Non-isothermal tensile tests during solidification of Al–Mg–Si–Cu alloys: Mechanical properties in relation to the phenomenon of hot tearing

An original set-up has been used to study the mechanical properties of aluminium alloys in tension during solidification with a high cooling rate (70 K/s). The mechanical behaviour of 6056 aluminium alloy with and without grain refiner has been investigated as well as that of mixtures between AA6056 and AA4047. The results show that the alloys exhibit a viscoplastic behaviour in the mushy state. A transition is observed between fracture in the mushy state and fracture in the solid state as a function of the displacement rate. This displacement rate at the transition depends on the cooling rate and on the composition of the alloy. The displacement before fracture is observed to be independent of displacement rate but to depend on the composition and on the solidification rate. Based on the observations a criterion for fracture in the mushy state is proposed. A simple rheological law describing the mechanical behaviour of the alloys is coupled to a finite element calculation giving the thermal field during the tensile test. This simulation is able to reproduce the mechanical response of the solidifying alloy during a non-isothermal test.

Intermediate temperature grain boundary phenomena in copper alloys

Doris Empl

Copper and copper alloys typically experience a sharp loss in ductility at intermediate temperature, induced by grain boundary damage and fracture processes. The susceptibility of the material to intermediate temperature embrittlement is, as a result, strongly affected by the presence, along the grain boundary, of segregated atoms or low-melting phase inclusions, the former potentially weakening atomic bonding and the second causing liquid metal embrittlement along the boundaries. This thesis is an exploration of underlying phenomena that govern the intermediate temperature embrittlement of Cu-Pb alloys, addressing in turn capillary phenomena and then their relation to the tensile strength and ductility of leaded copper alloys. The dihedral angle shown by intergranular lead inclusions in Cu-1Pb alloys is measured varying the purity of the metal and the temperature. Several measurement methods are used and compared, namely classical 2D methods based on metallurgical cross section analysis, and a 3D stereoscopic method that yields the true three-dimensional angle value for individual inclusions straddling a flat grain boundary (first presented by Felberbaum et al., J. Mat. Sci., vol. 40 (2005) p. 3121). Data confirm and extend earlier measurements using the newer 3D method. It is shown that a discrepancy found between literature data and stereoscopic 3D dihedral angle measurements is not caused by impurity effects, but the data indicate that its origin lies in a difference in average dihedral angle values measured between inclusions straddling two grains, and values found at inclusions located where three or more grains meet. The method is then extended towards the simultaneous measurement of relative grain boundary energy and all five macroscopic grain boundary degrees of freedom in a copper polycrystal containing quenched lead inclusions. Data are compared with a model for grain boundary energy in pure fcc metals. The comparison suggests (i) that relaxation can lower the grain boundary energy significantly when equal numbers of broken bonds are present on either side of the grain boundary and (ii) that lead segregation to grain boundaries affects the orientation-dependence of large-angle grain boundary energies in copper. A study of the tensile strength of two Cu-1Pb alloys of different purity and of different microstructure tested at 400 °C and at a strain rate of 10 s-1 is then presented. Data show that the alloy fracture stress is consistent with a Griffith-type fracture criterion that results from assimilation of the inclusions to a void. Measured fracture stresses suggest a 25-fold increase in fracture energy dissipation over surface tension contributions, likely caused by plastic deformation of the ductile copper phase. Tensile tests conducted at 400 °C and at a strain rate of 10-2 s-1 are then presented for a Cu-Ni-Sn-Pb alloy, which exhibits high mechanical strength at room temperature coupled with good machinability. Additions to this alloy of less than 1 wt% are investigated, namely Al, Mn, Zr, B or P. It is found that, unlike the other alloying additions explored, B and P improve the strength and ductility of the leaded alloy and also that these form second phase particles in the alloy; it is proposed that the improvement results from a capillary and mechanical interaction between these particles and molten lead inclusions in the alloy, consistent with the theory advanced to explain results found for the Cu-1Pb alloy. The thesis concludes with a preliminary exploration of grain boundary engineering strategies towards the improvement of the 400 °C strength of Cu-1%Pb, showing that marginal improvements in both strength and ductility can be achieved.

EPFL2009

Rheological Behavior of Al-Cu Alloys during Solidification: Constitutive Modeling, Experimental Identification, and Numerical Study

Jean-Marie Drezet

The rheological behavior of a solidifying alloy is modeled by considering the deforming material as a viscoplastic porous medium saturated with liquid. Since the solid grains in the mush do not form a fully cohesive skeleton, an internal variable that represents the partial cohesion of this porous material is introduced. The model parameters are identified using shear and compressive stress states under isothermal conditions on an Al-Cu model alloy. The model is partially validated with onisothermal conditions and we complete this study with tensile conditions. Such conditions, when applied on the mush, may lead to severe defects in many casting processes. The model has been implemented into a commercial finite-element code to simulate a tensile test. Comparison with experimental data shows that the model is able to reproduce the main features of a solidifying alloy under tension, although fracture is not directly addressed here. We show that two critical solid fractions must be introduced in the model to account for the rheology: the coherency solid fraction at which the mush acquires significant strength and the coalescence solid fraction at which solid grains start to form solid bridges.

2005

Constitutive behavior and fracture properties of tempered martensitic steels for nuclear applications

Raul Alejandro Bonadé

In this study, the tensile and fracture properties and the microstructure of the reduced-activation tempered martensitic steel Eurofer97 have been investigated. This technical alloy is a 9%Cr steel developed within the European fusion material research program. To a lesser extend, the plastic flow properties of a equiaxed ferritic Fe9%Cr model alloy were also studied for comparison with those of the tempered martensitic structure. The main objectives of this work are described hereafter: to correlate the microstructural features with the plastic flow properties measured by the tensile tests for both the Eurofer97 steel and the model alloy. The correlation established should be reflected in a physically-based model of plastic flow. to study the fracture properties of the Eurofer97 steel in details in the lower ductile-to-brittle transition region. to calculate with finite element modeling the stress fields at the crack tip. This information is further used in conjunction with a local approach model for quasi-cleavage to reconstruct the fracture toughness-temperature curve. Tensile tests were carried out at different imposed nominal strain rate at several temperatures from 77K up to 473K, on both the Eurofer97 steel and the Fe9%Cr model alloy. The temperature dependence of the yield stress was precisely determined. As expected for body-centered cubic (BCC) materials, a strong increase of the yield stress by decreasing temperature, below 200 K, was observed. At higher temperatures, the temperature dependence of the yield stress was found much weaker, being associated with the temperature dependence of the shear modulus. Efforts were made to analyze in details the post-yield behavior (strain-hardening) as a function of temperature. The post-yield behavior was modeled using the Kocks phenomenological model based on the competition of storage and annihilation of dislocations. While this model was originally developed for face-centered cubic (FCC) metals where the rate-controlling mechanism of dislocation motion is the dislocation-dislocation interaction, we used is model in the high temperature domain (T>200K) of BCC materials, to model the strain-hardening evolution of the Eurofer97 steel and the model alloy. The values obtained for the key parameters of the model, namely the dislocation mean free path and annihilation coefficient, were found consistent with the microstructural features. The parameters temperature-dependence observed was also consistent with the physics of two basic mechanisms of dislocation storage and annihilation (dynamical recovery). For the low temperature domain, the strain-hardening model was modified to account for the strong Peierls lattice friction, based on an original idea of Rauch. Transmission electron microscopy observations were done to characterize the undeformed microstructures and their evolution with strain. A clear correlation was established between the stress-dependence of the strain-hardening and the microstructures. Fracture toughness tests on the Eurofer97 steel were performed with 0.2T C(T) and 0.4T C(T) specimens in the lower transition. Five temperatures were selected at which many tests were repeated to determine the amplitude of the inherent scatter of quasi-cleavage. The temperature were chosen in such a way that the measure fracture toughness remain below 150 MPa m1/2, which correspond for the 0.4T C(T) at a M value larger than 70 at the highest temperature. Such a M value for 0.4T C(T) specimens is known to ensure enough constraint. An attempt to analyze the experimental data in the framework of the master-curve approach by following the ASTM E-1921 standard was done. It was clearly demonstrated that the assumed shape of the toughness-temperature curve as described in the ASTM E-1921 standard for the reactor pressure vessel steels is not adequate for the tempered martensitic Eurofer97 steel, which present a particularly steep transition. Our Eurofer97 fracture database was then compared to the existing one on another similar steel, the F82H. Differences in the amplitude of the scatter of both steels was found while the lower bound of the toughness-temperature curve, describing the 1% failure probability was shown to be the same. 2D finite element simulations of the compact tension specimens were performed at various temperatures using the constitutive laws determined previously. The stress field around the crack tip were calculated and used to determine a local criterion of quasi-cleavage. The criterion is defined by the attainment of a critical stress encompassing a critical area. The lower bound of the toughness-temperature curve was then successfully reconstructed by using this local criterion. Finally, the relationship between the critical area and the applied stress intensity factor for the C(T) specimen was shown to follow a power law whose coefficients are dependent on the real dimensions of the specimen. Such a relationship allows scaling the C(T) toughness data from one size to another in case of in-plane constraint loss.