Effect of solution cooling rate on the γ′ precipitation behaviors of a Ni-base P/M superalloy

The effect of cooling rate on the cooling γ' precipitation behaviors was investigated in a Ni-base powder/metallurgy (P/M) superalloy (FGH4096). The empirical equations were established between the cooling rate and the average sizes of secondary and tertiary γ' precipitates within grains and tertiary γ' precipitates at grain boundaries, as well as the apparent width of grain boundaries. The results show that the average sizes of secondary or tertiary γ' precipitates are inversely correlated with the cooling rate. The shape of secondary γ' precipitates within grains changes from butterfly-like to spherical with the increase of cooling rate, but all the tertiaryγ' precipitates formed are spherical in shape. It is also found that tertiary γ' may be precipitated in the latter part of the cooling cycle only if the cooling rate is not faster than 4.3℃/s, and the apparent width of grain boundaries decreases linearly with the increase of cooling rate.     

铬对铌微合金钢显微组织和热塑性的影响分析

It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.     

Effects of chromium on the microstructure and hot ductility of Nb-microalloyed steel

It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12 wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12 wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.     

Effect of manganese sulfide on the precipitation behavior of tin in steel

Tramp elements such as tin are considered harmful to steel because of hot brittleness they induce at high temperatures. Because tramp elements retained in steel scrap will be enriched in new steel due to the difficultly of their removal, studies on the precipitation behavior of tin are essential. In this study, the effects of different inclusions on the precipitation behavior of tin in steel were studied. The results show that the tin-rich phase precipitates at austenite grain boundaries in an Fe-5%Sn alloy without MnS precipitates, whereas Sn precipitates at the boundaries of MnS inclusions in steel that contains MnS precipitates. MnS is more effective than silicon dioxide or aluminum oxide as a nucleation site for the precipitation of the tin phase, which is consistent with the disregistry between the lattice parameters of the tin phase and those of the inclusions.     

A Cu-Cr alloy with nano and microscale Cr particles produced in a water-cooled copper mold

Microstructures have profound effects on the hardness and strength of Cu-Cr alloys. The microstructures of a Cu-Cr alloy cast in a water-cooled copper mold were studied in the present work. The scanning electron microscopy (SEM) results show that there are the copper matrix saturated with chromium, spherical precipitates of chromium separated from liquid phase during cooling before the initiation of solidification, and a eutectic phase in grain boundary areas. To investigate the effect of age-hardening treatment on the microstructures and properties of the material, some samples were subsequently age-hardened in a salt bath and investigated by transmission electron microscopy (TEM). The results show that coherent precipitates with the diameter of 11 nm are detectable in the samples before and after the age-hardening stage. Of course, the volume fraction of coherent precipitates is higher after the aging process.     

Carbides/nitrides precipitates in a C-Mn strip by CSP technology

The carbides/nitrides precipitates in ferrite grains, on grain boundaries and dislocations were investigated on a hot-rolled C-Mn strip (0.16wt%C-1.22wt%Mn-0.022wt%Ti) produced by the CSP (compact strip production) technology using TEM and X-ray energy dispersive spectroscopy. The Pickering’s equation for the contribution of precipitates to the yield stress was also discussed. It is shown that there are numerous fine and dispersive precipitates TiC in the ferrite grains, on the grain boundaries and dislocations. Also there are a small amount of coarser Ti(C, N) particles and TiC particles associated with MnS. Precipitation strengthening on steels produced by the CSP technology is significant.     

Dynamic process of trace boron non-equilibrium grain boundary segregation and the effect of cooling rate

The dynamic process of non-equilibrium grain boundary segregation of trace boron in Fe-40%Ni alloy during cooling and the effect of cooling rate were investigated by boron tracking autoradiography technique. The results indicate that during cooling process, the amount of segregated boron on grain boundary firstly increases fast, then enters a comparatively even increasing stage and increases rapidly again at the third stage. The details of each stage varied with cooling rate are explained. When thc segregation develops to a certain degree, the segregated boron atoms transform fiom solute status to precipitate status.     

Influence of MnS particles on the behaviors of grain boundary migration in Fe-3%Si alloys

The precipitation behaviors of MnS particles at 900℃ in a hot deformed Fe-3%Si alloy were observed statistically. The ratio of MnS particles on dislocations and in grain boundaries was calculated based on a model concerning the second phase precipitation in supersaturated solid solution. It was indicated that the precipitation of MnS particles on dislocations prevailed. The coarsening process of MnS particles in grain boundaries determined the boundary mobility during the secondary recrystallization. However, the density difference of precipitated MnS particles inside the grains on both sides of a boundary will determine the migration direction of the boundary as well, besides the grain size effect. It was observed that the densities of MnS particles in two neighboring grains were commonly different, and the boundary tended to move towards the area with lower particle density. The factors, e.g. dislocation densities in differently oriented grains will affect the density of precipitated particles, in which the Goss grains with higher particle density could grow more easily.     

Effect of long-term service on the precipitates in P92 steel

The precipitates in P92 steel after long-term service in an ultra-supercritical unit were investigated by field-emission scanning electron microscopy and transmission electron microscopy and were found to mainly consist of M₂₃C₆ carbides, Laves phase, and MX carbonitrides. No Z-phase was observed. M₂₃C₆ carbides and Laves phase were found not only on prior austenite grain boundaries, martensite lath boundaries, and subgrain boundaries but also in lath interiors, where two types of MX carbonitrides—Nb-rich and V-rich particles—were also observed but the “winged” complexes were hardly found. Each kind of precipitate within the martensite laths exhibited multifarious morphologies, suggesting that a morphological change of precipitates occurred during long-term service. The M₂₃C₆ carbides and Laves phase coarsened substantially, and the latter grew faster than the former. However, MX carbonitrides exhibited a relatively low coarsening rate. The effect of the evolution of the precipitate phases on the creep rupture strength of P92 steel was discussed.     

Effect of boron on hot strips of low carbon steel produced by compact strip production

The effect of boron on hot strips of low carbon steel produced by compact strip production (CSP) to reduce the strength to a certain degree was investigated, which is quite different from that of high-strength low alloy steel. The mechanical properties and microstructural evolution of the hot strip were studied using optical microscopy and tensile tests. By means of an electrolytic dissolution technique and Thermo-Cal calculation, the precipitates containing boron were analyzed and detected. From the electron backscattered diffraction analysis, it can be deciphered whether the microstructure has recrystallized or not. Furthermore, the effect of boron segregation on the recrystallization or non-recrystallization conditions can be distinguished. The segregation behavior of boron was investigated in boron-containing steel. The nonequilibrium segregation of boron during processing was discussed on the basis of the forming complexes with vacancies that migrate to the boundaries prior to annihilation, which was confirmed by the subsequent cold rolling with annealing experiments.     

奥氏体合金中溶质原子的晶界偏聚与晶界沉淀

用扫描电镜和透射电镜研究了晶界沉淀的形貌和化学组成,发现高温加热时生成富Ti 的厚片状晶界沉淀(MC);以后的低温加热时生成 Mo含量较高的薄片状晶界沉淀。不同冷却速度条件下的晶界沉淀(M_7C_3)的Cr 含量不同,加热后的冷却过程中,较低温度下析出的晶界沉淀(M_(23)C_6)中的Cr,Mn 含量较高。结果表明:溶质原子的错配度、点阵溶解度及沉淀温度对晶界沉淀产生影响。     

铁铬铝合金晶粒长大及合金中稀土元素

研究了不同稀土含量铁铬铝合金,在高温下晶粒长大及界面结构。结果表明:在1200℃以上高温,稀土组元(以La为主)对抑制合金的晶界长大有一定效果,La_2O_3可作为TiN相析出的形核中心,细小TiN质点可在晶界上出现,阻碍晶界迁移;在变温过程中,La向界而扩散,在晶界以La_2O_3形式出现,同时稀土加速氧穿过氧化膜向基体中渗透,导致Al_2O_3及La_2O_3在基体中形成。     

超纯奥氏体不锈钢00Cr25Ni22Mo2N非敏化态晶间腐蚀与磷(硅)的晶界偏聚

运用金相、电子显微镜观察及表面分析技术,研究了超纯奥氏体不锈钢00Cr25Ni22Mo2N制造的汽提管在尿素生产环境下的腐蚀形态和机理。结果表明:该钢种在生产条件下产生非敏化态晶间腐蚀。其特征是腐蚀介质不仅沿晶界向钢的内部渗入,而且同时向晶界的两侧扩展,因而在管断面上观察到的晶间腐蚀路径较宽,但深度较浅并呈现漏斗形貌。电子显微镜观察、二次离子质谱、俄歇能谱分析及计算表明,这种材料晶界不贫铬、无第二相沉淀,但有磷(硅)的高度偏聚。晶界区磷的含量(约25％)比晶内高约三个数量级。作者认为,磷的晶界偏聚造成的与晶内在腐蚀电解质溶液中的电位差,是这种超纯奥氏体不锈钢非敏化态晶间腐蚀的原因。     

SCR成形Al-1.5Mg-0.3Sc合金线材的组织与性能

采用单辊搅拌冷却技术(SCR)制备Al-1.5Mg-0.3Sc(质量分数,%)合金线材,对比相同成分常规铸造工艺制备的合金,对SCR成形合金线材组织与性能进行了研究.结果表明:SCR成形Al-1.5Mg-0.3Sc合金的晶粒较常规铸造工艺制备合金的晶粒细小;常规铸造工艺制备的Al-1.5Mg-0.3Sc合金,金相试样腐蚀后,在合金晶界处出现黑色的、结构不规则的层片状、有弓形边界的Al3Sc沉淀物;SA合金线材520℃在线固溶、320℃时效3 h,抗拉强度为304 MPa,延伸率为18.9%,合金线材具有较高的强度和良好的塑性.     

高强细晶IF钢微观形貌的研究

以新型高强细晶IF钢为研究对象,通过实验室冷轧和退火实验,研究了退火工艺对高强细晶IF钢微观形貌的影响.通过微观组织观察可以发现,化学成分的改善、轧制及退火工艺的控制可以使这种钢不仅具有细小的晶粒,而且存在10～40nm的细小析出物Nb(C,N);晶界附近析出物非常稀少,称之为PFZ(晶界无析出物区),且仅存在于晶界的一侧;部分三叉晶界处存在晶界合并现象.实验表明,由于这种钢中Nb析出物非常细小,使晶粒大大细化;同时由于PFZ带的存在,使这种钢具有较低屈强比及较高延伸率,且成形性能良好.     

基于Burton-Miller边界元法的不同类型单元计算精度对比

为了提高边界元法的计算精度和对具有复杂边界形状实际问题的应用能力,发展并应用非连续线性和二次边界单元进行数值计算.使用传统边界积分方程计算外声场,通过带有解析解数值算例,对比不同类型单元的计算精度,得到最有效的单元类型.然而使用传统边界积分法,在某些虚假特征频率处会产生解的非唯一性问题,Burton-Miller方法可以有效地克服这一问题.基于Burton-Miller法得到的非连续线性和二次单元的优化节点位置并不在勒让德多项式零点位置上,虽然表现得不像传统边界元法那样规律和统一,但是合适的经验值仍然被给出.     

沉积地球化学在层序地层分析中的应用

对于层序地层学研究来说最基本的、也是最关键的问题是层序的识别,而层序的识别包括两个方面的内容,一是层序界面的识别,二是层序内体系域构成的识别.层序界面和层序内体系域的识别除宏观的沉积记录分析方法外,还可通过微观的沉积地球化学分析资料来识别.研究表明,在层序界面的形成过程中,伴随着风化作用的进行,在层序界面上的Fe元素以高价Fe的氧化物形式出现;Al含量高,主要为Al的氧化物;K的含量大于Na元素的含量;Th/U比值很大.体系域与REE和微量元素、常量元素含量密切相关,LST(或SMST)期REE含量最低,TST期REE含量逐渐增高至最大,海泛期达到最高, HST期又逐渐下降.LST期,微量元素含量最低,TST期微量元素含量逐渐增高直至最高,HST期微量元素含量又逐渐下降.