The temperature factor in nitriding titanium alloys in a low-density dynamic nitrogen atmosphere

Summary A study has been made on the effects of isothermal hold time during nitriding of titanium alloys in a low-density dynamic nitrogen atmosphere on the nitride formation and gas saturation. To obtain a good hardened while retaining satisfactory strength and plasticity characteristics, the nitriding should be done in the temperature range corresponding to passive nitride formation.Translated from Fiziko-khimicheskaya Mekhanika Materialov, No. 5, pp. 76–79, September–October 1990.     

The tensile behaviour of rapidly solidified magnesium alloys

In this paper, the microstructure and tensile behaviour of three rapidly solidified magnesium alloys is reported. The alloy's composition, i.e. neodymium content, is observed to have an influence on tensile properties and fracture behaviour. The elastic modulus, yield strength and ultimate tensile strength of the alloy increase with an increase in neodymium content. The ductility of the alloys decreased marginally with an increase in neodymium content. The tensile fracture characteristics of the alloys is highlighted in the light of alloy composition and microstructural effects.     

Texture of surface layers obtained by ion nitriding of titanium alloys

Titanium metal and some of its alloys of the type Ti-Al-Me (Me=transition metal) were submitted to plasma nitriding. The resulting compound layer was examined by optical microscopy and by XR diffraction in order to establish both its microstructure and texture degree under different nitriding conditions. To this aim a texture evaluation method was employed, which was formerly devised and applied on surface layers obtained by boriding of steels. In these nitrided layers the Ti₂N nitride was clearly textured while the TiN did not show an appreciable degree of preferred orientation of crystals. The influence of process conditions and alloy composition on the compound layer texture is discussed.     

The effect of temperature on plasma nitriding behaviour of DIN 1.6959 low alloy steel

A series of experiments have been conducted on DIN 1.6959 low-alloy steel using a 5 kVA DC plasma nitriding apparatus with the aim of elucidating the role of treatment temperature in plasma nitriding process. Treatments were carried out in 75%N₂-25%H₂ atmosphere of 4 mbar for 5 h at temperatures ranging from 350 to 550 °C. Optical microscopy, scanning electron microscopy, X-ray diffraction, along with surface roughness and microhardness measurements were utilized to characterize the treated samples. The depth, microstructure, hardness profile and phase constituents of the nitrided layers as well as the surface roughness of the samples were assessed as a function of treatment temperature. The results suggested that the compound layers were mostly dual phase consisting of gamma prime and epsilon iron nitride phases. Increasing treatment temperature increases compound layer and diffusion layer thicknesses. However, maximum surface hardness and roughness were found on the samples treated at 500 and 550 °C, respectively.     

Mechanical behaviour of aluminium-lithium alloys

Aluminium-lithium alloys hold promise of providing a breakthrough response to the crying need for lightweight alloys for use as structurals in aerospace applications. Considerable worldwide research has gone into developing a range of these alloys over the last three decades. As a result, substantial understanding has been developed of the microstructure-based micromechanisms of strengthening, of fatigue and fracture as well as of anisotropy in mechanical properties. However, these alloys have not yet greatly displaced the conventionally used denser Al alloys on account of their poorer ductility, fracture toughness and low cycle fatigue resistance. This review aims to summarise the work pertaining to study of structure and mechanical properties with a view to indicate the directions that have been and can be pursued to overcome property limitations.     

Effect of chromium content on remarkably rapid nitriding in austenitic Fe-Ni-Cr alloys

Remarkably rapid nitriding which is independent of diffusion theory based on the thermal activation process, was observed during nitriding of austenitic Fe-Ni-Cr steels containing 16 and 19 mass% chromium. Increase of the chromium content in the alloys yielded increasing thickness of the nitrided layer, i.e. the internal nitriding theory did not hold in the nitriding. No rapid nitriding was observed in steels containing less than 13 mass% chromium. Hence the limiting concentration of chromium for the rapid nitriding will lie between 13 and 16 mass% chromium. A solution to the problem of abnormalities arising during nitriding of practical austenitic stainless steels which have been investigated since 1972, has been presented experimentally by nitriding various chromium-containing steels. Based on the experimental results, the origin of the rapid nitriding is discussed in connection with the free-energy function of Cr₂N and CrN to temperature. In particular, a plateau of nitrogen concentration measured in the nitrided layers leads to the conclusion that a forced nitrogen diffusion in the layer resulted in the rapid nitriding.     

Surface hardness behaviour of Ti-Al-Mo alloys

The microhardness characteristics of various micro-constituents formed in the Ti-Al-Mo alloys have been investigated. Four alloys having compositions, Ti-40Al-2Mo, Ti-42Al-2Mo, Ti-40Al-6Mo and Ti-42Al-6Mo, have been chosen for this purpose. All of these were heat treated at 1300°C and 1400°C for 1 h and water quenched. All the specimens after above heat treatments have displayed load independent Vickers hardness values (VHN) around 300 g of applied load. The average surface hardness characteristic of the alloys is largely found to be dictated by the phases that are present. The microstructural specific VHN values vary between 600 and 750. The indentation behaviour, however, is governed by the morphologies and length scales of microstructures. The most remarkable finding of the present study pertains to the formation of shear bands around the periphery of the indenter for a finer basket weave microstructure in the Ti-40Al-2Mo. The cluster of finely located slip steps was clearly seen. Such a report is lacking in literature in this class of alloys.     

Thermoelectric behaviour of amorphous magnetic alloys

The Thermoelectric emfs of thermocouples formed by amorphous METGLAS 2826 (Fe₄₀Ni₄₀P₁₄B₆) and METGLAS 2826B (Fe₂₉Ni₄₉P₁₄B₆Si₂) with standard thermocouple wires like copper, chromel, alumel, etc., were measured as a function of temperature between −196° C and 30° C to assess their suitability as thermoelectric temperature sensors. Thermoelectric emfs generated by METGLAS 2826/Cu and METGLAS 2826B/Cu thermocouples at −196° C are about an order of magnitude smaller when compared to thermal emfs of a standard copper/constantan thermocouple at the same temperature.     

The chemical behaviour of carbon fibres in magnesium base Mg-Al alloys

The chemical interaction between carbon fibres and Mg-rich Mg-Al alloys was studied at 723–1273 K using optical metallography, scanning electron microscopy and electron probe microanalysis. In a first stage, carbon fibres were heated at 723, 1000 and 1273 K with Mg-Al alloys of different compositions. Two carbide phases were identified at 1000 and 1273 K: an Al₄C₃ type phase with up to 6 wt.%Mg present in solid solution and a new ternary carbide with the chemical formula Al₂MgC₂ existing under two hexagonal crystalline varieties. In a second stage, the solid-liquid phase equilibria in the Al-C-Mg ternary system were experimentally established at 1000 K. At that temperature, all the Mg-Al alloys containing more than 19 ± 2 wt.%Al were observed to be in equilibrium with the Al₄C₃ type phase whereas Mg-Al alloys containing from 0.6 to 19 wt.%Al were found in equilibrium with the ternary carbide Al₂MgC₂. As for the Mg-Al alloys with an Al content lower than or equal to 0.6 ± 0.2 wt.%, they appeared to be in equilibrium with carbon. These thermodynamic principles being established, the extent, morphology and composition of the reaction zones formed in out of equilibrium conditions at the interface between carbon fibres and Mg-rich Mg-Al alloys were characterized. Attempts were made to determine the influence of different factors such as the fibre nature, the alloy composition, the heating time and the heating temperature on the formation and growth of the ternary carbide Al₂MgC₂. The observed changes were interpreted in terms of reaction mechanism and kinetics.     

Mechanical behaviour of partially crystallized amorphous alloys

Deformation and fracture of partially crystallized Ni₅₅Pd₃₅P₁₀ amorphous alloys have been investigated. The samples with a few percent crystallization show a fracture stress (175 kg mm⁻²) and apparent Young's modulus (19.5×10³kg mm⁻²) greater than those completely amorphous or partially crystallized 50%. On the other hand, the fracture strain of the former are lower than those of the two latter. A simulated model with mixtures of carborundum powder with grease, shows accord with morphological and mechanical aspects of the partially crystallized alloys. As expected, microcrystals embedded in an amorphous material act as obstacles to plastic flow.     

High temperature deformation behaviour of Al-Fe-V-Si alloys

Abstract

The high temperature deformation behaviour of three rapidly solidified dispersion strengthened Al-Fe-V-Si materials with three different compositions has been investigated. The three materials contain high volume fractions, ranging from 16 to 36%, of small round dispersoids. The stress exponent and the activation energy for deformation are anomalously high and are temperature dependent. The material with the highest volume fraction of dispersoids showed the highest stress exponent dependence on temperature. At the same time, microstructural characterisation revealed that this material becomes heterogeneous and loses creep resistance as a result of microstructural changes. A correlation is found between the stress exponent and the microstructural changes. A ‘skeleton line’is defined for creep behaviour analysis which allows comparison of the investigated materials and contributes to their characterisation.     

Tarnish and corrosion behaviour of palladium-silver alloys

Binary and commercial palladium-silver dental alloys show a greatest resistance to both tarnish and corrosion at compositions of between 50 and 75% Ag, most probably due to saturation of the d-electron shell in this composition range. The commercial Pd-Ag alloys have more corrosion resistance, but a lower tarnish resistance than the binary Pd-Ag alloys, due in part to the minor alloying with indium or zinc. The addition of copper increases the segregtion of silver, which adversely affects both tarnish and corrosion resistance. A solutionization heat treatment degrades tarnish resistance, but does not affect corrosion resistance.