喷丸强化对7050铝合金微观组织和耐磨性的影响

目前,有关增压喷丸对7050铝合金耐磨性能的影响研究鲜见报道。通过增压喷丸方式在7050铝合金表面制备纳米晶层;用X射线衍射仪、光学显微镜、透射电镜观察并分析了纳米晶层的组织结构,统计了晶粒尺寸大小和变形层的厚度;通过维式硬度计测量了样品喷丸前、后的显微硬度;采用钻盲孔法测试了残余应力的分布;在油润滑条件下对喷丸前、后样品进行了摩擦磨损试验,通过磨损失重和形貌对比分析了耐磨性能。结果表明:增压喷丸使7050铝合表面发生严重的塑性变形,变形层平均厚度约为140μm,表层纳米晶粒平均尺寸约为80 nm,硬度比基体提高了1.38倍,耐磨损性提高了1倍以上。试样表面经增压喷丸撞击产生的塑性变形引起的晶粒细化以及加工硬化现象的共同作用,是其显微硬度和耐磨性能提高的主要原因。     

超音速微粒轰击38CrSi钢表面纳米化的研究

采用超音速微粒轰击技术(Supersonic Fine Particles Bombarding,SFPB)对调质态合金钢38CrSi进行表面纳米化处理;利用X射线衍射、扫描电镜、透射电镜等分析技术研究不同工艺条件下表面纳米化层的微观组织结构特征.结果表明:经SFPB处理后,材料表层组织严重细化,并形成了纳米结构层(晶粒尺寸＜100nm),随处理时间的延长,最表面纳米晶的尺寸变化不大,纳米结构层的厚度有所增加;当处理时间为240s时,在最表面层形成了平均晶粒尺寸约为16nm的具有随机取向的等轴纳米晶.纳米结构层的晶粒尺寸随着距表面距离的增加而增大.在距表面约25μm处,存在着大量的由位错线和高密度的位错缠结分割的胞块,尺寸为80～100nm;分析表明位错运动是表面纳米化的主要原因.     

超音速微粒轰击45钢表面纳米化的研究

采用超音速微粒轰击技术(SFPB)对由铁素体和珠光体组成的45钢进行表面纳米化处理,在材料表面制备了纳米结构表层,利用X射线衍射、扫描电镜、透射电镜等分析技术研究了表面纳米结构层不同深度的微观组织结构特征.研究表明:经SFPB处理后,材料表层发生了严重的塑性变形,形成了由铁素体和渗碳体组成的纳米结构层;随着处理时间的增加纳米结构层的厚度由几微米增加到15 μm(晶粒尺寸＜100 nm);在材料的最表层形成了晶粒尺寸约15 nm的具有随机取向的等轴晶,纳米晶粒尺寸随着距表面距离的增加增大;在距表面约为15 μm处,存在平均晶粒尺寸约100 nm的等轴晶和具有相近尺寸的胞状结构;在约30 μm处,大量的高密度位错墙分别将铁素体相和珠光体相分割成尺寸在200～500 nm的胞状结构.分析表明45钢表面纳米化主要是位错运动的结果.     

喷丸处理对TiB_2/Al复合材料表面基体力学性能的影响

通过X射线衍射线形分析表征了喷丸表面的组织结构,利用原位拉伸X射线衍射应力分析研究了TiB2/6351Al复合材料喷丸表面基体的力学行为.结果表明,喷丸后复合材料表面基体的屈服强度提高了26%,整体强度提高约28%,显微硬度提高50%以上.喷丸前、后复合材料基体承载系数分别为81%和83%,喷丸后的基体承载系数略有提高.喷丸表面基体的晶块尺寸及位错密度分别约51 nm和3.05×1014 m-2,晶块细化及位错密度增高是导致表面基体力学性能提高的主要原因.     

高能喷丸对AZ31镁合金耐腐蚀性及硬度的影响

采用高能喷丸对AZ31镁合金棒材端面进行表面自纳米化处理,利用失重法研究了AZ31镁合金喷丸试样和未喷丸试样在中性5%NaCl溶液中的腐蚀行为。利用扫描电子显微镜(SEM)、能量色谱仪(EDS)对塑性变形层腐蚀后的表面形貌、元素分布进行了表征,利用微观硬度计测试了由喷丸表面到基体的硬度变化。结果表明,喷完后AZ31镁合金试样的腐蚀速率明显大于未喷丸的试样,随着腐蚀时间的延长,喷丸试样的腐蚀率急剧减小,然后缓慢降低,在喷丸表面形成了1层厚度约150μm的塑性变形区,在喷丸表面有裂纹存在。晶粒细化显著提高了母材表面的微观硬度,喷丸表面的微观硬度最高达到135HV,是母材的2倍多。     

大塑性变形制备超细晶/纳米晶Al-Mg铝合金的研究进展

近年来,大塑性变形(SPD)制备具有先进结构和功能的超细晶和纳米晶Al-Mg铝合金的研究取得了很大进展。SPD后,合金的晶粒显著细化、位错密度提高及有非平衡晶界和晶界偏析形成,这些微观结构导致合金的强度、硬度大幅提高。然而,SPD合金的塑性普遍较低。综述了SPD制备的Al-Mg铝合金在结构和性能方面的一些最新研究成果。     

爆炸和滚压强化对不锈钢表面层组织结构的影响

对奥氏体不锈钢表面进行了爆炸强化、滚压强化和喷丸强化。爆炸强化时表面层内出现形变孪晶栅栏,位错线具有方向性、呈密布排列,局部山现ε马氏体、α马氏体和奥氏体超细晶粒的区域;滚压强化后表面层内也有就、孪晶栅栏出现,但位错以胞状结构的形式分布在奥氏体基体上;喷丸强化的表面层内孪晶栅栏十分致密,位错线仍以方向性排布,并无位错胞出现。表面层内组织结构出现差异,主要与变形速度和变形量有关,高速和较大的变形量可诱发α,ε马氏体转变和形成超细奥氏体晶粒,慢速变形时能促使位错胞的形成。本文还讨论了孪晶栅栏的形成方式及其内部构造。     

Fe_(78)B_(13)Si_9纳米晶合金中的 Hall-Petch 关系

采用晶化法由 Fe_(78)B_(13)Si_9非晶态合金条带制备纳米晶合金,测定了纳米晶合金的显微硬度。晶粒尺寸在纳米数量级时,Hall-Petch 关系仍然成立。FeBSi 纳米晶合金的硬度值约为相同成分的粗晶粒状态合金的2倍。位错作用机制仍为强度提高的主要因素。     

表面纳米化对2024铝合金耐磨性能的影响

为了提高2024铝合金的耐磨性,采用超声波冷锻技术在2024铝合金表面进行纳米化处理,在其表面成功制备了纳米化处理层。利用透射电镜(TEM)和原子力显微镜(AFM)观察分析了试样表面的微观形貌、晶粒大小和粗糙度;利用显微硬度计分析了铝合金基体和纳米化处理层的硬度;采用高速往复摩擦磨损试验机研究了纳米化处理层在3.5%Na Cl溶液中的耐磨性。结果表明:经超声波冷锻处理后铝合金表面晶粒得到细化;纳米化处理层的平均表面粗糙度仅为5.50 nm;纳米化处理后的硬度为106.72 HV,是铝合金基体的1.36倍;纳米化处理后的摩擦系数由0.80降到0.65,磨损量也有所减少,磨痕深度也比铝合金基体的浅。综上可得:超声波冷锻技术提高了2024铝合金的耐磨性。     

第四代粉末高温合金热变形后的"项链"组织

利用扫描电镜、透射电镜及电子背散射衍射技术研究新型第四代粉末高温合金等温锻造后的"项链"组织,对其形成机理和消除方法进行了探讨。结果表明,实验合金经过多火次等温锻造后,锻坯大部分区域为再结晶后的等轴细晶组织;然而与模具接触的上下端面得到再结晶不完全的"项链"组织,在非等轴变形晶粒周围分布着大量细小的再结晶晶粒,变形晶粒内含有较高密度的小角度晶界,缠结了大量位错。对存在"项链"组织的试样进行不同温度(1080～1180℃)的退火处理,随着温度的升高,再结晶体积分数和再结晶晶粒尺寸均不断增大。合金锻坯经过1150℃再结晶退火后,可基本消除"项链"组织,获得组织较均匀的细晶盘坯,满足双组织热处理的要求。     

喷丸成形压力对7B50−T7751铝合金力学性能的影响

目的 研究7B50−T7751铝合金在不同喷丸成形压力下力学性能的变化规律,探究喷丸成形压力对材料表面形貌、疲劳寿命及静力性能的影响。方法 在不同的喷丸成形压力(0.42、0.50 MPa)下对7B50−T7751铝合金进行处理,分析材料的表面形貌。在此基础上,通过细节额定疲劳基准值和截止值进行计算,并进行压缩试验,结合铝合金材料在喷丸前后应变层的位错密度和形态,分析喷丸成形压力对合金材料疲劳寿命和静力性能的影响。结果 与未喷丸试件相比,在0.42 MPa的成形压力下,合金材料的疲劳寿命和静力性能均有所提高。喷丸成形之后,材料表层引入了一定深度的残余压应力层,形成位错密度较大的加工硬化组织,阻碍裂纹扩展,宏观上提高了材料的强度。在0.50 MPa的成形压力下,材料表面更加粗糙,裂纹易在晶粒连接薄弱处萌生,导致合金材料的疲劳寿命有所降低。结论 随着喷丸成形压力的增大,合金材料的疲劳寿命先增大后减小,抗压强度有所增大。在0.50 MPa的成形压力下,部分裂纹易于在弹坑边缘萌生,在一定程度上会降低合金材料的疲劳强度。     

工业纯铁自纳米化组织的EBSD分析

运用高能喷丸技术实现了工业纯铁表面自纳米化,利用电子背散射衍射（ElectronBackscatteredDiffraction,EBSD）对自纳米化组织的结构特征进行了分析,在此基础上探讨了工业纯铁自纳米化机理。结果表明,工业纯铁自纳米化组织由三个典型区域组成,分别含有大量残存住错,大角度晶界及大量小角度晶界。自纳米化组织含有再结晶织构和形变织构,但由于晶粒取向差不同,变形难易程度不同导致两种织构的比例不同。位错运动、晶粒取向差不同导致变形不同步及发生再结晶是工业纯铁表面自纳米化过程中晶粒细化的主要原因。     

Internal Friction and Elastic Study on Surface Nanocrystallized 304 Stainless Steel Induced by High—energy Shot Peening

The 304 stainless steel with nanostructured surface layer was successfully obtained by using the high-energy shot peening(HESP) method.The internal friction and Young‘s modulus of this kind of surface nanocrystallized material were dynamically measured by means of the vibrating reed apparatus.The results implied that different treatment time could induce different microstructure and distribution characteristic of defects in this kind of materials.It is also demonstrated that there is a transition layer between the nano-layern on surface and the coarse grain region inside.The transition layer obviously has certain influence on the overall mechanical properties.     

The investigation of internal friction and elastic modulus in surface nanostructured materials

In this paper, 304 stainless steel and pure Fe specimens, which were processed by high-energy shot peening (HESP) and ultrasonic shot peening (USP) respectively, were studied by the internal friction method. Measurements were carried out on a vibrating reed apparatus. The change of internal friction and elastic modulus shows that the treatment duration of specimen is not accompanied by the corresponding persistent increase of internal friction and elastic modulus. There is a transition layer from the top surface to the inside of the materials. Young’s modulus of surface shows obviously a fluctuation along the depth profile. The phenomena have never been shown by other measurement methods. The microstructure change should be related to some basic mechanism of surface layer formation. It may also explain why the improvement of mechanical properties in surface nanocrystallized materials does not simply correspond to the duration time of severe deformation.     

Thermal Stability of Surface Layer Microstructures of Commercially Pure Titanium Treated by High Energy Shot Peening

Commercially pure titanium was treated by high energy shot peening, and annealed at a series of temperatures. The surface layers are characterized by means of scan electronic microscope, X-Ray diffraction, transmission electronic microscope and micro-hardness testing machine. The results showed that microhardness of surface layers decreases with anneal temperature, the tendency of microhardness is similar to unannealed one, in other words, the more close to the surface, the more rapidly the hardness decreases, after reaches the depth of 50 μm, the decrease becomes steadily. But the sub-surface microhardness decreased suddenly over 500 ℃, From 550 ℃ to 650 ℃, the microhardness of surface layers almost unchanged.Observing by TEM and SEM, the grain sizes of pure titanium surface layers have increased below 500 ℃; Deformation twins begin disappearing obviously at 550 ℃; The nano-scaled grains within about 10 micrometers from surface existed even at 550℃.Surface nanocrystallization is well known as one of important methods to improve surface properties. The thermal stability of nanocrystalline microstructures was related to their preparation and application. The commercial pure Ti thermal stability of nanocrystalline and deformed microstructures induced by high-energy-shot-peening (HESP) technique was investigated. The nanostructured surface and deformed sub-surface layers of specimens were prepared through HESP treatment. The thermal stability was characterized through XRD analyses of surface layers, SEM and TEM microstructure observation and microhardness measurement of specimens annealed in different temperature in the air after HESP treatments. The results showed that after HESP treatment, the microhardness of surface layers increased with treatment time, especially in the rang of about 40 micrometers from the surface, the microhardness increase was obvious. The surface microhardness decreased gradually with annealing temperature, but the sub-surface microhardness decreased suddenly over 500℃. From 550 to 650 ℃, the microhardness of surface layers almost unchanged, and is still higher than that of the undeformed microstructure. SEM observation showed that at and below 500 ℃, the microstructure changes were not obvious. At 550 ℃, the grains in surface layers grew remarkably both in SEM and TEM images at 20 micrometers deep, and the deformed twins formed in HESP treatment could be seen in the subsurface. In addition, the TEM images showed that even at 550 ℃, the nanosized grains existed within 10 micrometers from surface.     

超声冲击纳米化对7B04高强铝合金疲劳性能的影响

探讨了超声波冲击表面纳米化作用后对7B04高强铝合金疲劳性能的影响,研究得到,应用超声波冲击表面纳米化技术,使7B04高强铝合金表面得到晶粒度10-50nm的纳米层,层厚约为20-50μm,并形成了由表面向里层的晶粒尺度从小到大的梯度结构,而且晶粒取向与冲击行走方向具有趋向一致性,超声波冲击作用于试样表面后形成了约为200Mpa的残余压应力层,使原来潜在的、或已存在的微小表面裂纹被压合,可提高7B04高强铝合金材料的疲劳寿命5-10倍。     

Combined effect of shot peening,subcritical austenitic nitriding,and cryo-treatment on surface modification of AISI 4140 steel

Shot peening is a simple but effective severe plastic deformation process to synthesize ultrafine grains in micro- to nanometer range on metallic surfaces. In this work, shot peening on AISI 4140 steel specimens was done in a novel centrifugal air blast shot peening reactor with shot velocity of 5.8?m/s for 3?h. Characterization of the shot peened surface (XRD, micro-hardness, SEM, and TEM) showed that surface undergoes significant plastic deformation with marked increase in microstrain of lattice, dislocation density, and surface hardness. XRD profiles and TEM analysis confirmed formation of ultrafine grain structure in the nanometer range. These specimens were then subjected to austenitic nitriding at 610°C for 4?h followed by cryo-treatment at???185°C for 32?h. Characterization of pre-shot peened nitrided and cryo-treated surfaces showed that there was marked improvement in surface hardness (from 695 to 797 HV_0.05) and effective case depth (from 19 to 54?µm) in comparison with un-shot peened nitrided and cryo-treated specimens. It was demonstrated that presence of ultrafine grain structure and austenitic phase during nitriding plays synergetic role to improve content and diffusion kinetics of nitrogen in AISI 4140 steel surface.     

Diffusion behavior of aluminum in the surface layer of iron processed by shot peening

An ultrafine-grained surface layer was fabricated on a pure Fe plate by the shot peening process. The average grain size within the surface layer of 10 μm thick is about 28 nm. The diffusion kinetics of Al was measured by secondary ion mass spectrometry within a temperature range of 300-380 °C. The diffusion coefficient of Al in the nanocrystalline layer is about 4 orders of magnitude higher than that in coarse-grained Fe. The activation energy of Al diffusion in nanocrystalline layer is 1.38 eV, which is much smaller than that reported for Al diffusion in the conventional Fe. The enhanced diffusivity of Al may originate from a considerable amount of nonequilibrium grain boundaries and a high density of dislocations in the surface layer processed by shot peening.     

Effect of ultrasonic shot peening on microstructure and properties of 301SS

Ultrasonic shot peening (USP) is an efficient way to improve the mechanical behavior of 301 stainless steel by inducing severe plastic deformation on its surface. However, this surface treatment induces complex microstructural evolutions, such as grain refinement and phase transformation. Therefore, a better understanding of those evolution mechanisms is critical to optimize the USP treatment. In this work, we rely on various electron microscope observations to compare a specimen before and after a 5-min shot peening treatment. We found an affected layer of ~450?µm with a significant increase in hardness on the top surface by a factor of 2.7 times. Inside this layer, we noticed a nanoscale grain layer of ~130?µm, the most strengthened layer, containing nanoscale grain of α′, with few γ and a large amount of low angle grain boundaries on the topmost. Afterward, we observed coarse grains with deformation twins, which seem to be a preferential site for martensite nucleation, especially at their intersection, and a high density of dislocation. We also conducted experiments to determine a possible scenario for the microstructural evolution, based on those observations.