Overview No. 1: The nucleation of cavities by plastic deformation

The strongly stress-sensitive and temperature-dependent creep behaviour of dispersion strengthened materials cannot be described satisfactorily by current creep laws. In this paper a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force. The approach is motivated by recent TEM observations and theoretical calculations which strongly suggest that the “classical” view, according to which particles merely force dislocations to climb around them, is inadequate. The creep equation is applied to a dispersion-strengthened superalloy, two aluminium alloys and bubble-strengthened tungsten. Practical conclusions, regarding the optimum dispersoid size and alloy development, are drawn.

https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/23870/1/ea199001.pdf

A new model-based creep equation for dispersion strengthened materials

The Nucleation of Cavities by Plastic Deformation

Mechanics of subsurface void nucleation in delamination wear

One of the important challenges facing the scientific and engineering community is the development of a reliable computational tool for predicting failure of structures under extreme and/or accidental loads. Considered in the present thesis is the process of formation of ductile cracks, which may be responsible for a catastrophic structural failure. Experimental, numerical and analytical studies are carried out on the prediction of ductile crack formation in uncracked bodies on an example of 2024-T351 aluminum alloy. A methodology for ductile crack formation suitable in industrial applications is developed through the introduction of different weighting functions for different stress triaxiality ranges where different ductile crack formation mechanisms occur ("shear fracture" in negative stress triaxialities, void nucleation, growth, and linkage in high stress triaxialities, a transition mode in intermediate stress triaxialities). A cut off value at the stress triaxiality of -1/3 is discovered. Effects of stress and strain ratios, specimen size and anisotropy are also quantified. It is found that the stress triaxiality and the equivalent strain are the two most important parameters governing ductile crack formation while other parameters are probably of secondary importance. A modification to the criterion is introduced to predict crack formation in a single reversal of straining. A case study of a solid aluminum 2024-T351 beam with a smooth round notch under 3-point bending is conducted. Other cases of component validations on different materials, structures and loadings performed by colleagues in the Impact and Crashworthiness Lab are summarized. It is concluded that the proposed approach of predicting ductile crack formation holds a promise of becoming a reliable and practical calculation tool for industrial applications. Thesis Supervisor: Tomasz Wierzbicki Title: Professor of Applied Mechanics

Prediction of ductile crack formation in uncracked bodies

The effect of dispersed oxide particles on the mechanical and high-temperature properties of a composite is closely related to their cohesion with the matrix and to their stability at elevated temperatures. Both these properties are affected by solutes in the matrix and the effect can be positive or negative in either case. The stability of the dispersion is reflected by its resistance to coarsening by Ostwald ripening. The coarsening rate is governed by several factors, most of which are affected by alloying additives in the matrix. The nature of this influence is discussed on the basis of results obtained on composites of alumina in matrices of nickel, iron, and iron–40% nickel with various minor alloying additions. Particle/matrix bonding is largely dependent on the magnitude of the interfacial energy between the oxide and the metal, which in turn is dependent on matrix alloying. The work of adhesion between matrix and particles has been determined by measuring contact angles at strain-induced...

Effects of Alloying on Structural Stability and Cohesion between Phases in Oxide/Metal Composites

The bonding strength, or ‘work of adhesion’, W, between particle and matrix in a number of austenitic and ferritic iron alloys has been determined from electron micrographs of the solid/solid contact angle, θ, at cavities formed by cold working and annealing. The critical strain to bring about cavitation, ec, was measured by plotting the strain corresponding to a sudden fall in rate of work-hardening. In deriving an equation to predict ec, account must be taken of the dependence of the equilibrium shape of the cavity on θ. Good correlation between experimental and predicted ec, as a function of W, was found. Brittle fracture of the matrix is likely to occur if θ is very low and the volume fraction is high. A criterion for brittle fracture, based on θ, is suggested which successfully predicts brittle fracture in one of the alloys studied.

E. Navara

Papers

Effects of Alloying on Structural Stability and Cohesion between Phases in Oxide/Metal Composites

The particle-to-matrix bond in dispersion-hardened austenitic and ferritic iron alloys*