Effect of grain growth and measurement on fracture toughness of silicon nitride ceramics

Two kinds of -Si₃N₄ powders with different properties (high purity and low purity) were pressureless sintered using MgAl₂O₄-ZrO₂ as sintering additives. The grain growth was controlled by sintering time and temperature. The fracture toughness was determined using indentation microfracture (IM) and single-edge-precracked-beam (SEPB) methods. A discrepancy of fracture toughness was found between the values obtained by these two methods, and the SEPB method provided higher fracture toughness than the IM method. Materials with more large elongated Si₃N₄ grains gave higher fracture toughness, R-curve behavior and larger discrepancy. This is contributed to the effects of grain bridging, pull-out and deflection by the large elongated Si₃N₄ grains. Comparing the specimens from two kinds of -Si₃N₄ powders with different purity level, both gave high fracture toughness of over 9 MPa·m^1/2 by the SEPB method, while the E10 samples have a little higher flexural strength.     

Vickers压入识别氮化硅断裂韧性

依据仪器化Vickers压入氮化硅断裂韧性实验获得的有关压痕裂纹参数,通过有限元数值分析方法识别出氮化硅的弹性模量和屈服强度,进一步采用虚拟裂纹闭合法确定其裂纹尖端的应力强度因子KI。以此为基础,与氮化硅断裂韧性标准值对比,分析有限元仿真KIC结果和基于L-E-M模型建立的3种典型陶瓷断裂韧性压入测试方法的准确度。结果表明:基于Vickers压入有限元数值分析结果的最大误差仅为2.38%,Anstis公式最大识别误差为2.65%,而Lawn公式和Miyoshi公式的识别误差的绝对值均超过10%,因此Vickers压入测试具有较高测试准确度。     

Correlation between microstructure evolution and cryogenic fracture toughness in aging ITER-grade 316LN weldments

In the present work, the correlation between microstructural evolution and fracture toughness in 316LN joints welded by Tungsten Inert Gas (TIG) was investigated. The effect of post-weld heat treatment (PWHT) on the microstructure and toughness was characterized. The welding process can significantly change the equiaxed grains of base metal to cells and dendrites, while the PWHT can increase the dendrite size, mitigate the texutre intensity, reduce the dislocation density, and slightly weaken the ultimate tensile strength of the joints. Fracture toughness tests reveal that the strain-induced martensitic transformation at cryogenic temperatures can remarkably deteriorate the fracture toughness. Due to the microstructural evolution during PWHT, the J-integral values at 77 K and 4.2 K decrese to 85% and 54% of those in the as-welded conditions, respectively. The fracture morphology of the as-welded joint shows a characterization of ductile fracture, while the PWHT joint features a mixture of ductile and brittle fracture.     

Self-monitoring of fracture and strain in titanium carbide reinforced silicon nitride ceramics

Si₃N₄ Ceramic Composites with TiC powder have been fabricated by gas-pressure sintering and their electrical conductivity has been investigated. The ceramic composites with different electrical resistivity consist of Si₃N₄ powder as an insulating matrix, and TiC as electrically conductive additive. Under tensile loading or compressive unloading, the R/R of TiC/Si₃N₄ composites reversibly increased. Under compressive loading, the R/R decreased gradually with the increasing of loading up to fracture. The results suggest the possibility of self-monitoring fractures and strains in the composites under tensile and compressive loading.     

Correlation between stretched zone size and fracture toughness

The topography of the stretched zone was measured from stereo electron micrographs. The stretched zone reflects crack tip blunting preceding fracture. The depth of the stretched zone of fourteen fracture toughness specimens of some Al-Zn-Mg alloys appeared to correlate reasonably with the critical crack tip opening displacement. The width of the stretched zone appeared to be in the order of magnitude of the ligament spacing in Krafft's model.     

Mixed-mode high temperature toughness of silicon nitride

Both opening-mode and mixed-mode fracture toughness tests were carried out at 1200 and 1300 °C on a sinter/HIP grade of silicon nitride. Data for pure opening loading (K _Ic) agree well with other experiments on the same material, which showed that the toughness was lower at 1000 °C than at room temperature, but increased as temperature increased above 1000 °C. The ratio of K _IIc/K _Ic was sufficiently insensitive to temperature that it can be considered to be constant. Results are discussed in the context of mechanisms that have been proposed to explain fracture toughness in silicon nitride.     

Comparison of fracture characteristic of silicon nitride ceramics with and without second crystalline phase

The comparison of microstructure and fracture characteristics of gas-pressure-sintered (GPS) Si₃N₄ ceramics with and without second crystalline phase at the junction regions was investigated by a combination of SEM, HRTEM and micro-indentation techniques. The bimodal microstructure composed of many large and fine rod-like Si₃N₄ grains was made by carbothermal reduction treatment (CRT). Most of triple regions were comprised of Y₃AlSi₂O₇N₂ as a second crystalline phase while all of grain boundaries and interfaces between Si₃N₄ and Y₃AlSi₂O₇N₂ phase were an amorphous layer of about 2 nm in thickness. Although a few large, rod-like Si₃N₄ grains were also observed in the sample without CRT, but most of junction regions existed with an amorphous phase. The value of fracture toughness in the sample with second crystalline phase at the junction regions by CRT was 6.6 MPa m^1/2. This was slightly higher than the value without second crystalline phase (5.1 MPa m^1/2). The main reason for the slight increase of the fracture toughness, even though showing well-developed bimodal microstructure, is the formation of the Y₃AlSi₂O₇N₂ phase at junction regions, which led the transgranular fracture of Si₃N₄ grains at local regions.     

Effect of microstructure on the relationship between fracture toughness and ductility

The relationship between fracture toughness, expressed as J_IC, and ductility, given by measurements of the bulge ductility, was studied experimentally for two microstructures of 1045 steel. These microstructures consist of annealed pearlitic structure with hardness R_c = 15 and a tempered matensitic structure with R_c = 23. J_IC and bulge ductility measurements were performed in the temperature range of −100-25°C. The tensile properties at these temperatures were also obtained. The results show that the variation in flow stress with temperature is similar for both microstructures. However, the flow stress is higher, and the bulge ductility is lower, at a given temperature, for the martensitic structure. Also shifts in the transition temperature from linear elastic to elastic-plastic behavior are observed. Previously developed models by the authors describing the variation of J^IC with temperature and ductility are used to account for the behavior of the different microstructures examined.     

Correlation between crack tortuosity and fracture toughness in cementitious material

This paper proposes a new technique for the evaluation of fractal dimension (D) of fracture surface and a quantitative correlation between D and fracture toughness of cementitious materials. The experimental program has been performed on compact tension (CT) specimens (600 × 525 × 125 mm) with three different aggregate sizes (d _max=4.7 mm, 18.8 mm and 37.5 mm). The fractal geometry concept is utilized in the evaluation of fracture surface roughness. To avoid indirect or destructive experimental procedures that are prohibitively laborious and time consuming, a new non-destructive technique is presented. Results of the analysis indicate that the concept of fractal geometry provides a useful tool in the fracture surface characterization. The results also suggest that the fracture toughness can be correlated with the fractal dimension of fracture surface.     

The relationship between fracture toughness and microstructure of fully lamellar TiAl alloy

Fully lamellar (FL) Ti–46.5Al–2Cr–1.5Nb–1V (at%) alloy is used to study the relationship between microstructure and fracture toughness. A heat treatment process is adopted to control the microstructural parameters of the studied alloy. Fracture toughness experiments and scanning electron microscope (SEM) in-situ straining experiments are carried out to determine the influence of lamellar spacing and grain size on the fracture toughness of FL TiAl alloys. It is found that ligament length depends on the lamellar spacing, and fracture toughness varies non-monotonously with the increase of grain size. The results are ascribed to the competition between the microcrack nucleation and microcrack propagation. Finally a semi-empirical relationship between the fracture toughness and microstructure parameters was established.