金属间化合物TiAl合金的缺口断裂机制研究

通过TiAl基合金两种组织的四点弯曲断裂实验，研究了TiAl基合金的缺口断裂机制。实验结果表明：缺口试样的断裂过程为几个沿层裂纹直接起裂于缺口根部，并沿着缺口根部的层间扩展；当遇到位向不利的晶粒时，裂纹便停止在这一晶粒前方，随着外加载荷的增加，即应力的增加，裂纹彼此连接并通过穿层裂纹而扩展；裂纹尖端超钝化、裂纹分叉、沿着层间偏转，形成显微裂纹区，裂纹停在障碍晶粒前的边界上为这种材料的韧化机制；裂纹扩展的驱动力是拉伸应力，缺口的存在增加了缺口前沿或者裂纹尖端的应力。     

全层TiAl基合金室温断裂机制的研究

通过拉伸、压缩、弯曲实验分析研究了全层(FL)组织TiAl基合金的断裂机制。研究发现:拉伸和压缩时材料抵抗裂纹的扩展能力不同,抗压强度远高于抗拉强度,这是由于两者的变形及断裂机制不同。TiAl基合金拉伸断裂机制为脆性解理断裂,压缩变形断裂是剪应力和正应力共同作用下的断裂,是准解理断裂。TiAl基合金的缺口弯曲断裂方式也为解理断裂,其断裂过程是先在缺口处产生微裂纹,一旦裂纹在缺口根部产生,由于材料已积累足够的能量使得材料快速失稳解理断裂。     

TiAl基合金双态组织平板拉伸连续卸载试验的研究

通过多次拉伸卸载试验对TiAl基合金在经历多次拉伸卸载以后宏观性能和微裂纹面密度的变化,以及前一阶段的损伤对随后阶段损伤产生的影响进行了详细的研究.研究表明:在载荷控制下的试验中,随着卸载应力的增加,裂纹面密度并没有增大,即用微裂纹表征的损伤程度并没有增加的趋势;整个多次拉伸卸载过程并不影响材料的弹性模量E;当拉伸到某一个应力下,宏观表现为断裂应力以,断裂应变‘f和单位面积断裂功W'开始减小.材料在载荷控制的拉伸中产生损伤的程度并不能用弹性模量E和裂纹面密度.来衡量.     

γ-TiAl基合金压缩损伤与断裂行为的研究

通过对γ-TiAl基合金压缩断裂及压缩卸载试验和试样断口与表面的扫描电镜(SEM)观察,分析压缩应力对裂纹产生、扩展及裂纹形态的影响,进而对该材料的压缩损伤与断裂行为进行较为深入的研究。压缩试验是室温下在Instron 1341试验机上进行的。结果表明,损伤起始于材料的塑性区载荷下降阶段,材料在断裂前发生很大的塑性变形,其压缩时有较大的塑性缓冲;随着压缩卸载应力的增大,观察到的试样表面裂纹依次增多或扩展增长,材料损伤的程度与压缩应力成正比。在压缩试样断口的中部发现存在的一个纵向韧带,当外加载荷增加,两个由压缩接触端面起裂的倾斜剪切裂纹扩展到试样中部,然后通过剪切穿过纵向韧带而连接,并诱发试样的完全脆性断裂。两个端面的切应力是裂纹形成的主要控制因素。该材料的压缩性能比拉伸性能更佳的主要原因是由于压缩时材料的损伤起始于塑性阶段,产生沿45°方向剪应力最大方向的剪切断裂和沿着压缩轴方向的准解理断裂的混合形式,而普通拉伸时材料损伤起始于弹性阶段,发生完全脆性解理断裂,在低应力下试样就会断裂。     

不同卸载应力对层状TiAl基合金损伤程度的影响

通过对层状TiAl基合金进行拉伸卸载试验，研究了不同预损伤对层状TiAl基合金断裂行为的影响。试验结果表明：随着损伤程度增加到一定程度，材料的弹性模量减小；随着预损伤程度的增加，裂纹面密度增大。通过统计分析发现裂纹面密度可以作为衡量损伤程度的损伤参量；但随着预损伤程度的增加，单位面积断裂功基本不变，进一步说明不同预损伤对层状TiAl基合金的最终断裂性能没有影响。     

铝合金在两种应力状态下损伤的有限元模拟

通过对铝合金（6063）进行缺口拉伸及纯剪切试验，研究了铝合金在这两种应力状态下的损伤没断裂机制。研究结果表明：缺口拉伸试验中，缺口根部产生相对较高的三轴应力，随着应力的不断升高，微孔洞的体积分数不断增大。当达到材料的临界孔洞体积分数时，试样断裂；纯剪切试验中，在材料内部几乎没有产生微孔洞而产生了剪切带。显微裂纹首先在剪切带中产生，随着裂纹的进一步扩展，最终导致试样断裂；用改进的Gurson模型和Johnson-cook模型分别模拟缺口拉伸和纯剪切试验，横拟的工程应力-应变曲城与试验的工程应力，应变曲线符台得很好。另外根据有限元模拟和试验数据还得出了6063（T6）铝合金缺口试样中微孔洞损伤的经验演化方程。     

γ-TiAl金属间化合物的压缩断裂行为研究

通过室温压缩试验,研究全片层γ-TiAl基合金在不同加载速度和不同卸载载荷下的压缩断裂行为。结果表明:随着加载速度的增加,γ-TiAl基合金试样的屈服强度及抗压强度相应增大;试样的最终断裂是通过裂纹的形核、扩展以及相互贯通而形成的,断裂面主要由剪应力形成的撕裂区和压应力形成的解理断裂区域组成,并且在不同加载速度下,断口也呈现出规律性的变化。在不同载荷加载-卸载-再加载的过程中,小载荷(4.67、9.42、18.94 k N)下卸载和加载的名义应力-名义应变曲线完全重合,大载荷(26.60、37.24、53.20 k N)下卸载后产生的不可逆应变依次增大;裂纹面密度随着卸载载荷的增大而逐渐增大,材料的损伤程度不断增加。     

TiAl基合金压缩状态下变形及损伤机制的研究

通过力学性能测试、扫描电镜观察以及有限元模拟计算的方法,研究了全层组织γ-TiAL基合金在压缩状态下的变形及损伤机制.结果表明:较小的加载卸载应力作用下,材料的压缩性能没有受到影响,直至卸载应力超过最大压缩应力之后,由于材料内部损伤的积累程度增大,在材料内部形成主裂纹,使得有效承载面积下降,后续再加载过程中材料的断裂应力整体下降.压缩状态下:首先,随着变形程度的增加,晶粒周围出现大量的滑移线及挤出脊,滑移线和挤出脊处出现较大的裂纹,试样表面产生平行于压缩轴方向的裂纹并迅速扩展,表面裂纹面密度明显增加,45°方向上的沿层裂纹扩展程度较大,但裂纹长度仅限于晶粒尺寸的大小(100～300μm).其次在压缩加载过程中,材料在较小的正应力作用下,观察得到表面萌生以下4种裂纹:平行于压缩轴方向的纵向沿层裂纹;与压缩轴方向成较小角度的纵向沿层裂纹;与压缩轴方向成较小角度的纵向穿层裂纹;纵向的穿晶裂纹.     

板条马氏体钢变形与断裂过程的原位观察

 在备有拉伸装置的扫描电镜上,原位观察了低碳板条马氏体钢的变形和断裂过程。结果表明,板条马氏体的变形是以滑移方式进行的,位错沿滑移面的滑移受阻,在试样表面留下呈波纹状的变形带。在应力峰值前后,主裂纹开始起裂;在主裂纹扩展过程中,在主裂纹前面的薄弱区域如夹杂等会先起裂形成小裂纹或空洞,随应力加大相邻的微孔聚合、连接长大成新裂纹;在断裂过程中,裂纹在板条束界发生转折。尽管原奥氏体晶粒尺寸小的试样起裂载荷大,不同晶粒尺寸马氏体组织的变形和断裂过程没有本质差别。     

K418合金缺口敏感性研究

通过对不同缺口半径的K418材料平板试样进行静拉伸实验宏观结果分析、组织观察及断口观察分析,研究了K418材料的缺口敏感性及断裂机制。结果表明:由于缺口根部的应力集中,试样的裂纹首先起裂在缺口根部,然后沿着缺陷处进一步扩展直至断裂。材料的抗拉强度随着材料缺口半径的增加及应力集中系数的降低而降低。缺口敏感性在缺口根部半径尺寸位于0.13~0.25 mm之间有一个临界值,当缺口半径尺寸等于或大于这一值时试样存在缺口敏感性,即缺口敏感性指标NSR值小于1;当缺口半径尺寸小于这一临界值时试样不存在缺口敏感度,即NSR接近1。K418材料的原始铸态晶粒尺寸粗大;枝晶偏析、缩松是其主要的缺陷组织。另外该材料的断裂即不是一般的典型脆性断裂,也不是典型的韧性断裂。它是韧脆混合的断裂,韧性断裂的特征表现为有一定的韧窝及撕裂棱。而脆性的特征表现为光滑明亮的解理小平面,同时在小平面内也出现二次微裂纹。同时在这种韧脆混合的断裂形态中也可以看到存在很多柱状树枝晶,疏松及枝晶偏析,这与材料在凝固时的条件有很大的关系。     

B级船板钢形变断裂过程的原位研究

 在扫描电子显微镜下观察分析了B级船板钢试样在原位拉伸过程中的断裂行为以及低温脆性断口中二次裂纹的扩展。结果表明：在拉伸变形过程中,微裂纹首先在试样缺口处形成,然后沿铁素体+珠光体界面扩展。加载过程中,多边形铁素体发生塑性变形,裂纹在基体内以“Z”字型扩展。在低温脆性断裂区,二次裂纹以穿晶方式通过铁素体基体,在扩展到珠光体区域时有时沿铁素体和珠光体的界面扩展,有时穿过珠光体区域扩展。     

Investigation on the fracture behavior of shape memory alloy NiTi

This work investigates the fracture behavior of shape memory alloy NiTi (50.7 at. pct Ni) at room temperature. Macroscopic mechanical tests, microscopic in situ observations of tensile fracture processes by scanning electron microscopy (SEM), and detailed analyses of fracture surfaces were carried out. The results reveal that specimens with different thicknesses show various shape memory effects and superelasticities. The main crack with a quasi-cleavage mode that combines cleavage with ductile tearing is initiated at the notch tip and is stress-control-propagated in line with the direction of the maximum normal stress. The microstructure has little effect on the direction of crack propagation, but coarser substructures show lower resistance to the crack propagation. In specimens with various types of notches, various notch acuities present different effects on the crack initiation and propagation and result in different fracture behaviors.     

Dynamic Fracture of a Zr-based Bulk Metallic Glass

In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m^1/2, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage.     

Further investigation of critical events in cleavage fracture of C-Mn base and weld steel

In this work, an investigation of the critical event in cleavage fracture of C-Mn base and weld steel was carried out. The fracture surfaces and the remaining cracks in the vicinities of notches and precracks in the double-notched specimens (DNBs) and precracked crack opening displacement (COD) test specimens unloaded prior to fracture were observed in detail. The results demonstrate that critical events are different in notched and precracked specimens even if they are made of identical materials. For the notched specimens with the radius of notch roots from 0.075 to 0.45 mm, critical events in cleavage fracture are the propagation of the ferrite grain-sized microcracks (FCs) into the neighboring ferrite grains. However, for the precracked specimens fractured at -110 °C, critical events are the propagation of the second-phase particle-sized microcracks (SCs) into matrix ferrite grains. For the precracked specimens tested at-70 °C, critical events could be the propagation of either SCs or FCs depending on the blunted width of precrack prior to fracture. The cleavage fracture at -196 °C is controlled by the nucleation of the microcrack; there were no remaining cracks found in specimens unloaded prior to fracture. With a drop in the test temperature or decrease in the radius of notch root, the length of the critical crack decreases.     

Study on notch fracture of TiAl alloys at room temperature

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.^[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.     

Relationship between fracture toughness and crack extension resistance curves (R curves) for Ti-6Al-4V alloys

The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack tip opening displacement (CTOD)) and microcrack extension resistance curves (R curves) of Ti-6Al-4V alloys were examined. At 0 °C, microstructure is the most influential factor in the toughness-strength relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless of strength (0.2 pct proof stress) and impurity content. At −196 °C, impurity content becomes a controlling factor in the toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD, irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing were investigated, and crack initiation (δ _i) and extension-resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0 °C, specimens with different microstructures and different impurity contents have almost the same δ _i. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance. At −196 °C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor for the fracture toughness of Ti-6Al-4V alloys.     

Fatigue Crack initiation and microcrack growth in 2024-T4 and 2124-T4 aluminum alloys

A metallograph was mounted directly on a closed-loop electrohydraulic testing unit and initiation of fatigue cracks was directly observed on polished notches at magnifications up to 800 times in aluminum alloys 2024 and 2124 in the T-4 condition. The latter is a high purity version of 2024 and contains considerably fewer constituent particles. At high stresses on the notch surface the fatigue cracks initiated on coarse slip lines in both alloys. At low stresses almost all of the cracks in 2024 initiated in the matrix adjacent to constituent particles. In 2124 at low stresses 50 pct of the cracks initiated near constituent particles and 50 pct in the matrix not near constituent particles. The probability that a constituent particle in 2024 initiates a fatigue crack falls off very rapidly as the particle size decreases below 6 μm. Growth of microcracks is impeded by grain boundaries. This research was done at Northwestern University.