P. Adeva

Microstructure and high temperature mechanical properties of tin

Abstract The mechanical properties of tin have been studied by tensile tests in the temperature range 293–463 K. Tensile tests were performed for cylindrical samples at a constant strain rate and varying strain rates during deformation. In-situ-tensile tests also were conducted in ribbon-form samples. At the strain rates studied, deformation takes place preferentially by slip, although some scattered twins also were observed at lower temperatures. Strong grain growth occurs at the higher temperatures. Microstructural observations of deformed samples show that dynamic recrystallization is not important in the temperature range investigated. The fracture surface of the cylindrical samples changes from a chisel type of fracture at the lower temperatures to a simple shear type of fracture at the higher temperatures. Both the tensile strength and ductility decrease with increasing temperature. An explanation is given for the loss of ductility at high temperatures. The activation energy for creep, obtained from strain-rate-change tests is 35 kJ mol−1 and the stress exponent is about 6. These values are related to a slip mechanism controlled by pipe diffusion.

Indentation creep of lead and lead-copper alloys

Stress exponent values have been determined in Pb and Pb-Cu alloys with small Sn, Se and Pd additions by indentation methods (long time hardness tests) to evaluate their applicability as compared with tensile tests. Homogeneous, fine grained alloys were obtained by induction melting and thermo-mechanical treatments. Grain size was 38–60 μm in alloys and 183 μm in pure lead. Stress exponent values, i.e. of 11–12 agree between different methods of derivation and, in fine grained material, with tensile methods. The largest differences in pure lead, i.e. 10–11 versus 7–8 are attributed to high strain rates when indentation size is comparable to grain size. In all cases indentation and tensile tests indicate the same deformation mechanism, namely slip creep. The indentation test is thus considered useful, within limits, to acquire information on the deformation mechanism.

Oxidation behaviour of a Ti–46Al–1Mo–0.2Si alloy: the effect of Mo addition and alloy microstructure

Abstract The influence of molybdenum addition and alloy microstructure on the oxidation behaviour of a Ti–46.8Al–1Mo–0.2Si alloy was studied in air at temperatures ranging from 600 to 900°C. The alloy produced by arc melting exhibited a structure of coarse lamellar grains in as-cast condition that transformed to a duplex microstructure after hot extrusion at 1300°C. Oxidation rate and scale spallation resistance were not affected by the type of microstructure. The effect of molybdenum addition was related to the formation and thickening of a protective continuous nitride layer. The low mass gain measured can be associated with the low oxidation rate of the nitride layer. Finally, the poor spalling resistance of the oxide scale at 900°C was attributed to the mismatch in thermal expansion coefficients of the oxide scale and the nitride layer.

Ball milling mechanical alloying in the Fe100−xSix system

Abstract Alloying and amorphization of ball-milled samples of nominal composition Fe100−xSix (x = 5–50) has been investigated by means of X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and magnetic measurements. An evaluation is made of the relative quantities of phases present in the as-milled samples.

Texture of magnesium alloy films growth by physical vapour deposition (PVD)

Abstract The texture developed by four physical vapor deposited Mg alloys grown at two collector temperatures and their correlation with the microstructure have been studied. The alloys Mg–2.3Zr, Mg–10.6Zr and Mg–12Ti (wt%) deposited at around 150°C exhibited elongated grains growing in the normal direction to the deposit surface, but tilted with respect to the Z axis, with pores and cracks at the boundaries. A strong fibre texture of the ( 0001 ) basal plane component forming the fibre axis and the normal to the substrate plane an angle of 24, 18 or 12° depending on alloying element concentration has been found. The Mg–14Ti–1Al–0.9Mn (wt%) alloy, deposited at higher temperature, consisted of a region of columnar grains growing from the collector side followed by a thin region of equiaxed grains. Furthermore, a fibre texture with two components, the ( 0002 ) basal plane and the ( 1120 ) and the ( 1010 ) components in the collector and free surfaces, respectively, were present. Differences found in this alloy with respect to the alloys deposited at lower temperature were related to the surface diffusion phenomenon of adatoms caused by a higher collector temperature. The change in the second texture component from the collector to the free surface can explained by the decrease in the deposit strain energy.

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni3Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl2O4, and an inner layer of Al2O3, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al2O3 layer at the grain boundaries which favor oxygen penetration through the grain interior.

Microstructural characterization of P/M Ni3Al consolidated by HIP

Abstract Rapidly solidified powder of Ni 3 Al doped with boron was produced by inert gas atomization and consolidated by hot isostatic pressing (HIP). Morphology and microstructure of the powder were studied. From the particle morphology, it could be deduced that the solidification time was similar at least to the time necessary for complete fragmentation of the liquid. The powder showed a two-phase microstructure that was finer the smaller the particle size. The presence of dendrites of NiAl (β) phase was consistent with the diagram proposed by Schramm and not with the traditional diagram of Singleton et al. The microstructure of the material consolidated at 1100°C and 1200°C was studied. A monophasic structure was observed after HIP, and no relevant microstructural differences were seen between the two temperatures used.

Extrudability of PM 2124/SiCp aluminium matrix composite

Abstracts are not published in this journal

Influence of powder particle size on the oxidation behavior of a PM Ni3Al alloy

The influence of particle size on the oxidationbehavior of Ni3Al prepared by powdermetallurgy (PM) was investigated in the temperaturerange of 535 to 1020°C for exposures up to 200 hr.Four alloys were obtained, each one processed with a differentpowder particle size fraction (<25, 25-50, 50-100,and 100-200 μm). For temperatures below 730°C,the scale consists of an outer NiO layer, a thindiscontinuous intermediate nickel layer, and an internaloxidation zone. The lowest oxidation rate corresponds tothe material with the smallest particle size. Thisresults from its higher grain-boundary density; the boundaries act as easy-diffusion paths foraluminum leading to the rapid formation of a continuousinner alumina layer. At temperatures above 730°C, athree layered scale is observed consisting of an outer NiO layer, an intermediate layer that,depending on temperature, consisted of a mixture ofnickel and aluminum oxides orNiAl2O4, and an inner layer ofAl2O3, which accounts for thehigher oxidation resistance. The oxidation attack is characterized byintrusions of the scale into the alloy, the intrusionnumber increasing as the particle size decreases. It isassumed that oxide particles and impurities present at the original particle boundaries facilitatealumina growth along these regions. Thus, the lowestoxidation rate for the highest temperature rangecorresponds to the largest particle-sizematerial.

Influence of rare-earth addition on the long-period stacking ordered phase in cast Mg–Y–Zn alloys

The microstructure and thermal stability of the Mg97Y2Zn1 (at.%) alloy, modified with the addition of 0.5 at.% of gadolinium or neodymium, have been examined by synchrotron radiation diffraction during in situ differential scanning calorimetry. The microstructure of the three alloys consists of magnesium dendrites with the Long Period Stacking Ordered (LPSO) phase at interdendritic regions. Rare-earth atoms substitute yttrium atoms in the LPSO phase, promoting the formation of the 14H structure. Lattice parameters of the LPSO do not change significantly with the rare-earth addition. However, they reduce the melting point of the LPSO phase, especially in the case of neodymium addition.