Temperature Dependent Deformation Mechanisms Research Articles

A novel D022-strengthened medium entropy alloy with outstanding strength-ductility synergies over ambient and intermediate temperatures

Precipitation-strengthened medium/high-entropy alloys (MEAs/HEAs) have great potential for high-temperature applications. In this study, we designed a novel Ni45.9Fe23Cr23V4Nb3Mo1B0.1 (at.%) MEA alloy, hardened by the D022 (Ni, Fe, Cr)3(Nb, V)-type nanoprecipitates, with an excellent strength-ductility combination from room to elevated temperatures. Specifically, the tensile strengths, at 700 and 800 °C, could be maintained as high as 845 and 589 MPa, respectively; meanwhile, elongations at all testing temperatures exceeded 25 % without any intermediate-temperature embrittlement. The temperature-dependent deformation mechanisms were unraveled using multi-scale characterizations, which involved profound slip planarities, such as stacking fault (SF) networks and deformation twins (DTs). Furthermore, the critical resolved shear stress (CRSS) to initiate SFs in both face-centered cubic (FCC) and D022 phases was evaluated, and the possible reasons for the origin of anomalous DTs at 800 °C were discussed in detail. The main findings demonstrate that the shearable D022 nanoparticles can provide the FCC matrix with considerable dislocation storage capacity, reinforcing strain hardening at ambient and intermediate temperatures. This work provides fundamental insights into the controllable design and deformation mechanisms of high-performance D022-strengthened MEAs/HEAs.

Comparison of temperature dependent deformation mechanisms of 8YSZ thermal barrier coatings prepared by air-plasma-spray and D-gun thermal spray: An in situ study

8 weight percent yttria stabilized zirconia (8YSZ) has gained widespread use in thermal barrier coatings for the hot sections of aero and power generation turbines due to its superb thermal and mechanical properties. In this study, in situ microcompression tests were conducted to evaluate the mechanical performance of 8YSZ coatings with dense vertically cracked (DVC) microstructures produced by detonation gun thermal spray to those deposited by air plasma spray (APS). At room temperature, the APS coatings showed high variability in fracture strength resulting from cracks and pores in the coating. DVC coatings, conversely, exhibited fracture strengths ranging from 3.9 to 6.6 GPa and less variability in fracture strength attributed to the relatively dense and less defective microstructure. At 500 °C, both coatings showed better consistency of fracture strength and enhanced deformability owing to deformable pores, ferroelastic domain switching, and dislocation activities.

Temperature-dependent deformation mechanisms and microstructural degradation of a polycrystalline nickel-based superalloy

The mechanical behavior of cast nickel-based superalloy Inconel 718 was investigated via hot tensile experiments at 650 °C, 800 °C, and 950 °C to understand the thermal instability of its metallurgic microstructure. Test specimens, which were machined from investment casting, were subjected to a standard heat treatment. Post-tensile testing, microstructure characterization, and fractography observations were conducted with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The resulting data revealed that the deterioration of toughness and ductility occurred at temperatures above 650 °C. Numerous shear bands and lattice mismatch suggested that plastic deformation through slip movement dominated the ductile fracture mode of the nickel-based superalloy at 650 °C. In contrast, surficial microporosity and grain boundary weakening caused early brittle rupture, before the material yielded, at 800 °C. Significant strain accumulation within grains contributed to plastic flow in the direction of the applied stress load, resulting in intergranular and transgranular fracture at 950 °C. Microstructural evidence displayed the interface debonding between the carbide and the matrix at 950 °C, indicating the temperature limitation of the carbide pileup.

Cu-SiO2 hybrid bonding simulation including surface roughness and viscoplastic material modeling: A critical comparison of 2D and 3D modeling approach

Cu-SiO2 direct hybrid bonding is considered as one of the key enabling technologies for 3D integration. Previous studies showed that the main process parameters influencing the bonding quality are temperature and annealing time, as well as the mechanical stress at the Cu-Cu interface. The latter is influenced by thermo-mechanical stress introduced by the coefficient of thermal expansion mismatch of SiO2 and Cu and by geometrical effects. The modeling approach of the present study aims to shed light on the influence of surface roughness on the contact area formation between Cu pads. Roughness profiles measured with atomic-force microscopy are directly used as input for the simulation. This introduces considerable computational effort when explicitly modeled within finite element simulation. A sub-modeling technique is used to reduce the numerical cost. The common 2D modeling approach is critically compared to full 3D modeling of the surface topography. The dominant micro-mechanical temperature dependent deformation mechanisms are taken into account by continuum mechanics material models from literature. Accordingly, the stress driven instantaneous dislocation glide and the diffusion triggered climb assisted dislocation glide are taken into account by corresponding plasticity and creep material models.

Temperature dependent deformation mechanisms of Al0.3CoCrFeNi high-entropy alloy, starting from serrated flow behavior

The deformation mechanisms of Al0.3CoCrFeNi high-entropy alloy (HEA) is investigated through studying the serrated flow behavior under different conditions. Results show that abnormal variation trends happen in the serration curves during compression, denoting a transition of the deformation mechanism with the temperature increasing. Microstructure characterization reveals this transition is a twining to slip deformation, which is evidenced by nanoscale twins at 600 °C but high density dislocations at 800 °C. This temperature dependent deformation behavior is thought to be related with the dislocation mobility and stacking fault energy (SFE).

Insights into the deformation behavior of the CrMnFeCoNi high-entropy alloy revealed by elevated temperature nanoindentation

Abstract

Temperature-dependent mechanical deformation of silicon at the nanoscale: Phase transformation versus defect propagation

This study uses high-temperature nanoindentation coupled with in situ electrical measurements to investigate the temperature dependence (25–200 °C) of the phase transformation behavior of diamond cubic (dc) silicon at the nanoscale. Along with in situ indentation and electrical data, ex situ characterizations, such as Raman and cross-sectional transmission electron microscopy, have been used to reveal the indentation-induced deformation mechanisms. We find that phase transformation and defect propagation within the crystal lattice are not mutually exclusive deformation processes at elevated temperature. Both can occur at temperatures up to 150 °C but to different extents, depending on the temperature and loading conditions. For nanoindentation, we observe that phase transformation is dominant below 100 °C but that deformation by twinning along {111} planes dominates at 150 °C and 200 °C. This work, therefore, provides clear insight into the temperature dependent deformation mechanisms in dc-Si at the nanoscale and helps to clarify previous inconsistencies in the literature.

Temperature dependent deformation mechanisms in pure amorphous silicon

High temperature nanoindentation has been performed on pure ion-implanted amorphous silicon (unrelaxed a-Si) and structurally relaxed a-Si to investigate the temperature dependence of mechanical deformation, including pressure-induced phase transformations. Along with the indentation load-depth curves, ex situ measurements such as Raman micro-spectroscopy and cross-sectional transmission electron microscopy analysis on the residual indents reveal the mode of deformation under the indenter. While unrelaxed a-Si deforms entirely via plastic flow up to 200 °C, a clear transition in the mode of deformation is observed in relaxed a-Si with increasing temperature. Up to 100 °C, pressure-induced phase transformation and the observation of either crystalline (r8/bc8) end phases or pressure-induced a-Si occurs in relaxed a-Si. However, with further increase of temperature, plastic flow rather than phase transformation is the dominant mode of deformation. It is believed that the elevated temperature and pressure together induce bond softening and “defect” formation in structurally relaxed a-Si, leading to the inhibition of phase transformation due to pressure-releasing plastic flow under the indenter.

In situ neutron diffraction study on temperature dependent deformation mechanisms of ultrafine grained austenitic Fe–14Cr–16Ni alloy

Using in situ neutron diffraction technique we investigated the temperature dependent deformation mechanisms in ultrafine grained (UFG) austenitic Fe–14Cr–16Ni alloy prepared by equal channel angular pressing. Tensile test studies show diminished ductility when testing temperature increased from 20 to 200°C. At 200°C, non-linear lattice strain deviation on [200] orientation proceeded plastic yielding by a large margin, accompanied by a greater distortion of crystal structure. In addition, the capability to accumulate dislocations was substantially reduced at 200°C as evidenced by lower dislocation density than that at 20°C. Dynamic recovery expedited at elevated temperature because of enlarged critical separation distance for annihilation of dislocation dipoles via climb. Calculations show that both high angle grain boundaries and thermal kinetic energy assisted the reduction of vacancy formation energy.

Single and multi-step phase transformation in CuZr nanowire under compressive/tensile loading

A novel stress induced martenistic phase transformation is reported in an initial B2-CuZr nanowire of cross-sectional dimensions in the range of 19.44 × 19.44–38.88 × 38.88 Å 2 and temperature in the range of 10–400 K under both tensile and compressive loading. Extensive Molecular Dynamic simulations are performed using an inter-atomic potential of type Finnis and Sinclair. The nanowire shows a phase transformation from an initial B2 phase to BCT (body-centered-tetragonal) phase with failure strain of ∼40% in tension, whereas in compression, comparatively a small B2 → BCT phase transformation is observed with failure strain of ∼25%. Size and temperature dependent deformation mechanisms which control ultimately the B2 → BCT phase transformation are found to be completely different for tensile and compressive loadings. Under tensile loading, small cross-sectional nanowire shows a single step phase transformation, i.e. B2 → BCT via twinning along {100} plane, whereas nanowires with larger cross-sectional area show a two step phase transformation, i.e. B2 → R phase → BCT along with intermediate hardening. In the first step, nanowire shows phase transformation from B2 → R phase via twinning along {100} plane, afterwards the nanowire deforms via twinning along {110} plane which cause further transformation from R phase → BCT phase. Under compressive loading, the nanowire shows crushing along {100} plane after a single step phase transformation from B2 → BCT. Proper tailoring of such size and temperature dependent phase transformation can be useful in designing nanowire for high strength applications with corrosion and fatigue resistance.

The Influence of Stacking Fault Energy on the Temperature Dependent Deformation Mechanisms in High Mn-N Austenitic Stainless Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009