Werner Skrotzki

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Abstract Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti–Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti–Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti–Nb alloys. In this work, the formation of martensites (α′ and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Development of texture and microstructure in extruded ionic polycrystalline aggregates

Abstract The crystallographic preferred orientation (here referred to as texture) developed in extruded ionic polycrystalline aggregates with the NaCl-structure is studied as a function of extrusion temperature and ionicity of the material. In pure alkali halides and silver chloride a 〈100〉〈111〉 double fibre texture is found for all extrusion temperatures investigated. In extruded natural rock salt the 〈100〉 fibre is replaced by 〉115〉 at or below room temperature. The temperature dependence of the intensity of preferred orientation together with microstructural investigations suggest the 〈100〉 and 〈115〉 components to be primarily due to dynamic recrystallization. The 〈111〉 deformation texture agrees with model calculations based on simultaneous slip on both the {110}〈110〉 and {100}〈110〉 slip systems, generally observed as primary and secondary slip systems in NaCl-type ionic crystals, respectively. Possible applications to the study of the diapirism of salt domes are discussed.

Defect structure and deformation mechanisms in naturally deformed augite and enstatite

Abstract The defect structure of naturally deformed augite containing enstatite lamellae has been investigated by conventional and high resolution transmission electron microscopy in order to obtain information on the deformation mechanisms of these important rock-forming minerals. The aggregate under investigation originates from a coarse-grained websterite dyke of the Balmuccia Massif which is located within the Ivrea Zone (NW-Italy). Dyke formation may have taken place during intrusion of the peridotite massif into the lower crust at temperatures of about 1000°C. Deformation of the pyroxenite dykes may be due to subsequent tilting of the Ivrea Zone at lower temperatures (about 650°C). Cooling of the augites led to exsolution of orthopyroxene, clinoamphibole and two generations of pigeonite lamellae. The microstructure observed in the augite matrix and the enstatite exsolution lamellae consists of dislocations, planar faults and subgrain boundaries. Burgers vectors and dislocation line directions in augite indicate the activation of the (100)[001], {110} 1 2 , {110} 1 2 1/2 , (100)[010], (010)[100], (010) and {110} slip systems, the first two being the most active. Dislocation reactions are common, as well as heterogeneous precipitation of pigeonite at [001] dislocations. The subgrain boundaries observed are {hkO} tilt boundaries with [001] tilt axis and (010) twist boundaries. Occasionally (100) twin lamellae exist. The dislocations in the deformed enstatite lamellae have [001], [010] and [100]Burgers vectors, as well as combinations of these. In general, all dislocations are dissociated. The subgrain boundaries observed are {hkO} tilt boundaries with [001] tilt axis. The planar faults produced by dissociation of [001], [010] and dislocations in augite are stacking faults (SFs) on (100), (010) and {110}, respectively, that of 1 2 and dislocations are chain multiplicity faults (CMFs) on (010). The planar faults produced in enstatite are either SFs on (100) or CMFs on (010). The SFs represent layers of proto- or clinoenstatite. The CMFs in both phases are amphibole-type defects, the formation of which requires a certain solubility of OH ions in the lattice. Broadening of the CMFs occurs by movement of partial dislocations along the fault leading to clinoand orthoamphibole lamellae. Diffusion of the necessary ions most probably is along the dislocation core of the partials. The main deformation mechanisms in augite and enstatite are dislocation creep and the stress-induced ortho/clinoenstatite inversion, respectively. Dislocation dissociation hinders dynamical recovery by climb and cross slip, exsolution leads to precipitation hardening. These processes may account for the different rheological behaviour between pyroxenes and olivine. A comparison of the microstructure in orthopyroxene lamellae deformed within the augite matrix with that in orthopyroxene grains deformed within an olivine environment shows that additional independent slip systems are activated if orthopyroxene is embedded in a less ductile matrix. Nevertheless, similar to clinopyroxenes, five independent slip systems required for full strain compatibility are not activated because slip on planes cutting the chains does not take place.

Growth of epitaxial SmCo5 films on Cr∕MgO(100)

Although the hard magnetic SmCo5 phase has very attractive and well-known intrinsic magnetic properties, it has not been grown as an epitaxial thin film, so far. This letter reports the epitaxial growth of SmCo5 films by pulsed-laser deposition on Cr(100) buffered MgO(100) single-crystal substrates. The phase purity, crystal structure, epitaxial relation to the substrate, and magnetic properties have been determined by careful energy-dispersive x-ray analysis, pole figure measurements, transmission electron microscopy, and vibrating sample magnetometry. Compared to the formerly studied Sm2Co7 films, the preparation of the SmCo5 phase improves the remanent magnetization by 38%.Although the hard magnetic SmCo5 phase has very attractive and well-known intrinsic magnetic properties, it has not been grown as an epitaxial thin film, so far. This letter reports the epitaxial growth of SmCo5 films by pulsed-laser deposition on Cr(100) buffered MgO(100) single-crystal substrates. The phase purity, crystal structure, epitaxial relation to the substrate, and magnetic properties have been determined by careful energy-dispersive x-ray analysis, pole figure measurements, transmission electron microscopy, and vibrating sample magnetometry. Compared to the formerly studied Sm2Co7 films, the preparation of the SmCo5 phase improves the remanent magnetization by 38%.

Strain induced superconductivity in the parent compound BaFe2As2

The discovery of superconductivity with a transition temperature, Tc, up to 65 K in single-layer FeSe (bulk Tc=8 K) films grown on SrTiO3 substrates has attracted special attention to Fe-based thin films. The high Tc is a consequence of the combined effect of electron transfer from the oxygen-vacant substrate to the FeSe thin film and lattice tensile strain. Here we demonstrate the realization of superconductivity in the parent compound BaFe2As2 (no bulk Tc) just by tensile lattice strain without charge doping. We investigate the interplay between strain and superconductivity in epitaxial BaFe2As2 thin films on Fe-buffered MgAl2O4 single crystalline substrates. The strong interfacial bonding between Fe and the FeAs sublattice increases the Fe-Fe distance due to the lattice misfit, which leads to a suppression of the antiferromagnetic spin density wave and induces superconductivity with bulk Tc≈10 K. These results highlight the role of structural changes in controlling the phase diagram of Fe-based superconductors.

Microstructure and texture in lherzolites of the Balmuccia massif and their significance regarding the thermomechanical history

Abstract The microstructure and crystallographic preferred orientation (here referred to as texture) in lherzolites of the Balmuccia massif have been investigated in order to unravel the thermomechanical history of this massif. Two deformation events may be recognized in the microstructure. In olivine the first deformation led to a coarse-grained dynamic recrystallization. The second deformation produced the subgrain and dislocation structure and a fine-grained dynamically recrystallized rim around the matrix grains. The subgrain boundaries are (100) and occasionally (001) tilt boundaries with variable tilt axis. The free dislocations are mainly screw dislocations with an [001] Burgers vector. An analysis of the dislocations bound in subgrain boundaries and the free dislocations yields {0 kl }[100] and { hk 0}[001] as main activated slip systems. The orthopyroxenes are not recrystaUized and show deformation-induced clinoenstatite lamellae. The texture of olivine is characterized by [010] perpendicular to the foliation and [100] parallel to the lineation. In the orthopyroxene [100] is normal to the foliation and [001] normal to the lineation. The results are comparable with those found in similar massifs except the texture in the orthopyroxene. Stress and temperature estimates based on the dislocation density, subgrain size, dynamically recrystaUized grain sizes and the ortho-clinoenstatite transformation yield ≈ 20 MPa and ≈ 1000°C for deformation event I and 300 MPa and 650°C for deformation event II. The first and second deformation events are interpreted as intrusion of mantle material into the lower crust and the tilting of the Ivrea zone, respectively. From the correlation of the texture and microstructure it is concluded that the texture in the olivine reflects the first deformation event. The texture of the relatively hard and therefore only weakly deformed orthopyroxene may be explained by external rotation in the ductile olivine matrix.

Effect of microstructure on the mechanical properties of as-cast Ti–Nb–Al–Cu–Ni alloys for biomedical application

The correlation between the microstructure and mechanical behavior during tensile loading of Ti68.8Nb13.6Al6.5Cu6Ni5.1 and Ti71.8Nb14.1Al6.7Cu4Ni3.4 alloys was investigated. The present alloys were prepared by the non-equilibrium processing applying relatively high cooling rates. The microstructure consists of a dendritic bcc β-Ti solid solution and fine intermetallic precipitates in the interdendritic region. The volume fraction of the intermetallic phases decreases significantly with slightly decreasing the Cu and Ni content. Consequently, the fracture mechanism in tension changes from cleavage to shear. This in turn strongly enhances the ductility of the alloy and as a result Ti71.8Nb14.1Al6.7Cu4Ni3.4 demonstrates a significant tensile ductility of about 14% combined with the high yield strength of above 820 MPa already in the as-cast state. The results demonstrate that the control of precipitates can significantly enhance the ductility and yet maintaining the high strength and the low Youngs modulus of these alloys. The achieved high bio performance (ratio of strength to Youngs modulus) is comparable (or even superior) with that of the recently developed Ti-based biomedical alloys.

Epitaxial growth,,of highly coercive Sm-Co thin films using pulsed laser deposition

Hard magnetic materials with a uniaxial magnetocrystalline anisotropy can be most efficiently used if the easy axis is well aligned along one crystallographic direction in the entire sample volume. Epitaxial growth is one suitable method to achieve this aim and therefore Sm–Co thin films on Cr-buffered single-crystal MgO (100) substrates were deposited. Pulsed laser deposition from elemental Sm and Co targets was used to prepare films of nominal Sm2Co7 stoichiometry. Pole figure measurements and magnetization measurements reveal an epitaxial growth of Cr on MgO onto which the c axis of the Sm–Co layer is aligned in plane, viz., MgO(001)[100]‖Cr(001)[110]‖Sm–Co(110)[001]. At higher deposition temperatures an additional Sm–Co (1 1 16) texture was observed. This corresponds to an additional epitaxial orientation relation, where the c axis is tilted 60° out of the substrate plane. However, at low deposition temperatures and with smaller pulse repetition rates a reduction in the amount of this unwanted compone...

Compression-induced texture change in NiMnGa-polymer composites observed by synchrotron radiation

Composites consisting of magnetic shape memory (MSM) particles embedded in a polyester matrix were prepared. Single-crystalline MSM particles were obtained by mortar grinding of melt-extracted and subsequently annealed Ni50.9Mn27.1Ga22.0 (at. %) fibers. The crystal structure of the martensite is tetragonal (5M) with c<a=b. Magnetic characterization of these composites shows indirect evidence for stress induced twin boundary motion in the MSM particles, as the compressed composite is easy to magnetize in the direction of compression and more difficult to magnetize in the perpendicular directions. The texture of all the embedded MSM particles is investigated before and after compression by means of synchrotron radiation. In the initial state, the MSM particles in the composite have a random texture, i.e., there is no preferred orientation of the c axis. After a 30% compression (height reduction), the MSM particles have a (004)-fiber texture in the direction of compression. This is unambiguous evidence for s...

Microstructure in hornblende of a mylonitic amphibolite

Abstract The microstructure in a dynamically recrystallized amphibolite of a deep crustal shear zone from the Ivrea Zone, NW Italy, has been investigated by conventional and high resolution transmission electron microscopy. The microstructure in the amphibole phase consists of recrystallized grains, subgrains, free dislocations and various stacking faults bounded by partial dislocations. The Burgers vectors of the dislocations are [001], [100], 1/2 <110> and very likely <101>. Dislocations with [100] Burgers vector and [001] line direction are mainly arranged in low angle tilt boundaries lying parallel to (100) planes. Faceting of these subgrain boundaries is produced by 1/2 <110> edge dislocations dissociated on (010). The free dislocations are mainly non-dissociated [001] and [100] dislocations gliding on (100) and (010) planes, respectively. It is assumed that [001] and [100] dislocations recombined to form <101> dislocations which subsequently dissociate in (010). The stacking faults extended on (010) planes either have pyroxene or sheet silicate character. They act as obstacles for dislocation motion on the main slip system (100)[001]. High resolution imaging of the dislocation cores suggests these areas consist of lower density material providing channelways for fast diffusion which may be responsible for the deformation-induced compositional changes found within amphibolite shear zones.