T6处理对挤压变形AI-- Mg-- Si （ -- Er） 合金低周疲劳行为的影响

为了明确T6处理对A1-O．8％Mg-0．6％Si（-0．5％Er）合金的循环应力响应行为和循环应力-应变行为的影响,进行了应变控制的室温低周疲劳实验。实验结果表明,挤压态Al-O．8％Mg-0．6％Si（-0．5％Er）合金在绝大多数应变幅下呈现出循环应变硬化,T6态Al-O．8％Mg-0．6％Si（-0．5％Er）合金的循环应力响应行为表现为循环应变硬化和循环稳定;挤压变形Al-O．8％Mg-O．6％Si（-O．5％Er）合金经过T6处理后,其循环变形抗力均显著提高;挤压变形Al-O．8％Mg-O．6％Si合金经过T6处理后,其应变疲劳参数K’显著提高,n’稍稍降低;挤压变形Al-O．8％Mg-O．6％Si-0．5％Er经过T6处理后,其应变疲劳参数K’与n’均明显提高。     

不同温度环境下Al–7Zn–2.5Mg–2.0Cu–0.1Zr–0.2Sc合金的力学行为研究

目的 研究不同时效时间对Al–7Zn–2.5Mg–2.0Cu–0.1Zr–0.2Sc合金强度的影响,以及室温和-40℃这2种温度环境对该合金疲劳行为的影响。方法 在不同时效时间下对Al–7Zn–2.5Mg–2.0Cu–0.1Zr–0.2Sc合金进行热处理,并采用透射电镜观察其显微结构以解释不同时效时间下强度变化的原因。在不同外加总应变幅的条件下,对T6态该合金进行低周疲劳实验,对比研究Al–7Zn–2.5Mg–2.0Cu–0.1Zr–0.2Sc合金在不同温度环境下的低周疲劳行为。结果 随着时效时间的延长,不同温度环境下Al–7Zn–2.5Mg–2.0Cu–0.1Zr–0.2Sc合金的屈服强度和抗拉强度都先升高后降低,-40℃环境下的屈服强度和抗拉强度均高于室温环境下的。在低应变幅时,合金的循环应力响应行为特征总体呈稳定趋势,在高应变幅时,合金的循环应力响应行为先表现为循环稳定特征,后表现为循环硬化特征。同一应变幅下,-40℃环境下合金的循环应力幅值高于室温环境下的,而合金的低周疲劳寿命则随着温度的降低而下降。此外,在室温和-40℃低周疲劳加载条件下,疲劳变形机制为平面滑移机制。当应变幅...     

挤压变形Al—0．8％Mg—0．6％Si—xSc合金的显微组织与力学性能

本文针对挤压变形Al—0．8％Mg—O．6％Si-xSc合金的显微组织和拉伸性能进行了研究,以确定稀土元素Sc和T6处理对该系合金性能的影响规律。结果表明,加入适量的元素Sc可以有效地细化挤压变形Al0．8％Mg-0．6％Si—xSc合金的组织,提高其室温抗拉强度、屈服强度和断裂伸长率;经过T6处理后,Al—08％Si-0．6％Si—xSc舍金的抗拉强度和屈服强度可得到显著提高;挤压变形Al—0．8％Mg～O．6％Si～xSc合金在拉伸加载条件下主要呈现韧性断裂特征。     

连续等通道转角挤压对Al-Ti-C合金组织与性能的影响EI北大核心CSCD

采用连续等通道转角挤压工艺,以连续的方式对Al-Ti-C合金进行多道次挤压,通过观察微观组织演化,探讨晶粒细化机理和力学性能变化。结果表明:连续等通道转角挤压工艺可有效细化Al-Ti-C合金微观组织,晶粒尺寸减小至1μm左右,形变诱导是变形过程中最主要的晶粒细化机制;高密度位错堆积引起Al基体和TiAl_(3)界面的裂纹以及TiAl_(3)内部的空洞产生,裂纹进一步扩展贯穿整个TiAl_(3)颗粒,最终导致第二相TiAl_(3)组织的细化,同时细小的第二相TiAl_(3)组织的钉扎机制和剪切机制促进了Al基体细化;连续等通道转角挤压1道次后,合金硬度提升最明显,与原始态相比提高59.2%;之后随挤压道次的增加,硬度提升的趋势变缓,合金塑性下降,韧性提高。     

800MPa级低合金高强度钢的循环软硬化特性

在不同应变级别控制下,研究了800MPa级低合金高强度钢10CrNiSMoV轴向加载低周循环软硬化特性,研究结果表明：应变加载级别不同,材料表现出不同的循环软硬化特性,当应变幅在0．4％～1．0％范围变化时,材料表现出循环软化的特性,且应变幅越大,循环软化速率越高;当应变幅在0．30％-0．40％范围变化时,材料表现出先硬化,再趋于循环饱和,再轻微循环软化直至断裂的特性。采用透射电镜观察了不同应变级别下试样断口的位错形态,用位错理论解释了产生上述现象的原因。     

工业纯钛低周疲劳应力—应变响应

测量了工业纯钛低周疲劳的应力－应力响应,选择动态模量ＤＭ与弹性模量ＥＭ之比值ＤＭ．ＥＭ衡量材料的相对“软”“硬”程度,发现在各个应变幅下ＤＭ．ＥＭ有向０．３５－０．４０靠近的趋势。讨论了新的参量ＤＭ．ＥＭ与循环硬化循环软件之间的关系。     

变形Al-Sr中间合金的变质遗传效应

经变形处理后的Al－5％Sr中间合金与未变形的相比,具有明显的变质效果差异在砂型条件下,用Al－5％Sr中间合金对共晶Al－Si合金进行变质处理,加入含锶量为0．1％的锭料,变质度为5级;加入变形处理的中间合金时,含锶量仅为0．05％时变质度为5级,含锶量为0．06％即达到最佳变质效果,为5.5级在金属型条件下,加入含锶量为0．06％的锭料与加入含锶量0.03％的变形处理的中间合金的变质效果相当,都为6级这种变质效果的差异是Al－5％Sr中间合金的组织遗传效应造成的     

500MPa级针状铁素体钢的低周疲劳行为

采用光滑圆柱疲劳试样,通过对500MPa级针状铁素体钢进行轴向总应变控制模式的室温低周疲劳实验,研究了实验钢的循环应力-应变行为、应变-寿命特性及循环应力响应。结果表明:当总应变幅为临界应变幅时(Δεt/2=0.35%),循环一开始就进入饱和状态,应力幅值保持不变;当总应变幅低于临界应变幅时(0.2%≤Δεt/2<0.35%),实验钢表现出循环软化特性;当总应变幅高于临界应变幅时(0.35%<Δεt/2≤0.6%),实验钢先表现出循环硬化,达到循环饱和后又表现出循环软化特性。疲劳断口的SEM分析表明,疲劳裂纹通常萌生于试样一侧的表面,具有多源性,且裂纹的扩展符合Laird的塑性钝化机制。     

ZTC4（Ti-6Al-4V）铸造钛合金的室温低周疲劳行为

研究了ZTC4钛合金应变控制的室温低周疲劳行为,对循环应力-应变和应变疲劳寿命数据进行了分析,通过双对数线性回归处理,得出了Manson-Coffin处理模型的疲劳参数。结果表明：ZTC4钛合金总应变幅在0.6%～0.8%时,材料存在轻度循环软化的现象;总应变幅为0.4%～0.5%时,循环初期表现出循环硬化的现象,而后循环软化。合金的疲劳裂纹萌生于试样表面,裂纹扩展区存在明显的疲劳条带,合金疲劳断口呈现韧性断裂特征。     

含Zr、Sc的Al-Zn-Mg-Cu合金的低周疲劳行为

利用透射电子显微镜和低周疲劳试验机研究了单级时效状态及回归再时效状态两种含Zr、Sc的Al-Zn-Mg-Cu合金的微观组织和低周疲劳性能。结果表明:单级时效基体析出相以η′相为主,晶界析出连续分布平衡相,并伴有晶间无析出带;回归再时效基体析出相略有长大,晶界析出相长大明显,无析出带变宽。低周疲劳加载条件下,合金在0.4%~0.7%外加总应变幅范围内表现出循环稳定性;在0.8%的应变幅下,呈现先软化后硬化。在0.4%~0.6%较低的外加总应变幅范围内,回归再时效合金表现出较高的低周疲劳寿命。两种状态合金的塑性应变幅和弹性应变幅与载荷反向周次之间均成直线关系,并可分别用Coffin-Manson公式和Basquin公式来描述。两种状态的合金的疲劳裂纹均萌生于试样表面,并以穿晶方式扩展。     

无铅焊料Sn-3.8Ag-0.7Cu的低周疲劳行为

测量了Sn-3.8Ag-0.7Cu无铅焊料试样的循环滞后回线、循环应力响应曲线、循环应力-应变和应变寿命关系,研究了焊料在总应变幅控制下的低周疲劳行为结果表明:该焊料合金在总应变幅较高(1%)时发生连续的循环软化,而在总应变幅较低(≤0.4%)时则表现为循环稳定.线性回归分析表明,该焊料的低周疲劳寿命满足Coffin-Manson经验关系式,由此给出了焊料在室温下的低周疲劳参数.采用扫描电镜观测和分析了焊料在疲劳前后的组织特征.     

Effect of Sc addition on low-cycle fatigue properties of extruded Al-Zn-Mg-Cu-Zr alloy

ABSTRACT

The influence of minor Sc addition on the low-cycle fatigue (LCF) properties of hot-extruded Al-Zn-Mg-Cu-Zr alloy with T6 state was investigated through performing the LCF tests at room temperature and air environment. The results indicate that two alloys show cyclic stabilisation, cyclic hardening and cyclic softening during fatigue deformation. The addition of Sc can significantly enhance the cyclic stress amplitude of the alloy. Al-Zn-Mg-Cu-Zr-Sc alloy shows higher fatigue lives at lower strain amplitudes, while has lower fatigue lives at higher strain amplitudes. For the two alloys, the density and movability of dislocations are related to the change of cyclic stress amplitudes. The existence of Al₃(Sc,Zr) phase can inhibit the appearance of cyclic softening phenomenon in the Al-Zn-Mg-Cu-Zr-Sc alloy.     

The cyclic strain resistance, fatigue life and final fracture behavior of magnesium alloys

This paper summarizes the results of a comprehensive study on the cyclic strain resistance, low-cycle fatigue life and fracture behavior of three rapidly solidification processed magnesium alloys. Test specimens of the magnesium alloy were cyclically deformed under fully-reversed total strain amplitude control straining, over a range of strain amplitudes, giving less than 10⁴ cycles to failure. The cyclic stress response characteristics, strain resistance and low-cycle fatigue life of the alloys are discussed in light of alloy composition. All three alloys follow the Basquin and Coffin-Manson strain relationships, and exhibit a single slope for the variation of cyclic elastic and cyclic plastic strain amplitude with reversals-to-fatigue failure. The cyclic stress response characteristics, fatigue life and final fracture behavior of the alloy are discussed in light of competing and synergistic influences of cyclic total strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructural feature interactions during fully-reserved strain cycling.     

Low cycle fatigue behavior of Cu-Cr-Zr alloy with different cold deformation

The present study addressed the cyclic deformation behavior and fatigue properties of Cu-0.69Cr-0.07Zr alloy with different cold deformation (ε = 64%, 75%, and 84%) using low cycle fatigue test. Low cycle fatigue tests were conducted under fully-reversed conditions at different total strain amplitudes. The microstructure changes and fatigue fracture characteristics were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The main findings suggest that the Bauschinger effect was significantly stronger with larger deformation at low total strain amplitude. And it was proved that the relationship between the total strain amplitude and the low cycle fatigue life of Cu-Cr-Zr alloy with different deformation can be expressed by the Manson–Coffin–Basquin formula. Further, the reason for the fatigue life was shorter and the cyclic softening rate decreased faster at high applied total strain amplitude was that the dislocation density decreased due to the rearrangement of the dislocations.     

Numerical simulations of cyclic behaviors in light alloys under isothermal and thermo-mechanical fatigue loadings

In this article, numerical simulations of cyclic behaviors in light alloys are conducted under isothermal and thermo-mechanical fatigue loadings. For this purpose, an aluminum alloy (A356) which is widely used in cylinder heads and a magnesium alloy (AZ91) which can be applicable in cylinder heads are considered to study their stress–strain hysteresis loops. Two plasticity approaches including the Chaboche’s hardening model and the Nagode’s spring-slider model are applied to simulate cyclic behaviors. To validate obtained results, strain-controlled fatigue tests are performed under low cycle and thermo-mechanical fatigue loadings. Numerical results demonstrate a good agreement with experimental data at the mid-life cycle of fatigue tests in light alloys. Calibrated material constants based on low cycle fatigue tests at various temperatures are applied to models to estimate the thermo-mechanical behavior of light alloys. The reason is to reduce costs and the testing time by performing isothermal fatigue experiments at higher strain rates.     

Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

Al-5.4Zn-2.6Mg-1.4Cu合金板材的低周疲劳行为

进行Al-5.4Zn-2.6Mg-1.4Cu合金板材的室温低周疲劳实验,对比研究了轴向平行于轧制方向(RD方向)和垂直于轧制方向(TD方向)试样的低周疲劳行为。结果表明：对于0.4%~0.8%的外加总应变幅,RD和TD方向合金试样的循环应力响应行为均呈现出循环稳定;对于相同的外加总应变幅,TD方向合金的循环应力幅值高于RD方向,而RD方向合金的疲劳寿命高于TD方向。对于RD和TD方向,Al-5.4Zn-2.6Mg-1.4Cu合金的塑性应变幅、弹性应变幅与载荷反向周次均呈线性关系。在低周疲劳加载条件下,裂纹在疲劳试样的自由表面以穿晶方式萌生和扩展。     

Cyclic shear deformation and fatigue of extruded Mg-Gd-Y magnesium alloy

Deformation and fatigue of extruded Mg-8.0 Gd-3.0 Y-0.5 Zr(GW83, wt%) magnesium(Mg) alloy were experimentally investigated under cyclic torsion using tubular specimen fabricated along the extrusion direction. The controlled shear strain amplitudes ranged from 0.606% to 4.157%. Twinning and detwinning of extension twins are observed to take place during cyclic torsion and the shear stress-shear strain hysteresis loops display a perfectly symmetric shape at all tested strain amplitudes. Marginal cyclic softening is observed when the shear strain amplitude is higher than 1.732%. The strain-life fatigue curve shows two kink points, corresponding to the shear strain amplitude of 1.040% and 1.732%, respectively.When the shear strain amplitude is higher than the upper kink point, early fatigue crack is found to initiate on the maximum shear plane. When the strain amplitude is lower than the lower kink point,fatigue cracking is parallel to the maximum tensile plane. At an identical equivalent strain amplitude,the fatigue life under pure shear is much higher than that under tension-compression. The fatigue life of extruded GW83 alloy is much higher than that of extruded AZ31 B alloy at the same plastic strain energy density.