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.     

β Phase decomposition in Ti–5Al–5Mo–5V–3Cr

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ～500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ～90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ～1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties.     

Ti–5Al–5Mo–5V–1Cr–1Fe合金高周疲劳性能及断裂行为的研究

研究了在不同应力集中系数K_t和应力比R下Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511)锻件和棒材纵向的室温高周疲劳性能及断口形貌。结果表明：应力比和应力集中系数是影响疲劳性能的两个重要因素。锻件和棒材的疲劳强度均随应力比R的增大而增大,随应力集中系数K_t的增大而减小。锻件的疲劳性能优于棒材,并且锻件的疲劳强度是棒材的1.08~1.57倍。     

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.     

ω-Assisted nucleation and growth of α precipitates in the Ti–5Al–5Mo–5V–3Cr–0.5Fe β titanium alloy

This paper discusses the structural and compositional changes at the nanometer scale associated with the nucleation and growth of α precipitates in the β titanium alloy Ti-5553 (Ti–5Al–5Mo–5 V–3Cr–0.5Fe) with ω precipitates acting as heterogeneous nucleation sites. The microstructural evolution in this alloy, during β-solutionizing, quenching and aging type heat-treatments, has been investigated by combining results from scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy, high-resolution TEM and three-dimensional atom probe (3DAP) tomography. Athermal ω precipitates form in this alloy on quenching from above the β transus temperature. On isothermal annealing at low temperatures, these ω precipitates coarsen to form chemically ordered ω precipitates, accompanied by the nucleation of the stable α phase. Annealing at higher temperatures leads to dissolution of ω and further growth of α precipitates accompanied by clustering of different α variants in self-accommodating morphologies. 3DAP results indicate that annealing at lower temperatures (～350 °C) leads to initial nucleation of α precipitates with a non-equilibrium composition, nearly identical to that of the β matrix. Subsequent aging at higher temperatures (～600 °C) leads to more pronounced partitioning of alloying elements between the two phases. These results indicate that the structural body-centered cubic to hexagonal close-packed transformation and the compositional partitioning of alloying elements occur in sequential steps, resulting in a mixed-mode displacive-diffusional transformation, similar to the bainite transformation in steels.     

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.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.     

Macrosegregation of ternary Al–4·5wt-%Cu–1·0wt-%Mg alloy in horizontal direct chill casting: implementation of non-equilibrium microsegregation model

Abstract

An extended and modified continuum mixture model for macrosegregation in solidifying alloys is applied to the case of horizontal direct chill casting (HDC) of a ternary Al–4·5Cu–1·0Mg (wt-%) alloy. A thermodynamic closure model for a Al–Cu–Mg alloy is coupled with an extended mixture continuum model to obtain steady state macrosegregation fields for the different casting parameters (e.g. casting speed, inlet superheat and inlet plenum position) in this study. Macrosegregation calculations were performed assuming that the solid forms a rigid and stationary network in all parts of the mushy zone. A microsegregation non-equilibrium model is used to calculate phase fraction compositions and phase distributions. The Cu exhibits more segregation compared to the previous study of a binary Al–Cu alloy, while Mg has a similar segregation profile in all cases but less pronounced. The phase compositions show that Cu is mostly distributed in secondary and ternary phases, while Mg is uniformly configured in the primary (Al) and ternary phase (Al₂CuMg). The primary Al phase is dominant (over 90%) in all cases, with highest peaks in depleted regions below the centreline of slab, except in the case where the inlet plenum is positioned on the bottom of the refractory plate due to a different flow pattern.     

Impurities in Al–5Ti–1B grain refiner rod

Abstract

Two different Al–5Ti–1B (wt-%) grain refiner rods (Rod A – a 'poor' refiner and Rod B – a 'standard' refiner) were examined using induced coupled plasma – optical emission spectroscopy (ICP-OES), computed tomography (CT) and scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy. CT with its digital geometrical processing allowed 3D images of non-destructed sections of the rod to be analysed. Results for Rod A revealed the presence of several types of inclusions up to several mm in size. Qualitative micro-chemical analysis using an SEM with EDX indicated that the inclusions were Ti, K and/or F rich. This would thus imply that the particles observed were, undissolved Ti/Al-Ti compounds namely Al₃Ti and/or Al₃Ti(Si), and potassium and fluoride spent salts namely KF-AlF₃ flux. These findings are indicative of an inefficient, non-optimal manufacturing process. An assessment of the grain refining efficacy of both the rods using the TP1 test showed the standard rod (Rod B) to be more effective in promoting refinement. Furthermore, the grain size observed for Rod A was not as consistent. Thus the quality of the grain refiner rod used in industrial practice is important as the grain size, feedability, cleanliness, uniformity of microstructure and integrity of the as-cast part may all be deleteriously affected.     

Microstructural characterisation and mechanical properties of Mg–xSn–5Al–1Zn alloys

Abstract

In this study, the microstructure and mechanical properties of as cast Mg–x Sn–5Al–1Zn alloys were investigated. The microstructures of the alloys were characterised by the presence of Mg₂Sn and Mg₁₇Al₁₂ precipitates. The greatest tensile strength and elongation were obtained at the alloy containing 5 wt-%Sn at room temperature. Microhardness of the alloys and volume fraction of the Mg₂Sn precipitates increased with increasing Sn content. Fractographic analysis demonstrated that dimple and cleavage facet were dominant mechanisms of these alloys tested at room and elevated temperature. The portion of cleavage facet was increased with the increment of Sn at the room and elevated temperatures.     

Grain refining action of Al–5Ti–C and Al–TiC master alloys with Al–5Ti master alloy addition for commercial purity aluminium

Al–Ti–C master alloys have a great potential as efficient grain refiners for aluminium and its alloys. In the present work, the Al–5Ti–C, Al–TiC and Al–5Ti master alloys have been successfully prepared by a method of liquid solidification reactions. While the Al–5Ti–C master alloy consists of some strip- or needle-like TiAl₃, and in addition to TiC particles in the Al matrix, the Al–TiC master alloy revealed the presence of only TiC particles, and the Al–5Ti master alloy consists of only some blocky TiAl₃ particles. A united refinement technology by Al–5Ti–C+Al–5Ti and Al–TiC+Al–5Ti master alloys was put forward in this paper. The blocky TiAl₃ particles in Al–5Ti master alloy can not only improve the grain refinement efficiency of Al–5Ti–C and Al–TiC master alloys but also reduce the consumption because the blocky TiAl₃ particles improve the grain refinement efficiency of TiC particles in Al–5Ti–C and Al–TiC master alloys.     

Corrosion resistance properties of hybrid organic–inorganic epoxy–amino functionalised polysiloxane based coatings on mild steel in 3·5%NaCl solution

Abstract

New hybrid organic–inorganic coatings have been developed by reacting a mixture of 3-glycidoxypropyltrimethoxysilane (GPTMS) and tetraorthosilicate (TEOS) with 2–4% aminoethylaminopropyl-methylsiloxane dimethylsiloxane (APDMS) copolymer as a modifier. The sol–gel polymerisation of the inorganic components was achieved by base catalysation using NaOH. The resultant base coating (CGA) was further modified using two different corrosion inhibitors: Moly-white 101-ED and Hfucophos Zapp yielding coatings (CGA-M) and (CGA-Z) respectively. The corrosion resistant efficiency of these coatings for the protection of mild steel sheets in 3·5%NaCl electrolyte was assessed using AC and DC electrochemical methods, notably, electrochemical impedance spectroscopy (EIS) and DC polarisation scans. Based on the results, the order of corrosion protection of the above coatings was in the order: CGA-M>>CGA-Z>CGA.     

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.     

Improved Corrosion Resistance of Selective Laser Melted Ti–5Cu Alloy Using Atomized Ti–5Cu Powder

The different types of metal powder used for selective laser melting(SLM) process would cause distinct corrosion behavior due to the uniformity of the obtained microstructure.The SLM-produced Ti–5Cu alloy using atomized Ti–5Cu metal powder(SLMed Ti–5Cu) in this work reveals a relatively uniform microstructure with overwhelming acicular α/α′ phase and shows great advantages on corrosion resistance compared with the SLM-produced Ti–5Cu alloy using the mixture powder(SLMedM Ti–5Cu).The effect of the micro-galvanic cells decreases due to the undetectable Ti_2Cu phase in the microstructure of the SLMed Ti–5Cu.An apparent passivation behavior was observed for SLMed Ti–5Cu instead of severe pitting phenomenon for the SLMed-M Ti–5Cu.The charge transfer resistance of SLMed Ti–5Cu in this work is 10.09 ± 2.63 MΩ cm~2, which is significantly higher than that of SLMed-M Ti–5Cu(4.76 MΩ cm~2).The above result indicates the atomized Ti–5Cu powder plays an important role in the formation of the uniform microstructure of SLMed product, thereby enhancing its corrosion resistance in Hank's solution at 37 ℃.     

A 1% magnetostrain in polycrystalline 5M Ni–Mn–Ga

Magnetostrain is achieved by twin boundary motion in magnetic shape memory materials. A 1% strain could be achieved in textured polycrystals of Ni–Mn–Ga at room temperature. Ni₅₀Mn₂₉Ga₂₁ was directionally solidified in order to obtain a texture parallel to the heat flow. The alloy was heat treated for chemical homogenization and stress relaxation in the austenite state. Thermomechanical training was employed to decrease the twinning stress and to enhance the strain by simplification of the variant microstructure. Investigations at slightly elevated temperatures were performed to increase the magnetostrain to 1%.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.     

Corrosion behaviors for peak-aged Mg–7Gd–5Y–1Nd–0.5Zr alloys with oxide films

The corrosion behaviors of T5 (225 °C, 6.5 h) and T6 (460 °C, 2 h + 225 °C, 12 h) peak-aged Mg–7Gd–5Y–1Nd–0.5Zr alloys with oxide films were investigated by optical microscope (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The weight loss rates and electrochemical tests were also analyzed. The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0 μm, respectively. The components of oxide films mainly consist of O, Mg, Y, Nd, and Gd, and the T6 oxide film results in surfaces with larger peaks than T5 oxide film. In addition, Y, Nd, and Gd peaks are all higher than those of Mg–7Gd–5Y–1Nd–0.5Zr alloys, but Mg peak is consistently far below than that of the alloys. The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities: T6 oxide film < T5 oxide film < T6 without oxide film < T5 without oxide film. The oxide films are compact to increase the corrosion resistance for Mg–7Gd–5Y–1Nd–0.5Zr alloys, which will provide a guiding insight into the corrosion and protection of Mg–RE alloys in atmospheric environments.     

Shape memory behavior of Ti–20Zr–10Nb–5Al alloy subjected to annealing treatment

The effects of annealing temperature on microstructures, phase transformation, mechanical properties, and shape memory effect of Ti–20Zr–10Nb–5Al alloy were investigated. X-ray diffraction(XRD) patterns show that the alloy is composed of single hexagonal ɑ'-martensite phase for both as-rolled sample and sample annealed at773 K for 30 min, while single orthorhombic ɑ' phase exists in the samples annealed at 873 and 973 K for30 min. The optical observations indicate that the alloy is recrystallized when annealed at 873 K, and the grain size of the sample annealed at 973 K is about five times larger than that annealed at 873 K. Both of the samples annealed at 873 and 973 K show almost the same reverse martensite transformation start temperature of 483 K as demonstrated by thermal dilatation tests. The critical stress values for martensite reorientation(σ_M) are 392 and 438 MPa for the alloys annealed at 873 and 973 K, respectively. The maximum shape memory strain is 2.8 %, which is obtained in the alloy annealed at 873 K due to the lower σ_M. Moreover,the sample annealed at 873 K exhibits larger tensile stress and tensile strain due to the smaller grain size.