Papers by Keyword: Creep Resistance

Abstract: The evolution of microstructure in low carbon heat resistant steel of P92-type modified with 3%Co was examined during creep tests at 923K for 500, 1500, 6000 and 16000 hours. After tempering at 1023K (750°C), the steel was composed of martensite lath structure with numerous precipitates of MX-type carbonitrides and rare M₂₃C₆-type carbides. The structural changes during creep tests are characterized by an increase in the sizes of laths and second phase particles. Moreover, the Fe₂W Laves-phase precipitates during long-term creep. The Laves-phase particles grow accordingly to power-law relationship of creep time.

Abstract: As a rapid deposition method, Electron Beam Physical Vapor Deposition (EBPVD) permits the fast fabrication of self-standing thick coatings with thickness in the range from several micrometers to hundreds of micrometers. In this paper, we present a new way to fabricate ferritic ODS alloy foil. By this method, 1.2 wt.% of yttria was successfully added to the ferritic base material. The base alloys are mainly composed of bcc structured Fe (Cr,Al) solid solution. The steady state creep rate of ferritic ODS is almost two magnitudes lower than that of FeCrAl alloy. Further thermo-mechanical treatment is expected to improve strength and ductility of both ODS and non-ODS material.

Abstract: The creep resistance of Elektron 21 magnesium alloy containing Zn, Nd, Gd and Zr has been investigated. Test has been conducted at 200°C, 225°C and 250°C with constant load amounts to 90, 120 and 150 MPa up to 100 hours. Some specimens cracked during the test. Metallographic and fractographic research has been performed in order to identify the microstructural changes occurring during the creep resistance test. Microstructure has been observed with light microscopy and scanning electron microscopy. Chemical composition of microstructural components has been investigated with energy dispersion spectroscopy. Research revealed presence of voids, microcracks and inclusions which can significantly influence creep resistance of material.

Abstract: Recent studies indicated that Mg-(7-11)Zn-4Al (the composition of the alloys in the present paper are given in weight percentage.) based alloys exhibited good creep resistance and additions of trace alkaline elements to the alloys further improved the mechanical properties at elevated temperatures. In order to optimize the zinc concentration and reveal the effect of small amount of alkaline element additions to Mg-Zn-Al ternary alloys on their microstructure and elevated temperature creep resistance, five alloys with compositions of Mg-7Zn-4Al, Mg-9Zn-4Al, Mg-11Zn-4Al, Mg-11Zn-4Al-0.2Ca and Mg-11Zn-4Al-0.6Ca, respectively, were prepared in the present investigation and the microstructure and creep properties of these alloys were studied.

Abstract: The aim of this paper is to investigate the microstructural characteristics and creep resistance of Mg-(8-12 Wt. %) Zn-(2-6 Wt. %) Al alloys in order to get a better overall understanding of these series alloys. The results indicate that the alloys studied with the ratio of Zn/Al concentration between 2 and 3 exhibit high creep resistances, and the alloy ZA124 with the continuous network of τ precipitate along grain boundaries shows the highest creep resistance. The creep resistance of alloys decreases significantly due to  eutectic phase,  eutectics and β compounds appearing, especially the creep properties of alloys containing  eutectic phase and β phase with lower Zn/Al concentration are obviously inferior to other alloys.

Abstract: The testing of creep resistance (for example σ_{1%/100000 h}) of metals has been required for a long time (about 10 years) and some methods have been worked out to decrease testing time. The application of these methods has some difficulties. Now we have worked out a new empirical method to reduce testing time, namely COD measuring at an elevated temperature. The paper presents this method, which makes it possible to determine creep properties by measuring the materials COD at an elevated temperature. To determine COD we use notched samples with different radii and we tear them at an elevated temperature. The determination of COD is based on the Czoboly Radon method. By opening three different radii notches we approximate to zero radius which gives the COD. Master Curves of COD and creep resistance connections were drawn at 500 °C, which is not dependent on materials and they can be used for determining Creep Properties from COD measured at 500 °C. This method can be used for the comparison of the creep resistance of materials and for that of the creep properties of different zones of welded joints. The method is found useful in technical practice.

Abstract: The search for alloys with improved high-temperature specific strength and creep-resistance properties for aerospace applications has led in the last decades to sustained research activities to develop new alloys and/or improve existing ones. Titanium and its alloys are excellent for applications in structural components submitted to high temperatures owing to their high strength to weight ratio, good corrosion resistance and metallurgical stability. Its high creep resistance is of great importance in enhancing engine performance. However, the affinity by oxygen is one of main factors that limit its application as structural material at high temperatures. Materials with adequate behavior at high temperatures and aggressive environmental became a scientific requirement, technological and economically nowadays. The objective of this work is the mechanical and microstructural characterization of the Ti-6Al-4V alloy after treatment by nitrogen Plasma Immersion Ion Implantation (PIII) process. The aim of this process is the improvement of superficial mechanical properties of the Ti-6Al-4V alloy. The selected alloy after ionic implantation process by plasma immersion was submitted to creep tests at 600 °C, in constant load mode at 250 and 319 MPa. The techniques used in this work were optical microscopy and scanning electronic microscopy. The fractograph analysis of the samples tested in creep shows narrowing phenomena and microcavities. The creep results show the significant increase of material resistance, it can be used as protection of oxidation in high temperatures applications.

Abstract: The QE22 cast magnesium alloy containing silver, rare earth elements and zirconium is characterized by high mechanical properties and creep resistance of up to 200 ° C. It is cast gravitationally into sand moulds and permanent moulds. After the casting process any possible defects appearing in the cast are repaired with the application of welding techniques. The repaired cast should possess at least the same properties as the one which does not require any repairs. The aim of this thesis was to determine the impact of the heat treatment on the microstructure of the QE22 alloy welded joint. The creep resistance of the welded joints was also analyzed.

Abstract: The effect of Ca and rare earths on compressive creep behavior of Mg-4Al alloy was investigated with a special apparatus. The microstructures were analyzed by OM, XRD, SEM and EDS before and after compressive creep test. The results reveal that a small amount of Ca was added into AE41 alloy in order to refine the crystalline structure and improve the creep resistance. Comparing with NdPr rare earths alloy, the creep resistance of LPC rare earths alloy decreases, but is still better than AE41 alloy. The as-cast microstructure of AE41 alloy is mainly composed of α-Mg matrix and Al11Nd3 phase. The acicular Al11Nd3 phase is prone to decompose at high temperature, which leads to the poor creep resistance of AE41 alloy. The acicular Al11RE3 phase is gradually replaced by Al2Ca and Al2RE with Ca addition into AE41. LPC rare-earth mixture is in cluster at grain boundaries so that the creep resistance is worse than that of alloy containing NdPr rare-earth mixture.

Abstract: Structural changes in a 9%Cr martensitic steel after 1%, 4% creep and creep rupture test at 650°C and stress of 118 MPa were examined. Heat treatment provided the formation of tempered martensite lath structure (TMLS) in the steel. The precipitations of second phase particles along block and lath boundaries provide effective stabilization of the TMSL under annealing/aging condition. This structure hardly changed under creep conditions in grip portion of crept sample. Significant coarsening of both the second phase particles and the martensite laths takes place in neck portion. In addition, the latter ones lose their original morphology and are replaced by large strain-induced subgrains. It should be noted that the increase of subgrain size is in almost direct proportion to the particle growth during the creep to 4% strain. The rapid growth of martesite laths followed by their evolution to deformation subgrains takes place within the tertiary creep regime.