Mechanistic insights into interface-facilitated dislocation nucleation and phase transformation at semicoherent bimetal interfaces

This article presents a nanoindentation study of polycrystalline and single crystals of yttria-doped zirconia with both tetragonal and cubic phases. Analysis of the deformation mechanisms is performed by both atomic force microscopy (AFM) and micro-Raman spectroscopy. Phase transformation from tetragonal to monoclinic phase is clearly distinguished on tetragonal crystals, whereas in cubic crystals the plastic deformation seems to be controlled by dislocation nucleation and interactions. AFM observations in tetragonal zirconia grains have shown that both grain size and autocatalytic transformation strongly influence the size of the transformed zone. Furthermore, the martensitic phase transformation seems to be also strongly dependent of the indenter shape. Experimental results suggest that a critical contact pressure is necessary to induce the phase transformation.

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Phase Transformation In Ti-6al-4v Alloys

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100096175 ◽

1972 ◽

Vol 30 ◽

pp. 584-585

Author(s):

Shiro Fujishiro

Keyword(s):

Phase Transformation ◽

Grain Boundary ◽

Cryogenic Temperature ◽

Β Phase ◽

Heat Treated ◽

Mixed Microstructure ◽

Ductile Behavior

The Ti-6 wt.% Al-4 wt.% V commercial alloys have exhibited an improved formability at cryogenic temperature when the alloys were heat-treated prior to the tests. The author was interested in further investigating this unusual ductile behavior which may be associated with the strain-induced transformation or twinning of the a phase, enhanced at lower temperatures. The starting materials, supplied by RMI Co., Niles, Ohio were rolled mill products in the form of 40 mil sheets. The microstructure of the as-received materials contained mainly ellipsoidal α grains measuring between 1 and 5μ. The β phase formed an undefined grain boundary around the a grains. The specimens were homogenized at 1050°C for one hour, followed by aging at 500°C for two hours, and then quenched in water to produce the α/β mixed microstructure.

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Microstructural and Microchemical Characterization of Nickel Sulfide Inclusions in Plate Glass

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100089123 ◽

1991 ◽

Vol 49 ◽

pp. 962-963

Author(s):

J. Cooper ◽

O. Popoola ◽

W. M. Kriven

Keyword(s):

Thermal Expansion ◽

Phase Transformation ◽

Glass Matrix ◽

Nickel Sulfide ◽

Plate Glass ◽

Thermal Expansion Mismatch ◽

Volume Increase ◽

Β Phase ◽

Microchemical Characterization

Nickel sulfide inclusions have been implicated in the spontaneous fracture of large windows of tempered plate glass. Two alternative explanations for the fracture-initiating behaviour of these inclusions have been proposed: (1) the volume increase which accompanies the α to β phase transformation in stoichiometric NiS, and (2) the thermal expansion mismatch between the nickel sulfide phases and the glass matrix. The microstructure and microchemistry of the small inclusions (80 to 250 μm spheres), needed to determine the cause of fracture, have not been well characterized hitherto. The aim of this communication is to report a detailed TEM and EDS study of the inclusions.

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HREM observation of dislocations near room temperature fractures in silicon

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100106521 ◽

1988 ◽

Vol 46 ◽

pp. 892-893

Author(s):

M. H. Rhee ◽

W. A. Coghlan

Keyword(s):

Cross Section ◽

Room Temperature ◽

Single Crystal Silicon ◽

Dislocation Nucleation ◽

Back Side ◽

Ion Milling ◽

Crystal Silicon ◽

Flat Surfaces ◽

Induced Fractures ◽

Vicker’S Microhardness

Silicon is believed to be an almost perfectly brittle material with cleavage occurring on {111} planes. In such a material at room temperature cleavage is expected to occur prior to any dislocation nucleation. This behavior suggests that cleavage fracture may be used to produce usable flat surfaces. Attempts to show this have failed. Such fractures produced in semiconductor silicon tend to occur on planes of variable orientation resulting in surfaces with a poor surface finish. In order to learn more about the mechanisms involved in fracture of silicon we began a HREM study of hardness indent induced fractures in thin samples of oxidized silicon.Samples of single crystal silicon were oxidized in air for 100 hours at 1000°C. Two pieces of this material were glued together and 500 μm thick cross-section samples were cut from the combined piece. The cross-section samples were indented using a Vicker's microhardness tester to produce cracks. The cracks in the samples were preserved by thinning from the back side using a combination of mechanical grinding and ion milling.

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AEM of reaction-bonded SiC

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100122125 ◽

1992 ◽

Vol 50 (1) ◽

pp. 344-345

Author(s):

K Das Chowdhury ◽

R. W. Carpenter ◽

W. Braue

Keyword(s):

Mechanical Properties ◽

Phase Transformation ◽

High Temperature ◽

Epitaxial Growth ◽

Liquid Silicon ◽

Structural Ceramic ◽

Few Data

Research on reaction-bonded SiC (RBSiC) is aimed at developing a reliable structural ceramic with improved mechanical properties. The starting materials for RBSiC were Si,C and α-SiC powder. The formation of the complex microstructure of RBSiC involves (i) solution of carbon in liquid silicon, (ii) nucleation and epitaxial growth of secondary β-SiC on the original α-SiC grains followed by (iii) β>α-SiC phase transformation of newly formed SiC. Due to their coherent nature, epitaxial SiC/SiC interfaces are considered to be segregation-free and “strong” with respect to their effect on the mechanical properties of RBSiC. But the “weak” Si/SiC interface limits its use in high temperature situations. However, few data exist on the structure and chemistry of these interfaces. Microanalytical results obtained by parallel EELS and HREM imaging are reported here.

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Phase transformation and layer evolution in molybdenum disilicide-silicon carbide nanolayered coatings

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100170426 ◽

1994 ◽

Vol 52 ◽

pp. 538-539

Author(s):

H. Kung ◽

T. R. Jervis ◽

J.-P. Hirvonen ◽

M. Nastasi ◽

T. E. Mitchell ◽

...

Keyword(s):

Phase Transformation ◽

High Temperature ◽

Oxidation Resistance ◽

Elevated Temperatures ◽

Matrix Material ◽

Molybdenum Disilicide ◽

High Temperature Strength ◽

Cross Sectional ◽

Good Oxidation Resistance ◽

Structural Applications

MoSi2 is a potential matrix material for high temperature structural composites due to its high melting temperature and good oxidation resistance at elevated temperatures. The two major drawbacksfor structural applications are inadequate high temperature strength and poor low temperature ductility. The search for appropriate composite additions has been the focus of extensive investigations in recent years. The addition of SiC in a nanolayered configuration was shown to exhibit superior oxidation resistance and significant hardness increase through annealing at 500°C. One potential application of MoSi2- SiC multilayers is for high temperature coatings, where structural stability ofthe layering is of major concern. In this study, we have systematically investigated both the evolution of phases and the stability of layers by varying the heat treating conditions.Alternating layers of MoSi2 and SiC were synthesized by DC-magnetron and rf-diode sputtering respectively. Cross-sectional transmission electron microscopy (XTEM) was used to examine three distinct reactions in the specimens when exposed to different annealing conditions: crystallization and phase transformation of MoSi2, crystallization of SiC, and spheroidization of the layer structures.

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Ferroelectric Domain Structure of Pb(Zr.52Ti.48)03

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s1431927600000933 ◽

1980 ◽

Vol 38 ◽

pp. 214-215

Author(s):

E.K. Goo ◽

R.K. Mishra

Keyword(s):

Phase Transformation ◽

Domain Structure ◽

Tetragonal Phase ◽

Ferroelectric Domain ◽

Cubic Phase ◽

Ion Milling ◽

Thin Foils ◽

Ferroelectric Domain Structure ◽

Ferroelectric Domains

Ferroelectric domains are twins that are formed when PZT undergoes a phase transformation from a non-ferroelectric cubic phase to a ferroelectric tetragonal phase upon cooling below ∼375°C.,1 The tetragonal phase is spontaneously polarized in the direction of c-axis, making each twin a ferroelectric domain. Thin foils of polycrystalline Pb (Zr.52Ti.48)03 were made by ion milling and observed in the Philips EM301 with a double tilt stage.

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