A Multi-scale Approach to Understanding the KJc(T) Brittle to Ductile Cleavage Transition in Ferritic Alloys
G. Robert Odette, M. Hribernik, T. Yamamoto and M. He
Department of Mechanical Engineering
University California at Santa Barbara
Santa Barbara, CA 93106
DRAFT - Extended Abstract
A Multi-scale Approach to Understanding the KJc(T) Brittle to Ductile Cleavage Transition in Ferritic Alloys
G. Robert Odette, M. Hribernik, T. Yamamoto and M. He
Department of Mechanical Engineering
University California at Santa Barbara
Santa Barbara, CA 93106
Our lecture overviews the physical processes that mediate cleavage fracture toughness from atomic to structural length scales. After a brief review of the ‘local approach’ to fracture, we use open questions about the so-called master curve method (MCM) as a framework to address cleavage mechanisms and models [1-5]. The MCM is based on the empirical observation that the toughness temperature KJc(T-To) curves of steels seem to have a remarkably invariant master curve (MC) shape, while the reference temperature, To, varies over a wide range, depending on the material microstructure and test conditions. Our objective is to understand and quantitatively model: 1) the shape of the toughness-temperature master curve over a wide range ofTo; 2) the effects of cracked body size, geometry on fracture toughness in the cleavage transition; and 3) loading rate and irradiation induced To shifts. These the fracture properties are linked to the material constitutive and local fracture properties and, hence, also to the underlying alloy microstructure.
For unirradiated alloys, with To < 0°C, an invariant MC shape is predicted by simple micromechanical models, assuming cleavage fracture occurs when an approximately temperature independent critical stress (*) contour encompasses a critical local volume (V*) of material in front of a blunting crack tip [4,5]. Continuum finite element (FE) methods are used to determine the loading (KJc) at cleavage as mediated by the alloy’s constitutive properties and local fracture properties, * and V*. At low To the shape of KJc(T) is governed almost entirely by the temperature dependence of y(T). However, irradiation hardening, y, increases To (To) which may reach values of more than 200°C. When To is in the athermal regime, where y only weakly depends on temperature, the assumption of a temperature independent * predicts changes (layovers) in the shape of KJc(T). This apparent contradiction with observation is resolved by a mildly temperature dependent *(T) at higher temperatures. The *(T) temperature dependence is, in turn, controlled by the micro-arrest toughness of the material controlling the conditions for the propagation of dynamic microcracks formed at brittle trigger particles in the high stress region of the crack tip process zaone. We hypothesize that the temperature dependence of micro-arrest toughness is controlled by atomic scale processes, and that this valves the much higher KJc(T) toughness at larger length scales, thus leading to an invariant MC shape.
In pursuit of independent support this hypothesis, we describe results of a recent experimental study of the temperature dependent initiation (KIc) and arrest toughness (Ka) of cleavage oriented (100)[010] and (100)[011] Fe single crystals, using specially designed composite specimen test techniques [6]. The Ka(T) is weakly temperature dependent below about -100°C, increasing from a minimum of 3.5 MPa√m, but rises more rapidly to 9 MPa√m at higher temperatures near 0°C. Static and dynamic single crystal KIc/d(T) curves were also measured over a wide range of loading rates from about 0.1 to 20,000 MPa√m/s. In all cases, the cleavage fracture dynamics were found to be controlled by nucleation of double kinks on screw dislocations, that also controls the flow stress dynamics. When plotted on a strain rate compensated temperature, T’, the arrest and initiation data overlap to form a Ka(T’) master curve.
The single crystal results were extended complex alloys by proposing that both the thermal and athermal contributions to y combine to control Ka(T) [unpublished]. The interplay between Ka and y is represented by a simple model Ka(T’)y(T) = C(y), with a minimum polycrystalline Ka of about 3.5 MPa, where C(y) is fitted to the single crystal data at -196°C and is a weak function of y. Using Ka(T) = C(y)/y(T) curves in the macroscopic KJc(T) model results in approximate master curve shapes over a wide range of To.
We also show the model properly predicts To due to irradiation and high strain rate induced increases in the average alloy flow stress between 0 and 10% plastic strain, fl, with a typical To/fl ≈ 0.7°C/MPa [3,4,7]. The use of fl, rather than y, accounts for the loss of strain hardening due of irradiation [7]. Finally, the fracture toughness both pertinent to structures and measured using small test specimens always depends on the size and geometry of the cracked body. Size and geometry effects arise from both statistical probabilities, related to the volume under high stress near a crack tip, and constraint loss associated with large amounts of deformation in small specimens and in the case of shallow surface cracks in structures. We describe micromechanical models that can be used to adjust the toughness measured using small specimens to both the intrinsic material KJc and the effective toughness pertinent to a structure [8,9].
Selected References
1. T. L. Anderson, Fracture Mechanics, Fundamentals and Applications 3rd Ed, CRC Press (2005)
2. ASTM E 1921-03, "Standard Test Method for Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range," Annual Book of ASTM Standards V3.01 (2005).
3. G. R. Odette and M.Y. He, “A Cleavage Toughness Master Curve Model”, J. Nuc. Mater. 283-287 (2000) 120
4. G. R. Odette and M. Y. He, “Micromechanical Modeling of Master Curve Temperature Shifts Due To Constraint Loss”, J. Nucl. Mater. 307 (2002) 1624
5. G. R. Odette, T. Yamamoto, H. J. Rathbun, M. L. Hribernik and J.W. Rensman, “Cleavage Fracture and Irradiation Embrittlement of Fusion Reactor Alloys: Mechanisms, Multiscale Models, Toughness Measurements and Implications to Structural Integrity Assessment”, J. of Nucl. Mater. 323 2-3(2003) 1243
6. M.Hribernik, “Cleavage Oriented Iron Single Crystal Toughness”, PhD Dissertation, Materials Department, University of California Santa Barbara (December 2006) papers in preparation – dissertation available on request.
7. G. R. Odette, M. Y. He, T. Yamamoto, “On the Relation Between Irradiation Induced Changes in the Master Curve Reference Temperature Shift and Changes in Strain Hardened Flow Stress”, J. Nucl. Mater (2007) in press but proof pages are available online:
8. H. J. Rathbun, G. R. Odette, T. Yamamoto and G. E. Lucas, “Statistical Stressed Volume and Constraint Loss Size Effects on Cleavage fracture Toughness in the Transition – A Single Variable Database, Engineering Fracture Mechanics, 73-1, (2006) 13
9. H. J. Rathbun, G. R. Odette, T. Yamamoto and G. E. Lucas, “Influence of Statistical and Constraint Loss Size Effects on Cleavage Fracture Toughness in the Transition – A Single Variable Database, Engineering Fracture Mechanics 73-1 (2006) 2723