INFLUENCE OF ALLOYING ELEMENTS（Nb，Mo，V） ON MICROSTRUCTURE OF Ti_3Al BASE ALLOYS

The microstructures of Ti-14Al-21Nb and Ti-14Al-20Nb-2Mo-3.2V(wt%) alloys have been investikated by transmission electron microscopy(TEM).The phase constitution and the orientation relationship between α2and βwere identified by means of the selected area diffraction and the micro-diffraction techniques.Results show that,compared to Ti-14Al-21Nb,the alloying elements Mo and V can greatly increase the fraction of β phase and improve significantly the microstructures of Ti-14Al-20Nb-2Mo-3.2V.The ordered B2 structure is also evident in the residual βgrains of the latter alloy.

Microstructures and mechanical properties of as-cast Mg-Sm-Zn-Zr alloys with varying Gd contents

The effect of Gd content on the microstructure and tensile properties of as-cast Mg-Sm-Zn-Zr alloy has been systematically investigated.In the Mg-3Sm-0.5Zn-0.5Zr alloy, the intermetallic compounds with multiple morphologies are identified as Mg_(3)Sm phase. In addition to Mg_(3)RE phase, Mg_(5)RE phase originated from Gd addition is observed in Gd-modified alloys. It should be noted that the lattice parameters of all the observed intermetallic compounds are significantly reduced by Zn segregation. The segregation behavior of Zn in Mg_(3)Sm phase is inhibited to some extent by Gd addition due to the electronegativity difference between Sm/Gd and Zn elements. In addition, the increased Gd content effectively leads to much more accumulation of solute atoms in front of the liquid-solid interface during solidification, which can prominently promote nucleation in liquid region and then refine grains. The tensile yield stress of the present alloys is thus improved with increasing Gd addition. Finally, Gd-modified alloys exhibit significantly age-hardening effect, which can be mainly attributed to the high-volume fraction and high density nano-scale precipitates.

Effect of gadolinium on aged hardening behavior,microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy

Mg-3.4Nd-0.1Zn-0.40Zr alloy samples with and without containing gadolinium(0.6%,mass fraction) were prepared by sand casting.The aged hardening behavior,solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM,XRD,TEM,hardness tests and mechanical property tests.The main research results are as follows.1) Compared with the alloy without the addition of gadolinium,the alloys with the addition of gadolinium shows the more remarkable age-hardening response.2) The as-cast microstructure of the alloy with and without containing gadolinium consists ofα-Mg grains with Mg_(12)Nd phase on the grain boundary.After solution heat-treatment,Mg_(12)Nd phase of the alloy without containing gadolinium is dissolved in the matrix,however,there is still discontinued Mgl2Nd phase at grain boundary of the alloy with containing gadolinium.The more finely dispersed precipitates in Mg matrix are formed in the alloy with containing gadolinium during age-treatment.3) The room temperature and high temperature mechanical properties of the alloy are satisfactory,withσ_b=280 MPa,σ_(0.2)=165 MPa at RT andσ_b=215 MPa,σ_(0.2)=155 MPa at 250℃.The high temperature mechanical properties decrease slightly with the increase of temperature.

Microstructural evolution of aluminum alloy 7B04 thick plate by various thermal treatments

The microstructure of an AA 7B04 alloy in the form of plate was investigated using differential scanning calorimetry(DSC)and TEM analysis technologies.Tensile properties and electrical conductivity of AA 7B04 under various heat treatment conditions were also presented.The results reveal that peak-aged microstructure contains GP zones and η' precipitates predominantly.After retrogressing and reaging(RRA),the η' and η precipitates disperse in the alloy matrix,and the η precipitates distribute coarsely and sparsely,decorating the grain boundaries,together with precipitate free zones(PFZs)around them.It is also shown that selecting of suitable heat treatments can provide optimal precipitates in matrix and at grain boundaries,which gives rise to a combination of high strength and stress corrosion cracking(SCC)resistance in such materials.

A MICROSTRUCTURAL ANALYSIS OF 7050 Al ALLOY

The microstructure of 7050 AI alloy in T74 condition has been studied by means of transmis- sion electron microscopy and dynamical diffraction simulation.It was found that the alloy studied contains mainly AI3Zr superlattice phase,η′-phase and Al_7Cu_2Fe constituent phase. The η′-phase has an orthorombic crystal structure with a=0.492 nm,b=0.852 nm and c=0.701 nm.The orientation relationship between the η′-phase and matrix was determined as(101)_m∥(010)_(η′)[111]_m∥[001]_(η′)

Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength.However,the mechanism has not been fully elucidated so far.In this work,the role of phase/grain bound-ary in the static globularization of TC17 alloy was systematically studied by setting differentαphase con-tent before annealing through low-and high-temperature deformation.Isothermal compression causes the parallel distribution and fragmentation of 3Dαplates and few globularαparticles are formed at a strain rate of 1 s^(-1).Post-deformation annealing promotes the static globularization ofαphase while it is affected by initialαphase content.After 730°C deformation,the development ofα/αinterface by absorbing dislocations promotes the formation of globularαgrains based on the nucleation of sepa-ratedαparticles and pre-recoveryαsubgrain during subsequent annealing.Theα/α/βandα/β/βtriple junctions formed due to highαcontent with about 36%volume fraction are favorable for the further nucleation and growth of globularαgrains by reducing interface energy,forming a 3D irregularαplate.Then nucleation and growth of theβphase dominate the microstructure evolution during subsequent an-nealing,resulting in the local dissolution of the plate and formation ofαrods.After 850°C deformation,theαphase tends to nucleate at theβ/β/βtriple junctions and grow into a lamellar shape along the high energyβ/βgrain boundary due to lowαcontent with about 7%volume fraction.Theαnucleation that maintains the Burgers orientation relationship(BOR)with the surroundingβphase grows along the habit plane and thickens slowly,resulting in the formation of a precipitatedαplate with a flat surface and the suppression of static globularization.The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.

Microstructure and nano-mechanical property of cold spray Co-base refractory alloy coating

Co-base refractory alloy coating was prepared on carbon steel substrate by cold spray technology; microstructure and nano-mechenieal property were examined by scanning electron microscope （SEM） and nano indenter individually. The results showed that about 250 μm Co-base refractory alloy coating could be deposited o11 steel substrate by cold spray technique, interface between coating and substrate was combined well, and the refractory alloy particle had a significant plastic deformation during deposition process; mixing Ni powders into Co-base refractory alloy powders could increase the density and decrease the nano-hardness of coating, the nano-hardness and elastic modulus of refractory alloy coating was higher than 6 GPa and 160 GPa, respectively.

Formation of the anomalous microstructure in the weld metal of Co-based alloy/AISI 410 stainless steel dissimilar welded joint

The Co-based alloy/AISI 410 stainless steel dissimilar welded joint was fabricated by the electron beam welding(EBW)technique.The anomalous microstructure containing the element transition zone(ETZ)and/or core of tail-like zone(CTLZ)is in the weld metal(WM)adjacent to the fusion line.The melting temperature difference between the WM and AISI 410 steel,melt stirring effect and element diffusion can trigger the formation of such anomalous microstructure.In particular,the larger distance of the region in WM away from the fusion line,the smaller CTLZ and larger ETZ occurred.Compared with the fine and ellipsoidal precipitates in the as-welded CTLZ,a large number of chain-type clustered precipitates were detected in the CTLZ and ETZ interface after the aging treatment at 566°C for 1000 h.The element diffusion under elevated temperature in WM is regarded as the crucial factor for such anomalous microstructure evolution during the aging treatment.

Ti-700合金显微结构的TEM研究

采用透射电子显微镜(TEM)研究了两相钛合金Ti-700在不同热处理制度下的显微组织和相结构。探讨了固溶温度,冷却方式以及时效处理对显微结构的影响。结果表明,在所选择的热处理制度下,合金为双态组织,除了基体(α+β)外,还发现了α2相以及调质结构。这些析出物对Ti-700钛合金的性能有一定影响。

钛合金电子束焊缝金属微观结构的TEM观察

本文对不同电子束扫描频率的 ＴＣ４ 钛合金焊缝金属微观结构进行了透射电镜观察。研究结果表明，电子束扫描频率对钛合金焊缝金属微观结构有很大影响。电子束扫描作用在焊接凝固前沿的动压力以及焊接凝固过程的稳定性的综合作用，直接影响 ＴＣ４

Microstructural Evolution of a Ni-base Alloy DZ468 Joint Bonded with a New Co-base Filler

Ni-base alloy DZ468 has been joined by transient liquid phase bonding technique with a newly developed Co-based filler. The microstructures of the Co-base filler and the joint, the effects of heat treatment on microstructure and hardness of the joint have been investigated by various experimental methods. Results show that the Co-base filler consists of γ, M2B, M5B3 and M23B6 phases. Because of the interdiffusion between the base metal and the filler, γ, MC, M5B3 and M23B6 phases are formed in the bonding zone. And localized liquidation of substrate occurs in the diffusion affected zone, with MC and M3B2 precipitating in this area. During heat treatment, the volume of the intermetallic phases in the bonding zone resulting from incomplete isothermal solidification decreases obviously. On the contrary, the width of the diffusion affected zone increases at the solution stage and subsequently decreases at the aging stages.

FORMATION OF SILICONIZED LAYER ON TiAl-BASED ALLOY AND THE OXIDATION RESISTANCE

The Ti-48Al alloy was pack siliconized with 15%Si+85%Al2O3. The microstructure of the siliconized coating on the TiAl-based alloy was analyzed and its effect on oxidation resistance was investigated. The specimens before and after cycle oxidation were examined by XRD and SEM equipped with XEDS. The results showed that the coating is composed of a thin Al2O3 outer layer and a composite inner layer of Ti5Si3 with an appropriate amount of Al2O3 dispersed in. Cycle oxidation tests showed that the high temperature oxidation resistance of TiAl-based alloy was greatly improved by forming such composite coating. No spaliation and crack happened and the weight gain was very small after cycle oxidation at 900℃ for 314h.

微量Ca添加对铸造Mg-Sm-Gd基合金显微组织与力学性能的影响

主要通过透射电子显微镜、高角度环形暗场扫描透射电子显微镜和拉伸测试研究微量Ca元素对Mg-Sm-Gd-Zn-Zr合金显微组织和力学性能的影响。结果表明,Mg-Sm-Gd-Zn-Zr合金中主要第二相为Zn元素富集的Mg_(3)RE相,Ca元素添加不仅可以细化晶粒,还可以促进Mg_(5)RE相和溶质原子富集的层错形成。此外,共格界面表明Mg_(3)RE相可以作为Mg_(5)RE相的形核位点,这些均有利于合金力学性能的提高。

Effects of samarium content on microstructure and mechanical properties of Mg–0.5Zn–0.5Zr alloy

Effects of samarium (Sm) content (0, 2.0, 3.5, 5.0, 6.5 wt%) on microstructure and mechanical proper-ties of Mg-0.5Zn-0.5 Zr alloy under as-cast and as-extruded states were thoroughly investigated. Results indicate that grains of the as-cast alloys are gradually refined as Sm content increases. The dominant intermetallic ph^se changes from Mg3Sm to Mg4iSm5 till Sm content exceeds 5.0 wt%. The dynami-cally precipitated intermetallic phase during hot-extrusion in dll Sm-containing alloys is Mg3Sm. The intermetallic particles induced by Sm addition could act as heterogeneous nucleation sites for dynamic recrystallization during hot extrusion. They promoted dynamic recrystallization via the particle stim-ulated nucleation mechanism, and resulted in weakening the basal texture in the as-extruded alloys. Sm addition can significantly enhance the strength of the as-extruded Mg-0.5Zn-0.5Zr alloy at room temperature, with the optimal dosage of 3.5 wt%. The optimal yield strength (YS) and ultimate tensile strength (UTS) are 368 MPa and 383 MPa, which were enhanced by approximately 23.1% and 20.8% com-pared with the Sm-free alloy, respectively. Based on microstructural analysis, the dominant strengthening mechanisms are revealed to be grain boundary strengthening and dispersion strengthening.

Creep-Induced Phase Instability and Microstructure Evolution of a Nearly Lamellar Ti–45Al–8.5Nb–(W, B, Y) Alloy

Microstructure degradation and stress-induced transformation of a high Nb-containing TiAl alloy with nearly lamellar microstructure during creep were investigated.Tensile creep experiments were performed at 800,850 and 900℃ under 150 MPa in air.Microstructures before and after creep tests were examined using scanning and transmission electron microscopy(SEM and TEM).Dislocations within the lamellar structure andβo(ω)region and twin intersection in massiveγgrains were investigated.Dislocation sliding played a critical role in the deformation ofωo phase,which preferentially occurred on the(0002)ωo plane.Possible deformation mechanisms were revealed.A stress-inducedγ→α2 phase transformation took place during the creep test at 850 and 900℃.α2 lamella could directly decompose into theωo phase at 850℃.The instability of high-temperature microstructure can weaken the creep resistance and promote the plastic deformation of the lamellar matrix,thus could be detrimental to the creep properties.The correlations between creep properties and microstructure instability were discussed.

定向凝固NiAl-Cr(Mo)-Hf合金微观结构的电子显微学研究

利用TEM研究了走向凝固方法制备的NiAl－28Cr－5．5Mo－0．5Hf（原子分数，％下同）共晶合金的微观组织与结构，以了解其复相强韧化机及其晶合金呈典型的胞状结构，并由层片状Cr（Mo）相，NiAl基体，不连续网状分布的Heusler相Ni2AlHf组成．NiAl和Cr（Mo）相存在固定的立方－立方取向关系，并共同沿（111）方向生长，两相间呈半共格界面，并存在三套规则的界面错配位错网．在Cr（Mo）相中，发现两种NiAl析出相，分别与Cr（Mo）相呈共格和半共格关系在NiAl基体中，也发现细小的Cr（Mo）相．Heusler相与NiAl或Cr（Mo）相不存在固定的取向关系，但在某些情况下发现二者也存在严格的立方－立方取向关系．

N36锆合金热挤压管坯显微组织研究

利用透射电镜(TEM)、X射线能谱仪(EDS)、扫描电镜(SEM)及其电子背散射衍射(EBSD)附件,研究N36锆合金热挤压管坯的显微组织。结果表明,N36合金热挤压管坯晶粒组织沿挤压方向被拉长而形成长条状组织,且长条状组织间有大量较小的再结晶晶粒及亚晶界。挤压坯组织中析出相主要是β-Zr及少量的β-Nb和Zr-Fe-Nb相,呈较为均匀弥散分布的特征。

新型TC21钛合金相变行为和相组成研究

采用X射线衍射(XRD)对TC21钛合金β热处理后,经不同冷却速率冷却及双重退火后的相结构进行了分析,并用扫描电镜(SEM)和透射电镜(TEM)对显微组织和相的形貌特征进行了观察研究。结果得出,TC21钛合金在β处理时经炉冷和水冷后,合金由α和β两相组成。空冷状态下,合金主相为α+β,并有少量Nb3Si相析出。实验证明,冷却速率主要影响α和β相含量的比例,相的组成基本不受冷却速率的影响。合金经970°,30min,空冷+600℃,4h退火后,仍由α相和β相组成,没有发现新的析出相,但观察到转变β相中存在次生析出的细小针状α相。

7050铝合金表面喷丸强化层内纳米结构微观组织结构分析

利用喷丸技术在7050铝合金表面制备了纳米结构表面层。采用EBSD、TEM和XRD等方法分析了表面纳米结构层的组织结构。试验结果表明，采用喷丸技术可以在7050铝合金表面制备出纳米结构表面层，表面层是由原始晶粒细化成由表及里梯度分布的不同取向纳米级结构构成，深度～150μm。7050铝合金表面纳米化使材料的表层残余应力变成拉应力，硬度明显降低，与基体相比，表层的硬度降低了～70％。

Co40NiCrMo合金强化机制

用电子拉伸试验机测定Co40NiCrMo高弹性合金固溶时效和冷拔时效样品的抗拉强度,用显微硬度计测定其硬度,用透射电子显微镜研究其固溶时效和冷拔时效的微观组织,并探讨该合金的强化机制。结果表明:冷拔引起的冷变形使Co40NiCrMo高弹性合金产生细小的形变孪晶,显著提高合金强度,并促使时效后的Cr、Mo、C等原子偏聚、形成铃木气团、有利于时效强化;但是未经冷变形,就没有这种孪晶,也不能时效强化。此外,孪晶形成的网络结构也可阻碍位错运动,从而提高合金强度。