Dendritic morphology evolution and microhardness enhancement of rapidly solidified Ni-based superalloys

The mechanisms of microstructure evolution and dendritic growth of Inconel alloys in a drop tube were investigated in this paper.The Vickers microhardness of the solidified alloy droplets was also measured to explore the effects of grain size and precipitated phase on the mechanical properties. From XRD results, the rapidly solidified Inconel 600, 617, 625, and 718 alloys are characterized by γ phase solid solution with fcc structure. Further analyses based on EDS and SEM reveal that Laves phase precipitates at the grain boundaries of γ phase in Inconel 718 alloy. With the decrease of droplet diameter(D), the dendritic morphology experiences a transformation of "orthogonal long dendrites→irregular stubby dendrites→equiaxed grains" in Inconel 600 alloy, "orthogonal long dendrites → stubby irregular dendrites" in Inconel 625 alloy and "orthogonal long dendrites→equiaxed grains" in Inconel 617 and 718 alloys. Although the dendrites become coarse locally, the dendritic grain size obviously reduces with the decrease of droplet diameter, leading to the increase of Vickers microhardness. For Inconel 718 alloy,the Vickers microhardness firstly decreases, when the droplet diameter decreases from 1000 to 900 μm, and then increases linearly with the decrease of droplet diameter. It is found that the Vickers microhardness of Inconel 625 alloy is the largest among the four alloys.     

Effect of homogenization treatment on microstructure and properties of Al-Mg-Mn-Sc-Zr alloy

The effect of homogenization on the hardness,tensile properties,electrical conductivity and microstructure of as-cast Al-6Mg-0.4Mn-0.25Sc-0.12Zr alloy was studied.The results show that during homogenization as-cast studied alloy has obviously hardening effect that is similar to aging hardening behavior in traditional Al alloys.The precipitates are mainly Al3(Sc,Zr)and Al6Mn.When homogenization temperature increases the hardness peak value is declined and the time corresponding to hardness peak value is shortened.The electrical conductivity of the alloy monotonously increases with increasing homogenization temperature and time.The decomposition of the supersaturated solid solution containing Sc and Zr which is formed during direct chilling casting and the precipitation of Al3(Sc,Zr)cause hardness increasing.The depletion of the matrix solid solubility decreases the ability of electron scattering in the alloy,resulting in the electrical conductivity increased.Tensile property result at hot rolling state shows that the optimal homogenization treatment processing is holding at 300-350 ℃ for 6-8 h.     

Effect of homogenization treatment on microstrucmre and properties of Al-Mg-Mn-Sc-Zr alloy

The effect of homogenization on the hardness, tensile properties, electrical conductivity and microstructure of as-cast Al-6Mg-0.4Mn-0.25Sc-0.12Zr alloy was studied. The results show that during homogenization as-cast studied alloy has obviously hardening effect that is similar to aging hardening behavior in traditional Al alloys. The precipitates are mainly Al3（Sc,Zr） and Al6Mn When homogenization temperature increases the hardness peak value is declined and the time corresponding to hardness peak value is shortened. The electrical conductivity of the alloy monotonously increases with increasing homogenization temperature and time. The decomposition of the supersaturated solid solution containing Sc and Zr which is formed during direct chilling casting and the precipitation of Al3（Sc, Zr） cause hardness increasing. The depletion of the matrix solid solubility decreases the ability of electron scattering in the alloy, resulting in the electrical conductivity increased. Tensile property result at hot rolling state shows that the optimal homogenization treatment processing is holding at 300-350℃ for 6-8 h.     

Evolution of carbides and carbon content in matrix of an ultra-high carbon sintered steel during heat treatment process

DTA, thermal expansion, XRD, and SEM were used to evaluate the effect of quenching temperature on the mechanical properties and microstructure of a novel sintered steel Fe-6Co-1Ni-5Cr-5Mo-1C. Lattice parameters and the mass fraction of carbon dissolved in the matrix of the steel quenched were investigated. It is discovered that the hardness of the steel increases with quench-ing temperature in the range of 840-900℃ and remains constant in the range of 900 to 1100℃. It decreases rapidly when the tem-perature is higher than 1100℃. The mass fraction of carbon dissolved in the matrix of the steel quenched at 840℃ is 0.38, but when the quenching temperature is increased to 1150℃, it increases to 0.98. The carbides formed during sintering are still present at grain boundaries and in the matrix of the steel quenched at low quenching temperatures, such as 840℃. When the quenching temperature is increased to 1150℃, most of the carbides at grain boundaries are dissolved with just a small amount of spherical M23C6 existing in the matrix of the quenched steel.     

Isothermal grain growth of reactive spray formed 7075 alloys in semi-solid state

The grain growth behavior in reactive spray formed 7075 2.91vol%TiC AI alloy was studied and compared with that of spray formed 7075 AI alloy at semi-solid state. The effects of in-situ TiC particles on the microstructure of spray formed 7075 AI alloy were also investigated. The specimens were heat-treated isothermally at various temperatures between the solidus and liquidus of 7075 A1 alloy for times in the range of 10-60 rain, then quenched in water. The microstructure of reheated specimens was characterized using scanning electron microscopy and optical microscopy. The grain size was measured using a mean linear intercept method. Results show that the in-situ TiC particles can effectively retard grain growth and refine the grain at a limited size. The grain growth exponent in Arrhenius equation increases from 2 to 3, which indicates that the in-situ TiC particles have the significant pinning effect on grain coarsening in the semi-solid state.     

Origin of the pre-peak in the structure factor of liquid Fe-4.30C-0.21Ce alloy

The liquid structure of Fe-4.30C and Fe-4.30C-0.21Ce alloys was studied by high temperature X-ray diffractometer. The results show that for Fe-C alloy the nearest neighbor distance of the eutectic alloy is 0.259-0.260 nm at the temperature range of 1200-1400℃, which increases to 0.269-0.271 nm with the addition of 0.21% (mass fraction) Ce in the Fe-C alloy at the same temperature range. There is a pre-peak at Q = 15.5 nm-1 on the original intensity curve and structure factor S(Q) of the liquid Fe-4.30C-0.21Ce alloy, which was caused by the Ce atoms in the C-Ce clusters. Combined with the shared face, the tetragonal structure can meet the requirement for the distance of Ce-Ce atoms. It also shows that the cluster size in the liquid Fe-4.30C-0.21Ce alloy increases with the decreasing temperature.     

Evolution of microstructure and high temperature tensile properties of as-extruded Ti Bw reinforced near-α titanium matrix composite subjected to heat treatments

The Ti Bw reinforced near-α titanium matrix composite(Ti-5.8 Al-3.4 Zr-4.0 Sn-0.4 Mo-0.4 Nb-0.4 Si-0.06 C) was successfully synthesized by powder metallurgy and hot extrusion route. The effects of solution and aging temperature on the microstructure and high temperature tensile properties of the composite were investigated. The results revealed that the fine transformed β phase can be obtained by the solution treatment at β phase region and aging treatment, no other precipitates were observed. The α2 phase(Ti3 Al) can be acquired when the solution treated at α+β phase region followed by aging treatment. With increasing the aging temperature from 500 to 700°C for 5 h, the size of α2 precipitates increases from about 5 to about 30 nm. The Ti Bw are stable without any interfacial reaction during the heat treatments. The high temperature tensile properties show that the composite performed by solution and aging treatment exhibits good strengthening effects. With increasing the aging temperature from 500 to 700°C, the strength of the composite increases at the expense of elongation due to the increment of α2 precipitates.The strength of the composite at 600°C increases by 17% to 986 MPa after 1000°C/2 h/AC and 700°C/5 h/AC heat treatment.     

Microstructure and mechanical properties of AZ91 magnesium alloy subject to deep cryogenic treatments

AZ91 magnesium alloy was subjected to a deep cryogenic treatment. X-ray diffraction （XRD）, scanning electronic microscopy （SEM）, and transmission electronic microscopy （TEM） methods were utilized to characterize the composition and microstructure of the treated samples. The results show that after two cryogenic treatments, the quantity of the precipitate hardening β phase increases, and the sizes of the precipitates are refined from 8-10μm to 2-4μm. This is expected to be due to the decreased solubility of aluminum in the matrix at low temperature and the significant plastic deformation owing to internal differences in thermal contraction between phases and grains. The polycrystalline matrix is also noticeably refined, with the sizes of the subsequent nanocrystalline grains in the range of 50-100 nm. High density dislocations are observed to pile up at the grain boundaries, inducing the dynamic recrystallization of the microstructure, leading to the generation of a nanocrystalline grain structure. After two deep cryogenic treatments, the tensile strength and elongation are found to be substantially increased, rising from 243 MPa and 4.4% of as-cast state to 299 MPa and 5.1%.     

Effect of one-step aging on microstructure and properties of a novel Al-Zn-Mg-Cu-Zr alloy

The effect of one-step aging temper on the mechanical properties, electrical conductivity and the microstructure of a novel Al-7.5Zn-1.6Mg-1.4Cu-0.12Zr alloy has been investigated. The results indicated that with elevating the aging temperature from 100℃ to 160℃, the aging response rate was greatly accelerated, and the UTS at peak aging condition decreased, while the corresponding TYS increased. However, the electrical conductivity of the alloy became higher. After aging for 24 h at 120℃, the peak UTS and TYS values were achieved as 591 MPa and 541 MPa, respectively; but the alloy achieved a lower conductivity, 20.4 MS/m. When T6 temper was performed at 140℃ for 14 h, the UTS decreased only by 1% of the former, whereas the TYS and the electrical conductivity increased obviously, which were up to 559 MPa and 22.6 MS/m, respectively. The major strengthening precipitates of the peak-aged alloy were GP zones and η′ phase. The precipitates in both the matrix and the grain boundary became coarser with rising aging temperature. There were obvious PFZs along the grain boundary both in T6 conditions aged at 140℃ and 160℃.     

Mechanism and Inhibition of Grain Coarsening of Al-Mg-Si Alloy in Hot Forming

A high-Ti 6061 alloy was rolled with strains up to 0.8-2.0 and at 350-550 ℃. Microstructures that developed during deformation and subsequent solution heat treatment (SHT) were observed by using optical and transmission electron microscopy. Microstructure evolution during SHT depends mainly on the initial rolling temperature, and it was found that the higher this temperature is, the coarser the grains are. After rolling at 400 ℃, well-defined cells and subgrains were formed, which induced further sites for recrystallization nucleation during subsequent SHT. The recrystallization mechanism was found to be subgrain rotation, with a final grain size smaller than 200 μm. Increasing the rolling temperature to 500 ℃ results in a low density of dislocations distributed uniformly in the deformed matrix and fewer nucleation sites during subsequent SHT. The recrystallization mechanism is grain boundary bulging, while the final grain size approaches several millimeters. Finally, a hot forming process of high-Ti 6061 alloy for inhibiting grain coarsening was proposed, and verified by experiments.     

轧制对AZ31/AZ91焊接接头组织及扩散的影响

为了理解异种镁合金焊接接头显微组织的变化规律,采用TIG焊接方法制备了AZ31/AZ91焊接接头,研究了轧制变形及热处理对AZ31/AZ91焊接接头显微组织及扩散系数的影响.结果表明,随着轧制变形量的增加,AZ31/AZ91熔合区的晶粒细化程度增大,但在随后的420℃×8 h固溶处理过程中,由于再结晶充分,因此晶粒逐渐变大.在420℃×2 h固溶处理过程中,Al的扩散系数随轧制变形量的增加而增大;在420℃×8 h固溶处理条件下,Al的扩散系数随轧制变形量的增加而减小.相同轧制变形量下,Al的扩散系数随着保温时间的增加而减小.     

Retrogression characteristics of a novel Al-Cu-Li-X alloy

Retrogression characteristics of a novel Al-Cu-Li-X alloy of 2A97 were studied by hardness testing, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The retrogression treatments of aging at 155°C for 12 h followed by aging at 220 and 240°C were chosen by determining the peak temperature of δ' precipitation at 230°C by DSC. The retrogression treatment at a lower temperature of 220°C causes the precipitation and coarsening of δ' and θ' phases in the matrix, resulting in an ...     

挤压温度对高纯铜组织演变规律的影响

为了研究挤压温度对高纯铜微观组织和变形行为的影响规律,通过反向挤压方式对高纯铜进行不同挤压温度下的挤压实验并观察了其显微组织.结果表明,随着挤压温度的升高,高纯铜的晶粒尺寸增大.当挤压温度为650 ℃时,挤压棒材的平均晶粒尺寸为36 μm;当挤压温度升至800 ℃时,挤压棒材的平均晶粒尺寸为51 μm.随着变形量的增加,当挤压温度为650~700 ℃时,压余变形区的平均晶粒尺寸趋向于由60 μm变为45 μm;当挤压温度为750~800 ℃时,平均晶粒尺寸则趋向于由90 μm变为75 μm.800 ℃挤压变形后晶粒内部出现大量以Σ 3特殊晶界为孪晶界的<111>60°退火孪晶.     

Ti-6Al-2Zr-15Ni合金固溶过程中Ti2Ni相的形貌变化

利用光镜、扫描电镜、透射电镜和X射线衍射术观察和分析了Ti-6Al-2Zr-15Ni合金在不同温度固溶处理后的显微组织．该合金的铸态组织由枝晶和枝晶间共晶组成,枝晶为Ti2Ni纤维和仅基体组成的共析组织,枝晶间共晶由棒状α相和Ti2Ni基体组成．固溶处理时Ti2Ni相的形貌和尺寸发生了明显变化．850℃以下固溶,随固溶温度的升高,纤维状的Ti2Ni发生了熔断,粒化和粗化等过程．900℃固溶时,枝晶内仪相转变为β相,Ti2Ni溶解,枝晶间α/Ti：Ni共晶组织转变为半连续Ti2Ni相分布于晶界．随温度的升高,晶界Ti2Ni球化,并在960℃时发生多边化,970℃时多边形Ti2Ni的棱角钝化．     

时效态Al-Zn-Mg-Cu-Zr-Sc合金的组织与疲劳性能

为了研究时效处理对Al-7.2Zn-2.5Mg-1.5Cu-0.08Zr-0.12Sc合金的组织与疲劳性能的影响,利用透射电子显微镜对合金的显微组织进行了观察分析,并针对不同时效状态的合金进行了低周疲劳实验.结果表明,经过150℃×6 h时效处理后,合金晶内析出相较少,晶界无析出相;经过150℃×36 h时效处理后,合金晶内析出相较为细小,并呈弥散分布,同时晶界析出断续分布的平衡相,并存在晶界无析出带;经过150℃×48 h时效处理后,合金的析出相均已长大,且晶界无析出带发生宽化.经过150℃×36 h时效处理后的合金,表现出了较高的循环变形抗力与较长的低周疲劳寿命;不同时效状态合金的塑性应变幅、弹性应变幅与载荷反向周次之间,以及循环应力幅与塑性应变幅之间均呈线性关系.     

-βSiC纳米颗粒对Ni-Co共电沉积合金结构和性能的影响

采用复合电沉积方法在Ni基体上制备了(Ni-Co)-SiC纳米复合镀层,该镀层的组织结构与电沉积Ni-Co合金(晶粒尺寸:110 nm)镀层不同,由Ni-Co固溶体基体(晶粒尺寸:50 nm)和均匀分布其中的SiC纳米颗粒(晶粒尺寸:45 nm)组成。SiC纳米颗粒的加入大大细化了镀层的组织,增加了镀层的显微硬度.电化学实验表明:SiC纳米颗粒使金属还原电位发生正移,降低了Ni-Co合金镀液的极化作用.     

退火温度对7022铝合金干摩擦性能的影响

对7022铝合金的不同温度退火试样进行干滑动摩擦磨损试验,用扫描电镜、显微硬度测试仪和三维形貌仪分析各试样的磨损机制.结果表明,退火温度对材料的显微硬度和摩擦磨损性能有明显影响,退火温度在200℃时,材料显微硬度和摩擦磨损性能最好,此温度下材料得到完全再结晶,且晶粒细化;摩擦磨损性能随着显微硬度的提高而减小.塑变磨损、磨粒磨损和疲劳磨损为7022铝合金的主要磨损机理.     

Mechanical properties and microstructure changes after long-term aging at 700℃ for a nickel-base superalloy

Mechanical properties and microstructure changes have been investigated on a new nickel-base superalloy after long-term aging at 700℃ It is found that the major precipitates of the tested alloy are MC, M23C6, M6C and in the course of long-term aging at 700℃. The carbides maintain good thermal stability with the aging time up to 5008 h. The growth rate of gamma prime precipitates is relatively high in the early aging period and then slows down. The coarsening behavior of gamma prime follows a diffusion-controlled growth procedure. The room temperature Rockwell hardness of the alloy aged at 700C increases slightly at the initial stage of aging, but it decreases with the prolonged time. It mainly depends on the size of gamma prime. In comparison with Nimonic     

Al-Zn-Mg-Sc合金连续冷却转变曲线的测定

测定了Al-Zn-Mg-Sc合金固溶处理后的连续冷却转变（CCT）曲线,通过动态电阻法测得冷却过程的电阻-温度曲线,根据曲线斜率的变化规律确定相变开始点、结束点以及临界冷却速度范围,绘制出该合金的CCT图,通过扫描电镜和透射电镜分析观察连续冷却过程的组织转变.结果表明,动态电阻法测得的CCT图是可信的;在470℃,保温1 h固溶处理后,抑制相变发生的临界冷速在2 168.0℃/min以下,但高于716.8℃/min,相变主要集中在150~420℃的温度区间发生;当冷却速度较慢时,平衡相η在晶内和晶界大量析出并逐渐长大和粗化,当冷却速度较快时,合金保持了较高的过饱和度,冷却到70℃以下仍有相变发生.     

Grain boundary characteristics and tensile properties of Ti14 alloy after semi-solid deformation

The microsuucture and room-temperature tensile properties of Til4,a new α+Ti2Cu alloy,were investigated after conventional forging at 950℃ and semi-solid forging at 1000 and 1050℃,respectively.Results show that coarse grains and grain boundaries are obtained in the semi-solid alloys.The coarse grain boundaries are attributed to Ti2Cu phase precipitations occurred on the grain boundaries during the solidification.It is found that more Ti2Cu phase precipitates on the grain boundaries at a higher semi-solid forging temperature,which forms precipitated zones and coarsens the grain boundaries.Tensile tests exhibit high strength and low ductility for the semi-solid forged alloys,especially after forging at 1000℃.Fracture analysis reveals the evidence of ductile failure mechanisms for the conventional forged alloy and cleavage fiacture mechanisms for the alloy after semi-solid forging at 1050℃.