Dacian Tomus

In situ lift-out dedicated techniques using FIB-SEM system for TEM specimen preparation.

The recent emergence of the focused ion-beam (FIB) microscope as a dedicated specimen preparation tool for transmission electron microscopy (TEM) has extended the reach of TEM to a wider variety of problems in materials science. This paper highlights three examples of using FIB-SEM lift-out techniques for preparing site-specific and crystallographic orientation-specific thin-foil specimens. An in situ lift-out technique used to extract thin foils from across a local grain boundary in bulk Al alloy and from individual fine Al atomised powder particles (down to 20μm in diameter) was performed with real-time secondary electron imaging within the chamber of a FIB-SEM system. In conjunction with electron backscatter diffraction (EBSD), the FIB is used for extracting TEM foil with a specific crystallographic orientation aligned normal to the broad plane of the foil. The above technique has been demonstrated using a dual-phase Ti-Si alloy for the exploration of orientation relationship between constituent phases. Furthermore, it is suggested that FIB is more applicable for preparing thin foils from hydrogen-sensitive metals (such as titanium alloys) than conventional thinning techniques, which tend to induce ambiguous artifacts in these foils.

Fabrication of shape memory TiNi foils via Ti/Ni ultrafine laminates

Abstract Thin foils of TiNi shape memory alloy were produced by a diffusion treatment of ultrafine laminates with 50 μm thick composed of pure Ti and Ni layers. The M s and transformation heat of the TiNi foil were similar to those of the bulk alloys. The foil exhibited the shape memory effect with the recovery strain of about 8.6×10 −3 .

Electron Beam Processing of Aluminium Alloys

Electron beam (EB) direct manufacturing is an additive near-net shape digital fabrication process developed recently. The process offers a promising route for the fabrication of intricate speciality aluminium alloy parts for aircraft and aerospace applications because of the excellent energy coupling between an electron beam and aluminium. As part of a fundamental study on EB manufacturing of Al alloys, this work investigates the effect of the EB processing parameters on the development of the molten pool and the solidification characteristics of Al 2219 and Al 6061 alloys. The samples were processed using a 50kV electron beam gun over a wide range of beam currents (10-40mA) and welding speeds (0.3-0.86m/min) in both the static and oscillation focus modes. In the static focus mode, the molten pool is wedge-shaped; while in the oscillation focus mode, the molten pool is hemispherical, wider and shallower. In both cases, the depth and width of the molten pool increase with increasing beam current but they are less affected by the moving speed of the EB gun in the range 0.3-0.86m/min. Electron beam re-melted and subsequently re-solidified Al 2219 and Al 6061 alloys show microstructural features distinct from those obtained under sand casting and direct chill casting conditions. In particular, fine intermetallic precipitates in the size range 100-200nm are prominent in the equiaxed grains formed in the re-solidified weld beads of Al 6061 compared to the coarse intermetallic particles up to 10m in size prior to EB processing. EB processing offers opportunities for aluminium alloy development.

A uniaxial tensile stage with tracking capabilities for micro X-ray diffraction applications

First results are presented for a uniaxial tensile stage designed to operate on a scanning micro X-ray diffraction synchrotron beamline. The new tensile stage allows experiments at typical loading cycles used in standard engineering stress–strain tests. Several key features have been implemented to support in situ loading experiments at the intragranular length scale. The physical size and weight of the load cell were minimized to maintain the correct working distance for the X-ray focusing optics and to avoid overloading the high-resolution raster scan translation stages. A high-magnification optical microscope and image correlation code were implemented to enable automated online tracking capabilities during macroscopic elongation of the sample. Preliminary in situ tensile loading experiments conducted on beamline 12.3.2 at the Advanced Light Source using a polycrystalline commercial-purity Ti test piece showed that the elastic–plastic response of individual grains could be measured with submicrometre spatial resolution. The experiments highlight the unique instrumentation capabilities of the tensile stage for direct measurement of deviatoric strain and observation of dislocation patterning on an intragranular length scale as a function of applied load.

Compaction of Ti-6Al-4V Powder by ECAE with Back-Pressure

Powder metallurgy is widely used to produce alloys with low cost of production. The main drawback using powders is the level of residual porosity of final product which often implies the application of a complicated and costly hot isostatic pressing process. However, this issue can be overcome by using equal channel angular pressing (ECAE) with back pressure (BP). The use of severe shear deformation, with imposed hydrostatic pressure, allows a reduction in the range of compaction temperatures compare to those used in conventional practice. The compaction of Ti-6Al-4V powder by the ECAE method has been investigated. The compaction has been performed at temperatures starting from room temperature (RT) and increasing up to 400°C with various back pressures ranging from 0 to 350MPa. A billet processed by ECAE with 43MPa back-pressure at 400°C was found to have improved relative density of 97.5% and increased Vickers hardness of 369HV, compared to values of 96.7% and 325HV respectively obtained at RT. A relative density of 98.2% and 426HV hardness were measured for billets processed with BP = 262MPa at 400°C. A fully compact billet was obtained by applying 350 MPa of BP at 400°C.

Use of residual hydrogen to produce CP-Ti powder compacts for low temperature rolling

Abstract The present work investigates the optimal level of residual hydrogen in partially de-hydrogenated powder to produce CP-Ti plate compacts using ECAP with back pressure which are subsequently rolled at low temperature. A comparative study of the compaction of two TiH2 powders and a CP-Ti powder, with particle sizes 150 μm, 50 μm and 45 μm respectively, has been carried out. The hydride powders have also been compacted in a partially de-hydrogenated state. The optimal level of residual hydrogen with respect to the density of the resulting compact and the associated mechanical properties has been defined. ECAP at 300°C produced compacts from these partially de-hydrogenated powders of 99.5 % theoretical density, while CP-Ti was compacted to almost full theoretical density under the same ECAP conditions. Therefore, the compaction of powder by ECAP does not benefit from temporary hydrogen alloying. These compacts then were rolled at temperatures ranging from room temperature to 500°C with an 80 % reduction in a single pass. Heat treatment after the rolling can modify the microstructure to improve the resulting mechanical properties and in this regard the temporary alloying with hydrogen has been observed to offer some significant benefits. It is shown that ECAP followed by low temperature rolling is a promising route to the batch production of fully dense CP-Ti wrought product from powder feedstock that avoids the need to subject the material to temperatures greater than 500°C. This low temperature route is expected to be efficient from an energy point of view and it also avoids the danger of interstitial contamination that accompanies most high temperature powder processing.

Microstructural Characteristics of Electron Beam Processed Al-2Sc

Plate samples of Al-2Sc were processed with an electron beam (EB) gun in a vacuum of 10-3 Pa at 50kV and 30 µA. EB processing suppressed the formation of primary Al3Sc, altered the morphology of the eutectic (Al)-Al3Sc, and increased the solubility of Sc in the eutectic (Al). In addition, its cyclic heating effect induced the precipitation of Al3Sc. EB processing resulted in more profound hardening than conventional high temperature solid solution and ageing.

Martensitic Transformation and Mechanical Behavior of TiNi Shape Memory Alloys after Severe Plastic Deformation

Thermal- and stress-induced martensitic transformation was investigated on TiNi shape memory alloys subjected to severe plastic deformation (SPD) by cold rolling. TEM observation revelaed the sample is a mixture of nanocrystalline and amorphous after 40% cold rolling. DSC analysis suggested that the martensitic transformation was suppressed when the thickness reduction was over 25% reduction. Aging at lower temperatures (573 ~ 673 K, 3.6 ks) restores the phase transformations, but to a limited extent. The stress-strain curves of nanocrystalline/amorphous TiNi are characterized by absence of stress-plateau and small hysteresis.

Property of TiNi Shape Memory Foils Produced by Ultrafine Laminates Method

Using the ultrafine laminate method, thin foils (50 µm) of Ni-rich TiNi shape memory alloys were produced. Overall composition of the Ti/Ni laminate is Ti-50.7%Ni. TiNi (B2) phase was obtained after different diffusion treatments at 1073 K for 36 ks. Aging treatment at 773 K for 3.6, 18, 36, 72 and 144 ks were also performed. Phase transformation behavior of aged foils changed from two-step, to three-step and then to two-step transformation, which is similar to the case of bulk TiNi alloys. Uniform distribution of Ti3Ni4 phase was observed for aged samples by transmission electron microscopy. Two-way shape memory effect of the aged TiNi foil was also demonstrated.

Practical considerations on the application of ultrasonic treatment to Mg-Al shape castings

This paper investigates the use of ultrasonic treatment on the grain refinement of Mg-Al alloys in castings similar to commercial gravity castings. It shows that it is a very effective form of grain refinement but it is limited spatially and by the advancing solidification front if partially-solidified product is washed into the die. It was found that the best way to obtain a homogeneous fine grain size through-out the casting was to combine a grain refining addition, SiC, with ultrasonic treatment during the initial stages of solidification.