一种含硼第三代镍基单晶高温合金的初熔及固溶处理研究

采用电子探针测试结合Thermo-Calc软件计算,研究了一种含0.001 wt%硼第三代单晶高温合金枝晶间区域的初熔过程。结果表明,初熔相为M2B,初熔温度为1265 ℃,远低于用差示扫描量热仪测得的共晶熔化温度1352 ℃。采用扫描电镜分析固溶处理过程中合金组织的演化过程,发现共晶组织中马赛克状共晶和扇状共晶率先溶解,粗大γ′最后溶解。初熔区域空冷可发生均质形核,重新凝固后的共晶组织呈圆形,初熔组织中粗大γ′组织较初始凝固的粗大γ′组织更细,而相比残余共晶组织,初熔组织中含有马赛克状共晶和扇状共晶。在通用分步固溶处理制度的基础上,通过低于硼化物初熔的首步处理和适当提高最终固溶温度（ST2）,可以有效避免初熔并完全消除共晶。     

一种含硼第三代镍基单晶高温合金的初熔及固溶处理

采用电子探针测试结合Thermo-Calc软件计算,研究了一种含0.001%(质量分数)硼第三代单晶高温合金枝晶间区域的初熔过程。结果表明,初熔相为M_2B,初熔温度为1265℃,远低于用差示扫描量热仪测得的共晶熔化温度1352℃。采用扫描电镜分析固溶处理过程中合金组织的演化过程,发现共晶组织中马赛克状共晶和扇状共晶率先溶解,粗大γ′最后溶解。初熔区域空冷可发生均质形核,重新凝固后的共晶组织呈圆形,初熔组织中粗大γ′组织较初始凝固的粗大γ′组织更细,而相比残余共晶组织,初熔组织中含有马赛克状共晶和扇状共晶。在通用分步固溶处理制度的基础上,通过低于硼化物初熔的首步处理和适当提高最终固溶温度(ST2),可以有效避免初熔并完全消除共晶。     

FACTORS INFLUENCED INCIPIENT MELTING IN Hf-BEARING DS Ni-BASE SUPERALLOYS

含Hf的镍基高温合金存在Ni_5Hf相,它是影响合金初熔的主要因素之一。当该相的量大于1.0v-％时,在加热示差热分析(DTA)曲线上的1135-1160℃处出现Ni_5Hf相的熔化峰。P.D法定向铸锭上、下端存在明显的Ni_5Hf偏析,因而对初熔的敏感性明显不同。 Ni_5Hf相的初熔促进了共晶γ′和M_3B_2的熔化。低碳与无碳含Hf合金的初熔倾向更大,缓冷组织和固溶处理的升温速度对初熔是敏感的。 高于3V-％的初熔量将会降低合金的持久强度,其断裂主要沿Ni_5Hf初熔区和共晶γ′之间的界面扩展。采用1150℃8h预处理能消除全部的Ni_5Hf相,有效地控制含Hf合金的初熔。     

影响含Hf定向凝固合金初熔的某些因素

含Hf的镍基高温合金存在Ni_5Hf相,它是影响合金初熔的主要因素之一。当该相的量大于1.0v-%时,在加热示差热分析(DTA)曲线上的1135-1160℃处出现Ni_5Hf相的熔化峰。P.D法定向铸锭上、下端存在明显的Ni_5Hf偏析,因而对初熔的敏感性明显不同。Ni_5Hf相的初熔促进了共晶γ′和M_3B_2的熔化。低碳与无碳含Hf合金的初熔倾向更大,缓冷组织和固溶处理的升温速度对初熔是敏感的。高于3V-%的初熔量将会降低合金的持久强度,其断裂主要沿Ni_5Hf初熔区和共晶γ′之间的界面扩展。采用1150℃8h预处理能消除全部的Ni_5Hf相,有效地控制含Hf合金的初熔。     

热处理过程中DD6单晶高温合金的组织演化规律与力学性能

研究了DD6单晶高温合金在热处理过程中的显微组织演化规律以及初熔组织的生成机理。通过研究不同固溶时效处理对γ′相形貌、尺寸分布和体积分数的影响且分析了完全热处理后合金的显微硬度和拉伸性能,从而确定了合金最佳的热处理工艺。结果表明,通过差热分析法和金相观察法确定合金的初熔温度在1300~1310 ℃。在1315 ℃固溶处理4 h,枝晶间/枝晶干γ′相尺寸趋于一致,呈立方状均匀排列。在固溶处理过程中,γ/γ′共晶组织熔化生成了不规则初熔组织。在不同的一次时效工艺下,1120 ℃时效4 h空冷后,γ′相立方度更好,尺寸分布更均匀。合金最佳的热处理工艺为1290 ℃×1 h+1300 ℃×2 h+1315 ℃×4 h, AC+1120 ℃×4 h, AC+870 ℃×32 h, AC。合金在完全热处理后,随拉伸温度从室温升高至850 ℃时,强度达到峰值,温度继续升高,强度下降;在760 ℃拉伸时塑性最差,随着拉伸温度从760 ℃升高到950 ℃,塑性提高。     

先进单晶高温合金均匀化-固溶热处理的设计（英文）

为设计更适用于先进单晶高温合金的均匀化-固溶热处理制度,研究了不同热处理温度和时间对一种先进单晶高温合金组织的影响。通过研究合金组织和元素分布发现,当温度直接升至γ′相溶解的实际起始温度1338℃时,合金不会发生初熔;当温度直接升至γ′相溶解的外推初始温度1350℃时,合金中出现了明显初熔,但初熔组织随着保温时间的延长逐渐减少;在1328℃固溶时,合金中虽然没有发生初熔,但均匀化效率明显降低。对实验结果进行了热力学和动力学计算与分析。结果表明,单晶高温合金的均匀化-固溶热处理窗口是一个动态的窗口,γ′相完全溶解温度和初熔温度均随着合金均匀化程度的提高而提高;高代单晶高温合金在均匀化-固溶热处理中,不须要将温度始终保持在铸态合金的初熔温度以下,只要保证温度低于合金所在均匀化状态对应的初熔温度即可;均匀化-固溶热处理中,提高每一台阶的温度可以得到的均匀化-固溶效果远优于延长热处理时间可达到的效果。根据实验及分析结果提出了一种适用于先进单晶高温合金的均匀化-固溶热处理制度设计方法,使合金在较短时间内得到了理想的合金组织和均匀化效果。     

先进单晶高温合金均匀化-固溶热处理的研究与设计

为设计更适用于先进单晶高温合金的均匀化-固溶热处理制度,研究了不同热处理温度和时间对一种先进单晶高温合金组织的影响。使用金相显微镜和场发射电子显微镜观察合金组织,使用电子探针测试合金元素分布,并分析试验结果。试验发现,当温度直接升至γ′相溶解的实际起始温度1338℃时,合金不会发生初熔;当温度直接升至γ′相溶解的外推初始温度1350℃时,合金中出现了明显初熔,但初熔组织随着保温时间的延长逐渐减少;当温度直接升至较低的1328℃时,合金中虽然没有发生初熔,但均匀化效率明显降低。结合先进单晶高温合金高熔点元素含量较高的特点对试验结果进行热力学和动力学计算与分析,结果表明,单晶高温合金的均匀化-固溶热处理窗口是一个动态的窗口,γ′相完全溶解温度和初熔温度均随着合金均匀化程度的提高而提高;高代单晶高温合金在均匀化-固溶热处理中,不须要将温度始终保持在铸态合金的初熔温度以下,只要保证温度低于合金所在均匀化状态对应的初熔温度即可;均匀化-固溶热处理中,提高每一台阶的温度可以得到的均匀化-固溶效果远优于延长热处理时间可达到的效果。根据试验及分析结果提出了一种适用于先进单晶高温合金的均匀化-固溶热处理制度设计方法,使试验合金在较短时间内得到了理想的合金组织和均匀化效果。     

固溶热处理对含碳镍基单晶高温合金AM3组织和偏析的影响

研究了不同固溶处理工艺对含0.045%（质量分数）碳的AM3镍基高温合金组织和元素偏析的影响。采用光学显微镜和扫描电镜对组织和γ''相进行了观察分析。采用电子探针对元素偏析进行了测试分析。结果显示,合金的初熔温度为1305 ℃,在对试样进行1305 ℃/6 h固溶处理前,对其先进行1300 ℃/3 h的均匀化处理可以减少初熔组织的出现,并能使得初熔温度提高约5 ℃。随着固溶温度的提高和固溶时间的延长,γ''相的体积分数和尺寸显著增大,元素Cr、Co、Mo、W和Al的偏析系数接近均值1。随着固溶时间的延长,初熔组织的出现阻碍了Ti的偏析。最佳的固溶处理工艺为1300 ℃/3 h+1305 ℃/6 h/空冷+1080 ℃/6 h+870 ℃/20 h/空冷。热处理后的AM3单晶高温合金枝晶组织完全消失,γ''相立方度更好,尺寸达到454 nm,并且体积分数为66.05%。元素Cr、Co、Mo、W、Al和Ti的偏析程度均有所降低。     

固溶温度对DD6单晶高温合金组织与性能的影响

针对DD6单晶高温合金进行了不同固溶温度的工艺试验,研究4种不同固溶温度对合金组织、性能的影响。结果表明:合金在1320℃及1330℃两个温度下固溶处理过程中发生初熔,经1300℃及1310℃固溶处理后未发现初熔组织。和1310℃相比,在1300℃固溶处理后仍有较多的残留共晶组织。合金固溶温度由1300℃升高至1310℃时,在合金后续时效处理时析出的γ'相更加均匀、细小。高温力学性能测试结果表明,经1310℃固溶处理并两次时效处理后,DD6合金的高温拉伸和持久断裂力学性能优于1300℃固溶处理和时效处理的DD6合金。     

固溶处理在镍基高温合金中作用的研究进展

随着镍基高温合金服役温度的不断提高,难溶元素含量的增多使合金固溶处理难度进一步增加。为了能更有效地制定出最佳的热处理制度,将固溶处理对镍基高温合金凝固组织、元素偏析以及力学性能的影响进行综合分析。结果表明,均匀化处理可以提高合金的初熔温度。适当提高固溶温度,延长固溶时间,增加固溶处理步骤可以显著减少合金元素的偏析,降低γ/γ′共晶组织、TCP相的体积分数,使碳化物转变为更稳定的类型,残余γ′相体积分数有所降低。但不同的固溶处理工艺对不同合金析出的γ′相尺寸、体积分数的影响仍存有差异,需对不同合金分别进行研究。新提出的斜坡固溶处理与重熔固溶处理均是保证在没有初熔组织存在的条件下,尽量提升固溶处理温度,使元素分布更均匀,并获得比传统逐步固溶处理更小的残余偏析,持久寿命明显增加。最后综述了固溶处理对合金微观组织及力学性能的影响机理,并指出今后的研究方向。     

铌在一种高合金化高温合金中的偏析与扩散行为

研究铌在一种高合金化镍基高温合金凝固过程中的偏析和均匀化过程中的扩散行为。铌严重偏析于枝晶间区域,其偏析系数高达4.30。许多富铌相,包括Laves相、δ相、（γ＋γ′）共晶、MC和M6C碳化物等在枝晶间析出。利用差热分析和在不同温度、时间的均匀化处理确定各种富铌相的溶解温度和枝晶偏析的消除程度。元素扩散计算表明：铌的扩散速率随着均匀化温度的提高而显著增加,从而有效地缩短均匀化时间。提出一种无初熔的三步均匀化制度,可以完全消除铌的枝晶偏析,获得均一的组织。     

Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron ContentEI北大核心CSCD

A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 gamma' phase. To achieve a uniform distribution of precipitated gamma' particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of gamma'/gamma eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and eta phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in gamma matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 degrees C and 1170 degrees C. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical gamma dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, gamma matrix and gamma'/gamma eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes gamma matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200. and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes gamma matrix outside the incipiently melted circle melt when the temperature is higher than 1210 degrees C. Especially, IMR swallows up plenty of gamma matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 degrees C which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.     

DD6单晶高温合金初熔组织演变机制研究

分析第二代单晶高温合金DD6在略高于固溶热处理窗口的温度1 330 ℃下保温的初熔组织演变。结果表明:保温0.5 h后，枝晶间发生明显初熔，并大量形成典型初熔组织，部分初熔组织中心形成显微孔洞；随保温时间延长，初熔逐渐消退并演变为正常合金组织；保温8 h后，初熔组织完全消失，仅留下初熔形成的微孔。应用热力学、动力学计算分析试验结果可知:DD6合金在铸态下枝晶偏析显著，枝晶间固相线温度明显低于枝晶干，在略高于枝晶间固相线的温度下保温，初始阶段枝晶间发生初熔；继续保温，元素均匀化程度改善，枝晶间固相线温度升高，初熔组织逐渐消退，最终演变为正常合金组织。     

Improving intergranular corrosion resistance of sensitized type 316 austenitic stainless steel by laser surface melting

An attempt was made to modify the surface microstructure of a sensitized austenitic stainless steel, without affecting the bulk properties, using laser surface melting techniques. AISI type 316 stainless steel specimens sensitized at 923 K for 20 hr were laser surface melted using a pulsed ruby laser at 6 J energy. Two successive pulses were given to ensure uniform melting and homogenization. The melted layers were characterized by small angle X- ray diffraction and scanning electron microscopy. Intergranular corrosion tests were carried out on the melted region as per ASTM A262 practice A (etch test) and electrochemical potentiokinetic reactivation test. The results indicated an improvement in the intergranular corrosion resistance after laser surface melting. The results are explained on the basis of homogeneous and nonsensitized microstructure obtained at the surface after laser surface melting. It is concluded that laser surface melting can be used as an in situ method to increase the life of a sensitized component by modifying the surface microstructure.     

固溶处理对铸造625合金显微组织和拉伸性能的影响

研究1050～1250℃ 固溶处理对铸造625合金显微组织和拉伸性能的影响.采用SEM、EDS、EPMA和DTA研究合金的显微组织及凝固特征.结果表明,合金的凝固顺序为L→L+γ→L+γ+MC→L+γ+MC+γ/Laves→ γ+MC+γ/Laves.经1225和1250℃固溶处理后,组织中Laves相发生初熔.经不同...     

磷硼复合添加对Inconel 718合金铸态组织及均匀化处理参数的影响(英文)

研究磷硼复合添加对Inconel 718高温合金铸态组织及均匀化处理参数的影响。结果表明:磷硼的加入促进块状Laves相的形成。由于硼在最后残余液相中大量富集,形成一种富含Nb、Mo和Cr的含硼相。根据DSC和电子探针分析结果,确定磷硼复合添加Inconel 718高温合金的凝固顺序为L→L+γ→L+γ+MC→L+γ+MC+Laves→γ+MC+Laves+含硼相。由于低熔点含硼相的存在,磷硼复合添加Inconel 718高温合金的均匀化处理温度应比标准Inconel 718合金低至少40°C。     

Effect of intermetallics on susceptibility of Mg alloy welds to liquation cracking

ABSTRACT

The circular-patch welding test was used to study the liquation and liquation cracking of AZ-series Mg alloys. A heat treatment was carried out on the as-received AZ91 alloy to dissolve the γ(Mg₁₇Al₁₂) particles before welding. The circular-patch welding test was then conducted on the heat-treated, as-received AZ91 alloy using AZ61 and AZ91 filler wires. The results showed that the susceptibility of AZ91 alloy to liquation and liquation cracking was significantly reduced by the dissolution of massive γ(Mg₁₇Al₁₂) particles via a heat treatment before welding as the liquation mechanism was changed. Both constitutional liquation and incipient melting occurred in the partially melted zone of the as-received AZ91 welds, while only incipient melting occurred in the heat-treated AZ91 welds.     

激光熔凝过程中低碳钢组织的形成及熔凝组织加热转变规律

基于激光熔凝处理的一维热场模型和金相分析，研究了低碳钢在激光熔凝过程中的组织形成规律。使用Gleeble1500型热力模拟机实现对激光熔凝处理试样不同保温时间的控温加热，研究了激光熔凝组织的加热转变规律。利用激光熔凝加后续热处理的方法在材料表面获得了细小等轴组织。     

Homogenizing Advanced Alloys: Thermodynamic and Kinetic Simulations Followed by Experimental Results

Segregation of solute elements occurs in nearly all metal alloys during solidification. The resultant elemental partitioning can severely degrade as-cast material properties and lead to difficulties during post-processing (e.g., hot shorts and incipient melting). Many cast articles are subjected to a homogenization heat treatment in order to minimize segregation and improve their performance. Traditionally, homogenization heat treatments are based upon past practice or time-consuming trial and error experiments. Through the use of thermodynamic and kinetic modeling software, NETL has designed a systematic method to optimize homogenization heat treatments. Use of the method allows engineers and researchers to homogenize casting chemistries to levels appropriate for a given application. The method also allows for the adjustment of heat treatment schedules to fit limitations on in-house equipment (capability, reliability, etc.) while maintaining clear numeric targets for segregation reduction. In this approach, the Scheil module within Thermo-Calc is used to predict the as-cast segregation present within an alloy, and then diffusion controlled transformations is used to model homogenization kinetics as a function of time and temperature. Examples of computationally designed heat treatments and verification of their effects on segregation and properties of real castings are presented.