10CrNiCu钢冷弯裂纹分析

10 CrNiCu钢冷弯裂纹有两种:一种宽长,另一种细小.宽长裂纹试样浅表层处出现粗晶区,粗晶内部为长条状或多边形状结构,长板条最长可达50μm.粗晶区硬度比钢板内部细晶区的硬度低42 HV0.1,浅表层与内部组织、性能的差异导致钢板弯曲时开裂.粗晶区原奥氏体晶界上出现了断续呈链状分布的铁素体组织,说明钢板回火时表面温度超高,导致浅表层组织相变,析出铁素体.细小裂纹是由钢板表面残留氧化铁皮引起的.     

The Effect of ITMT’s and P/M Processing on the Microstructure and Mechanical Properties of the X7091 Alloy

The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔK_th, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation (FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution, a weak texture, and a low concentration of oxides at grain boundaries. Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA.     

GH2132合金混晶条带组织分析及其对力学性能影响

 混晶条带是影响GH2132合金组织及性能稳定性的重要因素,因此针对合金冷拉棒材出现的混晶条带缺陷进行了试验研究。利用金相、EPMA、EBSD和TEM手段,结合热力学平衡相图与硬度测试,揭示了混晶条带的主要成因,并分析了其内部微观组织状态及混晶组织对显微硬度的影响。结果表明,混晶条带组织中细晶区晶粒尺寸普遍小于10 μm,粗晶区晶粒最大可超过60 μm,而元素偏析与冷拉变形是造成晶粒大小差异并形成混晶条带的原因。合金铸态组织中Ti、Mo、C、B元素均表现出正偏析,其中C、Ti元素的偏析程度较高,能够在枝晶间析出MC与M₃B₂相,并最终遗传至奥氏体晶界;晶界溶质富集不仅起到钉扎作用,阻止再结晶过程晶粒长大,还能够通过溶质拖拽作用降低晶界的迁移速率而阻碍晶界变形,最终溶质富集区域形成细晶区、贫化区域形成粗晶区,并沿合金棒材组织冷拉方向表现出混晶条带。另外,在冷拉变形过程中,由于晶体取向不一,在受同样拉拔力条件下晶粒变形的实际应变并不一致,这也会加剧混晶现象。微观组织观察显示冷变形后细晶区与粗晶区相比形变更为均匀,组织内平均位错密度更高,混晶区域内存在退火孪晶与形变孪晶,但细晶区孪晶密度更高。最终混晶区存在的晶界数量不同、位错密度变化、孪晶分布的梯度组织,导致细晶区显微硬度明显高于粗晶区的硬度梯度。     

Analysis of orientation clustering in a directionally solidified nickel-based ingot

Orientation imaging microscopy was used to study the field of lattice orientations in a directionally solidified ingot of nickel-based alloy in order to understand the evolution of grain structure and microtexture as a function of distance from the chilled surface. The lattice field is organized into equiaxed and randomly oriented grains at the chill plane. However, with increasing distance away from the chill plane, the microstructure organizes into increasingly coarse and ramified grain clusters in the region of columnar growth. The clusters comprise sets of grains connected by small-angle grain boundaries. Thus, the clustering of neighboring dendrites reflects a sharing of low-angle misorientations owing to the development of a strong 〈100〉 fiber texture and its concomitant, a profound change in misorientation distribution. The absence of any preferred orientation within a single cluster suggests that the clusters consist of separate grains. The radius of gyration and the fractal dimension have been used to characterize clustering in the microstructure. Clustering was found to increase with distance from the chill plane as the strength of the fiber texture increases. A critical misorientation angle was found at which the largest cluster percolates the microstructure. This critical angle decreases with increasing strength of the fiber texture.     

工业化制备7050铝合金厚板显微组织与力学性能

采用OM、TEM观察、室温拉伸试验,研究工业化制备大截面7050铝合金厚板微观显微组织和力学性能.结果表明:板材表层、1/4厚度层和芯部处均存在部分粗大晶粒组织以及晶粒尺寸分布不均匀现象,其中芯部粗大晶粒的尺寸、数量以及晶粒尺寸不均匀程度均高于1/4厚度层和表层;合金时效后晶内的析出相主要为η’相、η相以及少量GPⅡ区,表层晶粒内的析出相密度大于1/4层和芯部,且1/4层和芯部粗棒状的η相含量较高;合金的强度、硬度以及延伸率在厚度方向上呈梯度分布,表层硬度、抗拉强度（σ_b）、屈服强度（σ_0.2）和延伸率（δ）均最高,从表层到芯部,硬度、σ_b、σ_0.2以及δ均逐渐减小.      

10CrNiCu钢冷弯裂纹分析

对10CrNiCu钢板冷弯开裂进行分析，结果显示钢板浅表层出现粗晶区，在粗晶区原奥氏体晶界处分布有少量链状铁素体，粗晶区与钢板内部细晶区硬度差异较大，这是引起钢板冷弯开裂的主要原因。钢板表面残留的氧化铁皮是导致冷弯裂纹的另一诱因。     

低压烧结对硬质合金组织和性能的影响

通过配制不同粒度的WC粉末,分别在低压和真空条件下烧结制备WC-6Co硬质合金,采用扫描电镜分析、光学金相检测、显微硬度试验、钴磁检测、矫顽磁力检测和抗弯强度检测等方法,对比研究了低压烧结和真空烧结制备的硬质合金的显微组织和性能。结果表明,与真空烧结相比,低压烧结有效地降低了合金的孔隙度,增大了合金的密度,提高了合金的综合性能;低压烧结对合金的组织和性能的影响程度与原料WC粒度有关,低压烧结对粗颗粒WC为原料的合金的综合性能提高不明显。     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University     

Delamination of Pearlitic Steel Wires: The Defining Role of Prior-Drawing Microstructure

This article reports the occasional (< 10 pct of the actual production) delamination of pearlitic wires subjected to a drawing strain of ~ 2.5. The original wire rods which exhibited post-drawing delamination had noticeably lower axial alignment of the pearlite: 22 ± 5 pct vs 34 ± 4 pct in the nondelaminated wires. Although all wires had similar through-thickness texture and stress gradients, delaminated wires had stronger gradients in composition and higher hardness across the ferrite–cementite interface. Carbide dissolution and formation of supersaturated ferrite were clearly correlated with delamination, which could be effectively mitigated by controlled laboratory annealing at 673 K. Direct observations on samples subjected to simple shear revealed significant differences in shear localizations. These were controlled by pearlite morphology and interlamellar spacing. Prior-drawing microstructure of coarse misaligned pearlite thus emerged as a critical factor in the wire drawing-induced delamination of the pearlitic wires.     

The Mechanism of Grain Coarsening in Friction-Stir-Welded AA5083 after Heat Treatment

Friction stir welding (FSW) takes place in the solid state, thus providing potential advantages of welds of high strength and ductility because of fine microstructures. However, post-FSW heat treatment can create very coarse grains, potentially reducing mechanical properties. AA5083-H18 sheets were friction-stir butt welded using three sets of welding parameters representing a wide range of heat input. They were then heat treated for 5 minutes at 738 K (465 °C), producing grain sizes exceeding 100 μm near the top weld surfaces, with the coarse grains extending toward the bottom surface to various degrees depending on the welding parameters. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, inductively coupled plasma–mass spectrometry, and Vickers hardness testing were used to characterize the regions within welds. Particle pinning was determined quantitatively and used with Humphreys’ model of grain growth to interpret the behavior. The mechanism responsible for forming the large grains was identified as abnormal grain growth (AGG), with AGG occurring only for regions with pre-heat-treatment grain sizes smaller than 3 μm. Second-phase particle volume fractions and sizes, textures, and solute concentrations were not significantly different in AGG and non-AGG regions. Ultrafine grain layers with grain diameters of 0.3 mm were characterized and had high densities of pinning particles of MgSi₂, Al₂O₃, and Mg₅Al₈. Strategies to eliminate AGG by alloy and weld process design were discussed.     

Effect of initial microstructure on microstructural instability and creep resistance of XD TiAl alloys

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn+0.8 vol pct TiB₂) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed γ grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed γ grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.     

Microstructure Evolution of Surface Gradient Nanocrystalline by Shot Peening of TA17 Titanium Alloy

A surface gradient nanocrystalline structure (SGNS) was obtained by shot peening (SP) on the TA17 near α titanium alloy to improve its surface properties. The effect of shot peening time was investigated by characterizing the grain size of the surface nanocrystalline layer, the thickness of the severe plastic deformation (SPD) layer, the microstructure evolution of the SGNS and the hardness change. The experimental results show that the grains of TA17 titanium alloy can be refined to a nano-scale of about 22 to 26 nm when shot peened at 0.6 MPa pressure for 5 to 10 minutes. The thickness of the SPD layer increases from 55 to 88 μm with the SP duration from 5 to 10 minutes and tends to be saturated afterward. The SGNS is composed of a surface nanocrystalline layer and a transition layer. During the SP treatment, the coarse grains are first divided into small blocks by intersection of twins, then dislocation walls, dislocation tangles and dislocation bands lead to the formation of low-angle grain boundaries, which subdivide the subgrains into a finer scale until nanograins with a stable size are obtained. The topmost surface hardness can be improved significantly to twice the hardness of the matrix due to both grain refinement and work-hardening.

     

Macro- and Microstructure Evolution of 5CrNiMo Steel Ingots during Electroslag Remelting Process

A comprehensive mathematical model was established and used to simulate the macro and microstructure evolution during the production process of 5CrNiMo steel ingot by electroslag remelting （ESR） method. Along the ingot height, the macrostructure distribution characteristics changed from vertical, fine columnar grains to tilted, coarse columnar grains, and this transformation process occurred at the very beginning of ESR. In the cross section of the ingot, there were three grain morphology regions and two grain type transition regions from the outside to the center of the ingot. These regions were the fine columnar grain region, columnar competitive growth transition re gion, coarse columnar grain region, columnar to equiaxed grain transition （CET） region, and coarse equiaxed grain region. The influence of the remelting rate on the macrostructure and mlcrostructure was investigated using a series of experiments and simulations. The results showed that a low remelting rate could produce a small grain growth angle （GGA） ; the average secondary dendrite arm spacing （SDAS） firstly decreased and then increased as the remelting rate increased. An excessively high or low remelting rate can increase the GGA and average SDAS in ingots. Thus, the remelting rate should be controlled within a suitable range to reduce composition microsegregation and microshrinkage in the ingot to produce an ESR ingot with satisfactory hot forging performance.     

The recrystallization of commercially pure and doped tungsten wire drawn to high strain

The recrystallization processes in both undoped and doped tungsten wire after drawing to a true strain of 7.7 were examined by light microscopy and transmission electron microscopy. High angle grain boundary migration commenced at approximately the same temperature in both materials, but proceeded much more rapidly in the undoped wire, where the absence of a potassium bubble dispersion allowed a coarser, more equiaxed grain structure to form. No change from the (110) deformation texture was observed in either case. Recrystallization in the undoped wire was dominated at lower temperatures (1100 to 1200°C) by the growth of large grains into a much finer structure. As the annealing temperature was increased, this process was replaced by a general grain coarsening which eventually produced a relatively equiaxed recrystallized grain structure. It appeared probable that it was the second phase dispersion inhibition alone that prevented similar structural changes in the doped wire. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Microstructure Evolution and Wear Resistance Improvement of Ultrasonic Peened M50 Steel via Electromagnetic Shocking

Herein, the effect of electromagnetic shocking treatment (EST) on microstructure evolution and wear resistance of M50 steel treated by ultrasonic shot peening (USP) is investigated. The microstructure observation indicates that the EST promotes the precipitation of nanoscale carbides. The average grain size decreases slightly and the submicron grain layer increases after EST. The low-angle grain boundaries generated by USP will transform into high-angle grain boundaries, thereby refining the grains on the surface during EST. In addition, the surface hardness of USP-M50 steel decreases slightly, accompanied by the reduction of surface residual stress after EST. The wear resistance results indicate that the wear loss of the EST specimens decreases by 15.7% comparedwith the specimens after USP. The improvement of wear resistance induced by EST is attributed to the increased fine precipitations. These precipitations hinder the grinding of abrasive particles into the matrix and connect with wear debris and oxide particles to form well consolidated self-lubricating films to prevent wear losses. The refinement of surface structure is another important reason for the improvement of wear resistance.     

Microstructure and Vickers-hardness of 20CrMnTiH steel during hot compression testing

The isothermal hot compression tests of 20CrMnTiH steel were carried out by using Gleeble-3500 thermo-simulation-machine at deformation temperatures ranging from 1123 to 1423?K and at strain rates ranging from 0.01 to 1?s^?1. The microstructural characteristics after hot compression under various deformation conditions were described with optical microscope and EBSD method, meanwhile the Vickers-hardness (HV) corresponding to the samples was measured. The strain-induced fine and uniform recrystallised grains instead of the original coarse microstructure and higher hardness were obtained with increasing strain rate and decreasing deformation temperature. The average misorientation angle got increased and an intense α-fibre texture was dominated with increasing temperature, and then the angle decreased when the temperature increase to 1423?K. By introducing Zener–Hollomon (Z) parameter, the relationship of recrystallised grain size (D) and HV under different Z values were generated. The relationship of 20CrMnTiH steel recrystallised grain size (D) and HV was formulated based on the analysis of the experimental data, which is in agreement with Hall–Petch relationship.     

Cu-P-Cr-Ni-Mo耐候钢双相区多道次轧制模拟的组织演变

 为了实现Cu-P-Cr-Ni-Mo耐候钢的铁素体晶粒细化从而充分提高其强塑性,通过热模拟压缩试验,利用金相、SEM、EBSD等微观组织分析方法研究了其在双相区的多道次压缩变形过程中的组织演变。结果表明,试验钢在变形过程中,第二相（马氏体、贝氏体）呈条带状分布于铁素体基体上,随着道次增加,铁素体晶粒逐步细化,第5道次变形后得到1.8 μm左右的超细晶铁素体。前期铁素体晶粒细化的主要机制是形变强化铁素体相变,即多道次的累积大变形使组织内畸变能增大,铁素体形核点增多,促进铁素体快速析出,形成细小铁素体晶粒;后面几道次变形中,随着应变量继续增大,在铁素体晶粒内形成大量亚晶界,且亚晶界逐步累积扭转成大角度晶界,分割原来的粗大晶粒,发生铁素体连续动态再结晶细化。     

Fatigue-crack-propagation thresholds in a nickel-base superalloy at high frequencies and temperatures

Fatigue-crack-propagation (FCP) tests were conducted on the powder metallurgy nickel-base superalloy KM4 at temperatures of 20 °C, 550 °C, and 650 °C. Two different heat treatments were investigated, one yielding a relatively coarse grain size of 55 μm and another yielding a fine grain size of 6 μm. Tests were conducted at 100 Hz and 1000 Hz and at load ratios between 0.3 and 0.7. In the Paris regime, trends observed at high frequencies for KM4 were identical to those observed by earlier investigators at lower frequencies: coarse grains, low load ratios, low temperatures, and higher frequencies generally resulted in lower crack-propagation rates. However, in contrast to the Paris-regime behavior, thresholds were a complicated function of microstructure, load ratio, temperature, and frequency, and the only variable that resulted in a consistent trend in threshold was the load ratio. For example, thresholds increased from 100 to 1000 Hz for the fine-grained material at 550 °C, but decreased with the same frequency variation at 650 °C. One reason for this complexity was a change to intergranular fracture in the fine-grained microstructure at 650 °C, which was beneficial for high-frequency thresholds. Higher load ratios and lower frequencies promoted intergranular fracture. However, not all of the complexity could be explained by changing fracture mechanisms. Scanning electron microscope (SEM) stereofractography was utilized to determine quantitative measures of fracture-surface roughness. The most useful quantitative measure was found to be the standard deviation of the fracture-surface height, which is a physically meaningful length parameter and which corresponded to about half the grain size during room-temperature fatigue at near-threshold ΔK levels. The roughness of the fracture surface was found to increase as the load ratio was increased for both microstructures. For the coarse-grained microstructure, there was a direct correlation between fracture-surface roughness and FCP threshold over the entire range of temperatures, frequencies, and load ratios. However, measurements of closure loads indicated that roughness-induced closure was not the sole reason for the varying FCP thresholds.