垂直截面法测量的断口分形维数与7475 Al 合金超塑性的关系

本文利用垂直截面法测量了7475Al 合金在不同温度、不同应变速率及不同晶粒尺寸条件下超塑拉伸变形时断裂表面的分形维数。结果表明:断裂表面的分形维数越高,合金超塑拉伸破断时的延伸率越大。     

非定向有机玻璃拉伸断口形貌与拉伸温度相关性分析EI北大核心CSCD

为探究非定向有机玻璃断口定量表征方法及其断口形貌参数与拉伸温度的相关性,以航空有机玻璃YB-2为研究对象,首先采用体视显微镜和三维激光扫描仪测量了不同拉伸温度下的断口雾状区尺寸和表面粗糙度均值Ra,其次通过扫描电镜-盒维数法测算了断口雾状区的分形维数均值D,最后通过两个假设推算了有机玻璃断裂形成雾状区时消耗的能量,并与断口雾状区的分形维数进行联系。结果得出:当拉伸速率一定时,拉伸温度由-55℃提升至60℃,断口雾状区的尺寸J3从1.257 mm升高至4.978 mm,Ra从0.517μm降低到0.330μm,D从1.357升高至1.579,拉伸温度分别与断口雾状区尺寸、表面粗糙度、分形维数的拟合曲线的拟合度因子均高于0.9,拟合程度较高。研究表明:有机玻璃拉伸断口的形貌参数与其断裂条件存在一定的相关性,雾状区形成时消耗的能量与其分形维数呈正相关关系,该研究结果可为有机玻璃断口定量分析奠定一定的基础。     

陶瓷刀具材料断口形貌及裂纹扩展的分形特征

通过热压烧结工艺,制备了一种高性能的Si3N4基陶瓷刀具材料。利用X射线衍射仪和扫描电子显微镜分别对材料物相组成、微观形貌及裂纹扩展路径进行了分析。借助图像处理技术和分形理论,计算了断口形貌及裂纹扩展路径的分形维数,并揭示材料断裂机制。研究表明,Si3N4基陶瓷刀具材料表现为穿晶/沿晶的混合断裂模式,其裂纹扩展方式主要是偏转和桥联,断口形貌及裂纹扩展均具有明显的分形特征。当材料断口形貌越粗糙,裂纹扩展路线越不规则,分形维数值增大,表明断口微观结构的粗糙程度、裂纹扩展路线的不规则程度可用分形维数来刻画。     

Mg-3Al-6Zn-2Y合金在不同应变率加载下的力学性能研究

在不同应变率压缩与拉伸下,研究了Mg-3Al-6Zn-2Y合金的力学性能,发现两种条件下合金力学性能变化规律不同。压缩情况下,随应变率增大,合金的流变应力增大,极限强度、屈服强度、破坏应变先增大后减小,塑性先增大后减小;拉伸情况下,随应变率增大,合金的流变应力、极限强度、屈服强度先增大后减小,破坏应变减小,塑性减小。压缩情况下合金流变应力的应变率敏感性高于拉伸情况。     

Mg65CU20Zn5Y10多孔非晶的制备及性能

为提高镁基非晶合金的塑性应变能力,采用水冷铜模浇铸-拔丝造孔法制备了Mg65Cu20Zn5Y10多孔非晶合金,对Mg65Cu20Zn5Y10多孔非晶合金的组织、热稳定性和压缩性能进行了研究。结果表明,采用多孔芯水冷铜模制备出了直径6mm、长24mm的Mg65Cu20Zn5Y10多孔非晶合金;与镁基非晶合金相比,孔结构对其热稳定性没有影响;Mg65Cu20Zn5Y10多孔非晶合金在低应力作用下的塑性应变可达40%;Mg65Cu20Zn5Y10多孔非晶合金的断裂形貌呈脉状花样,属韧性断裂。     

粉末NBR-PVC共混型热塑性弹性体断裂面的分形维数

本文利用扫描电镜对粉末NBR-PVC共混型热塑性弹性体拉伸断面形貌进行了观察与分析;基于断面小岛周长-面积关系,测定了试样的断口分形维数,考察了NBR含量、拉伸断面分形维数、力学性能(拉伸强度、断裂伸长率、邵氏硬度)之间的关系。结果表明,NBR-PVC弹性体拉伸断面的分形维数随着NBR含量的增加而增加,当NBR含量超过29份后其分形维数呈下降趋势;分形维数与材料的拉伸强度、断裂伸长率和邵氏硬度变化关系一致。     

木材断裂解离分形特征与分形断裂力学模型研究

目的 以椴木为研究对象,研究冲击载荷作用下椴木试件的断裂解离形貌特征和断裂力学特性,建立适用于木材原料断裂解离的分形断裂力学模型,并对其断裂解离力学行为进行描述。方法 对椴木试件进行冲击加载试验,分析试件断口的形貌特征和断裂力学特性,构建适用于木材原料断裂解离的分形断裂力学模型。结果 椴木试件横向冲击断裂断口裂纹形状和断口形貌特征比纵向冲击复杂,横、纵向冲击断裂断口均具有分形特征;椴木试件纵向冲击断裂韧性均值是横向冲击断裂韧性均值的1.112倍,椴木试件横、纵向冲击断口的分形维数均值分别为2.063 5和2.075 1,椴木试件横、纵向冲击韧性与其断口分形维数之间存在线性正相关关系,拟合优度分别为0.778 7和0.812 2;构建的木材原料断裂解离临界解离应力和断裂韧性的分形断裂力学模型也适用于脆性材料。结论 在木材原料冲击断裂解离时,木材原料初始裂纹长度越短,断裂解离断口越粗糙复杂,木材原料断裂解离所需要的能量越大;当裂纹沿着与冲击加载力方向垂直成大约1.055rad方向扩展时所需的能量最小,木材原料最易沿该方向进行断裂解离。     

拉伸速率对Ni40 Ti50 Fe10合金环境氢脆的影响

研究了不同应变速率对Ni40Ti50Fe10合金环境氢脆的影响,并用扫描电子显微镜观察了拉伸断口的形貌.研究结果表明:Ni40Ti50Fe10合金在真空中的拉伸行为与应变速率无关,断口形貌主要是韧窝塑性断口.Ni40Ti50Fe10合金在空气中的塑性随着应变速率的降低而减小,断口形貌也从韧窝塑性断口转变为韧窝塑性断口和解理脆性断口的混合断口.Ni40Ti50Fe10合金在氢气中低应变速率拉伸时表现出明显的脆性,断口形貌主要为解理脆性断口.在高应变速率下(2×10-1s-1),Ni40Ti50Fe10合金在真空、空气和氢气中的塑性相近,说明在该应变速率下可以有效地抑制环境氢脆.Ni40Ti50Fe10合金的屈服强度不受应变速率和环境的影响.     

Mg-3Al-2Zn-2Y合金的动态力学性能研究

在不同应变率压缩与拉伸下,研究了Mg-3Al-2Zn-2Y合金的力学性能,发现2种条件下合金力学性能变化规律不同。压缩情况下,随应变率增大,极限强度与屈服强度先增大后减小,高应变率下(1300～4800s-1)的流变应力大于中低应变率(0.001～1s-1);在0.001～1450s-1拉伸下,随应变率增大,合金的流变应力呈增大趋势,极限强度、屈服强度增大,破坏应变先减小后增大。压缩情况下合金流变应力的应变率敏感性高于拉伸情况。     

3种镁合金高应变率压缩性能的比较

在应变率102～103s-1范围,研究比较了Mg-0.6Zr、Mg-3Al-6Zn-0.3Mn-2Y、Mg-8Zn-0.6Zr-5Y 3种镁合金的压缩性能及微观组织变化.研究发现,变形抗力、极限强度以及极限强度的应变率敏感性由大到小依次为:Mg-8Zn-0.6Zr-5Y＞Mg-3Al-6Zn-0.3Mn-2Y＞Mg-0.6Zr;塑性大小则与变形抗力大小相反;3种合金的微观组织应变率敏感性与力学性能应变率敏感性并不一致.     

热处理条件对锻造ZK60-Y镁合金力学性能的影响

研究了不同热处理条件下锻造ZK60-Y镁合金微观组织的变化对其力学性能的影响.结果表明,直接进行人工时效的合金具有优越的强度和塑性.XRD分析表明,析出相主要有Mg2Zn3、Mg24Y5、Zn2Zr3和w-Mg3Y2Zn3.Mg2Zn3和w-Mg3Y2Zn3等析出相的尺寸、数量及其在基体中的分布状态对合金的力学性能影响很大.锻造态下大块破碎呈带状分布的Mg3Y2Zn3相及T4和T6态下粗化呈片层状的Mg2Zn3相是合金力学性能降低的主要原因.细小呈带状分布的Mg3Y2Zn3相和细层片状分布的Mg2Zn3相及其在此状态下细小的晶粒使T5态合金具有优越的抗拉强度和塑性.     

Effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y alloy

This study was conducted to discuss the effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y (wt%) alloy, which was prepared by casting, and then homogenized and rolled at 200?°C. The rolling process was conducted with 10% reduction per pass and five different accumulated strains, varying from 10% to 70%. The results indicate that the as-cast and as-rolled Mg–8Li–3Al–2Zn–0.5Y alloys are composed of α-Mg, β-Li, AlLi and Al₂Y phases. After rolling process, anisotropic microstructure was observed. α-Mg phase got elongated in both rolling direction and transverse direction with the addition of rolling strain. Consequently, the strength of the alloy in both directions was notably improved whereas the elongation declined, mainly caused by strain hardening and dispersion strengthening. The tensile properties of the as-rolled alloys in the RD, no matter the YS, UTS or the elongation, are higher than those of the TD due to their larger deformation strain and significant anisotropy in the hcp α-Mg phase. In addition, the fracture and strengthening mechanism of the tested alloys were also investigated systematically.     

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.     

Quasicrystal-reinforced Mg alloys

The formation of the icosahedral phase (I-phase) as a secondary solidification phase in Mg–Zn–Y and Mg–Zn–Al base systems provides useful advantages in designing high performance wrought magnesium alloys. The strengthening in two-phase composites (I-phase + α-Mg) can be explained by dispersion hardening due to the presence of I-phase particles and by the strong bonding property at the I-phase/matrix interface. The presence of an additional secondary solidification phase can further enhance formability and mechanical properties. In Mg–Zn–Y alloys, the co-presence of I and Ca₂Mg₆Zn₃ phases by addition of Ca can significantly enhance formability, while in Mg–Zn–Al alloys, the co-presence of the I-phase and Mg₂Sn phase leads to the enhancement of mechanical properties. Dynamic and static recrystallization are significantly accelerated by addition of Ca in Mg–Zn–Y alloy, resulting in much smaller grain size and more random texture. The high strength of Mg–Zn–Al–Sn alloys is attributed to the presence of finely distributed Mg₂Sn and I-phase particles embedded in the α-Mg matrix.     

The microstructure and mechanical properties of as-cast Mg–4Y/Nd–2Zn alloys

Optical microscopy, scanning electron microscopy, X-ray diffraction and tensile testing were performed to investigate the microstructure and mechanical properties of as-cast Mg–4Y/Nd–2Zn alloys. The results show that the secondary dendritic arm spacing for the Mg–4Y–2Zn alloy is smaller than that for the Mg–4Nd–2Zn alloy, and that X-Mg₁₂YZn or W-Mg₃Zn₃Nd₂ form in Mg–4Y/Nd–2Zn alloys. The lamellar X phase distributes at the grain boundary, pointing into the grains, whereas the rod-like W phase preferentially segregates at the triangle junction of the grain boundary. The greater grain boundary strengthening effect and the smaller fragmentation effect of the brittle eutectic phases leads to the as-cast Mg–4Y–2Zn alloy having better comprehensive mechanical properties. The fracture mechanism for as-cast Mg–4Y/Nd–2Zn alloys is quasi-cleavage fracture.     

Microstructure and mechanical properties of LA51 and LA51–0.5Y alloys with different accumulated strains and rolling temperatures

Mg–5Li–1Al (LA51) and Mg–5Li–1Al–0.5Y (LA51–0.5Y) alloys were smelted and rolled with different accumulated strains (36% and 68%) and rolling temperatures (373 K and 573 K). The microstructure, mechanical properties, fracture morphology and texture of the specimens were investigated. Results show that, due to the PSN (particle stimulate nucleation) mechanism, the addition of 0.5 wt.% Y improves the deformation resistance and weakens the basal texture of LA51 alloy. The effect of Y on UTS (ultimate tensile strength) of as-rolled alloys is more obvious than that of the as-cast alloy. Accumulated strain and rolling temperature could influence the twinning number, slip systems and DRX (dynamic recrystallization), thus affecting the microstructure and mechanical properties of the alloy. Under the proper combination of the above factors, the as-rolled LA51–0.5Y alloy with an accumulated strain of 68% at 573 K possesses the best comprehensive mechanical properties.     

Strengthening mechanism of age-hardenable Al–xMg–3Zn alloys

The addition of 3?wt-% Zn to the traditional Al–Mg alloy doubled its strength. To understand the strengthening mechanism, the enhancement in strength with the variation in Mg content was studied. The grain boundary and solid solution strengthening decrease with the reduction in the Mg content; however, their contributions to the yield strength are insignificant. The contribution of precipitation strengthening to the yield strength is more than 80%; however, due to the unchanged precipitate characteristics, the strengthening effect changes slightly with the variation in the Mg content. The reduction in strength is primarily due to Mg solid solution strengthening. The variation in the ductility of Al–Mg–Zn alloys was studied by fracture analysis.     

合金成分对Mg-Zn-Y合金准晶形貌和体积分数的影响

利用铁模铸造法制备Zn/Y=6:1(原子比)的Mg-Zn-Y合金,通过XRD,SEM,EDS,TEM和DSC等研究合金成分对Mg-Zn-Y合金相组成、Mg_3Zn_6Y准晶相(准晶Ⅰ相)形貌和体积分数的影响。结果表明:Mg-Zn-Y合金的相组成、准晶Ⅰ相形貌、体积分数及其生成反应与合金成分密切相关。随着合金中Zn和Y元素含量的减少,准晶Ⅰ相的形成反应由单一的包晶反应到包-共晶反应再到完全共晶反应。当合金中Y含量≥7%(原子分数,下同)时,合金由(Mg, Zn)_5Y、准晶Ⅰ相、Mg_2Zn_3和Mg_7Zn_3相组成,且以叠层状形式分布在合金组织中。合金在凝固过程中通过包晶反应形成多边形块状准晶Ⅰ相;当Y含量<7%时,合金中除(Mg, Zn)_5Y、准晶Ⅰ相和Mg_7Zn_3相外,还析出了Mg相。当合金中Y含量在5%～7%时,准晶Ⅰ相通过包晶和共晶反应生成,以共晶反应为主。当Y含量≤4%时,准晶Ⅰ相完全通过共晶反应形成(Mg+I-phase)层片状共晶组织。所研究的合金中均生成了体积分数大于27%的准晶Ⅰ相,Mg₃₀Zn₆₀Y₁₀合金中准晶Ⅰ相的体积分数最高,约为77%。     

Development of high-performance quaternary LPSO Mg–Y–Zn–Al alloys by Disintegrated Melt Deposition technique

In the present study, new quaternary MgY_1.65Zn_0.74Al_0.53 and MgY_3.72Zn_1.96Al_0.45 alloys (wt.%) were synthesized employing the Disintegrated Melt Deposition (DMD) casting technique followed by hot extrusion. Microstructural characterization revealed the presence of 14H long-period stacking ordered structure (LPSO) and Mg₄Y₂ZnAl₃ phases aligned along the direction of extrusion in both alloys. Refined grains (⩽5 μm) due to the effect of dynamic recrystallization (DRX) were also observed to co-exist with larger worked grains (⩾20 μm) in the extruded microstructures. Compared to monolithic Mg, significant increase in the microhardness (∼67–88%), tensile yield strength (∼245–290%) and ultimate tensile strength (∼113–144%) were observed in the Mg–Y–Zn–Al alloys. Despite the significant increase in strength of materials, failure strains of both Mg–Y–Zn–Al alloys were comparable to monolithic Mg. Ignition temperatures of both Mg–Y–Zn–Al alloys were found to outperform commercially available AZ31, AZ80 and WE43 (high-temperature) Mg alloys, and the highest ignition temperature of 770 °C was achieved in the MgY_3.72Zn_1.96Al_0.45 alloy.     

Microstructure and mechanical properties of Mg–6Li–xAl–0.8Sn alloys

As-cast and as-extruded Mg–6Li–xAl–0.8Sn (x?=?0, 1, 3 and 5?wt-%) alloys were prepared. The microstructure and mechanical properties were investigated and discussed. The experimental results show that the Mg–6Li–0.8Sn alloy is composed of three phases: α-Mg, Mg₂Sn and Li₂MgSn. With the addition of Al, the test alloys display typical α-Mg?+?β-Li duplex structures. The new Mg₁₇Al₁₂ and LiMgAl₂ phases were found in the Mg–6Li–1Al–0.8Sn alloy. The lamellar-type AlLi phase was formed whereas the Mg₁₇Al₁₂ phase disappeared in Mg–6Li–3Al–0.8Sn alloy. The LiMgAl₂ phase vanished in the Mg–6Li–5Al–0.8Sn alloy. The mechanical properties of as-extruded alloys were remarkably improved. The as-extruded Mg–6Li–3Al–0.8Sn alloy exhibited the best mechanical properties, with a yield strength, tensile strength and elongation of 209.8?MPa, 242.6?MPa and 15.5%, respectively.