M. L. Sui

Bulk metallic glass formation in the binary Cu-Zr system

Using the Cu–Zr model system, we demonstrate that bulk amorphous alloys can be obtained by copper mold casting even in a binary metallic system. The narrow, off-eutectic, bulk-glass-forming range was found to require composition pinpointing to <1 at. %. A phase selection diagram is used to explain the success of our microstructure-based approach to pinpoint the best glass former in a given system. The implications of discovering simple binary bulk amorphous alloys are discussed, in terms of its impact on understanding the formation and physics of bulk metallic glasses.

Deformation-induced grain rotation and growth in nanocrystalline Ni

Nanobeam electron diffraction and a series of dark field images techniques were used to investigate the deformation mechanisms of nanocrystalline (nc) Ni in response to in situ tensile deformation under transmission electron microscopy (TEM). The experiments exhibit the complete processes of individual grain rotation and neighboring grain rotation/growth. Deformation-induced grain rotation and growth as one of plastic deformation mechanisms in nc materials was revealed. At the same time, these results were confirmed further by ex situ TEM observation on deformed sample and were also better understood by physical deformation model.

In situ observation of twin boundary migration in copper with nanoscale twins during tensile deformation

In situ transmission electron microscopy observations are reported of the dynamic process of twin boundary migration in Cu with nanoscale twins. The experiment provides the first direct evidence of twin boundary migration via Shockley partial dislocation emission from the twin boundary/grain boundary intersections, and reveals that such migration is the dominant deformation mechanism in the initial stage of plastic straining. The behaviour is discussed in comparison with molecular dynamics simulations and in terms of the unique characteristics of the sample microstructure.

Atomic-scale in situ observation of lattice dislocations passing through twin boundaries

In situ transmission electron microscopy tensile experiments were carried out to investigate lattice dislocation and twin boundary (TB) interaction in Cu with nanoscale growth twins. Results show that extended dislocations form inside thick twin lamellas and slip toward TBs. The extended dislocations shrink, combine, and redissociate when they pass through TBs, leaving behind detwinning partial dislocations at the TBs. The mechanism of the observed dislocation/TB interactions and the effect of the mechanism on mechanical properties are discussed.

Grain refinement at the nanoscale via mechanical twinning and dislocation interaction in a nickel-based alloy

A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.

Dynamical dislocation emission processes from twin boundaries

In situ tensile straining transmission electron microscopy investigation of electrodeposited copper with high density of nanoscale growth twins reveals that twin boundaries (TBs) can serve as dislocation sources. Atomic steps at TBs formed by sessile Frank partial dislocations are beneficial for TBs serving as dislocation sources. The underlying mechanism that includes dislocation reactions with TBs is discussed

Martensitic transformation from α-Ti to β-Ti on rapid heating

Instead of conventional quenching martensitic transformation from β-Ti to α-Ti, unusual martensitic transformation from α-Ti to β-Ti induced by rapid heating has been achieved simply by using an electric current pulse. A large amount of the high-temperature martensitic phase remained in a Ti–6Al–4V alloy. We verified that such transformation is structurally and kinetically achievable, which agrees well with the phenomenological crystallographic theory of martensitic transformation.

High-performance bulk Ti-Cu-Ni-Sn-Ta nanocomposites based on a dendrite-eutectic microstructure

Using a Ti-Cu-Ni-Sn-Ta alloy as an example, we demonstrate a strategy for the in situ formation of nanocomposite microstructures that can lead to simultaneous high strength and ductility. Our approach employs copper mold casting for the production of bulk alloys from the melt, and the solidification microstructure is designed to be composed of micrometer-sized ductile dendrites uniformly distributed inside a matrix of nanoscale eutectic reaction products. The nanostructured matrix is achieved at a relatively deep eutectic, which facilitates the formation of an ultrafine eutectic microstructure over a range of cooling rates. The multi-component recipe stabilizes a ductile solid solution as the toughening phase and helps to reduce the eutectic spacing down to nanoscale. The multi-phase microstructure (including phase distributions, morphologies, and interfaces) has been examined in detail using transmission electron microscopy (TEM) and high-resolution TEM. The metastable eutectic reaction and the nanoscale spacing achieved are explained using thermodynamic and solidification modeling. The benefits expected from the microstructure design are illustrated using the high strength and large plasticity observed in mechanical property tests. Our nanocomposite design strategy is expected to be applicable to many alloy systems and constitutes another example of tailoring the microstructure on nanoscale for extraordinary properties.

Formation of a Nanostructure in a Low-carbon Steel Under High Current Density Electropulsing

The microstructure of a low-carbon steel after high current density electropulsing treatment was characterized by high-resolution transmission electron microscopy. It was found that nanostructured gamma-Fe could be formed in the coarse-grained steel after the electropulsing treatment. The mechanism of the formation of a nanostructure was discussed. It was thought that change of the thermodynamic barrier during phase transformation under electropulsing was a factor that cannot be neglected. It was reasonable to anticipate that a new method might be developed to produce nanostructured materials directly from the conventional coarse-grained crystalline materials by applying high current density electropulsing.

Orientated nanometer-sized fragmentation of TiC particles by electropulsing

chinese acad sci, inst met res, shenyang natl lab mat sci, shenyang 110016, peoples r china.;zhang, w (reprint author), chinese acad sci, inst met res, shenyang natl lab mat sci, 72 wenhua rd, shenyang 110016, peoples r china