Effect of submicrocrystalline state on strength and impact toughness of low-carbon 12GBA steel

The effect of intense warm deformation on the structure and mechanical properties of low-carbon 12GBA steel was investigated. A submicrocrystalline (SMC) structure with an average element size of 0.3 ??m was formed in the steel by isothermal overall forging. The formation of the SMC structure resulted in a sharp increase in strength by a factor of two to three in relation to the initial coarse-grained state while retaining a sufficient level of plasticity and impact toughness. After further annealing, steels exhibit an improved set of properties; i.e., as the strength decreases slightly, the plasticity increases sharply. Impact tests at low temperatures have shown the significant advantage of the SMC state of the steel over the coarse-grained state in the impact toughness. It is established that the cold resistance in the SMC state increases because the crack propagation prevails in the overall sample fracture.     

Properties and character of crack growth in extrusions of an Al-Cu-Li alloy

Tests of specimens of Al-Cu-Li alloys have shown that fatigue cracks in these materials sometimes grow in directions different from the applied load. This article examines the effect of different stages of aging on the rate of fatigue-crack growth (FCGR) and direction of crack propagation in tests conducted on extrusions of alloy 1450 to determine fracture toughness (KID) and resistance to fatigue.All-Union Scientific Research Institute of Aviation Materials (VIAM). ITTs VUK, Prague. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 36–38, November, 1994.     

内含硬α夹杂钛合金轮盘裂纹扩展研究

为研究钛合金轮盘内部硬α夹杂疲劳裂纹扩展特性,对含预置硬α夹杂钛合金轮盘开展低循环疲劳裂纹扩展试验。结果表明:5229次循环后轮盘破裂;疲劳断口宏观、微观特征显示,预置硬α夹杂为本次疲劳破坏的疲劳源;在裂纹扩展前期,轮盘断口裂纹扩展速率较材料试验数据快;在裂纹扩展中期,断口裂纹扩展速率曲线呈对数线性关系;为了解决疲劳裂纹扩展后期疲劳条带不易识别的问题,使用等效裂纹扩展模型拟合断口裂纹扩展速率曲线,从而可以利用疲劳条带宽度来计算总寿命。同时,利用断口数据,提出和总结了预置硬α夹杂钛合金轮盘裂纹扩展特性仿真研究的方法。仿真研究显示:基于Paris公式建立裂纹扩展模型能较好地预测轮盘裂纹扩展特性;轮盘由于疲劳发生最终断裂破坏时,裂纹前沿的应力强度因子远大于断裂韧性,因此,不宜使用应力强度因子直接作为破裂准则。     

Effect of Hierarchical Microstructures of Lath Martensite on the Transitional Behavior of Fatigue Crack Growth Rate

In this study, the fatigue-crack growth (FCG) behavior of 20CrMTiH steel with different substructure sizes was investigated. The results showed that coarsen microstructures exhibit excellent growth resistance. Moreover, two transitional behaviors were observed in the FCG curves of all specimens. The first transition point occurs in the near-threshold regime, whereas the second transition point occurs in the Paris regime. A comparison of substructure size to cyclic plastic size showed that the block size is almost equal to cyclic plastic size at ?K_T1, indicating that block size is an effective grain size to control the first transitional behavior of fatigue-crack propagation, whereas the second transitional behavior is related to the packet width or grain size. According to the fracture morphology, the fracture mechanism above and below the transition point responsible for the above phenomenon were distinguished. In addition, two prediction models based on microstructure size were established for lath martensite to evaluate the threshold and stress intensity factor range at the transition point.     

Localization of macrodeformation in submicrocrystalline titanium

A study of the deformation behavior of coarse-grained and submicrocrystalline commercially pure titanium has been carried out. The main specific feature of the behavior of the SMC material is the presence in the loading diagrams of a stage of prefracture, where the deformation occurs virtually without strengthening. At this stage, zones of localized plastic flow with different levels of deformation accumulation are observed. The zone with the maximum amplitude of deformation remains immobile fixed and marks the place of future destruction. The remaining regions of localized deformation move with a velocity that is the greater, the further the zone is located from the zone of fracture. It has been established that in the SMC titanium the local loss and the global loss of stability of plastic flow occur simultaneously. In the coarse-grained material the zone of future fracture is revealed even prior to the beginning of the formation of a macroscopic neck, i.e., the local loss of stability occurs comparatively early, although at the global level the material continues to be deformed quasihomogeneously. The results obtained can be used for correcting regimes of pressure treatment of nanostructured and submicrocrystalline materials.     

Modeling crack growth from pores under compressive loading with application to metallic glasses

The mechanism of the fatigue-crack growth is essential to understand the fatigue and fracture behavior of bulk metallic glasses (BMGs) and is thus critical to predict the service lifetime of BMGs as potential engineering structural materials. Experiments indicate that fracture under compressive loading exhibits distinct behaviors different from that under tensile loading. A typical compression failure may initiate from micro porosity where cracks propagate in a direction generally parallel to the loading axis. Micromechanical stress analysis shows that pores cause axial tensile microcracks emanating from the pore. A simplified computational model based on the linear elastic fracture mechanics (LEFM) is proposed to investigate crack initiation and subsequent propagation under compressive load, where the effect of crack closure on mode-I fracture is considered. The stable crack length is characterized by a dimensionless fracture-mechanics quantity required to attain the associated crack length. The behavior of crack growth is examined based on the stress-intensity-factor (SIF) calculation, and its dependence on the loading and lateral confinement conditions is discussed.     

GH864合金蠕变/疲劳裂纹扩展速率及a-N曲线分析

研究了GH864合金不同保载时间下650℃蠕变/疲劳裂纹扩展行为,分析了裂纹扩展过程中蠕变和氧化的作用,以及a-N曲线的转折点含义。结果表明:保载5s时GH864合金以穿晶断裂为主,疲劳作用占主导;保载90s时GH864合金以沿晶断裂为主,蠕变作用占主导。利用Saxena模型可较好地表征本实验条件下650℃蠕变/疲劳交互作用的裂纹扩展速率曲线,可估算较高应力强度因子和较低应力强度因子的裂纹扩展速率。另外,用Saxena模型可求出蠕变和疲劳的表达式,对比分析高温蠕变/疲劳交互作用的裂纹扩展过程中蠕变和疲劳的作用及所占的比例。最后针对a-Ni/Nf、da/dN-a曲线及da/dN-N曲线变换中出现的拐点,结合断口形貌分析了转折点对应的含义。高温合金及其它材料的裂纹扩展速率曲线也适用于以上曲线分析方法。     

Strength properties and structure of a submicrocrystalline Al–Mg–Mn alloy under shock compression

The results of studying the strength of a submicrocrystalline aluminum A5083 alloy (chemical composition was 4.4Mg–0.6Mn–0.11Si–0.23Fe–0.03Cr–0.02Cu–0.06Ti wt % and Al base) under shockwave compression are presented. The submicrocrystalline structure of the alloy was produced in the process of dynamic channel-angular pressing at a strain rate of 10⁴ s^–1. The average size of crystallites in the alloy was 180–460 nm. Hugoniot elastic limit σ_HEL, dynamic yield stress σ_y, and the spall strength σ_SP of the submicrocrystalline alloy were determined based on the free-surface velocity profiles of samples during shock compression. It has been established that upon shock compression, the σ_HEL and σ_y of the submicrocrystalline alloy are higher than those of the coarse-grained alloy and σ_sp does not depend on the grain size. The maximum value of σ_HEL reached for the submicrocrystalline alloy is 0.66 GPa, which is greater than that in the coarse-crystalline alloy by 78%. The dynamic yield stress is σ_y = 0.31 GPa, which is higher than that of the coarse-crystalline alloy by 63%. The spall strength is σ_sp = 1.49 GPa. The evolution of the submicrocrystalline structure of the alloy during shock compression was studied. It has been established that a mixed nonequilibrium grain-subgrain structure with a fragment size of about 400 nm is retained after shock compression, and the dislocation density and the hardness of the alloy are increased.     

Corrosion fatigue crack propagation behavior of A7N01P-T4 aluminum alloy welded joints from high-speed train underframe after 1.8 million km operation

Using the potentiodynamic polarization analysis, the fatigue crack propagation behavior of A7N01P-T4 aluminum alloy metal inert gas welded joints cut from a high-speed train underframe after 1.8 million km operation was studied in air and in a 3.5 wt% NaCl solution. The fracture surface and crack growth path were analyzed using optical microscopy, scanning electron microscopy, and electron backscattered diffraction. The results reveal that the corrosion fatigue crack growth rate of an A7N01P-T4 welded joint in a 3.5 wt% NaCl solution is higher than that in air. Furthermore, the corrosion fatigue crack growth rate is noted to be the fastest in the heat-affected zone, followed by the base metal, whereas it is the slowest in the weld metal, which is consistent with the corrosion resistance of the A7N01P-T4 joints. The second phase is observed to exhibit a significant influence on the corrosion fatigue crack propagation path. The cracks are noted to grow toward the soft orientation and have obvious plastic deformation during the propagation process, which indicates that the anodic dissolution is the main cause of the corrosion fatigue crack growth.     

Ti8LC低成本钛合金疲劳裂纹扩展行为研究

采用双重退火对Ti8LC低成本钛合金进行热处理,测试其拉伸性能,并对其疲劳裂纹扩展行为进行研究,探索其疲劳断裂机制.结果表明,提高双重退火的第1次退火温度,强度略有升高,塑性降低.da/dN试样断口3个区特征明显:预裂区为解理断裂机制.稳态扩展区裂纹以条纹循环机制向前扩展,同时能观察到很多二次裂纹.快速扩展区的断口呈韧窝型断裂特征.该合金的疲劳裂纹扩展速率对退火温度不敏感.     

超声强化2D12铝合金疲劳断裂过程的微观分析

采用超声强化工艺对2D12铝合金进行表面处理,借助金相显微镜和扫描电镜,并结合原位跟踪测量裂纹长度的方法,对强化后的疲劳断裂行为进行了研究。结果表明,超声强化后2D12铝合金表面晶粒得到细化,疲劳源主要产生于试样表面,仅个别向内部转移,强化后疲劳寿命提升了约8倍。其原因一方面是强化过程在试样表面引入了残余压应力,由于裂纹扩展过程倾向于连接裂纹扩展路径上的缺陷,所以残余压应力的存在效降低了裂纹在两缺陷间的扩展速率;另一方面晶粒细化导致晶界密度增加,加强了对裂纹扩展的阻碍,从而有益于提高裂纹扩展寿命。     

车轮铸钢铁基合金涂层改善热疲劳损伤的强化机制

用扫描电镜分析研究了铁基合金涂层对车轮铸钢材料抗热疲劳损伤机制的影响.结果表明,铁基涂层试样表面出现一些腐蚀坑和微裂纹,但并未出现剥落现象,涂层有效地阻碍了微裂纹向基体的传播,较好地避免了基体材料的热疲劳损伤.而在同等条件下未涂层试样不论是表面还是断口处都已遭受了较为严重的破坏.     

Effect of High Strain-Rate Deformation and Aging Temperature on the Evolution of Structure,Microhardness, and Wear Resistance of Low-Alloyed Cu–Cr–Zr Alloy

The effect of the preliminary high strain-rate deformation, performed via the method of dynamic channel-angular pressing (DCAP), and subsequent annealings on the tribological properties of a dispersionhardened Cu–0.092 wt % Cr–0.086 wt % Zr alloy has been investigated. It has been shown that the surfacelayer material of the alloy with a submicrocrystalline (SMC) structure obtained by the DCAP method can be strengthened using severe plastic deformation by sliding friction at the expense of creating a nanocrystalline structure with crystallites of 15–60 nm in size. It has been shown that the SMC structure obtained by the high strain-rate DCAP deformation decreases the wear rate of the samples upon sliding friction by a factor of 1.4 compared to the initial coarse-grained state. The maximum values of the microhardness and minimum values of the coefficient of friction and shear strength have been obtained in the samples preliminarily subjected to DCAP and aging at 400°С. The attained level of microhardness is 3350 MPa, which exceeds the microhardness of the alloy in the initial coarse-grained state by five times.     

Influence of surface finish on fatigue cracking behavior of reactor pressure vessel steel in high temperature water

The low cycle fatigue behavior of a low‐alloy reactor pressure vessel (RPV) steel was investigated in high temperature water. Main attention was paid to the effects of surface finish of specimens on fatigue cracking behavior. It was found that the influence of surface finish on fatigue resistance of the steel was strain‐rate dependent in high temperature water. Pretty obvious degradation of fatigue resistance appeared at fast strain rate with rougher surface finish. At slow strain rate, surface circumferential scratches promoted crack initiation and propagation. The fracture surface showed relatively flat and slight crack‐arrest features. At fast strain rate, surface scratches also promoted crack initiation, but seemed not to dominate crack propagation. The fracture surface showed typical terraced and fan‐like features. The above fatigue cracking behavior can be rationalized by a strain‐rate dependent environmentally assisted cracking process of low‐alloy RPV steel in high temperature water.     

On the nature of anomalously high plasticity of high-strength titanium nickelide alloys with shape-memory effects: I. Initial structure and mechanical properties

This work presents the results of studies of the Ti_49.4Ni_50.6 alloy of enhanced purity with shapememory effects in an ordinary coarse-grained state with an average grain size of 20–30 μm or in a submicrocrystalline state with an average grain size of 0.2–0.3 μm. In this alloy the initial structure, phase composition, martensitic transformations, mechanical properties, and character of fracture have been investigated in a wide temperature range. It has been shown that upon cooling and mechanical tests at room temperature, the alloy exhibits highly reversible thermoelastic martensitic transformations. It has been established that the alloy exhibits high values of the strength and plastic properties and strain-hardening coefficients.     

Vibration fracture resistance of 5052H112 aluminum alloy processed by cold rolling and friction stirring

Hot-rolled Al–3Mg alloy processed by various cold-rolled reductions exhibited excellent vibration fracture resistance, however subsequent friction stirring did not endow better vibration fracture resistance than cold-rolled samples in spite of possessing microstructural refinement and homogenization. The D–N curve under a constant initial deflection amplitude (6.5 mm) also showed that the vibration fracture resistance of cold-rolled specimens increased as a result of the slower crack propagation rate. All samples used in this investigation showed a work hardening feature from the initial stage of the D–N curve, and the deflection amplitude tended to increase as the number of vibration cycles increased. During this ascending stage, slip band cracking and strain hardening in the vicinity of the main crack could be recognized. The D–N curve feature of the cold-rolled samples depicted a longer plateau region while the deflection amplitude was at its maximum. It is suggested that the dislocation tangles introduced by cold rolling can strengthen the matrix. As a result, the deflection amplitude drops could be associated with increasing the dislocation tangles due to prior cold rolling while the main crack achieved a critical crack length.     

Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution

The S–N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages: the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.     

损伤容限型TC4-DT钛合金近门槛区疲劳裂纹扩展行为研究

研究了双态组织、片层组织TC4-DT钛合金在近门槛区的疲劳裂纹扩展行为,通过扫描电镜观察裂纹扩展路径及断口微观特征,研究了等轴初生α相含量对TC4-DT钛合金在近门槛区疲劳裂纹扩展速率的影响,讨论了TC4-DT钛合金的疲劳裂纹扩展行为和断裂方式。结果表明:随着等轴初生α相含量的降低,TC4-DT钛合金在近门槛值区的da/dN-△K曲线逐渐向下偏折,裂纹扩展速率明显降低,在Paris区出现转折点现象且转折点对应的△Kt值逐渐增大;片层组织在近门槛区的裂纹扩展路径曲折,疲劳裂纹扩展速率显著降低,表现出更好的损伤容限性能。     

Na,K杂质及微量Ce对8090合金裂纹扩展阻力的影响

研究了含0.05%Ce及不同Na,K杂质含量的8090合金薄板裂纹扩展阻力,探讨了合金断裂韧性及裂纹起始扩展阻力随(Na+K)含量变化的规律和微量Ce的作用,结果指出:随(Na+K)含量增加,合金裂纹扩展阻力下降;若(Na+K)含量处于一定范围内时,合金中加入0.05%Ce会改善其断裂韧性及裂纹起始扩展抗力;当(Na+K)含量过高,Ce的这种有益作用便被掩盖,Na,K对合金韧性造成危害的原因之一是可以促使T_1等相沿晶界或亚晶界析出,并促使再结晶晶粒异常长大。     

Fatigue threshold and crack propagation in γ-TiAl sheets

The fatigue crack propagation behaviour of two different microstructures — a coarse-grained designed fully lamellar (DFL), and a fine-grained near γ (FG) — of a Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) alloy was studied. Both the threshold of stress intensity range and standard long crack growth behavior were determined. A special technique was applied to separate the different mechanisms — intrinsic and extrinsic effects — and their changes with crack length. The fatigue crack propagation rate of long cracks is much smaller in the DFL microstructure than in the FG microstructure at the same stress intensity range. The effective threshold of stress intensity range of both microstructures is about 1.7 MPa√m. The threshold of stress intensity range shows a strong R-curve behavior. In other words the propagation–non-propagation conditions of cracks are significantly influenced by the crack extension. The long crack thresholds of stress intensity range at the stress ratio 0.1 are relatively large; they are about 4.5 and 8 MPa√m in the DFL and the FG microstructure, respectively. The differences in the crack growth behavior between the two microstructures are mainly induced by extrinsic resistance mechanisms.