预蠕变对奥氏体不锈钢和黄铜应力腐蚀的影响

用恒载荷和恒位移试样分别研究了预蠕变对奥氏体不锈钢在成膜(42%MgCl_2)和不成膜(0.5mol/L HCl+0.5mol/L NaCl)溶液,以及黄铜在氨水溶液中应力腐蚀的影响结果表明,如用恒载荷试样,则无论是不锈钢还是黄铜,预蠕变对应力腐蚀断裂时间和门槛值均没有影响;如用恒位移试样,则预蠕变能使不锈钢和黄铜应力腐蚀门槛值成倍提高黄铜在氨水中加载阴极保护足够长时间后能抑制随后开路条件下的应力腐蚀,这归因于阴极极化改变溶液成分和pH值。

预蠕变损伤对高温疲劳宏观裂纹扩展的影响

针对ＳＵＳ３０４不锈钢光滑试棒预先导入１０７３ Ｋ·ｃｐ－ｔｙｐｅ条件下的预蠕变疲劳损伤，然后开小切口进行时间依存性 ９２３ Ｋ·ｃｐ－ｔｙｐｅ，１０７３ Ｋ·ｃｐ－ｔｙｐｅ及循环数依存性９２３ Ｋ·ｐｐ－ｔｙｐｅ的宏观裂纹扩展试验，考察了试棒内部因预蠕变疲劳而产生的大量的粒界微小裂纹对高温疲劳宏观裂纹扩展的影响．结果如下： １．预损伤加速了９２３ Ｋ·ｃｐ－ｔｙｐｅ下的蠕变裂纹扩展，对于同一蠕变Ｊ积分范围△Ｊｃ，损伤值越大，裂纹扩展速度ｄｌ／ｄＮ也越大．这种加速起因于主裂纹与微小裂纹的合体． ２．１０７３ Ｋ·ｃｐ－ｔｙｐｅ下的预损伤材料和处女材料的ｄｌ／ｄＮ在同一△Ｊｃ。下相等．即，损伤材料的裂纹扩展速度的上限值由１０７３ Ｋ·ｃｐ－ｔｙｐｅ下的处女材料的ｄｌ／ｄＮ－△Ｊｃ关系给出． ３．在９２３ Ｋ·ｐｐ－ｔｙｐｅ条件下，对于同一疲劳Ｊ积分范围△Ｊｆ，预损伤材料的ｄｌ／ｄＮ要比处女材料快１０倍左右．一般ｐｐ－ｔｙｐｅ的破坏形式为粒内破坏．预损伤材料的场合，因为试棒内部分布有大量的微小粒界裂纹，主裂纹便沿这些破坏阻抗最小的微小裂纹边合体边扩展，主要在粒界上扩展．即微小粒界裂纹是裂纹扩展阻抗减小的主因．

316LN奥氏体不锈钢在预蠕变条件下位错的变化

在320℃（核电主管道APl000近似运行温度）和600℃（液态用核电材料近似运行温度）条件下，分别在20和120MPa的应力下老化500和2000h进行316LN不锈钢预蠕变试验。通过光学显微镜（OM）、扫描电子显微镜（SEM）和透射电子显微镜（TEM）观测预蠕变后试样的微观组织。结果表明：在预蠕变阶段受到温度和应力作用，晶面滑移产生位错以及原有位错发生分解，被激活的位错在同一个平面内沿不同方向滑移移动，相遇时发生反应生成四边形或六边形的位错网络，随着温度以及应力增大位错网络消失。对于硬度变化而言随着温度从320℃升高到600℃时，显微硬度逐渐升高。

预蠕变对316不锈钢组织的影响

在300℃和600℃分别对316不锈钢在20~120 MPa的应力范围内预蠕变500 h和2 000 h,研究了预蠕变对316不锈钢组织的影响。结果表明,300℃时,位错在晶界处大量聚集,未发生动态再结晶,断口特征为塑性断裂。600℃时,发生动态再结晶,晶粒尺寸增加,位错逐渐消失,断口为蠕变孔洞及韧窝的混合状态。

预蠕变对应力腐蚀的抑制作用

早期工作认为,如预先蠕变足够长时间,则可抑制低碳钢在碳酸盐溶液中的应力腐蚀。对高强度钢,如加载后在干空气(如P_2O_5)中放置10min,则可抑制在水中的应力腐蚀。如果预先蠕变确实能抑制应力腐蚀或大幅度提高其门槛值,则在工业生产中为防止应力腐蚀提供了一个廉价而有效的手段。以前的工作仅是定性报道,本文将定量研究预蠕变对应力腐蚀性能的影响。

Effect of pre-deformation on aging creep of Al-Li-S4 alloy and its constitutive modeling

A set of uniaxial tensile creep tests at different pre-deformations, aging temperatures and stress levels were carried out for Al-Li-S4 alloy, and the creep behavior and the effects of pre-deformation on mechanical properties and microstructures were determined under basic thermodynamics conditions of aging forming. The results show that pre-deformation shortens the time of primary creep and raises the second steady-state creep rate. Then, the total creep strain is greater, but in the range of test parameters it is still smaller than that without pre-deformation. In addition, transmission electron microscopy（TEM） observation shows that pre-deformation promotes the formation of T1 phase and θ′ phase and makes them distribute more dispersively, while inhibits the generation of δ′ phase, which leads to the improvement of mechanical properties of the alloy. A unified constitutive model reflecting the effects of aging mechanism, stress levels and different pre-deformations was established. The fitting results agree with the experimental data well.

Effects of pre-deformation mode and strain on creep aging bend-forming process of Al-Cu-Li alloy

The bending deformation method was adopted to characterize the creep deformation behavior of Al-Cu-Li alloy in the creep aging forming(CAF) process based on a series of CAF tests, and the evolution laws of its mechanical properties and microstructures under different pre-deformation conditions were studied. The results show that the bending creep strain characterization method can intuitively describe the creep variation. With the increase of the pre-deformation strain, the creep strain of the specimen firstly increases and then decreases. The increase of the pre-deformation strain can promote the course of aging precipitation, and improve the formed alloy’s tensile properties at room temperature, the Kahn tearing properties, and the fatigue propagation properties. Pre-rolled specimens produce a slightly weaker work hardening than pre-stretched specimens, but they also create a stronger aging-strengthening effect;thus the strength, toughness and damage performance can be improved to some extent. Among all the types of specimens, the specimen with 3% rolling after CAF treatment has the best comprehensive mechanical properties.

Modification of the linear viscoelastic deformation prediction model of asphalt mixture

A deformation prediction model for the dynamic creep test is deduced based on the linear viscoelastic（LVE）theory.Then,the defect of the LVE deformation prediction model is analyzed by comparing the prediction of the LVE deformation model with the experimental data.To improve accuracy,a modification of the LVE deformation prediction model is made to simulate the nonlinear property of the deformation of asphalt mixtures,and it is verified by comparing its simulation results with the experimental data.The comparison results show that the LVE deformation prediction model cannot simulate the nonlinear property of the permanent deformation of asphalt mixtures,while the modified deformation prediction model can provide more precise simulations of the whole process of the deformation and the permanent deformation in the dynamic creep test.Thus,the proposed modification greatly improves the accuracy of the LVE deformation prediction model.The modified model can provide a better understanding of the rutting behavior of asphalt pavement.

Intelligent back analysis of geotechnical parameters for time-dependent rock mass surrounding mine openings using grey Verhulst model

In this paper,we present a new method of intelligent back analysis(IBA)using grey Verhulst model(GVM)to identify geotechnical parameters of rock mass surrounding tunnel,and validate it via a test for a main openings of−600 m level in Coal Mine“6.13”,Democratic People's Republic of Korea.The displacement components used for back analysis are the crown settlement and sidewalls convergence monitored at the end of the openings excavation,and the final closures predicted by GVM.The non-linear relation between displacements and back analysis parameters was obtained by artificial neural network(ANN)and Burger-creep viscoplastic(CVISC)model of FLAC3D.Then,the optimal parameters were determined for rock mass surrounding tunnel by genetic algorithm(GA)with both groups of measured displacements at the end of the final excavation and closures predicted by GVM.The maximum absolute error(MAE)and standard deviation(Std)between calculated displacements by numerical simulation with back analysis parameters and in situ ones were less than 6 and 2 mm,respectively.Therefore,it was found that the proposed method could be successfully applied to determining design parameters and stability for tunnels and underground cavities,as well as mine openings and stopes.

Effects of pre-compression on formation of dynamic precipitates and creep anisotropy of hot-extruded AZ91−2Y magnesium alloy

The creep anisotropy behavior under different stresses at 180℃ of hot-extruded AZ91−2Y magnesium alloy with pre-compression(PC)and without pre-compression(NPC)was studied.Microstructure,texture and mechanical properties of the alloy were examined by scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM)and tensile creep tests.The results revealed that the creep resistance was proportional to the volume fraction of spherical Mg_(17)Al_(12) precipitates.The dynamic precipitation of large volume fraction of lamellar Mg_(17)Al_(12) in NPC samples leads to the basaláañslip as the dominant creep mechanism,and the NPC samples have obvious anisotropy.In the PC samples,dynamic precipitation of large volume fraction of spherical Mg_(17)Al_(12) has inhibitory effect on the basaláañslip.The pyramidalác+añslip and twinning improve the creep anisotropy resistance significantly.