Crystallization of Ti33Cu67 metallic glass under high-current density electrical pulses

We have studied the phase and structure evolution of the Ti₃₃Cu₆₇ amorphous alloy subjected to electrical pulses of high current density. By varying the pulse parameters, different stages of crystallization could be observed in the samples. Partial polymorphic nanocrystallization resulting in the formation of 5- to 8-nm crystallites of the TiCu₂ intermetallic in the residual amorphous matrix occurred when the maximum current density reached 9.7·10⁸ A m^-2 and the pulse duration was 140 μs, though the calculated temperature increase due to Joule heating was not enough to reach the crystallization temperature of the alloy. Samples subjected to higher current densities and higher values of the evolved Joule heat per unit mass fully crystallized and contained the Ti₂Cu₃ and TiCu₃ phases. A common feature of the crystallized ribbons was their non-uniform microstructure with regions that experienced local melting and rapid solidification.PACS: 81; 81.05.Bx; 81.05.Kf.     

Elimination of oxide films during Spark Plasma Sintering of metallic powders: A case study using partially oxidized nickel

We present a case study addressing the issue of surface cleaning of the particles of metallic powders from oxide films during Spark Plasma Sintering (SPS). Based on the results obtained using an intentionally oxidized powder of carbonyl nickel (O/Ni(at.) = 0.2), we show that when the powder is heated in the SPS chamber but is not in direct contact with graphite foil commonly used in the SPS practice, the reduction of the oxide is limited to the surface layers of the powder bed. Comparative sintering experiments with graphite and copper foils indicate that a direct contact with a source of carbon is critical for the elimination of the oxide from the sintered compact. Compacts/regions of the compacts, in which nickel oxide NiO was reduced, showed more pronounced inter-particle necking than the regions maintaining the initial oxide concentration.     

MMAP|M|N queueing system with impatient heterogeneous customers as a model of a contact center

A multi-server queueing system with infinite buffer and impatient heterogeneous customers as a model of a contact center that processes incoming calls (priority customers) and e-mail requests (non-priority customers) is investigated. The arrival flow is described by a Marked Markovian Arrival Process (MMAP). The service time of priority and non-priority customers by a server has an exponential distribution with different parameters. The steady state distribution of the system is analyzed. Some key performance measures are calculated. The Laplace–Stieltjes transforms of the sojourn and waiting time distribution are derived. The problem of optimal choice of the number of contact center agents under the constraint that the average waiting time of e-mail requests does not exceed a predefined value is numerically solved.     

On Ground Water Flow to an Imperfect Gallery: Case of Evaporation from a Free Surface

The plane steady-state filtration in a phreatimetric formation to an imperfect gallery in the presence of evaporation from the free surface of groundwater is considered. In order to study the evaporation effect, a multiparameter mixed boundary-value problem of the theory of analytic functions is posed and solved by the Polubarinova-Kochina method. Based on the proposed model, we develop an algorithm to compute the seepage flow characteristics and provide the hydrodynamic analysis of the impact of the dependences of all the physical parameters of the scheme on the flow rate of the gallery and the ordinate of the exit point of the depression curve on an impermeable screen.     

Application of a spark plasma sintering facility for the heat treatment of compact and powder materials

Spark plasma sintering is widely used for the fast sintering of powders under controlled grain growth conditions and for simultaneous sintering of compounds and consolidation of powder materials. Spark plasma sintering facilities can be used as well for heat-treating powders and compact materials. In this study, an assembly without the upper punch has been used for heat treatment. It has been shown that annealing results depend on geometric features of the position of the sample in the graphite die. In a layer of a material heat-treated in a die without its upper punch, graded structures can be obtained. An example of the annealing of detonation coatings demonstrates the possibility of producing layers on substrates with compositions similar to those of compact materials produced by the spark plasma sintering of the starting powders.     

Formation of intermetallic phases during mechanical alloying and annealing of Cr + 20 wt % al mixtures

We have studied phase-formation processes in mixtures of Cr and Al (20 wt %) powders in the course of mechanical alloying (MA) and the phase transformations of the samples during subsequent annealing at temperatures of up to 800°C. The resultant x-ray amorphous intermetallic phases were identified by a differential dissolution method, which allows one to follow the formation of x-ray amorphous and partially crystallized phases. During MA of Cr + Al mixtures, the first to form is x-ray amorphous Cr₄Al, which then converts to partially crystallized Cr₂Al through reaction with aluminum. The peritectoid decomposition of Cr₄Al during heating of the MA samples is accompanied by heat release at 330–350°C. Heating to 420°C leads to the formation of Cr₅Al₈. At 800°C, Cr₅Al₈ reacts with Cr to form Cr₂Al.     

The influence of bridge groups nature in diepoxy compounds on kinetics of curing process by diamines and on physical and mechanical properties of forming polymers

Summary In the present work the initial monomer reactivity in curing reaction of epoxy compounds by aromatic diamines has been investigated. Also physical and mechanical properties of cured polumers of diglycidyl ethers 4,4-bisaromatic compounds with bridge groups have been examined.     

Retrial queuing system with Markovian arrival flow and phase-type service time distribution

We consider a multi-server queuing system with retrial customers to model a call center. The flow of customers is described by a Markovian arrival process (MAP). The servers are identical and independent of each other. A customer’s service time has a phase-type distribution (PH). If all servers are busy during the customer arrival epoch, the customer moves to the buffer with a probability that depends on the number of customers in the system, leaves the system forever, or goes into an orbit of infinite size. A customer in the orbit tries his (her) luck in an exponentially distributed arbitrary time. During a waiting period in the buffer, customers can be impatient and may leave the system forever or go into orbit. A special method for reducing the dimension of the system state space is used. The ergodicity condition is derived in an analytically tractable form. The stationary distribution of the system states and the main performance measures are calculated. The problem of optimal design is solved numerically. The numerical results show the importance of considering the MAP arrival process and PH service process in the performance evaluation and capacity planning of call centers.