Measuring and testing – Specimen stress or strain – or testing by stress or strain...
Reexamination Certificate
2001-02-09
2002-06-04
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
C073S800000
Reexamination Certificate
active
06397682
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to resistance of polycrystalline materials to intergranular degradation or failure (IGDF), and particularly to disruption of the material's random grain boundary network connectivity (RGBNC) structure as an indicator of the material's resistance to IGDF. This indicator may be used to assess the effectiveness of engineering processes to increase the material's IGDF resistance or as a diagnostic tool to detect possible onset of material failure due to IGDF.
DESCRIPTION OF RELATED ART
The phenomenon of stress corrosion cracking (SCC) in structural materials due to the collective actions of stress, material microstructure, and environment have been recognized for many years, and the mechanisms have been extensively investigated. The SCC process is believed to be governed by the subprocesses of crack initiation and crack propagation. The method of grain boundary engineering is currently seen as one means of modifying materials in order to increasing the SCC resistance of the grain boundaries.
A grain boundary is formed where two single-crystal grains in a polycrystalline aggregate meet. The boundary is characterized by its macroscopic and microscopic degrees of freedom. In its ideal form, the boundary is planar and defined by the misorientation of the grains on either side of the boundary (two degrees of freedom for the axis of misorientation and one for the misorientation angle) and the plane of the interface (two degrees of freedom). The rigid-body shifts, parallel and perpendicular to the boundary plane, comprise the three microscopic degrees of freedom. The general grain boundary is not planar and can take on curvatures consistent with the energetics of the system.
It is common practice to describe grain boundaries by the misorientation of one grain with respect to another. It is convenient to use the axis-angle notation to denote rotation axis and the rotation angle necessary to transform one into the other. Consider two crystal lattices misoriented with respect to each other and allowed to interpenetrate. At certain axis-angle pairs, the lattices form special patterns characterized by the coincident site lattice (CSL) notation (described by H. Grinrner,
Acta Crystallographica Section A—Foundations of Crystallography
, A30 (1974)680). In this notation, the misorientation is denoted as &Sgr;n where n is the reciprocal density of coincident lattice sites, n is always odd. The CSL notation is geometrical only and Ad disregards the plane of the grain boundary and the microscopic degrees of freedom. Although one would not expect macroscopic properties to correlate with &Sgr;n, however there is strong evidence that such a correlation exists for some properties.
Grain boundaries are often grouped into broad classes, such as low- and high-angle, twist and tilt, and special and random. The first class is based on structure and energy criteria while the second and third classes are strictly geometrical in nature. Conventionally, the delimiting angle separating low- from high-angle boundaries is 15 degrees for cubic crystals. This is approximately the angle where it is no longer possible to discern well-separated dislocations forming the boundary.
Strictly speaking, special boundaries (boundaries that have low &Sgr; and exhibit special properties) occur at well-defined misorientations, but it has been shown that boundaries near an exact &Sgr; misorientation can exhibit &Sgr;-like properties. The acceptance angle, &Dgr;&phgr;, over which boundaries exhibit &Sgr;-like properties is usually expressed as:
&Dgr;&phgr;=&Dgr;&phgr;
0
&Sgr;
−m
where the prefactor &Dgr;&phgr;
0
is 15 degrees, and m=1/2 according to the Brandon criterion [D. G. Brandon, Acta Metallurgica, 14 (1966) 1479].
Not all boundaries that meet this criterion exhibit special properties. Generally speaking, special boundaries are those boundaries with &Sgr;≦29. Other boundaries, including &Sgr;>29 are considered random. This arbitrary cut-off value of &Sgr;29 for cubic crystalline materials, was first suggested by Watanabe [Watanabe, T., J. Physique, 1985, 46(C4), 555]. The distribution of boundary types with respect to &Sgr; is called the grain boundary character distribution (GBCD).
Many important physical and mechanical properties of materials are intimately coupled to microstructural features such as chemistry, grain size and shape, texture, and the presence of second phases and precipitates. It is possible to tailor the microstructure of metals alloys through thermomechanical processing to obtain orders of magnitude improvement in resistance to corrosion, stress corrosion cracking, creep and possibly to irradiation assisted stress corrosion cracking. These processing methods have generically become known as grain boundary engineering.
In grain boundary engineering, properties such as those described above have been found empirically to correlate with the fraction of “special” boundaries in the microstructure. Palumbo (G. Palunbo, U.S. Pat. Nos. 5,817,193 and 5,702,543) has in described methods by which a material can be processed to increase the fraction of special grain boundaries in a microstructure. This typically involved sequential thermomechanical processing (TMP) where a material is deformed by a moderate amount, e.g. 20% and annealed at a relatively high temperature for a relative short time. The process of deformation and annealing is repeated until the desired special fraction is obtained.
In a few documented cases, intergranular stress corrosion cracking (IGSCC) has been observed to propagate along the interconnected random grain boundary network. Adams et al [Y. Pan, B. L. Adams, T. Olson, and N. Panayotou,
Acta Materialia
44 (1996)4685] have analyzed crack path dependence of IGSCC of alloy X-750. The study examined some 818 cracked triple junctions. The choice of which boundary the crack advances upon was studied as a function of misorientation and inclination relative to the stress axis. The general observation is that random boundaries are most susceptible to cracking when the direction of forward propagation of the crack lies within an angular range of ~20 degrees about the crack plane. Low angle (&Sgr;1) and &Sgr;3 boundaries are observed not to crack for any plane inclination. Some CSL boundaries lying in the range &Sgr;5-&Sgr;49 did crack; however, when the plane inclination was considered, boundaries whose planes lie sufficient close to the coherence plane(s) were observed not to crack. Watanabe [Watanabe, Res Mechanica, 1984, 11, pp 47-84] states that low-angle and coincidence high-angle boundaries are resistant to segregation-assisted IG fracture, whereas random high-angle boundaries are preferential sites for IG fracture in most situations.
It has been found that properties that are favorably influenced by grain boundary engineering tend to have percolative mechanisms, which depend on the topology of the grain boundary network. Wells et al. [Wells, D. B., Stewart, J., Herbert, A. W., Scott, P. M. and Williams, D. E., Corrosion, 1989, 45, 649], on the basis of a bond percolation formulation, suggested an appropriate statistical function that would describe when the assembly of grain boundaries in the microstructure attained a critical value of active segments. On the basis of these simulations, Wells predicted that the minimum fraction of random boundaries in a three-dimensional lattice structure that would lead to the formation of a one-dimensional continuous linear chain was 0.23. However, when a planar section, based on an approximation of the two-dimensional microstructure to a honeycomb network, was considered then this boundary fraction reached a value of approximately 0.65. This suggests that the probability of cracks propagating through the microstructure would be considerably reduced as the special fraction increases beyond 0.35.
Advances in the engineering of grain boundaries in materials have been facilitated in recent years by
King Wayne E.
Kumar Mukul
Schwartz Adam J.
Caress Virginia B.
Chang Randall W.
Daubenspeck William C.
Noori Max
The United States of America as represented by the Department of
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