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Zero-sum Flows in Graphs
,
M.Sc. Thesis
Sharif University of Technology
;
Akbari, Saeed
(Supervisor)
Abstract
For an undirected graph G, a zero-sum flow is an assignment of non-zero real numbers on the edges of G such that the total sum of all edges incident with any vertex of G is zero. A zero-sum k-flow for a graph G is a zero-sum flow with labels from the set {±1, . . . , ±(k ? 1)}. In this thesis for a graph G, a necessary and sufficient condition forthe existence of zero-sum flow is given. It has been conjectured that if a graph G has azero-sumflow, then it has a zero-sum 6-flow. Also, it has been proved that this conjectureand Bouchet’s Conjecture for bidirected graphs are equivalent. It is shown that the con-jecture is true for 2-edge connected bipartite graphs, and every r-regular graph...
Research on Influences of Coating on Low Cycle Fatigue Properties of Ni-Base Superalloy René 80
, Ph.D. Dissertation Sharif University of Technology ; Nategh, Saeed (Supervisor)
Abstract
In this thesis, based on the taking-off and landing of aircraft with GE 21-J85 turbine, the low cycle fatigue (LCF) test for superalloy used in first stage blades is considered. The LCF tests were performed on both coated and uncoated Rene 80 which are used in GE 21-J85 turbine at R = (min/max) = 0, strain rate of 2 10-3 s-1 and 760oC, 871oC and 982oC. To produce specimens, an investment casting method was used so that adequate amount of polycrystalline round bars could be obtained. The specimens were all homogenized and solution treated. After the inspection, some specimens were coated with pack cementation method in CODEP-B powder in H2 atmosphere. The coated and uncoated specimens all...
Dislocation network formation during creep in Ni-base superalloy GTD-111
, Article Materials Science and Engineering A ; Volume 339, Issue 1-2 , 2003 , Pages 103-108 ; 09215093 (ISSN) ; Sajjadi, S. A ; Sharif University of Technology
2003
Abstract
The Ni-base superalloy GTD-111 is used as a blading material in the first stage blades of high power gas turbines. The creep-rupture properties of the cast superalloy were studied over a wide range of temperatures and stresses. The observations of dislocation structures during steady-state creep confirmed that the creep mechanism was different in the high and low stress regions. The results showed that in the high stress region, shear mechanisms including stacking fault formation and anti-phase boundary creation were operative and in the low stress region, a by-passing mechanism occurred by either looping or dislocation climb and glide. With increasing exposure time in the high-temperature...
Microstructural evaluation and fracture mechanisms of failed IN-738LC gas turbine blades
, Article Case Studies in Engineering Failure Analysis ; Volume 1, Issue 2 , April , 2013 , Pages 85-94 ; 22132902 (ISSN) ; Khoshmanesh, R ; Nategh, S ; Davami, P ; Sharif University of Technology
2013
Effect of preweld microstructure on HAZ liquation cracking of rene 80 superalloy
, Article Materials Science and Technology ; Volume 23, Issue 5 , 2007 , Pages 547-555 ; 02670836 (ISSN) ; Kokabi, A. H ; Nategh, S ; Sharif University of Technology
2007
Abstract
The objective of this work was to study the microstructure of as cast, solution treated and overaged Rene 80 superalloy before and after welding treatment. Studies focused on identifying heat affected zone (HAZ) liquation cracking mechanism and the effect of preweld microstructure on it. The formation of HAZ liquation cracking or microfissuring is found to be strongly dependent on constitutional liquation of grain boundary precipitates. These liquated precipitates in the as cast state contain MC carbides and particularly γ/γ' eutectic islands. While in the overaged condition they consist of the MC carbides and borides. Also HAZ grain boundary liquation and subsequent resolidification occur...
Influence of filler wire and wire feed speed on metallurgical and mechanical properties of MIG welding–brazing of automotive galvanized steel/5754 aluminum alloy in a lap joint configuration
, Article International Journal of Advanced Manufacturing Technology ; Volume 82, Issue 9-12 , 2016 , Pages 1495-1506 ; 02683768 (ISSN) ; Paidar, M ; Khodabandeh, A ; Nategh, S ; Sharif University of Technology
Springer-Verlag London Ltd
2016
Abstract
In this research, the galvanized steel with thickness of 2 mm was joined to the 5754 aluminum alloy with thickness of 3 mm by the cold metal transfer MIG welding–brazing process. The effect of the filler wires (AlSi3Mn, AlSi5, and AlSi12) and wire feed speeds (4.7, 5, and 5.3 m/min) on metallurgical and mechanical properties have been discussed. According to the experimental results, thickness of intermetallic compound (IMC) layer which was formed along the interface during the MIG welding–brazing was varied by changing of parameters. In addition, the results indicated that by increasing of the wire feed speed, the thickness of IMC layer at first decreased and then increased. Results...
Join of two graphs admits a nowhere-zero 3-flow
, Article Czechoslovak Mathematical Journal ; Vol. 64, Issue. 2 , 2014 , pp. 433-446 ; ISSN: 00114642 ; Aliakbarpour, M ; Ghanbari, N ; Nategh, E ; Shahmohamad, H ; Sharif University of Technology
2014
Abstract
Let G be a graph, and λ the smallest integer for which G has a nowherezero λ-flow, i.e., an integer λ for which G admits a nowhere-zero λ-flow, but it does not admit a (λ − 1)-flow. We denote the minimum flow number of G by Λ(G). In this paper we show that if G and H are two arbitrary graphs and G has no isolated vertex, then Λ(G ∨ H) ⩽ 3 except two cases: (i) One of the graphs G and H is K2 and the other is 1-regular. (ii) H = K1 and G is a graph with at least one isolated vertex or a component whose every block is an odd cycle. Among other results, we prove that for every two graphs G and H with at least 4 vertices, Λ(G ∨ H) ⩽ 3
Influence of Rejuvenation Treatment on Microstructure and Creep Properties of Damaged IN738LC Superalloy
, Ph.D. Dissertation Sharif University of Technology ; Nategh, Saeed (Supervisor) ; Ekrami, Ali Akbar (Supervisor)
Abstract
The goal of the present study is to investigate the effect of rejuvenation treatment on microstructure and creep properties of damaged IN738LC superalloy. Virgin alloy was damaged by interrupted creep tests at 926°C and 170 MPa for 100, 200 and 250 hours. The results obtained showed that damaging the virgin alloy by interrupted creep tests resulted in microstructural deterioration (consisting of coarsening of ' precipitates, formation of continuous films of ' on grain boundary regions and formation of transition zone around MC carbides) and, consequently, degradation of creep properties. Rejuvenation treatment was carried out with the purpose of healing the damaged alloy and restoration of...
Morphological features of silicon rich phase in powder thixoformed spray atomized hyper-eutectic Al-Si alloy
, Article Semi-Solid Processing of Alloys and Composites 10 - Selected, peer reviewed papers from the 10th International Conference on Semi-Solid Processing of Alloy and Composites, S2P 2008, Aachen, 16 September 2008 through 18 September 2008 ; Volume 141-143 , 2008 , Pages 493-498 ; 10120394 (ISSN); 9771012039401 (ISBN) ; Aashuri, H ; Nategh, S ; Foroughi, A ; Narimannezhad, A ; Khosravani, A ; Sharif University of Technology
Trans Tech Publications Ltd
2008
Abstract
Microstructural evolution of the spray atomized and powder thixoformed hyper-eutectic A390 aluminum alloy was investigated. The spray atomized powder revealed homogeneous and very fine silicon particles distribution, due to the rapid solidification of the alloy. The semi-solid powders were extruded into a closed die cavity through a hole for the plastic deformation of the powder particles. A drop forge of 45kg weight at different heights was used in this investigation. Remarkable rearrangement and growth of the silicon rich phase was revealed in the final stage