Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement

Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl₃ and TiB₂ in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl₃ particles and then gradually expanded, which resulted in continuous refinement of TiAl₃ particles. In addition, micro-distribution uniformity of TiB₂ particles was improved due to the impingement of TiAl₃ particles to TiB₂ clusters during deformation. Excessively large sizes of TiAl₃ particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl₃ particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18–22 μm for TiAl₃ particles was suggested.     

Study on the grain refinement of A356 alloy by Al–3 wt-% VN master alloy

ABSTRACT

A novel Al–3?wt-% VN master alloy, mainly consisting of α-Al and VN phases, was successfully prepared by stir casting. The grain refinement performance of the master alloy on A356 alloy was then investigated. The results showed that the α-Al grain size of A356 alloy refined by Al–3?wt-% VN master alloy was 350?±?95?µm while that of A356 alloy treated by traditional Al–5Ti–B master alloy was 570?±?105?µm. Moreover, for A356 alloy with Al–3?wt-% VN addition, the good grain refining efficiency did not fade significantly within 30?min. The effectiveness of grain refinement might be attributed to VN particles, which acted as the heterogeneous nuclei of α-Al grains. Owing to the refinement strengthening, the yield strength, ultimate tensile strength and elongation of A356 alloy were improved.     

Influences of Preparation Conditions and Melt Treatment Procedures on Melt Treatment Performance of Al-5Ti-B and Al-10Sr Master Alloys

The influences of preparation conditions of Al-5Ti-B （as-cast and hot-rolled） and Al-10Sr （as-cast and hotextruded） and melt treatment procedures on the grain refinement and modification performance of A356 alloy are experimentally studied. For the two master alloys, the 50% reduction is sufficient to meet the demands of the efficient grain refinement and modification of A356 alloy. When Al-STi-B is introduced into the melt prior to degassing, the grain refinement efficiency of Al-5Ti-B will be greatly increased due to the better dispersity of TiB2 particles. Al-5Ti-B master alloy is less prone to affect the modification effect of Al-10Sr when they are used together.     

动态复合细化变质对A356铝合金显微组织的影响

为弥补Al-10Sr中间合金对A356铝合金变质处理的不足,采用自制的Al-5Ti-1B-1RE中间合金与A1-10Sr中间合金对A356铝合金进行动态复合细化变质处理,研究变质处理后合金的显微组织,并与理论计算结果进行了比较。结果表明:采用JJ-1型精密增力电动搅拌器对熔体进行强力搅拌、振动,动态复合细化变质不仅能使共晶硅相由粗大的板片状转变为细密的颗粒状,并在α-Al边界均匀析出,而且使α-Al相明显细化,力学性能显著提高,与约翰逊-梅尔方程理论对组织晶粒尺寸控制研究结果相一致;同时A356铝合金熔体吸气倾向显著减轻,与热力学近似计算方程和斯托克斯定律对除气机制进行定量计算研究结果相一致。     

Al-Ti-C和Al-Ti-B对7050铝合金微观组织与力学性能的影响

采用金相显微镜(OM)、扫描电镜(SEM)及能谱仪(EDS),结合拉伸力学性能与维氏硬度测试,研究了Al-5Ti-1B和Al-5Ti-0.2C晶粒细化剂对含Zr的7050铝合金铸态、均匀化态以及时效变形态的微观组织演变规律、第二相析出行为及力学性能的影响。结果表明:在7050合金中,Zr元素会使Al-5Ti-1B和Al-5Ti-0.2C均发生细化"中毒现象",降低晶粒细化剂的细晶效果;与Al-Ti-1B相比,增大Al-Ti-0.2C晶粒细化剂的添加量对于缓解"Zr中毒"现象,细化晶粒更有效,且能够提高合金强度与硬度,并使合金保持较好伸长率;同时,使用Al-5Ti-0.2C晶粒细化剂的7050合金,其第二相的分布较使用Al-5Ti-1B晶粒细化剂更加弥散、均匀。     

Microstructure and refinement mechanism of TiB2/TiAl3 in remelted Al-5Ti-1B system

In this work, we propose a new method (by remelting Al-5Ti-1B) to investigate the grain refinement mechansim. It is found that the morphology and size of TiAl₃ phase had little effect on the grain refinement of pure Al. Therefore, further experimental studies were carried out to understand the potency of TiB₂ particles. The high-resolution transmission electron microscopy (HRTEM) observation has confirmed the existence of an atomic layer on the surface of (0001) TiB₂, which is possibly a two-dimensional (2D) (1-12) TiAl₃. Crystallographic study indicates that it is a more suitable nucleation sites for α-Al than other particles. The TiB₂ particle with TiAl₃ 2D acts as the best effective nucleation sites for α-Al.     

Effects of Electromagnetic Stirring,Shearing, and Extrusion on TiB2 and TiAl3 Particles in Al–5Ti–1B(wt.%) Alloy

Electromagnetic stirring induced metal flow and led to homogenous dispersion of TiAl₃ particles. Fragmentation mechanism induced by electromagnetic stirring also contributed to TiAl₃ particle refining. TiAl₃ particle size decreased with the increases of stirring temperature and time. Shearing force among different melt layers under the shear action of roll in the roll-shoe gap increased with the decrease of casting temperature and the increase of melt viscosity, and the fragmentation of TiAl₃ phase became obvious correspondingly. Under the optimal process parameters, Al–5Ti–1B(wt.%) alloy wire with excellent inner microstructure and high surface quality was produced. The average sizes of TiAl₃ are less than 20 µm, and TiB₂ phases are less than 0.5 µm, respectively. Al–5Ti–1B(wt.%) alloy wire manufactured by present method has a high refining ability on pure aluminum and an excellent ability of refining effectiveness.     

Cyclic oxidation of aluminized Ti-14Al-24Nb alloy

Titanium aluminides are considered as replacements for superalloys in applications in gas turbine engines because of their outstanding properties. Ti₃Al has a superior creep strength up to 815° C, but has poor oxidation resistance above 650° C. Two approaches can be followed to improve the oxidation resistance of Ti₃Al above 650° C. One is alloying and the other obtaining a protective surface coating. Niobium was found to improve the oxidation resistance, when added as an alloying element. Recent investigations showed that a TiAl₃ surface layer considerably improves the oxidation resistance of titanium. In the present work, a TiAl₃ layer was obtained on a Ti-14Al-24Nb (wt%) alloy using a pack aluminizing process. The cyclic oxidation behaviour of aluminized and uncoated samples was evaluated.     

抗Zr“中毒”Al-Ti-B-C中间合金对7050铝合金力学性能的影响

通过场发射扫描电子显微镜(FESEM),X射线衍射仪(XRD),能量色谱仪(EDS)分析Al-5Ti-1B,Al-4Ti-1C和Al-5Ti-0.8B-0.2C中间合金的微观组织与物相组成,比较研究3种中间合金对7050铝合金晶粒尺寸与力学性能的影响。结果表明:Zr的存在削弱了Al-5Ti-1B和Al-4Ti-1C中间合金的细化效果,而对Al-5Ti-0.8B-0.2C中间合金细化效果影响较小。含掺杂型TiC粒子的Al-5Ti-0.8B-0.2C中间合金具有较好的抗Zr"中毒"能力,加入量为0.2%(质量分数,下同)时,含Zr7050铝合金平均晶粒尺寸由200μm细化至(60±5)μm,室温极限抗拉强度由405MPa提高到515MPa,提高了27.2%,伸长率由2.1%提高到4.1%。而加入0.2%的Al-5Ti-1B或Al-4Ti-1C中间合金时晶粒尺寸较粗大且分布不均匀,表现出明显的细化"中毒"。     

Fabrication of TiC/TiB/Ti3AlC2 phases reinforced coatings on Ti-6Al-4V substrate

Intermetallic matrix composite coatings reinforced by TiC, TiB₂, and Ti₃AlC₂ were fabricated by laser cladding the mixed power Ti, Al, and B₄C on the Ti-6Al-4V alloy. X-ray diffraction, scanning electron microscope, and energy dispersive spectroscopy were chosen to investigate the structures and morphologies of the coatings. Results showed that the coatings mainly consisted of the reinforcements of TiC, TiB₂, and Ti₃AlC₂ and the matrix of Ti₃Al, TiAl, TiAl₃, and α-Ti. The hardness and wear-resisting property of the prepared specimens of Ti-45Al-10B₄C and Ti-45Al-20B₄C were studied contrastively. It was found that the coating was metallurgical bonded to the Ti-6Al-4V substrate. The micro-hardness and dry sliding wear-resisting properties of the specimen of Ti-45Al-20B₄C were enhanced further. And the micro-hardness of Ti-45Al-20B₄C was from 900 HV_0.2 to 1225 HV_0.2. The wear-resisting property of Ti-45Al-20B₄C was four times as large as that of the Ti-6Al-4V alloys.     

Al-5Ti-0.25C细化剂对2024铝合金组织及力学性能的影响

研究了Al-5Ti-0.25C细化剂对2024铝合金铸态显微组织及力学性能的影响。试验结果表明:未添加细化剂时,2024铝合金显微组织呈粗大的枝晶状,平均尺寸约为150μm;添加Al-5Ti-0.25C后,晶粒为细小的等轴晶。本试验条件下,最佳的细化剂添加量为0.3%,此时,2024铝合金的平均晶粒尺寸为56μm,其力学性能得到显著提高,抗拉强度和延伸率分别为382 MPa、2.60%,与未细化试样相比增幅分别为12.4%、69.9%。     

Mechanical properties of rolled A356 based composites reinforced by Cu-coated bimodal ceramic particles

Three kinds of A356 based composites reinforced with 3 wt.% Al₂O₃ (average particle size: 170 μm), 3 wt.% SiC (average particle size: 15 μm), and 3 wt.% of mixed Al₂O₃–SiC powders (a novel composite with equal weights of reinforcement) were fabricated in this study via a two-step approach. This first process step was semi-solid stir casting, which was followed by rolling as the second process step. Electroless deposition of a copper coating onto the reinforcement was used to improve the wettability of the ceramic particles by the molten A356 alloy. From microstructural characterization, it was found that coarse alumina particles were most effective as obstacles for grain growth during solidification. The rolling process broke the otherwise present fine silicon platelets, which were mostly present around the Al₂O₃ particles. The rolling process was also found to cause fracture of silicon particles, improve the distribution of fine SiC particles, and eliminate porosity remaining after the first casting process step. Examination of the mechanical properties of the obtained composites revealed that samples which contained a bimodal ceramic reinforecment of fine SiC and coarse Al₂O₃ particles had the highest strength and hardness.     

Microstructural features of as-cast A356 alloy inoculated with Sr, Sb modifiers and Al-Ti-C grain refiner simultaneously

This article focuses on the microstructural refinement of as-cast A356 alloy, obtained by melt inoculation of the same. The inoculants used are Sr (modifier), Sb (modifier) and Al-Ti-C (grain refiner). Microstructural characterization of A356 alloy reveals that the Secondary Dendritic Arm Spacing (SDAS) of α-Al dendrites and size/morphology of eutectic silicon decrease by treating the melt with pre-defined amounts of Sr, Sb and Al-5Ti-2C grain refiner. Microstructure of the inoculated as-cast A356 alloy shows that, modified eutectic-silicon exhibits fine fibrous morphology at shorter melt holding, while the same exhibits fine lamellar morphology on longer melt holding.     

Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl₃, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al₄Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.     

The influences of the microstructure morphology of A356 alloy on its rheological behavior in the semi-solid state

In this paper, the rheological behavior of semi-solid A356 alloy with different solid morphology was studied with an improved static shear test method. The results indicated that the rheological behavior of the alloy was significantly influenced by the structural morphology of the alloy. The alloy had quite different rheological properties even though the same fraction of solid existed in the semi-solid state. The rheological behavior of the alloy fitted a five-element model (H₁–[N₁|H₂]–[N₂|S]) for the as-cast microstructure with developed primary (α–Al dendrites, whereas it fitted a six-element model ([H₁|S₁]–[N₁|H₂]–[N₂|S]) for degenerated dendritic or spheroidal primary α–Al, which had been obtained by electromagnetic stirring and spray deposition, respectively. Computation results showed that the deforming capability and shear rate of the semi-solid alloy increased remarkably with the change of primary α–Al from developed dendrites to degenerated dendrites, and then to spheroidal structures. On the other hand, the temperature dependence of the rheological properties of the semi-solid alloy with spheroidal or degenerated dendritic primary α–Al was much less than that with developed primary α–Al dendrites.     

Fabrication of low-cost Ti-1Al-8V-5Fe by powder metallurgy with TiH2 and FeV80 alloy

The low-cost Ti-1Al-8V-5Fe (Ti-185) alloy with a high strength is prepared by cold-compaction-and-sintering powder metallurgy process with low-cost titanium hydride (TiH₂) powders and FeV₈₀ master alloy powders. The use of simple technique process and cheap alloying elements can lead to the cost reduction for titanium alloys. The thermal decomposition of TiH₂-1Al-8V-5Fe is analyzed by thermal gravimetric analyses and differential scanning calorimetry simultaneous thermal analyzer. The shrinkage behavior of TiH₂-1Al-8V-5Fe during the sintering process is employed by the high-sensitivity dilatometer system. The microstructure of sintered Ti-185 consists of β-phase and lamellar α-phase. The results show that the sintered Ti-185 alloys have the relative density of 97.8%, homogeneous composition, and fine grains. The yield strength and the hardness are 1461?MPa and 40.1?±?1.0 HRC (unit of Rockwell hardness), which are better than that of as-cast Ti-185.     

An innovative method for the microstructural modification of TiAl alloy solidified via direct electric current application

Ti-48Al-2Cr-2Nb alloy solidified with the application of direct electric current has a refined and homogeneous microstructure without segregation. We observed an initial decrease followed by a subsequent increase in grain size and lamellar spacing, with the increase in current density. Similar trend can also be obtained by varying the amount of α2-phase(Ti_3Al). Using a directional solidification processing method,the columnar crystal microstructure transforms into an equiaxed crystal microstructure at a current density of 32–64 m A/mm~2. High dislocation density is also introduced with a minimum cross-sectional grain size of 460 μm at a current density of 64 mA/mm~2. The application of electric current alters the free energy of the critical nucleus and temperature via joule heating, causing a transformation from a columnar grain microstructure into an equiaxed grain microstructure. The increase in current density leads to a rise of the nucleation rate, and a resulting undercooling combined with temperature gradient contribute to growth of the primary phase, which finally results in grain coarsening at a critical current density of 96 mA/mm~2.The climb and cross-slip of dislocation and the migration of grain boundary ultimately create variable lamellae spacing of TiAl alloy.     

Microstructural investigation of a rapidly solidified Ti-6Al-4V-1B-0.5Y alloy

A Ti-6Al-4V-1B-0.5Y (wt%) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibre and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distribution in the (+) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phase in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB with a B27 structure, while the yttrium oxide has a cuboidal morphology and has been identified as bcc Y₂O₃. Detailed TEM examination also revealed that yttrium addition has a strong influence on the TiB morphology by comparing the microstructures of Ti-6Al-4V-1B alloys with and without yttrium addition. Under similar processing conditions, the TiB phase in the consolidated alloys without yttrium addition mainly has a nearly equiaxed morphology with a finer particle size, while the TiB phase in the consolidated alloy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.     

Effect of microstructural variables on tensile behaviour of A356 cast aluminium alloy

Abstract

The effects of microstructural variables, including secondary dendrite arm spacing (SDAS), the size of primary α phase, the aspect ratio of eutectic Si particle and the thickness of eutectic wall structure, on tensile behaviour of A356 cast aluminium alloy, were quantitatively identified using linear regression analysis method. For systematic microstructural control of A356 specimen, directional solidification method was used with different solidification rates of 5, 25, 50 and 100 μm s^?1 respectively. The linear regression analysis suggests that each microstructural variable affects tensile strength and tensile elongation of A356 cast aluminium alloy in a similar fashion. The change in tensile behaviour with varying microstructural variables in A356 cast aluminium alloy is discussed based on fractographic and micrographic observations.     

Hot deformation behavior of Cu-bearing antibacterial titanium alloy

We investigated the deformation behavior of a new biomedical Cu-bearing titanium alloy (Ti-645 (Ti-6.06Al-3.75V-4.85Cu, in wt%)) to optimize its microstructure control and the hot-working process. The results showed that true stress–true strain curve of Ti-645 alloy was susceptible to both deformation temperature and strain rate. The microstructure of Ti-645 alloy was significantly changed from equiaxed grain to acicular one with the deformation temperature while a notable decrease in grain size was recorded as well. Dynamic recovery (DRV) and dynamic recrystallization (DRX) obviously existed during the thermal compression of Ti-645 alloy. The apparent activation energies in (α?+?β) phase and β single phase regions were calculated to be 495.21?kJ?mol^?1 and 195.69?kJ?mol^?1, respectively. The processing map showed that the alloy had a large hot-working region whereas the optimum window occurred in the strain rate range of 0.001–0.1?s^?1, and temperature range of 900–960?°C and 1000–1050?°C. The obtained results could provide a technological basis for the design of hot working procedure of Ti-645 alloy to optimize the material design and widen the potential application of Ti-645 alloy in clinic.