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Transitions in Creep Mechanisms of Z...

Kombaiah, Boopathy.

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Transitions in Creep Mechanisms of Zirconium Alloys.

紀錄類型:	書目-電子資源 : Monograph/item
正題名/作者:	Transitions in Creep Mechanisms of Zirconium Alloys./
作者:	Kombaiah, Boopathy.
面頁冊數:	229 p.
附註:	Source: Dissertation Abstracts International, Volume: 76-11(E), Section: B.
Contained By:	Dissertation Abstracts International76-11B(E).
標題:	Materials science. -
電子資源:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3710641
ISBN:	9781321866230

Transitions in Creep Mechanisms of Zirconium Alloys.
Kombaiah, Boopathy.

Transitions in Creep Mechanisms of Zirconium Alloys. - 229 p.

Source: Dissertation Abstracts International, Volume: 76-11(E), Section: B.

Thesis (Ph.D.)--North Carolina State University, 2015.

Zirconium and its alloys have a rich history of serving light and heavy water reactors as structural materials such as fuel claddings and grids for several decades owing to their attractive properties, such as low absorption cross section for thermal neutrons, good aqueous corrosion resistance and sufficient strength at high temperature. Creep of zirconium alloys is regarded as a primary performance degrading mechanism that can deter the structural integrity of reactors and also limit their operational temperatures resulting in a process with lesser efficiency. In view of this, understanding of the creep mechanisms along with their constitutive equations to define the creep rates under varying stress and temperature conditions becomes essential not only for predicting the life of the zirconium claddings but also for developing more creep-resistant microstructures.

ISBN: 9781321866230Subjects--Topical Terms:

543314
Materials science.

Transitions in Creep Mechanisms of Zirconium Alloys.
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100 1 $a Kombaiah, Boopathy. $3 3184001
245 1 0 $a Transitions in Creep Mechanisms of Zirconium Alloys.
300 $a 229 p.
500 $a Source: Dissertation Abstracts International, Volume: 76-11(E), Section: B.
500 $a Adviser: Korukonda Linga Murty.
502 $a Thesis (Ph.D.)--North Carolina State University, 2015.
520 $a Zirconium and its alloys have a rich history of serving light and heavy water reactors as structural materials such as fuel claddings and grids for several decades owing to their attractive properties, such as low absorption cross section for thermal neutrons, good aqueous corrosion resistance and sufficient strength at high temperature. Creep of zirconium alloys is regarded as a primary performance degrading mechanism that can deter the structural integrity of reactors and also limit their operational temperatures resulting in a process with lesser efficiency. In view of this, understanding of the creep mechanisms along with their constitutive equations to define the creep rates under varying stress and temperature conditions becomes essential not only for predicting the life of the zirconium claddings but also for developing more creep-resistant microstructures.
520 $a This research work was undertaken to investigate the creep behavior of Zirclaoy-4 sheet and HANA-4 tubing with a focus on uncovering the creep rate-controlling mechanism(s). To this end, creep tests on standard specimens were conducted to measure the steady state creep rates at various stresses and temperatures from which two important creep mechanistic parameters of stress exponent ( n) and creep activation energy (Qc) were evaluated. Possible creep mechanisms have then been short-listed based upon the values of n and Qc. Furthermore, detailed transmission electron microscopy analyses of the dislocation structures in the crept specimens as well as validation of the experimental results with the predictions by standard creep models were undertaken to ascertain the underlying rate controlling mechanisms.
520 $a At first, uniaxial creep tests were performed on recrystallized Zircaloy-4 tensile specimens at the stress range of, 2.56 x 10--4 E2.8 x 10--3E, where E is the elastic modulus, at three temperatures: 500°C, 550°C and 600°C. Transitions in creep mechanisms were identified as n changed from a lower value of 1 through 4.5 to higher values of 9.3-11 with increasing stress where the values of Qc values were calculated to be 196 kJ/mol, 247 kJ/mol and 220-242 kJ/mol respectively. TEM analyses of the deformed specimens revealed a dislocation structure with sub-boundaries formed by edge and mixed dislocations and dense hexagonal network of screw dislocations respectively in intermediate and high stress regimes, i.e., with n=4.5 and n=9.3-11. Furthermore, the experimental creep results were compared with the predictions of the Coble creep, Weertman-climb and Friedel-cross slip models to check their applicability to Zircaloy-4. Based on the preceding analyses, Coble creep, climb of edge and mixed dislocations and cross-slip of screw dislocations were then identified as the rate-controlling mechanisms respectively for n=1, n=4.5 and n=9.3-11 regimes of Zircaloy-4.
520 $a Secondly, biaxial creep tests on HANA-4 tubes, Nb-added Zircaloy-4 were conducted using internal pressurization of closed-end tubes over a range of hoop stresses, 8.38 x 10--5 E 2.87 x 10--3E, at three different temperatures: 400°C, 450°C and 500°C. Based on the variance in n with respect to the applied stress, three regimes have been identified: a stress exponent close to 1 at low stresses increased to 3 at the intermediate stresses, which became 4.5 at high stresses. An activation energy value of 226 kJ/mol was evaluated for the n=3 and n=4.5 regimes, which lies close to the activation energy for self-diffusion (QL) in alpha-Zr alloys. Coble creep, climbing of dislocations to bypass the beta-Nb precipitates presented in the zirconium matrix and dynamic recovery by edge dislocation climb are proposed as the rate controlling mechanisms in the n=1, n=3 and n=4.5 regimes of HANA-4 respectively based on the collective examination of the TEM analyses and the model predictions.
520 $a Finally, the effect of hydrogenation of the biaxial creep behavior of HANA-4 was studied using internal pressurization of closed end tubes by applying a range of hoop stresses (27 MPa to 156 MPa) at two temperatures: 400°C and 500°C. Two HANA-4 tubes were hydrogenated using the electrolytic method respectively with &sim;387 ppm and &sim;715 ppm of hydrogen while the HANA-4 tubes without hydrogenation were utilized as controlled specimens. Further, high temperature XRD experiments were conducted on the hydrogenated HANA-4 specimens to detect the dissolution limit of the hydride phase. Based on the results of the creep tests and the XRD experiments we find that hydrogen fully present in the solid solution at the creep temperature enhances the creep rate of HANA-4, whereas hydrogen present as both hydride and dissolved hydrogen reduces the creep rate regardless of the applied stress. The rationale behind the creep behaviors of hydrogenated HANA-4 has been suggested from models.
590 $a School code: 0155.
650 4 $a Materials science. $3 543314
650 4 $a Nuclear engineering. $3 595435
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710 2 $a North Carolina State University. $3 1018772
773 0 $t Dissertation Abstracts International $g 76-11B(E).
790 $a 0155
791 $a Ph.D.
792 $a 2015
793 $a English
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=3710641