Band 93, Heft 3

The intention of this study was to investigate the mechanical properties of an engineering γ -TiAl-based alloy with fully lamellar microstructure under quasistatic and dynamic compression at room temperature and strain rates ranging from 5.0 × 10 –3 s –1 to 4.0 × 10 3 s –1 The microstructure of undeformed and deformed specimens was investigated by means of optical and electron microscopy. The results of the compression tests exhibit a slight strain rate sensitivity of the postyield stress. The fracture behavior of the studied γ -TiAl alloy strongly depends on the applied strain rates. Transmission electron microscopy observations conducted on deformed specimens revealed the existence of a high number of tangled ordinary dislocations, curved superdislocations and mechanical twins. The analysis of the deformation texture confirms the high activity of superdislocations and mechanical twinning during dynamic compression testing.

The phase equilibria in the ternary system Fe –Gd – Zr at 1073 K have been investigated by X-ray diffraction and electron probe microanalysis. The isothermal section consists of terminal solution phases bcc (Fe), hcp (Zr), and hcp (Gd) (bcc = body-centred cubic, hcp = hexagonal close-packed), binary compounds FeZr 2 and FeZr 3 , quasibinary compounds (Fe, Zr) 17 Gd 2 , Fe 17 (Gd, Zr) 2 , Fe 23 (Gd, Zr) 6 , Fe 3 (Gd, Zr), and Laves phase Fe 2 (Gd, Zr). No ternary compounds were detected. The isomorphous phases Fe 23 Gd 6 –Fe 23 Zr 6 and Fe 2 Gd–Fe 2 Zr do not form continuous solid solutions. The Fe 3 (Gd, Zr) phase shows the largest homogeneity region (0–12 at.% Zr). In the compound Fe 17 Gd 2 , Zr atoms can substitute for both Gd and Fe atoms. This results in the formation of two distinct phases (hexagonal and rhombohedral), respectively. The experimental results have been used to obtain a self-consistent thermodynamic description of the ternary system Fe – Gd–Zr.

The phase equilibria in the Fe –Co –Ti system were investigated at 1073 K by means of X-ray diffraction, transmission electron microscopy, and in situ energy-dispersive spectroscopy. Four three-phase and eleven two-phase microstructures, consisting of disordered solid solutions, B2 ordered intermetallics, Laves phases, and L1 2 ordered phase, were observed. On the basis of the experimental results and the chemical analyses of the coexisting phases, an isothermal section at 1073 K is proposed, which is similar to the section at 1173 K reported in the literature.

The copper concentration inside a Guinier-Preston (GPI) zone was studied by means of an extended Hückel molecular orbital (EHMO) calculation. The present EHMO calculation revealed that the Cu concentration inside a GPI zone depends on the size of the zone. The GPI zone comprises both Cu and Al in a small GPI zone, whereas the Cu concentration inside the GPI zone increases to nearly 100 at.% Cu in a large GPI zone. A compressive lattice distortion, which is associated with the zone, increases the stability, but when a GPI zone is 5 nm in diameter, the stabilizing effect of the zone is estimated to be not higher than 10% of the chemical energy due to Cu coalescence.

Transmission electron microscopy (TEM) imaging, X-ray energy-dispersive spectroscopy (EDS), and selected-area diffraction (SAD) were used with the aim of identifying the particles present in an AA 6013 alloy (Al –Mg –Si – Cu), that was developed during the last years as an alternative for the AA 6061 alloy (lower Cu content) in the automotive industry. The particles found can be classified in three different groups: silicon particles, two kinds of α(AlFeMnSi) particles, and small structural precipitates. The main interest of the AA 6013 alloy is the combined beneficial influences of medium-sized α(AlFeMnSi) dispersoids and structural precipitates. The dispersoids improve the fracture toughness by limiting grain growth. The β" phase (precursor of the β Mg2Si phase) and Q ' (precursor of the Q–Al 5 Cu 2 Mg 8 Si 7 quaternary phase) lead to a higher yield strength than for other 6XXX alloys.

The results of P and C redistribution measured in steel weldments are reported. Four sorts of steels were used in steel weldments preparation. The experimental specimens, having a diameter of 12 mm and a thickness of (4 + 4) mm were prepared in the form of sandwiches using electric shocks. The welded specimens were isothermally heated in the temperature range 500– 1100 °C. The concentration of P and C was measured by the use of an analytical complex JEOL JXA 8600/KEVEX. The measured curves N (P, x, t ) and N (C, x, t ) revealed uphill diffusion in the investigated steel weldments. The diffusivities D (P) and D(C), and the concentrations N(P, 0) ± and N(C, 0) ± at the weld interface x = 0 were evaluated from the measured concentration profiles N (P, x, t ), N(C, x, t). The obtained D and N(0) ± values made it possible calculate the diffusion ( β P i , β C i ) $\left( \beta _{\text{P}}^{i},\beta _{\text{C}}^{i} \right)$and thermodynamic (εPi,εCi) $\left( \varepsilon _{\text{P}}^{i},\varepsilon _{\text{C}}^{i} \right)$interaction parameters; i ≡ C, Mn, P, Cr, Ni.

TiC/Ti composites were produced by nonconsumable arc-melting technology utilizing the self-propagation high-temperature synthesis reaction between Ti and graphite. X-ray diffraction was used to identify the phases in the composite. The microstructures of the composites were observed by optical microscopy and transmission electron microscopy. The results show that there are two phases in the composite: TiC and Ti. Reinforcements are distributed uniformly in the matrix alloy. The interface between reinforcement and Ti alloy is very clean. There are high-density dislocations around the TiC particles due to the difference in coefficient of thermal expansion between TiC and Ti. The mechanical properties of the composites improve significantly due to the incorporation of reinforcement. The addition of Al not only strengthens the matrix alloy by solid-solution strengthening, but also improves the mechanical properties of the composites by refining the reinforcement and matrix alloy. The strengthening mechanisms mainly include the following factors: (a) bearing load by the TiC particles, (b) refinement of the grain size, and (c) intrinsic strengthening of the matrix alloy by high-density dislocations.

Employing X-ray diffraction, scanning electron microscopy and differential scanning calorimetry techniques, the effect of the aluminium concentration on the glass forming ability (GFA) of Zr 65 Ni 10 Cu 25– x Al x alloys were systematically studied. The experimental results show that with increasing Al concentration the supercooled liquid region of the as-quenched ribbons of Zr 65 Ni 10 Cu 25– x Al x alloys increases and the critical cooling rate required to avoid crystallization during the rapid solidification for these alloys decreases, indicating that the GFA of Zr 65 Ni 10 Cu 25– x Al x alloys is significantly improved by the addition of Al. The effect of Al on the GFA of Zr 65 Ni 10 Cu 25 – x Al x alloys is discussed from the micromechanism point of view.

The ε ↔ γ reversible transformation has been studied in Fe-14 wt.% Mn and Fe-21 wt.% Mn alloys which are known to exhibit a weak shape memory effect. The internal friction changes and the associated elastic modulus changes were measured using the high frequency (40 kHz) composite oscillator technique in the temperature range 20 – 350 °C in which the ε ↔ γ transformation occurs. The strain-induced amplitude dependent internal friction was measured in order to evaluate changes in the dislocation structure that occur during the transformations and to identify the transformation mechanism involved. The study revealed that the propagation of the ε / γ interface boundary during the ε ↔ γ transformation is a thermally activated relaxation process obeying the Debye relaxation equation. The internal friction data were used to determine the activation energy of the process responsible for the martensitic transformation. The damping peak is observed at temperatures where both phases coexist and is due to the motion of the ε / γ interface. The ε / γ interface consists of Shockley partial dislocations moving in groups and causing a minimal macroscopic strain. The ε / γ interface-related dislocation motion is, therefore, the cause of the internal friction peak. The internal friction is associated with a step-like change in the elastic modulus which is due to the ε / γ interface related relaxation and the difference in the modulus of the ε and γ phases.

The high-temperature strain partitioning in duplex stainless steel was studied by means of single-pass hot-rolling tests at 1040 and 1245 °C. By varying the temperature and the amount of reduction during the hot-rolling pass the influence of the austenite volume fraction and the strain could be investigated. By means of optical microscopy and application of the law of mixtures the partitioning of strain between the ferrite matrix and the austenite islands was obtained. A distinct strain partitioning was observed. At 1245 °C, the ferrite-austenite strain ratio increased from 2.8 at 30% rolling reduction to 4.2 at 80% reduction. In addition, single-hit hot-torsion tests were performed on single-phase ferritic and austenitic materials, as well as on the duplex stainless steel in order to study the stress partitioning between the phases. The austenite-ferrite stress ratio was found to increase up to 3.5 at 1200 °C. The torsion tests confirmed that the deformation behaviour of the duplex stainless steel is mainly determined by the deformation behaviour of the ferritic matrix, in particular during the early stages of deformation. The strain concentration in the ferrite was found to result in a delay of the dynamic recrystallisation of the austenite.

The high cycle fatigue (HCF) behaviour and load sequence effects in HCF of two microalloyed steels are presented. Dislocation structures that resulted from HCF loading were examined. The fatigue limits of the two microalloyed (MA) steels (non-failure after 2×10 6 cycles) correlated well with the cyclic yield strength. The results of two-step loading showed that in both the steels after an initial life fraction of approximately 0.4, the residual life deviated significantly from that predicted by the Palmgren-Miner rule. Crack initiation lives of the MA steels had been determined from the two-step loading experiments. Fatigue functions, which can be used for life prediction under complex loading conditions using the consecutive Wöhler curve approach, were determined from the two-step loading tests. Dislocation cell structures were observed near the fracture surface in the HCF regime. At the fatigue limit, dislocation dipoles and loops were seen.

The intermetallic compound that appears during the soldering reaction between pure Sn and Ni substrate at temperatures ranging from 250 to 400 °C is analyzed to be Ni 43 Sn 57 A similar reaction at the Sn– 58Bi/Ni interface results in the formation of the Ni 41.5 Sn 57.9 Bi 0.6 phase. At the Sn/Ni and Sn–58Bi/Ni interfaces, each kind of intermetallic compound formation exhibits a similar scallop-type morphology. However, the growth rate of intermetallic compounds at the Sn/Ni interface is greater than that at the Sn– 58Bi/Ni interface. The growth kinetics for both cases is diffusion controlled. The activation energy for the intermetallic growth at the Sn/Ni interface is 43.7 kJ/mol, which is higher than that at the Sn–58Bi/Ni interface (28.3 kJ/mol).