Fracture Toughness Determination with the Use of Miniaturized Specimens

Aluminum-lithium alloys have a low density and high strength to weight ratio. They are being developed for the aerospace industry.The high strength of Al-Li can be attributed to precipitation hardening. Unfortunately when aged, Al-Li aquires a low ductility and fracture toughness. The precipitate in Al-Li is part of a sequence SSSS → Al3Li → AlLi A description of the phases may be found in reference 1 . This paper is primarily concerned with the Al3Li phase. The addition of Zr to Al-Li is being explored to find the optimum in properties. Zirconium improves fracture toughness and inhibits recrystallization. This study is a comparision between two Al-Li-Zr alloys differing in Zr concentration.Al-2.99Li-0.17Zr(alloy A) and Al-2.99Li-0.67Zr (alloy B) were solutionized for one hour at 500oc followed by a water quench. The specimens were then aged at 150°C for 16 or 40 hours. The foils were punched into 3mm discs. The specimens were electropolished with a 1/3 nitric acid 2/3 methanol solution. The transmission electron microscopy was conducted on the JEM 200CX microscope.

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Microstructural and fractographic characterization of B4C-Al cermets tested under dynamic and static loading

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100154780 ◽

1989 ◽

Vol 47 ◽

pp. 562-563

Author(s):

Gyeung Ho Kim ◽

Mehmet Sarikaya ◽

D. L. Milius ◽

I. A. Aksay

Keyword(s):

Thin Film ◽

Mechanical Properties ◽

Fracture Toughness ◽

Static Loading ◽

Carbon Deposition ◽

Full Density ◽

Unique Combination ◽

Strong Component ◽

Reaction Products

Cermets are designed to optimize the mechanical properties of ceramics (hard and strong component) and metals (ductile and tough component) into one system. However, the processing of such systems is a problem in obtaining fully dense composite without deleterious reaction products. In the lightweight (2.65 g/cc) B4C-Al cermet, many of the processing problems have been circumvented. It is now possible to process fully dense B4C-Al cermet with tailored microstructures and achieve unique combination of mechanical properties (fracture strength of over 600 MPa and fracture toughness of 12 MPa-m1/2). In this paper, microstructure and fractography of B4C-Al cermets, tested under dynamic and static loading conditions, are described.The cermet is prepared by infiltration of Al at 1150°C into partially sintered B4C compact under vacuum to full density. Fracture surface replicas were prepared by using cellulose acetate and thin-film carbon deposition. Samples were observed with a Philips 3000 at 100 kV.

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Characterization of interfaces in CVD-coated nicalon fiber-reinforced SiC

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100154755 ◽

1989 ◽

Vol 47 ◽

pp. 556-557

Author(s):

K.L. More ◽

R.A. Lowden

Keyword(s):

Mechanical Properties ◽

Fracture Toughness ◽

Chemical Vapor ◽

Chemical Vapor Infiltration ◽

Frictional Stress ◽

Fiber Reinforced ◽

Pull Out ◽

Carbon Coated ◽

Fiber Matrix

The mechanical properties of fiber-reinforced composites are directly related to the nature of the fiber-matrix bond. Fracture toughness is improved when debonding, crack deflection, and fiber pull-out occur which in turn depend on a weak interfacial bond. The interfacial characteristics of fiber-reinforced ceramics can be altered by applying thin coatings to the fibers prior to composite fabrication. In a previous study, Lowden and co-workers coated Nicalon fibers (Nippon Carbon Company) with silicon and carbon prior to chemical vapor infiltration with SiC and determined the influence of interfacial frictional stress on fracture phenomena. They found that the silicon-coated Nicalon fiber-reinforced SiC had low flexure strengths and brittle fracture whereas the composites containing carbon coated fibers exhibited improved strength and fracture toughness. In this study, coatings of boron or BN were applied to Nicalon fibers via chemical vapor deposition (CVD) and the fibers were subsequently incorporated in a SiC matrix. The fiber-matrix interfaces were characterized using transmission and scanning electron microscopy (TEM and SEM). Mechanical properties were determined and compared to those obtained for uncoated Nicalon fiber-reinforced SiC.

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Phase separation in sol gel-derived ceramic films of silica-zirconia, alumina-zirconia, and titania-zirconia system

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100170967 ◽

1994 ◽

Vol 52 ◽

pp. 646-647

Author(s):

J. Tong ◽

L. Eyring

Keyword(s):

Fracture Toughness ◽

Phase Separation ◽

Sol Gel ◽

Great Influence ◽

High Strength ◽

Chemical Durability ◽

Separation Method ◽

Toughening Mechanism ◽

Ceramic Films ◽

Zirconia Toughened Alumina

There is increasing interest in composites containing zirconia because of their high strength, fracture toughness, and its great influence on the chemical durability in glass. For the zirconia-silica system, monolithic glasses, fibers and coatings have been obtained. There is currently a great interest in designing zirconia-toughened alumina including exploration of the processing methods and the toughening mechanism.The possibility of forming nanocrystal composites by a phase separation method has been investigated in three systems: zirconia-alumina, zirconia-silica and zirconia-titania using HREM. The morphological observations initially suggest that the formation of nanocrystal composites by a phase separation method is possible in the zirconia-alumina and zirconia-silica systems, but impossible in the zirconia-titania system. The separation-produced grain size in silica-zirconia system is around 5 nm and is more uniform than that in the alumina-zirconia system in which the sizes of the small polyhedron grains are around 10 nm. In the titania-zirconia system, there is no obvious separation as was observed in die alumina-zirconia and silica-zirconia system.

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