Selection of risk response actions with consideration of secondary risks

Secondary risk in project risk management refers to the risk that arises as a direct result of implementing a risk response action (RRA). It is important for project managers (PMs) to consider the effects caused by the secondary risks in the process of RRA selection. The purpose of this paper is to propose an optimization method to address the problem of selecting risk response actions (RRAs) with consideration of secondary risk which is seldom considered in the existing studies. The optimization model aims to minimize the total risk costs with time constraint being placed on the project makespan. By solving the model, an optimal set of RRAs along with the earliest start time for each activity can both be obtained. The results show that secondary risk plays an important role in the process of RRA selection. Project managers should allocate more budget for responding the project risk when the secondary risk is considered, and consider all factors relating to both time and cost so as to select appropriate RRAs to mitigate primary risk and secondary risk.     

Effect of heat treatment on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy

In this study, effect of various aging tempers (T6, T73 and RRA treatment) on the microstructure and mechanical properties of the spray-deposited Al–10.8Zn–2.8Mg–1.9Cu alloy was studied using high-resolution electron microscopy, selected area diffraction, and tensile tests. The results indicate that the two types of GP zones, GPI and GPII, are major precipitates for the alloy under T6 condition. No clear precipitation free zone was observed, and the grain boundary precipitates were continuous. Under two-step aging condition, the GP zones and η′ are major precipitates for the alloy, the discontinuous grain boundary precipitates are favorable to SCC resistance in over-aged condition, which reduces its strength 58 MPa (about 7%) compared to the peak-aged condition. After retrogression and re-aging treatment, the grain boundary precipitates are discontinuous, which is closed to that resulting from T73 temper. RRA treatment decreased ultimate tensile strength 25 MPa (about 3%) in values compared with the alloy at T6 condition.     

Al-6.1Zn-2.6Mg-1.6Cu 铝合金的回归再时效热处理

The effects of the retrogression temperature and time of retrogression and re-aging heat treatment(RRA) on the hardness and electrical conductivity of Al-6.1Zn-2.6Mg-1.6Cu aluminum alloy were studied. Samples were pre-aged at 120℃ for 24h as the first-stage treatment. Then, retrogression was performed at a temperature range of 170～250℃ for times of between 1min and 180min, followed by re-aging at 120℃ for 24h. Hardness (H) and electrical conductivity (EC) measurements were used to characterize the samples after RRA treatment. Analysis of the results shows: (1)The re-aging treatment at 120℃ for 24 h increases both H and EC of the retrogressed alloy in the RRA process;(2) RRA with retrogression at higher than 200℃ result in EC higher than that of peak-aged, but H lower; The change of H and EC with respect to retrogression temperature (T) and time (t) can be seen as functions of H (t) = H0 A1e(-t/s) ,EC(t) =A(1-e(-k·(t-Xc)));(3) RRA treatments with retrogression at 190℃ for 4～30min result in H and EC which are both higher than those of the peak-aged temper, and retrogression at 190℃for 30min is the industrial application that yields H of 190 HV and EC of 33.5%IACS.     

Effect of residual hydrogen content on the tensile properties and crack propagation behavior of a type 316 stainless steel

The tensile properties and crack propagation rate in a type 316 austenitic stainless steel prepared by vacuum induction melting method with different residual hydrogen contents (1.1–11.5 × 10⁻⁶) were systematically investigated in this research work. The room temperature tensile properties were measured under both regular tensile (12 mm/min) and slow tensile (0.01 mm/min) conditions, and the fracture properties of the tensile fractures with both rates were analyzed. It shows that the hydrogen induced plasticity loss of stainless steel strongly depends on the tensile rate. Under regular tensile condition, there is no plastic loss even when the hydrogen content is up to 11.5 × 10⁻⁶ while in the slow tensile condition, the plastic loss can be clearly identified rising with the increasing H contents. The fatigue crack propagation rate was tested at room temperature, and the crack growth rate formula (Paris) of the 316 stainless steels with varied H contents were obtained. The fatigue crack propagation rate test shows that the crack growth rate of the 316 stainless steel with 8.0–11.5 × 10⁻⁶ hydrogen is significantly higher than that of benchmark steel.     

含Sc7075合金的回归热处理

通过金相显微镜,扫描电镜,能谱(EDS)分析仪对含Sc的7075铝合金的组织结构进行了分析,并研究了其力学性能。结果表明：含0.22%Sc的7075铝合金经过优化回归热处理(120℃×16 h+160 ℃×15 min+120 ℃×16 h)后,合金的力学性能得到了显著提高。测得合金的抗拉强度、屈服强度及伸长率分别为714 MPa、645 MPa和10.8%,较T6时效分别提高了18.2%、14%和6.5%。     

时效制度对LC52铝合金组织与性能的影响

研究了在单级时效、双级时效、回归再时效 (RRA)处理条件下 ,LC5 2铝合金的微观组织、室温力学性能、抗剥落腐蚀性能和应力腐蚀开裂敏感性。结果表明 ,LC5 2铝合金经 12 0℃× 2 4h +2 5 0℃× 2min(油浴 ) +12 0℃×30h回归再时效处理后 ,具有良好的综合性能     

LC52铝合金抗剥落腐蚀性能研究

测定了LC52铝合金时效后的剥落腐蚀性能，并对该合金各种状态下的显微组织进行了TEM分析，研究了影响该合金腐蚀性能的主要因素以及显微组织与性能之间的关系。结果表明，LC52铝合金经RRA处理后，具有良好的抗剥落腐蚀性能。测试室温下不同时效状态LC52铝合金试样的恒电位极化曲线，发现可以以合金的极化电阻定性地描叙合金的抗剥落腐蚀性能，且当合金的极化电阻越大时，合金的抗剥落腐蚀性能越好。     

Retrogression and re-aging heat treatment of Al-6. 1Zn-2. 6Mg-1.6Cu aluminum alloy

The effects of the retrogression temperature and time of retrogression and re -aging heat treatment （RRA） on the hardness and electrical conductivity of Al-6. 1Zn-2.6Mg - 1.6Cu aluminum alloy were studied. Samples were pre-aged at 120℃ for 24 h as the first-stage treatment. Then, retrogression was performed at a temperature range of 170 ～250℃ for times of between 1 min and 180 min, followed by re - aging at 120 ℃ for 24 h. Hardness （H） and electri- cal conductivity （EC） measurements were used to characterize the samples after RRA treatment. Analysis of the results shows： （ 1 ） The re - aging treatment at 120 ℃ for 24 h increases both H and EC of the retrogressed alloy in the RRA process ; （2） RRA with retrogression at higher than 200℃ result in EC higher than that of peak - aged, but H lower; The change of H and EC with respect to retrogression temperature （T） and time （t） can be seen as functions of H （t） = H0 ＋A1e^（-t/s） , EC （t） =A （1 -e^{-k·（t-xc））） ; （3） RRA treatments with retro- gression at 190 ℃ for 4 ～ 30 min result in H and EC which are both higher than those of the peak-aged temper, and retrogression at 190℃ for 30 min is the industrial application that yields H of 190 HV and EC of 33.5% IACS.     

Al-Zn-Mg-Cu系高强铝合金研究进展

简要评述了Zn、Mg、Cu、Cr、Zr等主要元素及稀土对Al—Zn—Mg—Cu系合金性能的影响，介绍了热处理工艺和进展以及制备Al—Zn—Mg—Cu系合金的新技术，指出了现存的问题并展望了发展趋势。