High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that favour the primary creep regime. The study has revealed the detailed structure of extended a2〈112〉 dislocations as they shear the γ′ precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a〈1 1 2〉 dislocations was also found to distort the γ/γ′ interface, changing its appearance from linear to a "saw tooth" pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a2〈110〉 dissociated dislocations were also observed in the γ channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the γ′ and intrinsic stacking faults in the γ, and to make estimates of their energies

The effect of grain size on the twin initiation stress in a TWIP steel

The influence of grain size on the twinning stress of an Fe-15Mn-2Al-2Si-0.7C Twinning Induced Plasticity (TWIP) steel has been investigated. Five grain sizes were obtained using a combination of cold rolling and annealing. Electron backscatter diffraction (EBSD) analysis revealed that the material exhibited a typical cold rolled and annealed texture. Tensile testing showed the yield stress to increase with decreasing grain size, however, the ductility of the material was not substantially affected by a reduction in grain size. Cyclic tensile testing at sub-yield stresses indicated the accumulation of plastic strain with each cycle, consequently the nucleation stress for twinning was determined. The twin stress was found to increase with decreasing grain size. Furthermore, the amount of strain accumulated was greater in the coarser grain material. It is believed that this is due to a difference in the twin thickness, which is influenced by the initial grain size of the material. SEM and TEM analysis of the material deformed to 5% strain revealed thinner primary twins in the fine grain material compared to the coarse grain. TEM examination also showed the dislocation arrangement is affected by the grain size. Furthermore, a larger fraction of stacking faults was observed in the coarse-grained material. It is concluded that the twin nucleation stress and also the thickness of the deformation twins in a TWIP steel, is influenced by the initial grain size of the material. In addition grain refinement results in a boost in strength and energy absorption capabilities in the material

Prediction of Mechanical Behaviour in Ni-Base Superalloys Using the Phase Field Model of Dislocations

The "Phase-Field Model of Dislocations" (PFMD) was used to simulate shearing of gamma-prime precipitate arrays in single crystal turbine blade superalloys. The focus of the work has been on the cutting of the L12 ordered precipitates by a{111} dislocation ribbons during Primary Creep. The Phase Field Model presented incorporates specially developed Generalised Stacking Fault Energy (γ-surface) data obtained from atomistic simulations. The topography of this surface determines the shearing mechanisms observed in the model. The merit of the new γ-surface, is that it accounts for the formation of extrinsic stacking faults, making the model more relevant to creep deformation of superalloys at elevated temperatures

Superelastic load cycling of Gum Metal

The superelastic beta titanium alloy, Gum Metal, has been found to accumulate plastic strain during tensile load cycling in the superelastic regime. This is evident from the positive drift of the macroscopic stress vs. strain hysteresis curve parallel to the strain axis and the change in its geometry subsequent to every load-unload cycle. In addition, there is a progressive reduction in the hysteresis loop width and in the stress at which the superelastic transition occurs. In situ synchrotron X-ray diffraction has shown that the lattice strain exhibited the same behaviour as that observed in macroscopic measurements and identified further evidence of plastic strain accumulation. The mechanisms responsible for the observed behaviour have been evaluated using transmission electron microscopy, which revealed a range of different defects that formed during load cycling. The formation of these defects is consistent with the classical mathematical theory for the bcc to orthorhombic martensitic transformation. It is the accumulation of these defects over time that alters its superelastic behaviour

Functional stability of a ferromagnetic polycrystalline Ni2MnGa high temperature shape memory alloy

Electrocaloric Ni 2 MnGa is of interest for solid state refrigeration applications, as well as a high temperature thermal shape memory alloy. Here, polycrystalline Ni 54 Mn 25 Ga 21 is examined using in situ synchrotron X-ray di raction. The initial martensite ( M f ) and austenite ( A f ) finish temperatures were found to be 232 C and 298 C respectively. M f was observed to decline by 8 C / cycle and A f increased by 1 C / cycle. Both below and surprisingly, above the Curie temperature, the application of an e.m.f. was found to a ect the lattice parameters measured. A change in the thermal expansion of the two phases was found around the Curie temperature

The dynamic behaviour of a twinning induced plasticity steel

The influence of strain rate on the twinning behaviour and microstructure of an Fe–15Mn–2Al–2Si–0.7C twinning induced plasticity (TWIP) steel has been investigated. A Hopkinson pressure bar setup was used in addition to blast testing to perform the high strain rate testing. The yield stress exhibited a positive strain rate sensitivity with increasing strain rate. However, the failure strain of the material was relatively unaffected. Post-deformation microscopy indicated that deformation twinning was less profuse at higher strain rates. Electron backscatter diffraction also indicated the activation of multiple twin systems at strain rates below 1000 s−1 although this did not occur at the higher strain rates tested. A large intragranular misorientation was found to exist in the material tested at lower strain rates indicating a relatively larger dislocation density existing in the material tested at lower strain rates. In addition selected grains in the blast tested material exhibited a 'wavy' structure which was determined not to be due to a phase transformation. It is suggested that this was caused by the complex loading experienced by the material during testing. High resolution transmission electron microscopy also indicated a large density of intrinsic stacking faults in the material subjected to blast testing

In situ micropillar deformation of hydrides in Zircaloy-4

Deformation of hydrided Zircaloy-4 has been examined using in situ loading of hydrided micropillars in the scanning electron microscope and using synchrotron X-ray Laue microbeam diffraction. Results suggest that both the matrix and hydride can co-deform, with storage of deformation defects observed within the hydrides, which were twinned. Hydrides placed at the plane of maximum shear stress showed deformation within the hydride packet, whilst packets in other pillars arrested the propagation of shear bands. X-ray Laue peak broadening, prior to deformation, was associated with the precipitation of hydrides, and during deformation plastic rotation and broadening of both the matrix and hydride peaks were observed. Post-mortem TEM of the deformed pillars has indicated a greater density of dislocations associated with the precipitated hydride packets, while the observed broadening of the hydride electron diffraction spots further suggests that plastic strain gradients were induced in the hydrides by compression

Precipitation of the ordered α2 phase in a near-α titanium alloy

Precipitate evolution in a near-α alloy was studied using transmission electron microscopy (TEM) and correlative atom probe tomography (APT) after ageing at 550-700 for times up to 28 days. It is found that precipitation occurs much faster and is more prolific in samples heat treated at higher temperatures. Particles were spherical after ageing at 550 °C, while after ageing at 700 °C they become ellipsoids with the major axis lying close to the [0001] direction. At longer ageing times, the α2 precipitates were found to contain greater amounts of Sn + Si, indicating that Sn and Si are stronger Ti3(Al,Sn,Si) formers than Al