Synopsis: 

Our research program is focused on the fundamental understanding of the mechanical response of metallic alloys, composites and coatings through elucidation of deformation and fracture mechanisms via in situ nanomechanical characterization and electron microscopy.

This mechanistic understanding is crucial to the development of predictive models of the mechanical behavior of advanced metallic materials with ultra-high flow strengths without loss in ductility and toughness. In particular, we look to develop the scientific underpinnings of the design of next-generation structural materials that, for example, are high strength and yet light-weight, or achieve high strength without loss in electrical conductivity, or are high strength and tolerant to damage in extreme conditions of temperature, particle radiation or environmental exposure.

 Current research interests:

1- Mechanics of Nanolaminate and Nanotwinned Metals

 There is plenty of room at material interfaces! Nanolaminates are compositionally-modulated assemblies of two or more dissimilar materials that exhibit unprecedented properties due to nanoscale confinement and unique atomic arrangements at interphase boundaries. For example, Cu/Nb semi-coherent interfaces exhibit multiple atomic structures without any significant change in interface energy. These interface boundaries resist morphological evolution at elevated temperatures and are relatively weak in shear. The low shear strength leads to strong interactions with glide dislocations. The dislocation stress field causes localized shear of the interface and attraction of the dislocation into and core spreading along the interface plane. Dislocations with delocalized cores are strongly pinned in the interface leading to a high interface barrier for slip transmission and unusually high yield strengths. These interfaces are also effective sinks for point defects leading to attraction, absorption, and annihilation of irradiation-induced vacancies and interstitials. Nanolaminates designed with such interfaces have resulted in ultra-high strengths without loss of plastic deformability, high thermomechanical stability and high resistance to radiation damage. Our group conducts research on a family of nanolaminate model systems synthesized by ‘bottom-up’ physical vapor deposition (e-beam evaporation or magnetron sputtering) or ‘top-down’ accumulative roll bonding approaches.

 Nanotwinned metals are chemically homogeneous but crystallographically-modulated nano-confined structures resulting from profuse twin nucleation during film growth. Nanotwinning in metals such as Cu leads to ultra-high strengths while preserving the electrical conductivity since coherent twin boundaries are strong obstacles to glide dislocations due to the geometric mismatch in slip systems but weak scattering sites for electrons. 

 STEM images taken from samples after nano-pillar compression tests (1) localized shearing in a 3 nm Cu/Mo multilayers (2) kink band formation in Cu/Mo nanocomposite with lateral concentration modulation (3) uniform deformation in Cu/Mo nanocomposite with bicontinuous interpenetrating morphology. 

 2- Metallic Thin Films with Controlled Architectures

The goal of this project is to create nano-metallic materials that exhibit ultra-high strengths and by virtue of their bicontinuous, intertwined architecture, resist flow localization.  We are using an iterative design process that integrates theory/computation, synthesis, and structural characterization. Model material systems under investigation include co-sputtered Cu-Mo, Cu-Ag-Mo, and Al-Si with bicontinuous intertwined morphology.

Single phase metallic “mesh” structures in thin films are also being explored for optical coatings. Our group explores the physics of morphological evolution and nanomechanics of sputtered films with nano-patterned morphology.

 To improve the functionality of the next generation of thin film technologies, moving away from monolithic thin film designs towards 3D morphologies using non-equilibrium synthesis techniques is required. Changes in the deposition conditions during co-sputtering of immiscible materials, manipulate the nanoarchitecture of thin films which will provide for unique and improved functionality.

The goal of this project is to develop a fundamental understanding of plastic flow behavior in high-strength metallic composites containing disparate (soft/hard) phases synthesized via laser-based additive manufacturing. In particular, we seek to elucidate the role of the microstructural scale, morphology and interphase boundary structure and crystallography in enabling plastic co-deformability in composites containing relatively soft and hard phases in mono-, bi- or mixed-modal microstructures. 

Model material systems under investigation include additively manufactured Al-based binary and higher-order eutectics. Laser processing enables nanoscale eutectic microstructures of varying morphologies and modalities. 

The interlamellar spacing of Al-Al2Cu eutectic was reduced from microscale (~1-3 μm) in the as-cast material to nanoscale (~20-100 nm) after laser surface remelting.

Bimodal eutectics (coarse binary eutectic, Al-Al2Cu, and fine matrix ternary eutectic, Al-Al2Cu-Si) formed by laser surface melting on Al-Al2Cu-Si ternary eutectic alloy.

Selective Laser Melting of 316 Stainless Steel alloys 

4- Dislocation-Interface Interactions

The need to develop predictive models of mechanical behavior that can accelerate the discovery, design and development of novel high-strength, light-weight Mg- and Al-based alloys requires quantitative, in situ straining TEM characterization. Our group uses state-of-the-art in situ nanomechanics in TEM to elucidate and quantify the mechanisms of dislocation interactions with key microstructural features such as precipitates, and grain and interphase boundaries. The vision is to couple atomic-scale imaging, in situ straining and quantitative stress measurement in the understanding of dislocation interactions to integrate with dislocation theory and simulations in developing the predictive capability.

Interaction of Glide Dislocations with Extended Precipitates in Mg-Nd alloys (More information is available here)

Our research is sponsored by:

• Department of Energy, NNSA (National Nuclear Security Administration), SSAA  (Stewardship Science Academic Alliances) Program
• National Science Foundation-Designing Materials to Revolutionize and Engineer our Future (NSF-DMREF) Program
• Department of Energy, Office of Basic Energy Sciences (DOE-BES)
• Light-weighting Innovations For Tomorrow (LIFT), a ManufacturingUSA Institute managed by Office of Naval Research
• PRedictive Integrated Structural Materials Science (PRISMS), a DOE-BES Software Center for Predictive Theory and Modeling
• Ford –UM Alliance
• Modumetal, Inc.
• Guardian Industries, Corp.

Current Collaborators:

• Professors J.E. Allison, L. Qi, E. Kioupakis, V. Sundararaghavan, University of Michigan
• Professor J. Wang, University of Nebraska-Lincoln
• Professor M.J. Demkowicz, Texas A&M University
• Professors D. Farkas, W. T. Reynolds, M. Murayama, S.G. Corcoran, Virginia Tech
• Dr. N.A. Mara, Dr. N. Li, Mr. J.K. Baldwin, CINT, Los Alamos National Lab, NM
• Professor I.J. Beyerlein, University of California-Santa Barbara
• Dr. N.A. Mara, University of Minnesota
• Professor Avinash Dongare, U-Conn