Volume 97, Issue 2

Details of phase separation in the microstructure of amorphous Si – (B) –C–N ceramics derived from polymers have been resolved using the recent results of structural investigations. The formation of an amorphous phase built of atomic compounds SiC i /4 N (4 – i )/3 and consequently located along the composition line between SiC and Si 3 N 4 in the ternary Si–C–N phase diagram demonstrates a generic feature of phase separation in all these materials. The amorphous carbon phase separates as a counterpart in the micro-structure of Si –C–N ceramics, and in the case of Si –B– C–N ceramics such counterpart represents B–N–C domains of the composition (BN) c C y located along the tie line C–BN in the ternary B–C–N phase diagram. The effect of phase separation has been also pondered as a source of exceptional material properties.

An improved method is presented for determination of solidification curves, i. e., solid fraction versus temperature, for commercial Mg alloys using heat-transfer modeled differential thermal analysis (DTA) curves. A better simulation of the measured DTA signal is attained through an independent measurement of the time constant as function of temperature for the applied equipment. This enables a better desmearing of the DTA signal. Challenging Mg alloys could be appropriately handled by redesigning the tantalum encapsulation. Due to high oxygen affinity and vapor pressure of the investigated magnesium alloys, this special adaptation of the DTA setup using sealed Ta capsules was indispensable for generation of reproducible and reliable data.

The Mg–Nd system was critically assessed by means of the CALPHAD technique. The solution phases (liquid, body-centered cubic, double hexagonal close-packed and hexagonal close-packed) were modeled with the Redlich –Kister equation. The intermetallic compounds Mg 41 Nd 5 , Mg 3 Nd and MgNd, which have a homogeneity range, were treated as the formulae (Mg, Nd) 41 (Mg, Nd) 5 , Mg 3 (Mg, Nd) and Mg(Mg, Nd) by a two-sublattice model with Mg and Nd or Mg on the first sublattice, and Mg and Nd on the second one, respectively. The compound Mg 2 Nd was treated as a stoichiometric compound. A set of self-consistent thermodynamic parameters of the Mg–Nd system was obtained.

Solid-state reactions, induced by ion-beam mixing (IBM) and thermal annealing, in Ni/Si multilayered films (MLF) with an overall stoichiometry of Ni 2 Si, NiSi and NiSi 2 , and with a constant Ni sublayer thickness (nominally, 3.0 nm), were studied by optical and magneto-optical spectroscopies as well as X-ray diffraction (XRD). The layer mixing was performed with Ar + ions of an energy of 80 keV and a dose of 1.5 × 10 16 Ar + /cm 2 . It was shown that the IBM leads to structural changes in the Ni/Si MLF, which cannot be easily detected by XRD but are recognized by optical tools. An annealing at 1073 K of the Ni/Si MLF with an overall stoichiometry of NiSi and NiSi 2 induces formation of predominantly the η-NiSi and the NiSi 2 phases, respectively. IBM of all the investigated Ni/Si MLF leads to the formation of regions with a short-range order of the crystalline NiSi silicide, and of Ni 2 Si (and/or Ni 3 Si) additionally for the Ni/Si MLF with an overall stoichiometry of Ni 2 Si.

The phase equilibria of the Co–Cu –Ti system at 850 °C are investigated using a high-efficiency diffusion couple and six key ternary alloys. The samples are examined by means of optical microscopy, scanning electron microscopy, and electron probe microanalysis. At 850 °C, no ternary compound is found. The Ti 3 Cu phase is a metastable phase. The solubilities of Cu in Ti 2 Co, TiCo, TiCo 2 (c) and TiCo 3 were determined to be 8.7, 35.8, 2.1, and 5.4 at.%, respectively. And the solubilities of Co in Ti 2 Cu, TiCu, Ti 3 Cu 4 , and TiCu 2 were measured to be 6.6, 0.7, 2.4, and 10.2 at.%, respectively. The following 9 three-phase equilibria are observed: (Ti) + Ti 2 Co + Ti 2 Cu, Ti 2 Co + Ti 2 Cu + TiCo, Ti 2 Cu + TiCu + TiCo, TiCu + Ti 3 Cu 4 + TiCo, Ti 3 Cu 4 + TiCu 2 + TiCo, TiCu 2 + TiCo + (Cu), TiCo + TiCo 2 (c) + (Cu), TiCo 2 (h) + TiCo 3 + (Cu), and TiCo 3 + (Co) + (Cu).

The effects of an external dc electric field of 0 – 5 kV/cm, applied at 475 – 550 °C during solution heat treatment (SHT) to AA6111, on the subsequent natural aging kinetics and volume fraction of precipitated clusters were determined employing resistivity. The field increased the asquenched resistivity and that during natural aging, the effect being significant from 0 to ≈0.5 kV/cm and increasing only slightly, if at all, thereafter. An Avrami-type analysis of the natural aging kinetics gave n = 0.59 ± 0.2 and k = (1.2 ± 0.8) · 10 – 2 min –1 , relatively independent of SHT temperature and field strength. The field increased the volume fraction of precipitated clusters which occurred during natural aging. This was attributed to the increase in solubility during SHT produced by the field. Tensile properties equivalent to those obtained by the SHT at 550 °C without a field were obtained at 500 °C with a field of only 200 V/cm, representing a reduction of 50 °C in the nominal SHT temperature.

The present work was undertaken to prepare graphitic Cu–Cr –Zr matrix composites in order to develop new materials for application under sliding contact in an electrical field. The electrical sliding wear behavior of the graphite (Gr)/CuCrZr composite was investigated from a microstructural viewpoint, compared with the matrix alloy. The results indicate that by using the water atomized Cu–Cr–Zr powder and copper-coated graphite particles as starting materials, compact and homogeneous microstructure can be obtained in the composites. Fine and coherent particulate precipitations are formed in the Cu–Cr–Zr matrix during the aging treatment process, which help to improve the mechanical properties. Under the electrical sliding wear condition, incorporation of graphite in the Cu–Cr–Zr matrix not only decrease the wear loss of the sliding pair but also maintain steady-state friction coefficient. The main reason is that the graphite film that smears on the contacting surfaces can alleviate materials loss induced by electric arc discharge and adhesion wear. Proper aging treatment helps to make full use of the solid lubricant effect of the graphite particles.

An earlier study of the creep behaviour of Elektron 21 alloy has been extended to 290 °C (0.61 T m ; T m : melting temperature). The combined results confirm a stress exponent of creep rate close to 6, but with an activation energy for creep of (310 ± 20) kJ/mol for 240 to 290 °C and 40 to 90 MPa. Possible mechanisms giving rise to this behaviour are discussed.

In this paper, we attempt to apply the modified hardening theory to estimate the creep resistance, which is considered as a key factor for controlling the creep deformation mechanism in cast nickel-base superalloys. It is suggested that, when the applied stress is high enough for the dislocations to cut into the γ ′ particles, the creep resistance is almost a constant and independent of applied stress. At low applied stress, creep deformation is mainly controlled by dislocations climb, where the creep resistance has two components of a threshold stress and a friction stress. The model is verified for two cast nickel-base superalloys DZ17G and IN738. The results of detailed calculations are in good agreement with the experimental data.

In this study an attempt has been made to identify analytical methods for detecting magnesium nitride, and document in which part of the magnesium recycling process this phase is concentrated. The samples were taken from a continuous fluxfree recycling session, where an AZ91D magnesium alloy was remelted and purified by blowing nitrogen through the melt. Exothermic reactions took place when magnesium nitride reacted to ammonia and Brucite and also when aluminium nitride reacted to ammonia and Gibbsite in a moisture-bearing environment. Because of this, it was essential to avoid any contact with moisture during the whole sampling and preparation process. The samples were exclusively handled in an argon atmosphere. Due to their crystallinity and Raman activity the nitrides were detected with X-ray diffraction and Raman spectroscopy, respectively. With the scanning electron microscope it was difficult to analyze small concentrations of nitrides since large, stable crystals were absent and the carbon coating absorbs the nitrogen Kα line. The Mg-alloy was found to be free of impurities. Although nitrides were expected to be concentrated in the slag because of their greater density, they were found adhering at the nitrogen bubbles and accumulating in the dross. Large amounts of oxides were concentrated in the slag.

Ultrafine grained magnesium alloy was synthesized via mechanical milling of AZ91 chips. Mechanical property measurement revealed enhanced yield strength of 470 MPa after mechanical milling. The increase in yield strength is associated with reduction in grain size which restricts twining and dislocation gliding. The present investigation demonstrates that magnesium alloys can be cost-effectively recycled through the process of mechanical milling.

The sigma phase (σ) formation in long-term aged polycrystalline K44 superalloy has been studied as a function of aging temperature and time. The σ phase in K44 alloy forms mainly in dendrite cores with a few at interdendritic regions. The kinetics of σ formation can be described by the Johnson–Avrami –Mehl equation that yields activation energies of more than 130 kJ/mol in an early stage and less than 100 kJ/mol in steady state. Tensile experiments at room temperature and 1173 K and endurance experiments at 1173 K and 274 MPa were performed to test the effect of σ phases on these properties and no remarkable harmful effect is found. Moreover, the effect of long-term aging on the mechanical properties is realized mostly through the variation of γ ′, not σ phase formation.