B.L. Mordike

Structural aspects of high performance Mg alloys design

Magnesium–gadolinium binary alloys exhibit good mechanical properties and high creep resistance comparable to or better than commercial WE type (Mg–Y–Nd–Zr) alloys. Combining scandium and manganese with a particular rare earth element (R.E.–Gd, Y, Ce) has a beneficial effect on the creep behaviour of complex Mg–R.E. alloys, at lower R.E. contents than in WE type alloys. They stabilise high creep resistance up to high temperatures (above 300°C) by precipitation of the stable phase Mn2Sc and by precipitation of basal plates of a Mn and R.E.-containing hexagonal phase.

Microstructure evolution in isochronally heat treated Mg-Gd alloys

High thermal stability and good mechanical properties are crucial for the wider future application of magnesium alloys. One of the most promising directions is the alloying of Mg with heavy rare earth elements such as Gd, Dy, Tb, etc. Three squeeze cast Mg–Gd binary alloys (up to 15 wt% Gd) have been investigated after a solution heat treatment by isochronal annealing up to 500 °C using electrical resistivity and hardness measurements. The microstructural development during this treatment, responsible for the observed changes, was observed by transmission electron microscopy. The decomposition of α′-Mg supersaturated solid solution in Mg–14.55 wt% Gd with increasing heating temperature is as follows: β″ (D019) metastable phase β′ (c-b.c.o.) metastable phase β(Mg5Gd f.c.c.) stable. Peak hardening is achieved by a heat treatment resulting in precipitation of β′ phase in the shape of fine plates parallel to all three 21-1-0 planes of the α-Mg matrix. At higher temperatures (above 280 °C) coarsening occurs and only one orientation of β′ plates remains. The decomposition of Mg–4.47 wt% Gd and Mg–9.33 wt% Gd alloys differs from that of Mg–14.55 wt% Gd by the absence of the β′ phase. Um das Anwendungsfeld fur Magnesiumlegierungen zu erweitern, ist die Entwicklung von Legierungen mit hoher thermischer Stabilitat bei gleichzeitig guten mechanischen Eigenschaften erforderlich. Eine der vielversprechendsten Entwicklungen ist das Zulegieren von schweren Seltenen Erden wie z. B. Gd, Dy und Tb. Nach einer Homogenisierungsbehandlung wurden drei presgegossene binare Mg–Gd-Legierungen (bis zu 15 Gew.% Gd) mit Hilfe elektrischer Widerstandsmessungen und Hartemessungen hinsichtlich ihres Verhaltens bei isochronen Gluhungen bis zu 500 °C untersucht. Die wahrend dieser Warmebehandlung eingetretenen mikrostrukturellen Veranderungen wurden mittels transmissionselektronenmiskroskopischer Untersuchungen verfolgt. Mit der Gluhungstemperaturerhohung bilden sich in einer Mg–14,55 Gew% Gd-Legierung folgende Phasen: β″ (D019) metastabile Phase β′ (c-b.c.o.) metastabile Phase β (Mg5Gd f.c.c.) stabil. Die hochsten Hartewerte sind mit einer Warmebehandlung erreichbar, die zur Bildung der β′-Phase in Form feiner Plattchen parallel zu allen drei 21-1-0-Ebenen der Mg-Matrix fuhrt. Bei hoheren Temperaturen (uber 280 °C) bleibt nur eine Orientierung der β′-Phase erhalten und es tritt eine Vergroberung ein. Die korrespondierenden Entmischungsvorgange in einer Mg–4,47 Gew.% Gd- und einer Mg–9,33 Gew.% Gd-Legierung ist durch die Abwesenheit der β′-Phase gekennzeichnet.

Plasma immersion ion implantation of stainless steel: austenitic stainless steel in comparison to austenitic-ferritic stainless steel

Abstract It has been shown previously in the literature that plasma immersion ion implantation (PHI) can increase the wear resistance of austenitic stainless steel without losing its corrosion resistance. In this work, the effect of PHI treatment on the microstructure and the properties of an austenitic (X6CrNiTi1810, AISI 321) and a duplex austenitic-ferritic (X2CrNiMoN2253, AISI 318) stainless steel has been studied and the results compared. Three different treatment temperatures and treatment times were used. The microstructures were studied by optical metallography and glancing angle X-ray diffraction (XRD). The formation of expanded austenite was observed in both steels up to treatment temperatures of 400 °C. The ferrite in the duplex austenitic-ferritic steel was also transformed to expanded austenite. At 500 °C, a surface layer consisting of CrN was formed on the duplex austenitic-ferritic steel whereas the modified layer on the austenitic steel was still expanded austenite with a small amount of CrN precipitation. Elemental depth profiling by sputtered neutral mass spectrometry (SNMS) revealed a similar treatment depth for both materials up to 400 °C, which was a function of treatment temperature and time. A pin on disc tribometer was used to determine the tribological behaviour. A change in the wear behaviour was observed and the wear depth decreased relative to untreated material. This was due to an increase in the surface hardness and a decrease in the coefficient of friction. The decrease in wear depth correlated with the thickness of the modified layer. The best results were found with the duplex austenitic-ferritic steel at a treatment temperature of 500 °C and can be attributed to the formation of a CrN layer. Corrosion tests have shown that good corrosion resistance was preserved up to 400 °C for both materials with only a small decrease being observed. This is due to nitrogen remaining in solid solution without CrN-precipitation. At a treatment temperature of 500 °C, the corrosion resistance decreased dramatically, especially for the duplex austenitic-ferritic steel where a layer of CrN was formed. These results show the capability of PIII treatment to increase the wear resistance of these stainless steels without losing their good corrosion performance. This may allow the use of such steels in applications where the poor wear resistance of the untreated material would normally prohibit their use. In comparison to the austenitic steel, the duplex austenitic-ferritic steel performed better after PIII treatment. For an optimum surface treatment, it is necessary to consider the substrate material as well as the treatment parameters.

Laser surface engineering of a magnesium alloy with Al+Al2O3

The present study concerns an attempt to improve the wear resistance of a commercial magnesium alloy, MEZ (rare earth 2 wt.%, Zn 0.5 wt.%, Mn 0.1 wt.%, Zr 0.1 wt.%) by dispersion of Al2O3 particles and alloying with aluminium on the surface by laser surface engineering. Laser processing was carried out with a 10-kW continuous wave CO2 laser by melting and subsequent feeding of Al+Al2O3 particles (in the ratio of 3:1) on the surface of MEZ. Following laser processing, a detailed microstructural and phase analysis of the surface modified layer were carried out. The microhardness of the surface layer was measured as a function of laser parameters and wear resistance property was evaluated in details. Microhardness of the surface layer was significantly improved to as high as 350 VHN as compared to 35 VHN of the substrate. The optimum processing region for formation of a homogeneous microstructure and composition for laser surface modification of MEZ with Al+Al2O3 was established. The wear resistance of the laser surface modified samples was considerably improved as compared to the as-received specimen.

Friction and wear behavior of Ti following laser surface alloying with Si, Al and Si+Al

Abstract This study concerns the friction and wear behavior of Ti following laser surface alloying (LSA) with Si, Al or Si+Al. The said tribological characteristics of the laser-alloyed samples, subjected to the earlier determined optimum conditions of LSA, were investigated in terms of the variation of wear depth as a function of load and time using a computer-controlled reciprocating ball-on-disc wear testing machine fitted with an oscillating hardened steel ball. A detailed post wear microstructural analysis was conducted to determine the mechanism of wear and role of alloying elements in improving the resistance to wear. It appears that LSA with Si is more effective in improving the wear resistance of Ti than that by Si+Al or Al alone. The enhanced wear resistance in Si surface alloyed samples has been attributed to the presence of uniformly distributed Ti 5 Si 3 in the alloyed zone (AZ).

Laser produced functionally graded tungsten carbide coatings on M2 high-speed tool steel

Abstract The objective of the investigation was to produce functionally graded, carbide alloyed multilayer coatings on M2 high-speed steel by laser alloying with direct injection of WC powder into the melt pool. Single layer coatings with a wide alloying range corresponding to 12–58 wt.% W and 1.3–4.3 wt.% C, respectively, were produced by varying laser beam power and beam traverse velocity. Depending on the alloying degree, four different types of structures were observed in laser alloyed coatings; they were characterized by scanning electron microscopy and X-ray microanalysis. Multiple laser alloying with beam power decreasing at each successive stage was used for producing a triple-layer coating with tungsten content increasing from layer to layer and reaching 75 wt.% in the upper layer. The observed hardness was in the 1100–1200 HV range for single layer coatings with 40–50% W and as high as 1600 HV in the upper layer of a triple coating with 75% W. The coating with 58 wt.% W showed wear resistance five times as high as compared with the unalloyed laser-melted M2 steel.

Development of Mg-Sc-Mn alloys

Abstract Scandium is a potential alloying element for improving the high temperature properties (above 300°C) of magnesium alloys. Scandium ( T m =1541°C) increases the melting point of the magnesium solid solution, diffuses slowly in magnesium and has a lower density than other potential alloying additions (3 g cm −3 ). Based on thermodynamic equilibrium calculations the alloys MgSc6Mn1 and MgSc15Mn1 were prepared using the squeeze casting technique. The alloys showed a strong annealing response due to the formation of Mn 2 Sc precipitates. The low diffusivity of the alloying elements retards the overageing process. The existence of the Mn 2 Sc precipitates was confirmed by X-ray diffraction and energy dispersive X-ray microanalysis. The ultimate tensile strength of the new alloys was somewhat lower than the corresponding values for the high creep resistant conventional alloy WE43 (Mg–4.0wt.% Y–1.0wt.% Heavy Rare Earths–2.25wt.% Nd–0.5wt.% Zr) whereas the tensile yield strength was comparable to WE43 at higher Sc content. The alloys showed a low elongation to fracture due to a strongly localised plastic deformation. Creep tests of the material in the as cast condition resulted in secondary creep rates which were comparable to WE43 at higher stresses but significantly lower at lower stresses. A T5 heat treatment of the new alloys led to creep rates which were up to two orders of magnitude lower than for WE43. Hence it is possible to increase the operating temperature by 50°C for the new alloys.

Laser composite surfacing of a magnesium alloy with silicon carbide

The present study concerns an attempt to improve the wear resistance of a recently developed magnesium alloy (MEZ) by formation of a SiC reinforced composite layer on the surface by laser surface engineering. Laser processing was carried out with a 10 kW continuous wave CO 2 laser by melting and subsequent feeding of SiC particles on the surface of MEZ. Following laser processing, detailed microstructural and phase analysis of the composite surfaced layer were carried out and correlated with the laser parameters. The microhardness of the surface layer was measured as a function of laser parameters and wear resistance was evaluated in details. Microstructure of the composite surfaced layer mainly consists of uniform dispersion of SiC particles in grain refined MEZ matrix. The volume fraction of SiC particles was found to vary with laser/process parameters. Microhardness of the surface layer was significantly improved to as high as 270 VHN as compared to 35 VHN of the substrate. The wear resistance of the composite surfaced samples was considerably improved as compared to the as-received specimen.

Laser surface alloying of Ti with Si, Al and Si+Al for an improved oxidation resistance

This study concerns laser surface alloying (LSA) of Ti with Si, Al or both to enhance the oxidation resistance of pure Ti. A detailed investigation on the effect of process parameters (laser power, interaction time, etc.) on microstructure, composition, phase distribution and hardness was carried out to determine the optimum LSA conditions to form a crack-free and uniform alloyed zone (AZ). Following LSA under such optimum conditions, the kinetics and mechanism of cyclic oxidation behavior between room temperature and 923:1023 K were extensively studied to identify the scope and role of Si and Al in enhancing the oxidation resistance of the substrate. It appears that LSA with Si or Si Al is more effective than that with Al alone in improving oxidation resistance of Ti. This enhanced oxidation resistance has been attributed to the presence of uniformly distributed Ti5Si3 in the AZ underneath a SiO2:Al2O3 rich superficial oxide scale.

Thermal diffusivity and thermal conductivity of MgSc alloys

In this work the temperature and concentration dependence of the thermal diffusivity of MgSc alloys is reported. Measurements were carried out from 20° up to 300°C and from 3.2 up to 19.0 wt.% Sc using the flash method. The thermal conductivity was calculated as the product of density, specific heat and thermal diffusivity. The thermal conductivity of MgSc alloys increases with increasing temperature and decreases with increasing concentration of Sc. The concentration dependence of all thermophysical properties (thermal diffusivity, thermal conductivity, density and specific heat) is also presented. The results are discussed together with the electrical resistivity data from the literature. The electronic and phonon thermal conductivity is calculated using the Wiedemann-Franz law.