail.com

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Papers by Liang Tian

Universal nature of the saddle states of structural excitations in metallic glasses

by Liang Tian and Robert Ritchie

Materials Today Physics, 2021

A widely used schematic picture of the potential energy landscape (PEL) for liquid and glass give... more A widely used schematic picture of the potential energy landscape (PEL) for liquid and glass gives an impression that the pathway of moving from a valley to another through a saddle point is predetermined. However, in reality the pathway is much more stochastic and unpredictable because thermal history is wiped out at the saddle point and the pathway down is randomly chosen. Here we explain this puzzling behavior through the study of local structural evolutions in the β relaxation process by atomistic simulations of structural excitations for metallic glasses. We find that the saddle states in the PEL show universal melt-like features in short-range order and atomic dynamics, independent of thermal history and composition. We propose that the short-lived melting at the saddle point is responsible for wiping out the prior thermal history. This explains why the activation and relaxation stages of the β process are decoupled. The findings highlight the importance of understanding the nature of the saddle states in elucidating the system dynamics, and pose a question on the current view on the system evolution in the PEL.

Effects of cluster expansion on the locations of phase transition boundary as a first step to quantify uncertainty in first principles statistical mechanics framework

Computational Materials Science, 2021

Predicting phase diagrams from first principle calculations eliminates the need of tedious experi... more Predicting phase diagrams from first principle calculations eliminates the need of tedious experimental trials and errors. Fully automating first principle phase diagram calculations without any sort of human intervention has been a long daunting task and troubling scientific communities for decades. This grand problem remains not fully resolved, largely due to the vastly high-dimensional parameter space associated with density functional theory, cluster expansion, lattice Monte Carlo, and the substantial uncertainty propagating through a set of complex simulations. As a first step to tackle this grand problem, we reported a first demonstration of how sensitive phase boundary locations can be to various cluster expansion fittings and input DFT training data. To the best knowledge of the authors, this study reported the first ever attempt to quantify uncertainty in first principles statistical mechanics framework. In addition, a semi-automated phase transition detection algorithm has been devised in this paper to deal with the associated statistical errors and uncertainties from Monte Carlo method and its predecessor DFT calculations and cluster expansions. This algorithm has been applied in a classical cluster expansion Mg-Cd binary alloy system to detect phase transitions at various locations of phase diagram using different cluster expansions to demonstrate its predictive power and quantify uncertainties. The results suggested that using Chebyshev basis function shifted the transition locations toward the dilute solid solution phase and expanded the phase stability of concentrated ordered phases to wider composition range. The addition of perturbed defect configurations into training data set lowered the order-disorder transition temperature to be away from true transition temperature, suggesting the transition nature is indeed configurational disorder dominated rather than defect assisted. We found that the weighting has negligible effect on the transition locations, except for the case of using Chebyshev basis function to fit all non-weighted configurations that can be susceptible to Monte Carlo sampling hysteresis.

Identifying flow defects in amorphous alloys using machine learning outlier detection methods

Scripta Materialia, 2020

Shear transformation zones (STZs) are widely believed to be the fundamental flow defects that dic... more Shear transformation zones (STZs) are widely believed to be the fundamental flow defects that dictate the plastic deformation of amorphous alloys. However, it has been a long-term challenge to characterize STZs and their evolutions by experimental methods due to transient nature. Here we first introduced a consistent, automated, robust method to identify STZs by linear based machine learning outlier detection algorithms. We exemplify these algorithms to identify the atoms of STZs in Cu64Zr36 metallic glass sys- tem, and verify this data-driven model with a physical based model. It is revealed that the average STZ size slightly increases with decreasing cooling rate.

Latest Developments in Modeling and Characterization of Joining Metal Based Hybrid Materials

by Shahin Khoddam and Liang Tian

Advanced Engineering Materials, 2018

Advanced materials consist of several materials systems that exhibit complementary properties for... more Advanced materials consist of several materials systems that exhibit complementary properties for multi‐purpose applications. Joining of dissimilar materials is a critical and challenging advanced manufacturing technique to develop novel hybrid materials with properties fully transferred. The “bonding strength” of a joint is crucial for its integrity and performance. The bonding strength is affected by a range of parameters that can be better understood, controlled, and optimized via both experimental and analytical approaches. In this paper, the authors review the theoretical and experimental studies of the interface inside several metal based composites. The scope includes interface bonding's critical parameters, characterization techniques of joining processes, potential applications, and their future perspectives. The review is significant to develop advanced manufacturing techniques for heterogeneous materials and to design innovative heterogeneous systems for various medical, electrical, electronics, industrial, and other daily life applications that involve the broad range of “joining” processes.

ART_data_analyzer: Automating parallelized computations to study the evolution of materials

Software X, 2019

The kinetics and dynamic evolution of material structures need a comprehensive understanding of t... more The kinetics and dynamic evolution of material structures need a comprehensive understanding of the potential energy landscape at current sample state. The Activation–Relaxation Technique (ART) is an efficient way to probe the potential energy landscape by sampling a large amount of events (a single event involves initial, saddle and final state) from which a statistical distribution of activation energy barrier can be extracted. However, there has been a lack of a user-friendly toolkit to automate the parallelization of running of ART simulations and post-processing of data from ART simulations to extract useful physics information and insights. The ART_data_analyzer Python package has been developed to serve this purpose and fill in this gap for the broad community of scientific researchers interested in the kinetics and dynamic transitions of material structures. As a demo, we utilized this software package to demonstrate the user-friendly workflow of studying ZrCuAl metallic glass sample prepared by molecular dynamics.

High temperature damping behavior and dynamic Young's modulus of magnesium matrix composite reinforced by Ti 2 AlC MAX phase particles

A B S T R A C T The evolutions with the temperature of damping capacities and dynamic Young's mod... more A B S T R A C T The evolutions with the temperature of damping capacities and dynamic Young's moduli of ternary Ti 2 AlC particles reinforced AZ91D composites were investigated. This work has highlighted that Ti 2 AlC significantly contributes to the damping capacity and to dynamic moduli of composites. With increasing temperature, especially above 200 °C, the interfacial damping capacity becomes dominant and dynamic moduli of composites decrease. These trends were revealed from the transformed tensile fracture mechanisms. At room temperature, Ti 2 AlC particles delaminated without any decohesion between Ti 2 AlC particles and the Mg matrix. However, the interfacial decohesion occurred above 200 °C. One explanation of this change of decohesion mechanism could be ascribed to the inherent structure of the Ti 2 AlC MAX phase and to the formation of a robust amorphous magnesium layer at the interface between Ti 2 AlC and magnesium matrix that was evidenced by HRTEM. This amorphous layer further gives rise a high activation energies (H) of damping peaks for composites, which was calculated to 128 kJ/mol. This value is higher than those calculated for SiC or graphite reinforced AZ91D composites, making Ti 2 AlC particles particularly interesting for application in reinforcing magnesium composites .

Self-lubricate and anisotropic wear behavior of AZ91D magnesium alloy reinforced with ternary Ti 2 AlC MAX phases

The dry sliding wear behavior of Ti 2 AlC reinforced AZ91 magnesium composites was investigated a... more The dry sliding wear behavior of Ti 2 AlC reinforced AZ91 magnesium composites was investigated at sliding velocity of 0.5 m/s under loads of 10, 20, 40 and 80 N using pin-on-disk configuration against a Cr15 steel disc. Wear rates and friction coefficients were registered during wear tests. Worn tracks and wear debris were examined by scanning electron microscopy, energy dispersive X-ray spectrometry and transmission electron microscopy in order to obtain the wear mechanisms of the studied materials. The main mechanisms were characterized as the magnesium matrix oxidation and self-lubrication of Ti 2 AlC MAX phase. In all conditions, the composites exhibit superior wear resistance and self-lubricated ability than the AZ91 Mg alloy. In addition, the anisotropic mechanisms in tribological properties of textured Ti 2 AlC-Mg composites were confirmed and discussed.

A Review on the Strengthening of Nanostructured Materials

Nanostructured materials, whose characteristic microstructure size is under 100 nm, can be either... more Nanostructured materials, whose characteristic microstructure size is under 100 nm, can be either single-phase nanocrystalline materials or multi-phase nanocomposite materials. Nanocrystalline materials can also be treated as nanocomposites with grain interior as matrix and grain boundary as secondary phase. The strengthening models of nanostructured materials resemble those strengthening models of conventional composite structures, but have substantial deviations from conventional strengthening mechanisms due to their distinctive nanoscale structure and the complex hierarchy of their nanoscale microstructure. This paper reviewed the current progress in developments of strengthening models for nanostructured materials with emphasis on single-phase nanocrystalline and multi-phase nanocomposite materials, which would help guide the design of new nanostructured materials and other similar nanoscale composite structures. Furthermore, practical large scale industrial applications of high strength nanostructured materials require these materials to possess decent formability, ductility or other functional properties to satisfy both structural and multifunctional applications. Therefore, the latest developments of novel nanostructured materials are discussed to highlight their potential of overcoming the strength ductility trade-off and strength-conductivity trade-off by various approaches. Their complex and distinctive nanoscale microstructure suggests the potential challenges and opportunities in developing new strengthening models for designing future advanced nanostructured materials with unprecedented properties.

A Short Review on Mechanical Behavior of Nanocrystalline Materials

Nanocrystalline materials, as a new generation of advance materials, have outstanding mechanical ... more Nanocrystalline materials, as a new generation of advance materials, have outstanding mechanical properties such as high strength and hardness, low elastic modulus, good ductility and excellent fatigue and wear resistance. In this paper, an overview of the synthesis and mechanical properties of nanocrystalline materials is provided. Furthermore, the grain size dependent plastic deformation mechanism is discussed to correlate with their mechanical behavior.

Tensile Properties of High-purity Ca Metal

Recent interest in using calcium (Ca) as a reinforcement metal in Al/Ca metal-metal composites pr... more Recent interest in using calcium (Ca) as a reinforcement metal in Al/Ca metal-metal composites prompted this study of the mechanical properties of high-purity Ca metal. Previously reported measurements of Ca’s mechanical properties were performed on Ca of relatively low purity (~95 at%). Ca used in this study was purified by sublimation to reduce O, N, and C concentrations, yielding 99.95% purity metal for fabrication of tensile test specimens. Yield strength, ultimate tensile strength, ductility, and strain rate sensitivity of high-purity Ca were measured at both 77K and 295K for annealed and cold-worked Ca. Annealed samples were found to be more strain-rate sensitive than as-swaged samples. Both as-annealed and as-swaged Ca samples were stronger and more ductile at 77K than at 295K, behavior that seems to be supported by molecular dynamics simulations of perfect Ca single crystals.

A dislocation-based, strain–gradient–plasticity strengthening model for deformation processed metal–metal composites

by Liang Tian and A. Russell

Deformation processed metal–metal composites (DMMCs) are high-strength, high-electrical conductiv... more Deformation processed metal–metal composites (DMMCs) are high-strength, high-electrical conductivity composites developed by severe plastic deformation of two ductile metal phases. The extraordinarily high strength of DMMCs is underestimated using the rule of mixture (or volumetric weighted average) of conventionally work-hardened metals. In this article, a dislocation- density-based, strain–gradient–plasticity model is proposed to relate the strain-gradient effect with the geometrically necessary dislocations emanating from the interface to better predict the strength of DMMCs. The model prediction was compared with the experimental findings of Cu–Nb, Cu–Ta, and Al–Ti DMMC systems to verify the applicability of the new model. The results show that this model predicts the strength of DMMCs better than the rule- of-mixture model. The strain-gradient effect, responsible for the exceptionally high strength of heavily cold worked DMMCs, is dominant at large deformation strain since its characteristic microstructure length is comparable with the intrinsic material length.

Phase field study of interfacial diffusion-driven spheroidization in a composite comprised of two mutually insoluble phases

The phase field approach is a powerful computational technique to simulate morphological and micr... more The phase field approach is a powerful computational technique to simulate morphological and microstructural evolution at the mesoscale. Spheroidization is a frequently observed morphological change of mesoscale heterogeneous structures during annealing. In this study, we used the diffuse interface phase field method to investigate the interfacial diffusion-driven spheroidization of cylindrical rod structures in a composite comprised of two mutually insoluble phases in a two-dimensional case. Perturbation of rod radius along a cylinder’s axis has long been known to cause the necessary chemical potential gradient that drives spheroidization of the rod by Lord Rayleigh’s instability theory. This theory indicates that a radius perturbation wavelength larger than the initial rod circumference would lead to cylindrical spheroidization. We investigated the effect of perturbation wavelength, interfacial energy, volume diffusion, phase composition, and interfacial percentage on the kinetics of spheroidization. The results match well with both the Rayleigh’s instability criterion and experimental observations.

A deformation-processed Al-matrix/Ca-nanofilamentary composite with low density, high strength, and high conductivity

by Liang Tian and I. Anderson

Light, strong materials with high conductivity are desired for many applications such as power tr... more Light, strong materials with high conductivity are desired for many applications such as power transmission conductors, fly-by-wire systems, and downhole power feeds. However, it is difficult to obtain both high strength and high conductivity simultaneously in a material. In this study, an Al/Ca (20 vol%) nanofilamentary metal- metal composite was produced by powder metallurgy and severe plastic deformation. Fine Ca metal powders (~200 μm) were produced by centrifugal atomization, mixed with pure Al powder, and deformed by warm extrusion, swaging, and wire drawing to a true strain of 12.9. The Ca powder particles became fine Ca nanofilaments that reinforce the composite substantially by interface strengthening. The conductivity of the composite is slightly lower than the rule-of-mixtures prediction due to minor quantities of impurity inclusions. The elevated temperature performance of this composite was also evaluated by differential scanning calorimetry and resistivity measurements.

The microstructure-strength relationship in a deformation processed Al–Ca composite

An Al-9 vol% Ca composite was produced by powder metallurgy and deformation processing. The Al–Ca... more An Al-9 vol% Ca composite was produced by powder metallurgy and deformation processing. The Al–Ca composite was extruded, swaged and wire drawn to a deformation true strain of 13.8. Both Al and Ca are face-centered cubic, so the Ca second phase deformed into continuous, nearly cylindrical filaments in the Al matrix. The formation of intermetallic compounds, filament coarsening, and spheriodization at elevated temperature was observed by scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction. Both the thickness and spacing of the Ca filaments decreased exponentially with increasing deformation. The ultimate tensile strength of the composite increased rapidly with increased deformation, especially at high deformation processing strains. The relation between deformation true strain and ultimate tensile strength is underestimated by the rule of mixtures; a modified Hall–Petch barrier strengthening model was found to fit the data better.

Production of fine calcium powders by centrifugal atomization with rotating quench bath

by Liang Tian and A. Russell

Recently, a novel Al/Ca composite was produced by severe plastic deformation of Al powders and Ca... more Recently, a novel Al/Ca composite was produced by severe plastic deformation of Al powders and Ca granules for possible use as a high-voltage power transmission conductor. Since the strength of such composites is inversely proportional to the Ca filament size, fine Ca powders (less than ~ 250 μm) are needed to achieve the desired high strength for the powder metallurgy production of an Al-matrix composite reinforced by nano-scale Ca filaments. However, fine Ca powders are not commercially available. Therefore, we have developed a method to produce fine Ca powders via centrifugal atomization to supply Ca powder for prototype development of Al/Ca composite conductor. A secondary goal of the project was to demonstrate that Ca powder can be safely prepared, stored, and handled and could potentially be scaled for commercial production. Our results showed that centrifugal atomiza- tion can yield as much as 83 vol.% Ca powder particles smaller than 250 μm. The mean particle size sometimes matches, sometimes deviates substantially from the predictions of the Champagne & Anger equation likely due to unexpected secondary atomization. The particle size distribution is typical for a ligament-disintegration atom- ization mode. Scanning electron micrographs showed that the morphology of these Ca powders varied with pow- der size. Spark testing and auto-ignition tests indicated that the atomized powders were difficult to ignite, providing confidence that this material can be handled safely in air.

Modeling the electrical resistivity of deformation processed metal–metal composites

by Liang Tian and I. Anderson

Acta Materialia, 2014

Deformation processed metal–metal (matrix–reinforcement) composites (DMMCs) are high-strength, hi... more Deformation processed metal–metal (matrix–reinforcement) composites (DMMCs) are high-strength, high-conductivity in situ com- posites produced by severe plastic deformation. The electrical resistivity of DMMCs is rarely investigated mechanistically and tends to be slightly higher than the rule-of-mixtures prediction. In this paper, we analyze several possible physical mechanisms (i.e. phonons, inter- faces, mutual solution, grain boundaries, dislocations) responsible for the electrical resistivity of DMMC systems and how these mech- anisms could be affected by processing conditions (i.e. temperature, deformation processing). As an innovation, we identified and assembled the major scattering mechanisms for specific DMMC systems and modeled their electrical resistivity in combination. From this analysis, it appears that filament coarsening rather than dislocation annihilation is primarily responsible for the resistivity drop observed in these materials after annealing and that grain boundary scattering contributes to the resistivity at least at the same magnitude as does interface scattering.

Prospects for novel deformation processed Al/Ca composite conductors for overhead high voltage direct current (HVDC) power transmission

by Liang Tian and A. Russell

In the quest for more efficient long-distance power transmission, High Voltage Direct Current (HV... more In the quest for more efficient long-distance power transmission, High Voltage Direct Current (HVDC) transmission offers a reliable, cost-effective alternative to HVAC for long distance overhead transmission. Development of stronger, more conductive, and lighter conductor materials could lower HVDC construction costs, reduce energy losses, and increase reliability. In this paper, we describe a new deformation processed Al/Ca composite conductor now in development that has the potential to offer substantially lower density, higher strength, and higher conductivity for HVDC than today’s conductor materials. This Al/Ca composite has shown promising early corrosion and elevated temperature performance test results, but additional research is needed to determine its fatigue and creep strengths, as well as its susceptibility to electrotransport phenomena and other factors. The design process, production methods, and physical properties of the new composite conductor are discussed. An economic and thermal analysis of this conductor material is also presented.