Characterization Of Deformation Mechanisms Research Articles

Deformation mechanisms in the α phase of the Ti-6Al-2Sn-4Zr-2Mo titanium alloy: In situ experiments and simulations

Titanium alloys are widely employed in aerospace applications due to an outstanding combination of properties. The variety of loading conditions and microstructures encountered in industrial components is urging the development of microstructure sensitive modeling capabilities. In particular, reliable predictions require a good knowledge of operating deformation mechanisms. The present study aims at providing a thorough characterization of deformation mechanisms in the near-α Ti-6Al-2Sn-4Zr-2Mo alloy using experiments and simulations. The sequential activation of deformation mechanisms in the α phase was monitored in situ during a tensile test carried out in a scanning electron microscope using a combination of slip traces analysis and electron back-scattered diffraction. Basal slip is activated first, and prismatic slip activity, which proceeds at a higher macroscopic stress level, is needed for a significant creep/relaxation to occur. While little evidence of 〈a〉-type pyramidal slip was found, 〈c + a〉 pyramidal slip involved first-order pyramidal planes and operates at stress levels near or above the 0.2% proof stress. Atomic force microscopy characterization of the features of the different slip modes revealed that 〈a〉 slip is coarser and wavier than 〈c + a〉 slip. Twinning, which is usually neglected in such alloys within this strain regime, was observed to be slip stimulated at a plastic strain as low as 0.5%. Crystal plasticity parameters leading to an accurate simulation of the activation sequence of deformation mechanisms were then determined. For this purpose, critical resolved shear stress values were derived for the different slip and twinning modes using specific approaches and subsequently validated using crystal plasticity simulations based on fast-Fourier transforms.

Characterization of Deformation Mechanisms in Mg Alloys by Advanced Acoustic Emission Methods

Adaptive sequential k-means (ASK) analysis of acoustic emission (AE) data was used to analyze the sources of AE during compression of three AZ31 magnesium samples with different initial texture. The results were compared to the classical hit-based approach. Observation of the deformed microstructure shows that the ASK analysis can distinguish very well between the signal originating in deformation twinning and dislocation slip. Moreover, together with microstructural analysis, the ASK algorithm revealed another source of AE for one of the samples, which was shown to be the double twinning.

Novel Characterization of Deformation Mechanisms in a Ni-base Superalloy Using HAADF Imaging and Atomic Ordering Analysis

Novel Characterization of Deformation Mechanisms in a Ni-base Superalloy Using HAADF Imaging and Atomic Ordering Analysis

Molecular dynamics based study and characterization of deformation mechanisms near a crack in a crystalline material

Modeling crack propagation in crystalline materials is a challenging enterprise due to complexities induced by the interaction of the crack with various deformation mechanisms such as dislocation, micro twin, stacking faults etc.. As a first step toward the development of physics-based models of deformation in the presence of a crack, this paper proposes a comprehensive approach based on molecular dynamics simulations of a crystalline material with an embedded crack. The MD-based framework invokes a sequence of four tasks to accomplish the overall goal, viz. (i) MD simulation, (ii) characterization of atomic-level crack and deformation mechanisms, (iii) quantification of atomic-level deformation mechanisms and crack, and (iv) response analysis. Effective characterization methods like CNA, DXA and deformation gradient analysis followed by quantification are able to delineate the crack length/opening, dislocation structure and microtwins at a high resolution. Interactions of the crack with the dislocation networks and microtwins under mode I loading conditions are investigated for different lattice orientations. Crystal orientation has significant effect on the mechanisms activation and evolution. An important study is made through partitioning of the total energy into recoverable elastic energy, defect energy and inelastic dissipation, and correlating them with deformation characteristics such as dislocation density and twin volume fraction. Finally, a simple mechanistic model of deformation is developed, which associates dislocation density evolution with the stress–strain response in a crystalline material in the presence of a crack. Results show good quantitative agreement of material softening and hardening behavior with direct MD simulation results. The model can be further used to estimate the range of strain-rates that may be applied for physically meaningful MD simulations.

Characterization of deformation mechanisms during low cycle fatigue of a single crystal nickel-based superalloy

The deformation and fracture mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated during low cycle fatigue (LCF) tests at tem- peratures of 750, 850, and 950 C under strain-controlled R = 0. It was found that LCF lives at 750 and 850 C were similar and longer than those at 950 C. The specimens tested at 750 and 850 C showed fatigue crack initiation at internal pores, and their failure occurred by cracking at persistent {111} slip bands. On the other hand, at 950 C the crack initiated at the oxide-layered surface and propa- gated along \100( c channel until fracture. At the two lower temperatures, a/2\110( dislocations with low den- sity was rarely present within c channels, and a/3\211( partial dislocations were occasionally seen to shear c 0 leaving superlattice stacking faults behind. At 950 C, homogeneous deformation was produced by perfect dislo- cation movements of cross-slip and climb in the c channel and a limited c 0 shearing by superdislocation was observed. At total strain range lower than 0.6%, well-developed polygonal dislocation network formed at rafted c 0 interface. Comparison of dislocation structures revealed that load- controlled LCF tests lead to more severe deformation to specimens than strain-controlled tests.