Evaluation of solidification cracking susceptibility of Fe–18Mn–0·6C steel welds

Solidification cracking susceptibilities of high Mn steel welds were evaluated in the present study. A longitudinal Varestraint technique was utilised to assess the solidification cracking behaviours of the fusion zone. High Mn steel welds were more susceptible to solidification cracking than 304 and 202 austenitic stainless steel welds, however, they were less susceptible than 310S austenitic stainless steel welds. Extensive segregations of Mn and C took place at the dendritic and grain boundaries in the weld metal, and accordingly contributed to the increase of the hot cracking susceptibility of high Mn steel by the enlargement of solidification temperature range. Further, continuous γ-(Fe,Mn)₃C eutectic phases formed at 1090°C along the grain boundary primarily resulted in the increase of solidification cracking sensitivity in high Mn steel.     

On microstructure and mechanical properties of linear friction welded dissimilar Ti–6Al–4V and Ti–6·5Al–3·5Mo–1·5Zr–0·3Si joint

Abstract

Linear friction welding of dissimilar titanium alloys Ti–6Al–4V (TC4) and Ti–6·5Al–3·5Mo–1·5Zr–0·3Si (TC11) was achieved. Microstructural examination showed that the joint has a clearly identified weld zone and a thermomechanically affected zone on both TC4 and TC11 sides with a clearly identified weld line. In the weld zone of TC4, superfine α grains are dispersed in the β matrix, while in that of TC11, a few recrystallised α grains are observed along the β boundaries. In the thermomechanically affected zone of TC4, both deformed and recrystallised grains exist in the same area, while that of TC11 has a deformed α+β structure. The tensile strength of the joint is comparable to that of the parent TC4 where fracture occurs. The joint microhardness is well related to its microstructure.     

Microstructure and mechanical properties of spot welded Al–5·5Mg–0·3Cu alloy

Abstract

The influences of spot welding on the microstructure and mechanical properties of an Al–5·5Mg–O·3Cu alloy have been investigated. Results showed that dendrites were formed with porosity and cracks in the nugget. Grain boundary melting occurred in the heat affected zone and wide grain boundaries appeared. The alloy exhibited low hardness in the nugget centre. Tensile cracks propagated at the edge of the nugget and mixed rupture with dimples and intergranular fracture occurred. Fatigue fracture initiated at the edge of the nugget and propagated perpendicularly to the tensile axis. Transgranular fracture with striations was also observed.     

Effect of microstructure on tensile and fatigue properties of cast Mg–10Gd–2Y–0·5Zr alloy

Abstract

The microstructure and its effect on tensile properties and fatigue properties of a Mg–10Gd–2Y–0·5Zr (wt-%) cast alloy have been studied. The microstructures of as-cast, solution treated and T6 treated specimens were examined by optical and scanning electron microscopy (SEM). Tensile properties and fatigue properties of the specimens were determined and fractography was carried out. The SEM examination showed that the precipitates after T6 treatment were mainly distributed at grain boundaries, which accounts for the intergranular brittle fracture observed. The average grain size of the specimens measured after solution treatment varied from 87 to 128 μm. The mechanical tests showed that the tensile strength and low cycle fatigue strength increase with decreasing average grain size, whereas high fatigue strength is less sensitive to grain size. The fractography indicated that ductile and brittle fracture patterns coexist.     

Effect of surface oxidation on corrosion mechanism of Co–20·8Ni–6Al–10Cu–11Fe cobalt alloy in molten cryolite

In this paper, the effect of surface oxidation on corrosion behaviour of Co–20·8Ni–6Al–10Cu–11Fe alloy in molten cryolite is investigated. The samples were produced by casting and then were oxidised at 1000°C for 10, 30 and 70 h respectively. The oxide layers were studied by scanning electron microscope (SEM) and X-ray diffraction (XRD). To determine the corrosion behaviour of the oxidised samples, they were exposed to molten cryolite at 930°C for 20 h. After corrosion, the samples were studied by SEM. The results showed that all the samples corroded, but the sample oxidised for 70 h, was more stable than the other and the other one that oxidised for 10 h, corroded severely.     

Microstructure of Rapidly Solidified Alloys of the Sn–Zn–Bi–In System

The results of a study of the phase composition and microstructure of foils of Sn–8.0 Zn–3.0 Bi–X In (X = 1.5, 2.5, 4.5, 9.0) (wt %) alloys formed by rapidly quenching from the melt at a cooling rate of up to 5 × 10⁵ K/s have been presented. The dependence of the phase composition of the rapidly quenched foils on the concentration of In has been determined. It has been shown that, in rapidly quenched foils, crystallization occurs with the formation of supersaturated solid solutions based on β-Sn and γ phase (Sn₄In). The mechanisms and rates of decomposition of the supersaturated solid solutions at room temperature have been established. The specific features of the formation of the microstructure of the foils have been discussed. The grain structure has been studied by the electron back-scatter diffraction (EBSD) method; the formation of an elongated shape of grains and the high specific surface area of small-angle boundaries has been explained.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.     

The influence of Cr alloying on microstructures of Fe–Al–Mn–Cr alloys

The effects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12α-Mn and DO₃ phase-field regions.     

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27?µm, associated with cooling rates ～2°C?s^?1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70～80% associated with higher UTS (～80?MPa) as compared with the other alloys examined.     

Structure–Property–Functionality of Bimetal Interfaces

Interfaces, such as grain boundaries, phase boundaries, and surfaces, are important in materials of any microstructural size scale, whether the microstructure is coarse-grained, ultrafine-grained, or nano-grained. In nanostructured materials, however, they dominate material response and as we have seen many times over, can lead to extraordinary and unusual properties that far exceed those of their coarse-grained counterparts. In this article, we focus on bimetal interfaces. To best elucidate interface structure?Cproperty?Cfunctionality relationships, we focus our studies on simple layered composites composed of an alternating stack of two metals with bimetal interfaces spaced less than 100?nm. We fabricate these nanocomposites by either a bottom?Cup method (physical vapor deposition) or a top?Cdown method (accumulative roll bonding) to produce two distinct interface types. Atomic-scale differences in interface structure are shown to result in profound effects on bulk-scale properties.     

Modeling of Oxidation Kinetics of Y-Doped Fe–Cr–Al Alloys

Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of -Al₂O₃forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be 3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.     

Corrosion resistance of Mg–8·8Li alloy compared with AZ91

Abstract

The superlight Mg–Li alloys exhibit good formability but poor corrosion resistance due to the alloying of lithium. In this paper, the corrosion behaviour of Mg–8·8Li alloy was investigated by electrochemical measurements, corrosion morphology observations and weight loss determination. The results indicate that Mg–8·8Li alloy displays worse corrosion resistance than AZ91D alloy in the early stages of corrosion. However, the corrosion resistance of AZ91D alloy declines and becomes worse than Mg–8·8Li alloy with increasing exposure time. In addition, it is found that a 250°C annealing treatment has a detrimental effect on the corrosion resistance of Mg–8·8Li alloy.     

Effect of platinum group metal addition on microstructure and corrosion behaviour of Ti–47·5 at-%Al

Plain and alloyed titanium aluminides of composition Ti–47·5 at-%Al were prepared with the addition of 1·0 at-% platinum group metals (PGMs). The as cast alloys were subjected to potentiodynamic scans in 5, 15 and 25 wt-%HCl solutions at room temperature, and the PGM containing alloys were assessed for their abilities to spontaneously passivate by cathodic modification. Plain titanium aluminide had a duplex microstructure consisting of lamellar (α₂ and γ alternating lamellae) and γ-TiAl phase grains. The introduction of 1·0 at-%PGMs (platinum, palladium and iridium) led to the formation of a new phase, developing more in the γ-TiAl phase grains and a general improvement of corrosion resistance by increasing the corrosion potential to nobler values. Platinum group metal additions to plain TiAl resulted in the corrosion potentials falling in the passive region of plain TiAl, indicating spontaneous passivation of PGM alloyed TiAl in 5 and 15 wt-%HCl solutions. In 25 wt-%HCl solution, the addition of PGMs shifted the cathodic process in the transpassive or active region of plain TiAl, resulting in either case in the dissolution of the alloy due to the absence of an extended passivation region. The cathodic modification of PGM alloyed TiAl occurred as a result of PGM accumulation on the surface of the TiAl alloys, which simultaneously improved the hydrogen evolution efficiency and inhibited anodic dissolution.     

Modelling of the age hardening behaviour of Al–Mg–Si alloys

In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.     

Friction stir welding of thick plates of Al–Mg–Sc alloy

A technology is developed for single-pass friction stir welding (FSW) of 11- and 35-mm-thick plates of Al–Mg–Sc alloys. The microstructural and mechanical heterogeneity of the welded joints is investigated. The welded joints obtained under the optimum welding conditions are free from macrodefects. The strength of the welded joint equals 98% of the strength of the parent metal, which is higher than the strength of fusion-welded joints. It is concluded that the FSW of thick plates of Al–Mg–Sc alloy can be used efficiently in practice.     

Microstructural design of hardfacing Ni–Cr–B–Si–C alloys

This work reports the procedure for selection of alloying elements to refine the microstructure of hardfacing Ni–Cr–B–Si–C alloys by providing in situ formed nucleation agents. It is concluded that the refining element should be able to spontaneously produce precipitates at high temperatures with little solubility in their Cr-rich counterparts. After exploring the theoretical backgrounds on how to select the refining element, Nb and Zr were selected and the phase formation reactions of Zr- or Nb-modified Ni–Cr–B–Si–C alloys were calculated using Thermo-Calc® simulations. Detailed microstructural analyses of the rapidly solidified samples deposited from the modified alloys showed that addition of Nb in specific quantities induces a significant microstructural refinement in the original Ni–Cr–B–Si–C alloy without deteriorating its high hardness. The Nb-modified alloy could be used to further investigate the viability of microstructural refinement as an effective toughening mechanism for Ni–Cr–B–Si–C and similar alloy systems.