Solidification characteristics of Al–9·6Mg alloy solidified under high pressure (4 GPa)

Abstract

The present paper studies influences of high pressure up to 4 GPa at 1273 K on the microstructures and the phases of Al–9·6Mg alloy by means of optical microscopy (OM), X-ray diffraction (XRD), energy dispersive X-ray microanalysis (EDX), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results showed that fine dendrite with long primary arm formed at 4 GPa in comparison with the coarse discontinuous dendrite under normal pressure. Specially, a new high pressure metastable phase, Al_(63·7)Mg_(36·3), was produced besides a small amount of β-Al₃Mg₂ phases at 4 GPa, which was not found in other solidification conditions. Al_(63·7)Mg_(36·3) phase has bcc structure with the lattice constant a=8·495 Å. Evolution mechanism of microstructures and phases was studied in the present paper. Furthermore, Mg concentration in α-Al phase has increased by 42%. Correspondingly, the lattice constant of α-Al phase has increased by 0·44%. The nanohardness of α-Al phase in the alloy increased under high pressure, whereas that of the elastic modulus decreased.     

Columnar to equiaxed transition during growth of a univariant eutectic

Abstract

In this work the transition from columnar to equiaxed growth is studied during the solidification of the univariant eutectic L→ α(Al) + θ-Al₂Cu in the ternary Al–Cu–Ag system. The experimental technique used is upwards solidification with a controlled cooling rate. In the lower part of the sample where the temperature gradient is high, a cellular eutectic is observed. When the temperature gradient decreases, the morphology first changes into two-phase dendrites and finally into an equiaxed eutectic. When the amount of Ag is increased, the formation of eutectic colonies becomes easier. Also, a higher cooling rate facilitates the formation of eutectic colonies. The addition of TiB₂ does not influence the nucleation of eutectic colonies. Although the composition is on the eutectic groove, primary α(Al)-dendrites are found in the samples with TiB₂. This leads to segregation effects due to the density difference between α(Al) and the liquid.     

Electrochemical boriding of titanium for improved mechanical properties

In this study, we introduce a rapid boriding technique that can produce very thick titanium diboride (TiB₂) layers on titanium substrates. We also discuss the effects of process duration on boride layer thickness, chemistry and, morphology. In our experiments, the boriding of commercial purity titanium (CP-Ti) substrates was carried out in an electrochemical cell in which we used a mixture of sodium tetraborate and sodium carbonate as the base ingredients of molten electrolyte at 950 °C and at current density of 300 mA/cm². The titanium test pieces were attached to the cathode holder of the electrochemical cell while a graphite crucible served as the anode. Both TiB and TiB₂ phases were detected by X-ray diffraction method even after 5 min of treatment. Scanning electron microscopy (SEM) images verified that the production of 4.5 µm thick TiB₂ layers was feasible after boriding for an hour. The boride layers consisted of a homogeneous TiB₂ phase on the top and TiB whiskers toward the substrate. The micro-indentation studies indicated that the layer on top has hardness values as high as 40 GPa. The main advantages of this technique are its ability to produce very thick and hard TiB₂ quickly and to create no gaseous emissions or solid wastes during or after the treatment.     

Phase equilibria in TiB2-reinforced high modulus steel

An application of thermodynamic calculations to the phase equilibria in TiB₂-reinforced high modulus steel (HMS) is described. Titanium diboride (TiB₂) has been verified as the best reinforcement for improving isotropic Young’s modulus of steels. The TiB₂ particles can be synthesized in-situ in a ferrite matrix using a combination of ferrotitanium and ferroboron powders. The calculations reveal that impurities, such as oxygen, could promote the iron boride (Fe₂B) formation in a Fe-TiB₂ pseudobinary system, especially for the HMS processed through this powder metallurgical technique. Modification of the steel matrix of HMS by an addition of ferrite formers is encouraged based on the calculated (Fe-TiB₂)-X diagrams that show no significant change in the stability of TiB₂. In the development of HMS with an Fe-Cr matrix, the phase constitution was predicted successfully, and the highest Young’s modulus is reached with a proper control of titanium content.     

Phase equilibria in TiB2-reinforced high modulus steel

An application of thermodynamic calculations to the phase equilibria in TiB₂-reinforced high modulus steel (HMS) is described. Titanium diboride (TiB₂) has been verified as the best reinforcement for improving isotropic Young’s modulus of steels. The TiB₂ particles can be synthesized in-situ in a ferrite matrix using a combination of ferrotitanium and ferroboron powders. The calculations reveal that impurities, such as oxygen, could promote the iron boride (Fe₂B) formation in a Fe-TiB₂ pseudobinary system, especially for the HMS processed through this powder metallurgical technique. Modification of the steel matrix of HMS by an addition of ferrite formers is encouraged based on the calculated (Fe-TiB₂)-X diagrams that show no significant change in the stability of TiB₂. In the development of HMS with an Fe-Cr matrix, the phase constitution was predicted successfully, and the highest Young’s modulus is reached with a proper control of titanium content.     

Effects of trace TiB2 on microstructure in cast titanium alloys

Abstract

Effects of trace TiB₂ on solidification microstructure of Ti–6Al–4V alloy were investigated. The result shows that the microstructure of the ingot was refined by the added TiB₂. The grain size and α lath size were reduced gradually with the increase in TiB₂. The grain size is reduced by about an order of magnitude with an addition of 0·96 wt-%TiB₂, and the dendrite appears in the ingot. Such a tendency is similar to the change of the microstructure with boron addition. Ti–6Al–4V–B phase diagram and growth restriction factor Q are applied to analyse the influence mechanism of trace TiB₂ on the microstructure.     

TiN-TiB2 composite coatings reactively produced by electrothermally exploded powder spray

Composite coatings composed of titanium nitride, TiN, and diboride, TiB₂, were reactively produced by the electrothermally exploded powder spray technique, in which feedstock powder was prepared from titanium and boron nitride particles. The microstructure of the coating was composed of titanium-ceramic particles the size of which were on the order of several nanometers to a few hundred nanometers. Such reactive thermal spraying brought base-metal saturation into a coating layer at the early stages of coating formation. The ceramic composite spray using feedstock of TiN and TiB₂ particles preferentially brought a new phase of cubic titanium boronitride together with TiN and TiB₂ into a coating. On comparing such a coating to one produced by the conventional method, the reactive thermal spray coating was richer in TiN and TiB₂ due to the excess nitrogen in the feedstock.     

The effects of vacuum annealing on the film properties of titanium boride (TiBx) grown on (100)Si substrate

In order to investigate the effects of vacuum annealing on the properties of titanium boride films (TiBx) on a (100)Si substrate, TiBx/Si samples were prepared by the co-evaporation process and then annealed in the temperature range of 300≈1000°C. The interfacial reaction of TiBx/Si systems and the thermal stability of non-stoichiometric TiBx films (0≤B/Ti≤2.5) were investigated by means of sheet resistance, x-ray diffraction, transmission electron microscopy, x-ray photo-electron spectroscopy, and stress measurement. For TiBx samples with a ratio of B/Ti≥2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with the ratio of B/Ti<2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide (self-aligned silicide) process. The sheet resistance and the film stress in the Ti/Si and TiB_x/Si systems are explained well by the solid phase reactions.     

Copper-titanium diboride coatings obtained by plasma spraying reactive micropellets

Electrotribological applications require materials with both high electrical conductivity and wear resisance. For this purpose, a copper- base plasma sprayed coating containing titanium diboride particles was developed. The process for fabricating this CU- TiB₂ coating consists of plasma spraying reactive powders that contain a Cu- Ti alloy and boron. The reaction between the copper alloy and boron proceeds in different steps going from solid- state diffusion of titanium and copper to the synthesis of TiB₂ in a liquid below 1083 ‡C. Plasma sprayed copper coatings contain finer TiB₂ crystals than Cu- TiB₂ materials synthesized in a furnace at 1200 ‡C. Coatings with 25 vol% TiB2 have hardnesses that are comparable to Cu- Co- Be and Cu- Ni- Be alloys and to Cu- W and Cu- Mo alloys used in spot welding. Their low electrical resistivity of 52 ΜΩ cm could be increased by lowering the oxygen content with coatings and controlling the formation of TiB₂ clusters, the titanium content in solution in copper remaining low after the synthesis reaction.     

Microstructure and mechanical properties of diffusion bonded joints between titanium and stainless steel with copper interlayer

Abstract

The solid state joining of titanium to stainless steel with copper interlayer was carried out in the temperature range of 850–950°C for 7·2 ks in vacuum. The interface microstructures and reaction products of the transition joints were investigated with an optical microscope and a scanning electron microscope. The elemental concentration of reaction products at the diffusion interfaces was evaluated by electron probe microanalysis. The occurrence of difference in intermetallics at both interfaces (SS/Cu and Cu/Ti) such as CuTi₂, CuTi, Cu₄Ti₃, χ, FeTi, Fe₂Ti, Cr₂Ti, α-Fe, α-Ti, β-Ti, T₂(Ti₄₀Cu_60?xFe_x; 5<x<17), T₄(Ti₃₇Cu_63?xFe_x; 5<x<7) and T₅(Ti₄₅Cu_55?xFe_x; 4<x<5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were detected by X-ray diffraction technique. The maximum tensile strength of ～91% of Ti strength and shear strength of ～74% of Ti strength along with ～ 7·2% ductility were obtained for the joint bonded at 900°C due to better coalescence of mating surfaces. At a lower joining temperature of 850° C, bond strength is poor due to incomplete coalescence of the mating surfaces. With an increase in the joining temperature to 950°C, a decrease in bond strength occurred due to an increase in the volume fraction of brittle Fe–Ti base intermetallics.     

Development and growth of boron-modified and germanium-doped titanium-silicide diffusion coatings by the halide-activated,pack-cementation method

A halide-activated, cementation pack has been developed to codeposit either silicon and boron or else silicon and germanium in a single processing/reaction step to grow Ti-silicide diffusion coatings on commercially pure (CP) titanium, Ti-22Al-27Nb, and Ti-20Al-22Nb. Since boron is nearly insoluble in TiSi₂, a TiB₂ layer is localized at the surface of the B-modified silicide coatings. The thickness of the TiB₂ layer is controlled by the choice of boron activity and halide activator in the pack. Germanium is soluble in the Ti-silicide layers but inhomogeneously distributed in the Ge-doped silicide coating. The germanium content is controlled by choices of the Si-to-Ge ratio and the halide activator in the pack. The growth kinetics for the five-layered B-modified silicide coatings are generally similar to the undoped silicide coatings. The growth mechanism for the five-layered Ge-doped silicide coatings is generally different from the undoped silicides. The growth of dual-layer Ti-boride coatings was also studied.     

Effects of electrochemical boriding process parameters on the formation of titanium borides

In this study, the boriding of titanium via molten salt electrolysis was investigated in borax based electrolyte at various current densities (50–700 mA/cm²), temperatures (900–1200°C) and process durations (1 min-4 h). Thin film XRD results revealed that two main titanium boride phases TiB₂ and TiB was formed even after 1 minute of process time. Scanning electron microscopy (SEM) images conducted on the cross-sections demonstrated a bilayer boride structure composed of a continuous uniform TiB₂ phase and TiB whiskers that grew below the TiB₂ layer toward the substrate. Dependence of boride layer thickness and morphology on the process parameters was evaluated. The results of the study showed that temperature of boriding was the most critical parameter both on boriding rate and morphology of the boride layer. It was possible to grow 8 μm thick TiB₂ layer on titanium in 30 minutes of boriding at 1200°C.     

Reactive spark plasma sintering of TiB2–SiC–TiN novel composite

The effects of adding SiC as a reinforcement and TiN as an additive on TiB₂-based composites fabricated by the spark plasma sintering (SPS) technique were investigated. SPS was implemented at the sintering conditions of 1900 °C temperature, 7 min holding time and 40 MPa pressure. Adding these two secondary phases had noticeable effects on the microstructure of TiB₂-based composites. A relative densities of 99.9% was obtained for TiB₂–SiC–TiN composite. Detection of in-situ formed phases and investigation on them were done using SEM, XRD, EDS and thermodynamic assessment. These evaluations proved the formation of in-situ phases of TiC, BN nano-platelets, TiSi and B₄C in the TiB₂-based composite codoped with SiC and TiN. Formation of these in-situ phases had fascinating effects on the sinterability and ultimate microstructure of titanium diboride.     

Densification of titanium diboride by hot isostatic pressing and production of near-net-shape components

Hot isostatic pressing (HIP) was applied to the production of titanium diboride (TiB₂) parts. Cylinders were first produced to select the best starting powder (of two possible choices) and the processing conditions. Transverse strength, hardness, and toughness measurements were carried out on the densified products. Results were equivalent or better than data published in the literature, showing that HIP is an efficient method of processing ceramic powders. Constitutive equations representing the rheology of porous materials are presented and applied to the selected titanium diboride powder. Hot pressing and sinter forging tests were carried out to obtain the parameters of the constitutive equations. These equations were used in a finite-element program to simulate the forming of TiB₂ crucibles by HIP. These parts were actually processed using graphite or titanium inserts to produce a hollow, cylindrical part with a closed end. Observed and calculated final shapes were compared, showing good agreement. In addition, the finite-element program allowed the calculation of residual stresses after processing, of eventual remaining porosity. It then became possible to optimize processing routes, can, and insert geometries.     

激光焊接颗粒增强铝基复合材料中TiB2颗粒的演变行为（英文）

研究大功率激光器焊接TiB2颗粒增强铝基复合材料时TiB2粒子的演变行为。采用X射线衍射(XRD)、扫描电镜(SEM)及能谱(EDS)分析焊缝内粒子的物相、热力学过程及形貌特征;同时对TiB2和铝基体的界面反应进行讨论。结果表明:当TiB2团簇尺寸大于激光光斑直径时,焊缝中部的TiB2粒子会熔融在一起,较大尺寸的TiB2会发生断裂;当与铝熔体接触后,熔化后的TiB2粒子会与Al发生反应生成Al3Ti和AlB12,并且焊缝中部的界面反应比焊缝边缘的剧烈。     

Corrosion of alumina/titanium diboride composites in neutral and acidic chloride solutions

This research concerns the study of the corrosion behaviour of monolithic titanium diboride and the composites 50%Al₂O₃/50%TiB₂ (50/50) and 70%Al₂O₃/30%TiB₂ (70/30) in 3.5% NaCl solutions, at pH 7 and 3 (adjusted by HCl additions). The test temperature is 45 ± 1°C. Both electrochemical and chemical techniques are applied. Scanning electron microscope (SEM) observations are also used to investigate the corrosion attack morphology. Polarization curve recording evidences that at short immersion times both composites exhibit a passive behaviour. The stability of these passive conditions decreases at increasing volume fractions of titanium diboride in the material and is minimum in the monolithic material. The analyses of the aggressive solutions during 30 days of immersion of 50/50 under free corrosion conditions are in agreement with the hypothesis that at both pH values passivity is due to the presence of a titanium‐containing insoluble surface film, probably constituted by hydrated titanium oxide. SEM observations evidence the presence of a third phase in the composites, beside titanium diboride and alumina, that is constituted by an aluminum borate, formed during sinterization. This phase undergoes a chemical attack during exposures to the aggressive solutions, under free corrosion conditions. The relative importance of chemical towards electrochemical corrosion, as assessed by coupling potentiostatic tests and chemical analyses, increases when passive conditions are maintained.     

Sintering of Al2O3–TiB2 nano-composite derived from milling assisted sol–gel method

Synthesis and sintering of an alumina /titanium diboride nano-composite have been studied as an alternative for pure titanium diboride for ceramic armor applications. Addition of TiB₂ particles to an Al₂O₃ matrix can improve its fracture toughness, hardness and flexural strength and offer advantages with respect to wear and fracture behavior. This contribution, for the first time, reports the sintering, microstructure, and properties of Al₂O₃–TiB₂ nano-composite densified with no sintering aids. Nano-composite powder was produced by combination of sol–gel and mechano-chemical methods. The densification experiments were carried out using both hot pressing and pressureless sintering routes. In the pressureless sintering route, a maximum of 92.3% of the theoretical density was achieved after sintering at 1850 °C for 2 h under vacuum. However, hot pressing at 1500 °C for 2 h under the same condition led to achieving a 99% of the theoretical density. The hot pressed Al₂O₃–TiB₂ nano-composites exhibit high Vickers hardness (16.1 GPa) and a modest indentation toughness (~ 4.2 MPa.m^1/2).     

Effects of cooling rate and initial composition on the solidification path and microstructure of Al-Cu-Si alloys

The influences of cooling rate and initial composition on solidification path, microstructure and hardness of Al-Cu-Si alloys were investigated. The results indicate that the solidification paths of alloys Al-3.7Cu-6.5Si, Al-5.2Cu-2.6Si and Al-15.1Cu-6.2Si were (L + α-Al)→(L + α-Al+ β-Si)→(L + α-Al+ β-Si+ θ-Al₂Cu), and the solidification paths of alloys Al-4.6Cu-0.9Si, Al-24.2Cu-3.9Si and Al-26.9Cu-2.1Si were (L + α-Al)→(L + α-Al+ θ-Al₂Cu)→(L + α-Al+ β-Si+ θ-Al₂Cu). Furthermore, it is found that the further the initial compositions from the binary eutectic trough and the slower the cooling rate was, the second dendrite arm spacing was larger and the volume percent of phase θ-Al₂Cu was fewer, and then influence the hardness. There were three kinds of (α-Al+ θ-Al₂Cu) binary eutectic, two kinds of (α-Al+ β-Si+ θ-Al₂Cu) ternary eutectic and one kind of (α-Al+ β-Si) binary eutectic morphologies existed in the solidification specimens.     

Boron Solubility in Silicide Ti5Si3

Alloys of the Ti-Si-B system in the Ti₅Si₃ vicinity melted in an arc furnace from pure components were studied after annealing at 1850 °C for 3 h by means of XRD analysis, scanning electron microscopy with electron probe microanalysis (SEM/WDS), and ¹¹B solid state nuclear magnetic resonance (NMR). The phase based on Ti₅Si₃ (D8₈ crystal structure of the Mn₅Si₃ or Hf₅CuSn₃ type), Ti₅Si₄ silicide (Sm₅Ge₄ crystal structure type) and TiB₂ diboride were identified in the alloys. The NMR spectra contained two well-separated narrow peaks (at δ = 9 and δ = 274 ppm; Δν = 18 and Δν = 14 kHz) related to the B atoms located in the crystal lattices of TiB₂ and solid solution based on Ti₅Si₃ respectively, which stand out against a broad peak (δ = 150 ppm, Δν = 120 kHz) from B atoms which lack for a long-range order environment. The ¹¹B spectra were used to determine boron contents in the constituents. The B content of the Ti₅Si₃-based phase determined from the integral intensity of proper narrow peaks is in agreement with SEM/WDS data (both about 5 at.%).     

Microstructures and mechanical properties in laser beam welds of titanium matrix composites

Laser beam welding (LBW) was used to 2 mm thick titanium matrix composites (TMCs) sheets and microstructure, hardness and tensile properties of butt joints were studied. Welded joints without defects were obtained indicating that LBW is a suitable processing method for TMCs. The results reveal that the fusion zone completely consists of α′ martensite causing an increase of more 27% in hardness compared with that of base metal. The heat affected zone consists of a mixture of α′ martensite and primary α phases. Large gradients of microstructures and hardness are found over the narrow heat affected zone dependent on the β-transus temperature during weld cooling. TiB_w with smaller sizes redistribute at grain boundaries in the weld. The joints show excellent strength and they can reach the full strength compared with the base metal with sound welding parameters, which is ascribed to the presence of α′ martensite and refinement of TiB_w in the weld.