Gao, Qian.2006-07-282006-07-2819971997http://hdl.handle.net/10012/160An overview of the toughening mechanisms in the intermetallic-base in-situ composites is presented. Based on the literature review and preliminary research, the two phase ((3 + y) region of Ni-Al system was chosen as a model in situ composite to study fracture toughness of the in-situ NiAl-NhAl intermetallic composites and explore the fracture toughening mechanisms in these intermetallic materials. The composition ranges investigated were 25-35 at.% Al for both as-solidified and as heat-treated composites. To evaluate fracture toughness, a three point bending of Chevron notched beam (CNB) specimens were used. The values of fracture toughness were calculated either directly from the maximum load at unstable crack propagation or by using a modified J integral approach. Compressive testing was also carried out to obtain yield strength of tested in-situ intermetallic composites. Micromechanical properties of individual phases were probed by Vickers microhardness testing. The relationship between fracture toughness (Kn77r, Kn-c> and volume fraction of second phase Vd, in the following form: Kh·c=f(V/) has been established. Also, boron-doped (0.2 and 0.4 at.%) Ni3Al was fabricated. Fracture mechanisms and boron effect on fracture toughness of the NiJAl phase were explored. The obtained results of fracture toughness (Kr-.m Kfo.:c) are compared with the existing models, which describe the second phase toughening mechanisms, and rule of mixtures (ROM). Weibull analysis is also applied for the analysis of the fracture toughness distribution of the investigated Ni3AVNiA1 in-situ composites. The important features of the K-/la and J-lla curves by a CNB bend test have been explored in this research. The stress intensity factor K decreases with increasing crack extension (ful) and a PLATEAU usually appears with increasing of the crack extension only until the critical crack extension (&z,J, then K starts to increase with increasing crack extension, forming a very special shape which can be called "HOOP HEAD". Particularly. a critical value (hvc) of the fracture energy for a CNB test can be simply calculated by a horizontal line tangent to the ''HOOP HEAD". It is shown that fracture toughness of Ni3AVNiAI increases with increasing volume fraction of NhAl in the in-situ composites according to a general formula K,-..c=6.l + 0. 7V/-75 (Mpa✓m) (where Vd - volume% of NiJAl). In some NiJAVNiAI composite alloys the NisAh fine particles are formed (so-called "mat-like structure") which exhibits very high Vickers microhardness (==690 kg/mm2). The significant yield strength of =ll50 kg/mm2 in the aged N~5_9Al~.1 in-situ composite is also attributed to this needle-like structure of NisAl3. It is wonh of pointing out that a very high yield strength ( O'ys == l 150Iv1Pa) is combined in aged alloys with a reasonable value of fracture toughness ( = 13 MPa✓m). It indicates that such a new promising alloy can be yielded by an economic and simple casting method followed by a proper heat treatment as shown in this research. The highest Weibull's modulus m = 23.8 for N~3_1Al36.3 (==17 vol% NhAI) indicates that this alloy is a very reliable material for engineering design even with lower fracture toughness value (K,m/v = 8 MPa✓m). The lowest Weibuirs modulus m == 5.8 for Nh32A126.s (=99 vol Ck NhAI) means that the fracture toughness of this alloy is highly variable and no single value for K w be. d il lmt Can asstgne eas y.application/pdf14076991 bytesapplication/pdfenCopyright: 1997, Gao, Qian.. All rights reserved.Harvested from Collections CanadaDoctoral Thesis