Conference Programme

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I-02: Extreme Fracture and Toughening Mechanisms
Monday, 19/Jun/2017:
4:00pm - 6:15pm

Session Chair: Arief Budiman, Singapore University of Technology & Design
Session Chair: Yong-Wei Zhang, Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR)
Location: Rm 305

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4:00pm - 4:30pm

Deformation and Failure Behavior of Nanostructures

Yong-Wei ZHANG

Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), Singapore

Nanostructures are instrumental in creating many interesting engineering applications that require lightweight, high strength, and damage-tolerance. The reliability of nanostructures is often dictated by the mechanical properties of their individual components, such as nanowires, nanopipes, connections, and their defects, such as notches and voids. An in-depth understanding on the effects of intrinsic factors, such as grain boundaries and surface roughness, and extrinsic factors, such as size, shapes and man-made notches, on the deformation mechanisms and failure patterns is of great importance in designing and fabricating nanostructures with high reliability. We note, however, that the effects of defects, such as notches and voids and testing conditions, such as loading rate and testing temperature, on the failure behavior of nanolattices remain largely unexplored.

In this talk, we will report our recent study on the effects of intrinsic factors such as grain boundaries and surface roughness and extrinsic factors such as sizes, shapes and man-made notches, on the plasticity and failure of nanostructures, using both nano-mechanical testing and computer simulations. We will then report our recent work on the effect of temperature and component size on the deformation and failure of nanolattices under compression using both nano-mechanical testing and molecular dynamics simulations. Finally, we discuss the effect of notch on the failure behavior of nanolattices under tension using both nanomechanical testing and finite element modeling. These works explored the size effect, testing conditions and flaw sensitivity of nanostructures and their components using both experiments and simulations, and demonstrated various interesting and unique features of the architected nanomaterials.

4:30pm - 5:00pm

Strengthening and Toughening Mechanisms in Graphene-Al Nanolaminated Composite Micro-Pillars

Qiang GUO, Siwen FENG, Zan LI, Genlian FAN, Zhiqiang LI, Di ZHANG

State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, China

Bulk graphene-reinforced Al matrix composites of various reinforcement concentrations were fabricated via a modified powder metallurgy approach. These composites possess a nanolaminated, brick-and-mortar architecture, where layers of ~200nm-thick pure Al platelets are stacked in a staggered arrangement, and are separated by graphene sheets, each containing several graphene monolayers. The composite containing 1.5 vol. % graphene were shown to have an uniaxial tensile strength of 302±3MPa, about 50% higher than that of unreinforced Al matrix prepared using the same fabrication route (201±6MPa). Moreover, the composite possess a uniform elongation of 3.4±0.2%, only slightly lower than that of the Al matrix (4.3±0.4%), and have a significantly lower strain hardening capability. Combined with post-mortem and in situ transmission electron microscopic (TEM) analysis, our findings were interpreted in terms of the uniform distribution of graphene in the Al matrix, the effective load transfer between the graphene sheets and Al platelets, and the interaction between mobile dislocations and the graphene-Al interfaces.

5:00pm - 5:15pm

Ductile or Superelastic Deformation in CeO2-ZrO2 Nanoparticles under In-situ Compression Tests

Zehui DU1, Xiaomei ZENG1,2, Chee Lip GAN1,2

1Temasek Laboratories @ NTU, Nanyang Technological University, Singapore; 2School of Materials Science and Engineering, Nanyang Technological University, Singapore

Nano- and microscale CeO2-ZrO2 (CZ) shape memory ceramics (SMCs) that exhibit highly repeatable superelastic deformation are strongly demanded for smart micro-electro-mechanical systems (MEMS), sensing, actuation and energy damping applications. In this work, we have developed highly monodisperse CZ nanoparticles with single crystal or oligocrystal structures and used in-situ micro-compression method to study its deformation and fracture behavior. We found that the oligocrystal particles have a much higher probability to break, but exhibit a ductile fracture behavior with large plastic strain (> 10%). Single crystal particles are more resilient, exhibit highly reproducible superelasticity through over five hundred strain cycles (up to 4.7%) with the dissipated energy up to ~40 MJ/m3 per cycle. We attribute these distinct behaviors to the difference in the grain boundary area and surface roughness of the two types of particles, while the Hertzian contact geometry between particle and compression punch exacerbates the effects. Such speculation has been proved by analysis of the mechanical strength and yield strain data using Hertzian contact theory and CBD model (Chang, J. Tribol. 109 (1987) 257). For those single crystal particles, we have further studied the effects of cycling and testing temperature (between 25-400°C) on their superelasticity properties.

5:15pm - 5:30pm

Crack Propagation Through Heterogeneous Material Systems

Siu Sin QUEK, Nathaniel K.C. NG, Sridhar NARAYANASWAMY

Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), Singapore

The strengths of heterogeneous materials (e.g. polycrystalline materials, multiphase alloy structures, composites, etc.) are highly dependent on the underlying microstructure. While the mismatch of material properties between phases/grains/defects play an important role in determining the effective strength, the interfacial properties between them can also contribute to damage progression or crack propagation. Using a regularized approximation to the variational formulation for fracture we study the effects of microstructure on crack nucleation and propagation. The regularization introduces a diffused interface between the crack and uncrack material, where the interfacial width sets a length scale for crack nucleation. The approach is first validated numerically with classical results of crack deflection at a bi-material interface. We then simulate crack nucleation and propagation through a brazed joint, which consists of a complex alloy microstructure of multiple phases formed from the brazing process. These studies enrich our understanding of microstructure-property relationships, which, in turn is important in determining processing parameters for microstructure engineering.

5:30pm - 5:45pm

Fracture Toughness to Delamination Judgment during Scratch Tests

Chia-hua CHANG2, Po-Jen WEI1

1Hysitron, Inc, United States; 2National Sun Yat-sen University, Taiwan

The scratch test has been widely used to understand the delamination force between two different materials in the semiconductor device and display device industry. Usually, the discontinued burst in the normal force-normal displacement or the coefficient of friction-normal displacement curve was regarded as the point which layer delamination happened. But for some brittle or well-bonded films, the discontinued burst caused by the layer delamination was hard to distinguish. Therefore, the increment of the displacement accomplished by the fracture toughness changed in this article would use to distinguish the delamination point and this method would not influence by the loading rate differences.

5:45pm - 6:00pm

High Cycle Fatigue Behavior of Ultrafine Grained Al 2014 Alloy Processed through Cryorolling

Amit JOSHI1, Yogesha K K1, Ravi raj VERMA1, Jayaganthan R2

1Indian Institute of Technology Roorkee, India; 2Indian Institute of Technology Madras, India

High cycle fatigue behavior of bulk ultrafine grained Al 2014 alloy processed by cryorolling is studied in the present work. Post deformation heat treatment after cryorolling in the temperature range of 1000C to 2500C has also been investigated in order to optimize high cycle fatigue properties of bulk UFG Al 2014 alloy. The mechanical characterization has been performed by conducting tensile testing, Vickers hardness test, and high cycle fatigue testing and microstructural characterization is carried out through optical, TEM and FESEM for the deformed and post deformed samples. The improvement in high cycle fatigue properties of cryorolled (CR) Al 2014 alloy is observed due to formation of ultrafine grain microstructure as compared to coarser grain solution treated (ST) alloy while the post deformed annealed samples at 1000C exhibited the maximum high cycle fatigue properties due to evolution of metastable semi coherent θ' phase (Al2Cu) which facilitates the improved crack growth resistance as observed from fractographic observations after fatigue testing.

6:00pm - 6:15pm

Hybrid Al Sheets with Improved Mechanical Properties by Multi-layer Accumulative Roll-Bonding Process

Chayong LIM1, Seonghee LEE2, Beomsuk HAN3

1Korea Institute of Materials Science, South Korea; 2Mokpo National University, South Korea; 3Korea Automotive Technology Institute, South Korea

Several different kind of Aluminum alloys were processed by multi-layer accumulative roll-bonding (ARB) for ultra grain refinement and high strengthening. Several Al sheets (AA1050, AA5052, AA6061) with 1 mm thickness, 30 mm width and 300 mm length, were first degreased and wire-brushed for sound roll bonding. The different sheets were then stacked on top of each other and rolled by 50% reduction rate without lubrication at room temperature. The bonded sheet was cut into two pieces of the same dimensions and the same procedure was repeated up to 6 cycles. Microstructural evolution of bonded sheets with the number of the ARB cycles was investigated by optical microscopy (OM), transmission electron microscopy (TEM), and electron back scatter diffraction (EBSD). The grain size decreased gradually with the number of ARB cycles. The strength of the bonded sheets increased with the number of ARB cycles. The improved strength and elongation were obtained by applying annealing treatment.

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