Microstructure and mechanical properties of spark plasma sintered Ti-Mo alloys for dental applications

Ti-Mo alloys with various Mo contents from 6wt% to 14wt% were processed by spark plasma sintering based on elemental powders. The influence of sintering temperature and Mo content on the microstructure and mechanical properties of the resulting alloys were investigated. For each Mo concentration, the optimum sintering temperature was determined, resulting in a fully dense and uniform microstructure of the alloy. The optimized sintering temperature gradually increases in the range of 1100–1300℃ with the increase in Mo content. The microstructure of the Ti-(6–12)Mo alloy consists of acicular α phase surrounded by equiaxed grains of β phase, while the Ti-14Mo alloy only contains single β phase. A small amount of fine α lath precipitated from β phase contributes to the improvement in strength and hardness of the alloys. Under the sintering condition at 1250℃, the Ti-12Mo alloy is found to possess superior mechanical properties with the Vickers hardness of Hv 472, the compressive yield strength of 2182 MPa, the compression rate of 32.7%, and the elastic modulus of 72.1 GPa. These results demonstrate that Ti-Mo alloys fabricated via spark plasma sintering are indeed a perspective candidate alloy for dental applications.     

Microstructure evolution and mechanical properties of a large-sized ingot of Mg-9Gd-3Y-1.5Zn-0.5Zr (wt%) alloy after a lower-temperature homogenization treatment

In this paper, a large-sized ingot of Mg-9Gd-3Y-1.5Zn-0.5Zr (wt%) alloy with a diameter of 600 mm was successfully prepared by the semi-continuous casting method. The alloy was subsequently annealed at a relatively low temperature of 430℃ for 12 h as a homogenization treatment. The microstructure and room-temperature mechanical properties of the alloy were investigated systematically. The results show that the as-cast alloy contained a mass of discontinuous lamellar-shaped 18R long-period stacking ordered (LPSO) phases with a composition of Mg₁₀ZnY and an α-Mg matrix, along with net-shaped Mg₅(Y,Gd) eutectic compounds at the grain boundaries. Most of the eutectic compounds dissolved after the homogenization treatment. Moreover, the amount and dimensions of the lamellar-shaped LPSO phase obviously increased after the homogenization treatment. The structure of the phase transformed into 14H-type LPSO with composition Mg₁₂Zn(Y,Gd). The mechanical properties of the heat-treated large-sized alloy ingot are uniform. The ultimate tensile strength (UTS) and tensile yield strength (TYS) of the alloy reached 207.2 MPa and 134.8 MPa, respectively, and the elongation was 3.4%. The high performances of the large-sized alloy ingot after the homogenization treatment is attributed to the strengthening of the α-Mg solid solution and to the plentiful LPSO phase distributed over the α-Mg matrix.     

均匀化处理温度对6082铝合金组织和性能的影响

采用硬度、电导率测试、金相显微镜、X线衍射、扫描电镜、透射电镜和能谱分析技术,研究均匀化温度对合金组织和性能的影响.研究结果表明:铸态合金由α-Al固溶体和非平衡共晶相组成;490～510℃均匀化,Mg2Si相从过饱和固溶体中析出,在510℃以上均匀化,随着温度的升高,Mg2Si又逐步回溶到基体中,560℃均匀化,Mg2Si相和过剩单质Si完全溶解;随着均匀化温度的升高,非平衡析出物鱼骨状共晶形态逐渐消失,针状β-AlMnFeSi溶解、断裂,转变为具有更高(Mn+Fe)/Si比值颗粒状α-Al(MnFe)Si相,析出相在高温均匀化过程中聚集、球化;560℃均匀化,析出物的连续网状结构转变成链状结构,析出物演化为等轴粒状α-Al(MnFe)Si相.均匀化过程中合金中析出弥散α-Al(MnFe)Si相;在490～560℃保温6h均匀化处理,温度升高,合金的硬度和电导率分别升高和降低.     

Effects of Ru on the microstructure and phase stability of a single crystal superalloy

Two experimental single crystal superalloys, the Ru-free alloy and the Ru-containing alloy with[001] orientation, were cast in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Ru on the microstructure and phase stability of the single crystal superalloy were investigated. γ' directional coarsening and rafting were observed in the Ru-free alloy and Ru-containing alloy after long-term aging at 1070℃ for 800 h. Needle-shaped σ topologically close packed (TCP) phases precipitated and grew along the fixed direction in both the alloys. The precipitating rate and volume fraction of TCP phases decreased significantly by adding Ru. The compositions of γ and γ' phases measured using an energy-dispersive X-ray spectroscope (EDS) in transmission electron microscopy (TEM) analysis showed that the addition of Ru lessened the partition ratio of TCP forming elements, Re, W and Mo, and decreased the saturation degrees of these elements in γ phase, which can enable the Ru-containing alloy to be more resistant to the formation of TCP phases. It is indicated that the addition of Ru to the Ni-based single crystal superalloy with high content of the refractory alloying element can enhance phase stability.     

Effects of Ru on microstructure stability and mechanical properties of Ni_3Al single crystal alloy

The effect of Ru on microstructure stability and stress rupture properties of a Ni_3Al single-crystal alloy was investigated. The experimental results showed that the addition of 2%Ru(mass fraction) improved the microstructure stability due to the restraint of harmful Y-NiMo phase formation during the thermal exposure at the high temperature above 1 000℃.And the reason may be that the addition of Ru increased the degree of Mo supersaturation in bothγandγ' phases,and hence suppressed the precipitation of ...     

Mg-6Zn-(Al,Ca)镁合金显微组织及力学性能

利用WD-10A电子拉伸试验机和RD2-3型蠕变试验机对Mg-6Zn-(Al,Ca)系列镁合金的力学拉伸强度及高温蠕变性能进行测试,研究其显微组织对高温性能的影响,并利用金相显微镜、扫描电镜、X射线衍射分析等方法进行分析表征.研究结果表明,Mg-6Zn-2Al-0.3Mn(ZA62)具有较好的综合力学性能,其铸态显微组织主要由α-Mg基体和致密片状Mg51Zn20共晶相组成.少量Ca加入ZA62合金后,抑制了Mg51Zn20相的析出,并代之形成了含Ca的MgZn相和τ相.随Ca加入量增加,晶间相的数量增加,合金组织中出现另一热稳定四元Mg-Zn-Al-Ca化合物相.当w(Ca)>0.5%时,合金晶粒显著细化.随Ca含量增加,合金常温拉伸强度和塑性呈下降趋势,基体显微硬度减小.加入Ca提高了合金高温拉伸强度,改善了合金蠕变性能.在175℃/70 MPa条件下,合金蠕变性能受合金晶粒尺寸影响,随晶粒尺寸的减小,蠕变变形量增加.     

Deformation and damage behaviors of as-cast TiAl-Nb alloy during creep

By means of creep properties measurement and microstructure observation,the deformation and damage behavior of an as-cast TiAl-Nb alloy during creep at temperature near 750°C were investigated.The results showed that the microstructure of the alloy consisted of lamellarγ/α_2 phase,and the boundaries consisted ofγphase located in between lamellarγ/α_2 phases with different orientations.In the latter stage of creep,the dislocation networks appeared in the interfaces of lamellarγ/α_2 phases due to the coarsening of them,which made the coherent interface transforming into the semi-coherent one for reducing its adhesive strength.The deformation mechanism of the alloy during creep was twinning and dislocations slipping within lamellarγ/α_2 phases.In the later period of creep,significant amount of dislocations plied up in the interfaces of lamellarγ/α_2 phases,which may cause the stress concentration to promote the initiation and propagation of the cracks along the lamellarγ/α_2interfaces perpendicular to the stress axis.Wherein,some cracks on the various cross-sections were connected by tearing edge along the direction of maximum shear stress,up to the creep fracture,which is considered to be the damage and fracture mechanism of alloy during creep at 750°C.     

Mössbauer and electron microscopy study of martensitic transformations in an Fe-Mn-Mo alloy

The kinetic, morphological, crystallographic, and magnetic characteristics of thermally induced martensites in Fe-13.4wt% Mn-5.2wt% Mo alloy were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Mössbauer spectroscopy. The experimental results reveal that two types of thermal-induced martensites, ? (hcp) and α′ (bcc) martensites, are formed in the as-quenched condition, and these transformations have athermal characters. Mo addition to the Fe-Mn alloy does not change the coexistence of ? and α′ martensites with the Mn content between 10wt% and 15wt%. Besides, Mössbauer spectra reveal a paramagnetic character with a singlet for the γ (fcc) austenite and ? martensite phases and a ferromagnetic character with a broad sextet for the α′ martensite phase. The volume fraction of α′ martensite forming in the quenched alloy is much more than that of the ? martensite.     

Effects of P addition on the microstructure and mechanical properties of Mg–5%Sn–1.25%Si alloy

The effects of Al-P addition on the microstructure and mechanical properties of as-cast Mg–5%Sn–1.25%Si magnesium alloy were investigated. The results show that the phases of the as-cast alloy are composed of α-Mg, Mg2 Sn, Mg2 Si, little P, and AlP. The Chinese character shape Mg2 Si phase changes into a granular morphology by P addition because AlP can act as a heterogeneous nucleation core for the Mg2 Si phase. When 0.225wt% of Al–3.5%P alloy is added, the mechanical properties of the Mg–5%Sn–1.25%Si alloy are greatly improved, and the tensile strength increases from 156 to 191 MPa, an increase of 22.4% compared to the alloy without P addition. When the amount of Al–3.5%P reaches 0.300wt%, a segregation phenomenon occurs in the granular Mg2 Si phase, and the tensile strength and hardness decrease though the elongation increases.     

Ni3Ta合金的微观组织结构和相变特性

采用光学显微镜、背散射电子图像、X射线衍射、电子探针、差示扫描量热法和透射电子显微镜研究了Ni2Ta合金的微观组织结构和相变特性.结果表明:Ni2Ta合金在经过1 200 C保温4h的热处理后,主要由大量Ni2Ta相和少量Ni2Ta析出相组成,其中Ni2Ta相有单斜和四方两种结构.单斜Ni2Ta相为典型的细小板条状马氏体形状,其板条宽度为0.1～0.3μm,且存在以(001)晶面为孪品面的典型孪品结构.Ni3Ta合金在升温和降温过程中存在单斜Ni2Ta相和四方Ni3Ta相的可逆相变,相变开始温度分别约为310和245℃.另外,在升温过程中还存在单斜Ni3Ta相向正交Ni3Ta相的转变,其相变开始温度约为310℃,但降温过程中并不存在由正交Ni3Ta相到单斜Ni3Ta相的逆转变.     

Effects of Mo addition on microstructure of a 4th generation Ni-based single crystal superalloy

The effects of Mo addition on the microstructure of a 4th generation Ni-based single crystal superalloy were investigated. Mo addition significantly promoted elements Mo, W and Re partition into γ phase and enhanced absolute lattice misfit at 1100 ?°C. The increase of Mo concentration from 2 ?wt% to 4 ?wt% also decreased the content of eutectic islands and the segregation ratios of other alloying elements in the as-cast state, especially Re and W. After heat treatments, the size of γ′ phase and width of γ channels decreased slightly with higher Mo content. More Mo additions slightly enhanced the segregation behavior of Re while reducing the segregation behavior of Mo. However, it revealed that the primary and secondary dendrite arm spacings were barely affected by Mo addition.     

高速钢相分析的计算机模拟

阐述了利用平衡计算对合金进行相分析计算机模拟的依据。对W6Mo5Cr4V2,W10Mo4Cr4V3Al和Mo5Cr4V3种高速工具钢在淬火加热态和退火加热态的组织进行了相分析，其结果表达了这3种钢在上述状态下的相组成及其变化规律。     

700℃以上超超临界电站锅炉过热器管材用典型镍基合金的平衡析出相规律

利用热力学计算软件Thermo--Calc及镍基合金数据库,计算了三种700℃以上超超临界电站用过热器管道材料Inconel740、Inconel617和GH2984合金的热力学平衡相图,并对比了三种材料主要析出相的析出行为.计算结果表明:三种合金主要的析出相包括γ、γ'、碳化物、σ、η、δ、μ及α--Cr等,凝固过程中Mo、Nb和Ti元素偏析严重,会降低合金的初熔点,因此后期均匀化退火处理十分重要.另一方面,750℃时Inconel740合金γ'相析出量大于另外两种合金,并且Al和Ti含量对γ'相和η相析出行为有较大影响.碳化物的计算表明,Inconel617合金一次碳化物与另两种合金不同,并且其二次碳化物的析出温度范围最大.GH2984合金中Fe含量较大时会导致σ相出现,对合金的性能产生不利影响.     

Microstructure and stress rupture properties of polycrystal and directionally solidified castings of nickel-based superalloys

A new directionally solidified Ni-based superalloy DZ24, which is a modification of K24 alloy without rare and expensive elemental additions, such as Ta and Hf, was studied in this paper. The microstructure and stress rupture properties of conventionally cast and directionally solidified superalloys were comparatively analyzed. It is indicated that the microstructure of K24 alloy is composed of γ, γ′, γ/γ′ eutectics and MC carbides. Compared with the microstructure of K24 polycrystalline alloy, γ/γ′ eutectic completely dissolves into the γ matrix, the fine and regular γ′ phase reprecipitates, and MC carbides decompose to M₆C/M₂₃C₆ carbides after heat treatment in DZ24 alloy. The rupture life of DZ24 alloy is two times longer than that of K24 alloy. The more homogeneous the size of γ′ precipitate, the longer the rupture life. The coarsening and rafting behaviors of γ′ precipitates are observed in DZ24 alloy after the stress-rupture test.     

Microstructure and improved mechanical properties of ultrafine-grained Mo-12Si-8.5B (at.%) alloys with addition of ZrB2

Multi-hierarchical Mo-12Si-8.5B-x Zr B_2(x=0,0.5,1.0,1.5,2.5 wt%)alloys consisting of three ultrafine-grained(UFG,0.47–0.81μm)phases of Mo_5Si B_2(T2),Mo_3Si and Mo solid solution(α-Mo)were prepared by mechanical alloying following hot pressing.Microstructure observations showed that the intermetallic phases(Mo_3Si and T2)distributed dispersedly in the continuousα-Mo matrix associated with the homogeneously embedded nanoscaled particles(10–225 nm)in the grain interiors and at the grain boundaries.The Mo-12Si-8.5B-x Zr B_2alloys exhibited monotonically increasing compressive strength to 3.13 GPa with increasing content of Zr B_2,and the fracture toughness increased about 27%and reached at 11.5 MPa m~(1/2)at 1.0 wt%Zr B_2,rendering the Mo-12Si-8.5B-1.0 wt%Zr B_2alloy possessing the best combined mechanical properties of high strength and high toughness.The underlying reason for the superior mechanical properties of the Mo-12Si-8.5B-x Zr B_2alloys is that the dispersedly distributed nanosized particles in the UFG multi-phased-matrix can not only effectively block the dislocation motion to increase the strength but also store the dislocations to increase the strain hardening ability during mechanical deformation.     

时效处理对2205 DSS组织及力学性能的影响

首先对2205 DSS进行了1 100℃固溶处理,随后将试样分别在650,700,750,800,850和900℃下进行不同时间的时效处理,探究2205 DSS中σ相的析出规律及其对材料组织和力学性能的影响.研究结果表明:2205 DSS中σ相的析出分为有碳化物伴随和无碳化物伴随两种方式,前者发生在α-γ相界上,后者则主要发生在α相的晶内和晶界;2205 DSS在850℃时效时σ相的析出行为最严重;在析出σ相后,合金元素Cr和Mo在各相中会发生不同程度的偏聚;2205 DSS中析出少量的σ相对材料的塑性影响不大,但会显著降低材料的冲击韧性,而σ相的大量析出则会使两者均发生严重恶化;σ相的析出对材料的屈服强度影响不大,对材料的抗拉强度有略微的提高作用.     

挤压比对低温挤压ZA15锌合金组织和力学性能的影响

利用拉伸试验和扫描电镜,研究了在150℃,挤压比对反向挤压ZA15锌合金的微观组织和力学性能的影响.结果表明:随着挤压比的增加,ZA15锌合金室温抗拉强度有所提高,但都在150 MPa以下.其伸长率在160%~180%,具有室温超塑性.这主要是由于均匀化后形成的(α+η)片层共析组织经塑性变形后转变成以η相为基体,α相呈粒状弥散分布组织.这意味着采用低温常规挤压制备ZA15锌合金即可获得室温超塑性,同时,其力学性能也能够满足热喷涂ZA15锌合金线材的新标准要求.     

Effect of thermal exposure on the stress-rupture life and microstructure of a low Re-containing single crystal alloy

In this paper, the stress-rupture tests of a low Re-containing single crystal alloy IC21 before and after thermal exposure at 1100 ℃for various periods of time were conducted under the test condition of 1100 ℃/137 MPa, and the microstructure of the tested specimens was characterized by SEM and TEM. The experimental results showed that the stress rupture life of this alloy was over 150 h after the standard heat treatment of1320 ℃, 10 h/AC t 870, 32 h/AC, however the stress rupture life decreased with the increase of exposure time due to the microstructure degradation. The TEM analysis revealed that the interface mismatch dislocation networks were well established. It was observed that these mismatch networks could form at 1100 ℃ even after thermal exposure for 1 h without the external stress, which is quite different from that in the traditional single crystal superalloys.     

Microstructure and mechanical properties of Nb–Mo–ZrB2 composites prepared by hot-pressing sintering

Nb–Mo–ZrB₂ composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB₂ were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB₂ composites increases with increasing ZrB₂ content; Nb–Mo–30vol%ZrB₂ had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB₂ composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB₂ content. Finally, the fracture modes of the Nb–Mo–ZrB₂ composites are also discussed in detail.     

Influence of hot-rolling on the microstructure and mechanical properties of a near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

In this work, the 90° clock rolling and the uni-directionally rolling processes at high temperature were carried out on the near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr titanium alloy cutting from an ingot, respectively. The corresponding microstructures were quantitatively characterized, and its effect on the dynamic mechanical properties and fracture mechanism were emphatically investigated. It was found that after 90° clock rolling, the microstructure composed of equiaxed primary α phase(α_p) with an average size of about 2 ?μm and the β transformed regions containing the acicular secondary α phase(α_s) with an average thickness of about 50 ?nm and the separated β phase was obtained. However, in the uni-directionally rolled titanium alloy, no acicular α_s was observed, and the corresponding microstructure consisted elongated lamellar α phase (average thickness: about 1.3 ?μm), few equiaxed α phase (average grain size: about 300 ?nm) and the inlaid β phase. The microstructural difference of the hot-rolled titanium alloys was closely related to the deformation process. Moreover, a great number of α_p and α_s in the 90° clock rolled titanium alloy effectively enhanced the strength, and the dynamic compressive strength reached to 1730 ?MPa. Furthermore, equiaxed α_p was conducive to the homogeneous deformation, which counteracted the localized deformation caused by acicular α_s to a certain extent and made the 90° clock rolled titanium alloy exhibit an acceptable critical fracture strain of about 10.5%. Moreover, the fracture microstructures showed that the main failure mode of the 90° clock rolled and the uni-directionally rolled titanium alloy were ductile fracture and brittle fracture, respectively.