Effect of Gd on microstructure and refinement performance of Al-5Ti-B alloy

Al-5Ti-B and Al-5Ti-B-Gd master alloy refiners were fabricated by fluorine salt casting method.The microstructure and phase constitution of the master alloys were investigated by scanning electron microscopy (SEM),transmission electron microscopy (TEM) and X-ray diffraction (XRD).The results show that Al-Ti-B alloy refiner consists of Al_(3)Ti phase and TiB2 phase.After Gd is introduced into the intermediate alloy,Ti_(2)Al_(20)Gd phase appears in the alloy,the size of Al_(3)Ti is significantly reduced,and Ti-Al-Gd phase is found in the edge of Al_(3)Ti phase.At the same time,some independent Ti-Al-Gd phases appear in local areas,which are Ti_(2)Al_(20)Gd phase determined by micro-area electron diffraction analysis.Analysis and calculation results of the high-resolution images of the Ti_(2)Al_(20)Gd/Al structure show that there is no other compound at the junction between the Ti_(2)Al_(20)Gd phase and Al,and Ti_(2)Al_(20)Gd phase has a great difference in atomic space with the α-Al,which cannot be directly used as heterogeneous nucleus.But,after being decomposed in the aluminum melt,the Ti_(2)Al_(20)Gd phase can promote the refinement effect of the refiner.In the Al-Ti-B-Gd master alloy,there are many dispersed Al_(3)Ti particles with a size of less than 1 μm,which can promote the Al-5Ti-B refining effect.

Refining Effect of Boron on Hypoeutectic Al-Si Alloys

Several concepts of the grain refinement mechanism of B on hypoeutectic Al-Si alloys have been adopted: the refining effect of B on the a-AI and eutectic Si with the different additions of Al-B master alloys made at 850℃ was investigated; and the Al-B master alloys formed under different temperature conditions have been studied to explore the morphologies of AIB2 particles; slowly cooled sample with addition of Al-B was made to explore the refinement mechanism. AI-B master alloy can refine not only a-AI, but eutectic Si. Theoretical analysis indicates that, although AIB2 does not take part directly in the nucleation process in pure Al in the presence of Si, it provides a substrate for precipitation of a small content of Si from which a-At will grow without any undercooling. When the temperature decreases to eutectic line, AIB2 subsequently nucleates eutectic Si; AIB2 particles appear in two different morphologies, namely, hexagonal platelet and tetradehedron morphology which depend on the processing temperature conditions.

Effect of Extrusion Ratio on the Microstructure and Mechanical Properties in an Al-Cu-Mg-Ag Alloy

In order to examine the effect of extrusion ratio on the microstructure and mechanical behavior in Al-Cu-Mg-Ag alloy, the Al-6.3 Cu-0.48 Mg-0.4 Ag alloy was subjected to extruding with different extrusion ratios of 17, 30 and 67. The results indicate that the grains are refined and the strength is improved effectively with increasing extrusion ratio. However, further investigation shows that the extrusion ratio of 30 is more effective than the lower extrusion ratio（17） and the higher extrusion ratio（67） to refine the grains in the T6-temper alloy. Moreover, the sample with an extrusion ratio of 30 obtains more precipitates and superior mechanical properties after T6 treatment. This study supports the idea that there exists a critical extrusion ratio for grain refinement and improvement of mechanical properties for the T6-temper alloy. Recrystallization and precipitation during T6 treatment were introduced to explain the effects of extrusion ratio on the microstructure and mechanical properties of the Al-Cu-Mg-Ag alloys.

Influence of C/Ti stoichiometry in TiC_x on the grain refinement efficiency of Al–Ti–C master alloy

TiC_x contained Al-Ti-C is a kind of grain refiner for Al alloys. In this work, the influence of C/Ti stoichiometry, i.e. the x value in TiC_x on grain refinement efficiency was investigated. TiC_x particles have been obtained in five Al-5Ti-m C（m = 0.1, 0.5, 0.8, 1, 1.25） master alloys and the x values were measured to be0.72, 0.75, 0.79, 0.81 and 0.8, respectively. It was found that the refinement performance of the master alloys had a close relationship with the x value of TiC_x . The Al-5Ti-m C alloy with lower-x TiC_x shows better refinement efficiency and anti-fading capability. It is supposed that TiC_x particles with lower x are more preferred to release Ti atoms during nucleating process and have a better Ti-absorbing capability.This contributes to the Ti-rich zone formation at TiC_x /melt interface, thus enhancing the refinement and anti-fading capability.

Structural evolution of Al–8%Si hypoeutectic alloy by ultrasonic processing

High intensity power ultrasound was respectively introduced into three different solidification stages of Al–8%Si hypoeutectic alloy, including the fully liquid state before nucleation, the nucleation and growth process of primary α（Al） phase and L →（Al） ＋（Si） eutectic transformation period. It is found that both the primary α（Al） phase and（Al ＋ Si） eutectic structure were refined by different degrees with various growth morphologies depending on the ultrasonic treatment stage. Based on the experimental results,the cavitation-induced nucleation due to the high undercooling caused by the collapse of tiny cavities was proposed as the major reason for refining the primary α（Al） phase. Meanwhile, obvious eutectic morphological change was observed only when ultrasound was directly introduced in the eutectic transformation stage, in which typical divorced eutectics and（Al ＋ Si） eutectic cells with symmetrical flower shape were formed at the top of the alloy sample. The introduction of ultrasound in each solidification stage also improves the yield strength of Al–8% Si alloy to a diverse extent.

Effects of Nd on microstructure and mechanical properties of as-cast Mg-8Li-3Al alloy

Effects of Nd on microstructure and mechanical properties of as-cast Mg-8Li-3Al alloy were investigated by OM,X-ray diffraction（XRD）,EPMA,scanning electron microscopy（SEM） and energy dispersive spectroscopy（EDS）.The results showed that the dendrites sizes of α phase were decreased by the Nd addition.When the amount of addition Nd was 1.6 wt.%,the alloy with the smallest α phase was obtained.The refining mechanism mainly owed to the increasing constitutional supercooling at the solidification front.Furthermore,the compound Al 2 Nd generated by the reaction of Al and Nd,which distributed at the phase boundary and inside β phase,could also restrain the growth of α phase.Nd could improve the tensile strength and elongation of Mg-8Li-3Al alloy,however,excessive Al 2 Nd might also become crack source and decrease the elongation.

Influence of the Accumulative Roll Bonding Process Severity on the Microstructure and Superplastic Behaviour of 7075 Al Alloy

The 7075 Al alloy was processed by accumulative roll bonding （ARB） at 350 ℃ using 2：1, 3：1 and 4：1 thickness reductions per pass （Rp） up to 8, 6 and 3 passes, respectively. Microstructural examinations of the processed samples revealed that ARB leads to a microstructure composed of equiaxed crystallites with a mean size generally lower than 500 nm. It was found that, due to both the stored energy through- out the processing and the particle pinning effect, the alloy is affected by discontinuous recrystallisation during the inter-pass heating stages, the precise microstructural evolution being dependent on Rp. Me- chanical testing of the ARBed samples revealed that the main active deformation mechanism in the ARBed samples in the temperature range from 250 to 350 ℃ at intermediate and high strain rates is grain bound- ary sliding, the superplastic properties being determined by both the microstructure after ARB and its thermal stability.

Effect of Sr on Grain Refinement of Mg–3%Al Alloy Containing Trace Fe by Carbon-Inoculation

Magnesium（Mg）-3% aluminum（Al）（in weight） alloy was modified by carbon（C） inoculation combining with strontium（Sr）.The effects of trace 0.1% iron（Fe） addition and operating sequence of carboninoculation and Fe addition on the grain size of Mg-3%Al alloy were studied.The results reveal that the Sr addition could effectively suppress grain-coarsening resulted from the inclusion of Fe in the carboninoculated Mg-Al alloy.Sr addition could contribute to the formation of the duplex-phase particles that Al-C-rich phases coated on Al-Fe or Al-C-Fe-rich phases,regardless of the Fe addition sequence.These duplex-phase particles should be the potent substrates for α-Mg grains.Consequently,Sr addition could effectively subsidize the inhibiting effect of Fe on grain refinement and the active nuclei were maintained.In other words,Sr plays a counter role in the poisoning effect of Fe on the microstructure of Mg-3%Al alloy.

Grain Refining Efficiency of SHS Al-Ti-B-C Master Alloy for Pure Aluminum and Its Effect on Mechanical Properties

A new Al-5Ti-0.75B-0.2C master alloy was successfully prepared by self-propagating high-temperature（SHS）reaction from an Al-Ti-B_4C system with molten Al.Microstructure and phase characterization of the prepared Al-5Ti-0.75B-0.2C master alloy show that the nearly spherical TiC particles,hexagonal or rectangular TiB_2 particles,and blocklike TiAl_3 particles distribute uniformly in the aluminum matrix.Grain refining test on commercial pure aluminum indicates that Al-5Ti-0.75B-0.2C master alloy exhibits a better grain refining performance than Al-5Ti-lB master alloy.By addition of 0.2 wt%Al-5Ti-0.75B-0.2C master alloy,the average grain size of a-Al can be effectively refined to160 ± 5 μm from about 3000 μm,and the tensile strength and elongation are increased by about 20%and 14.1%due to the grain refinement.