B. Chmiela

Effect of Heat Treatment on Chemical Segregation in CMSX-4 Nickel-Base Superalloy

Superalloys display a strong tendency toward chemical segregation during solidification. Therefore, it is of great importance to develop appropriate techniques for the melting and casting of superalloys. Elements partitioning between the γ and γ′ phases in single crystal superalloys have been investigated by several authors using electron probe microanalysis (Hemmersmeier and Feller-Kniepmeier Mater Sci Eng A 248:87-97, 1998; Kearsey et al. Intermetallics 12:903-910, 2004; Kearsey et al. Superalloys 2004, pp 801-810, 2004; D’Souza et al. Mater Sci Eng A 490:258-265, 2008). We examined the effect of the particular stages of standard heat treatment (solution treatment and ageing) applied to CMSX-4 single crystal superalloy on chemical segregation that occurs between dendrites and interdendritic areas. Dendritic structures were observed using a scanning electron microscope. Analyses of the chemical composition were performed using energy dispersive x-ray spectroscopy. The obtained qualitative and quantitative results for the concentrations of elements enabled us to confirm the dendritic segregation in as-cast CMSX-4 superalloy. The concentrations of some refractory elements (tungsten, rhenium) were much greater in dendrites than in interdendritic areas. However, these differences in chemical composition gradually decreased during heat treatment. The results obtained in this study warrant further examination of the diffusion processes of elements during heat treatment of the investigated superalloy, and of the kinetics of diffusion.

Phase Identification in Nickel-Based Superalloys Using EBSD/SEM and Electron Diffraction in STEM

Aero engine turbine blades made of nickel-based superalloys are critical components in flight safety. Therefore, it is very important to make sure that the chemical composition, phase composition and microstructure are suitable. However, due to their chemical compositions, superalloys are prone to many transformations and the formation of deleterious phases, which deteriorate the mechanical properties. Hence, investigations concerning the structural stability and phase identification—especially topologically close-packed phases (TCP)—are necessary. Because the volume fractions of these phases are generally small, phase identification should be performed by nanodiffraction techniques in a scanning transmission electron microscope (STEM) and electron backscatter diffraction in a scanning electron microscope (EBSD/SEM). These methods complement each other, but each of them is characterized by different difficulties and limitations. In this paper we present the possibilities and limitations of phase identification in single crystal CMSX-4 superalloy after long thermal exposure.

Phase analysis in duplex stainless steel: comparison of EBSD and quantitative metallography methods

The purpose of the research was to work out the qualitative and quantitative analysis of phases in DSS in as-received state and after thermal aging. For quantitative purposes, SEM observations, EDS analyses and electron backscattered diffraction (EBSD) methods were employed. Qualitative analysis of phases was performed by two methods: EBSD and classical quantitative metallography. A juxtaposition of different etchants for the revealing of microstructure and brief review of sample preparation methods for EBSD studies were presented. Different ways of sample preparation were tested and based on these results a detailed methodology of DSS phase analysis was developed including: surface finishing, selective etching methods and image acquisition. The advantages and disadvantages of applied methods were pointed out and compared the accuracy of the analysis phase performed by both methods.

The Influence of Tin on the Microstructure and Creep Properties of Mg-5Al-3Ca-0.7Sr-0.2Mn Magnesium Alloy

The paper presents results of microstructural investigations and creep properties of Mg-5Al-3Ca-0.7Sr-0.2Mn (ACJM53) and Mg-5Al-3Ca-0.8Sn-0.7Sr-0.2Mn (ACTJM531) magnesium alloys in as cast condition. The microstructure of the ACJM53 consists of α-Mg, (Mg,Al)2Ca - C36, Al3Mg13(Sr,Ca), Al2Ca - C15 and AlxMny. Additionally, the CaMgSn phase is observed in the ACTJM531 magnesium alloy. The addition of 0.8 wt% tin reduces the tensile strength at ambient temperature and creep resistance at 180°C.

Analysis of Stray Grain Formation in Single Crystal CMSX-4 Superalloy

Abstract Modern single crystal (SX) turbine blades are fabricated by directional solidification using a grain selector. The grain selection process was investigated by numerical simulation and verified by the experiment. A coupled ProCAST and cellular automaton finite element (CAFE) model was used in this study. According to the latest literature data, we designed the grain selector. Simulation confirmed an optimal grain selection efficiency of the applied selector geometry. The obtained experimental results reveal the possibility of stray grain formation in SX castings with a designed selector, in contrast to the simulation results.

Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE2Zr2O7 TBC

Modern aero engine turbine blades made of nickel-based superalloys are covered by thermal barrier coatings (TBC) for thermal and oxidation protection. A new generation of TBCs consist of a bond coat (thin layer of MCrAlY, where M may be Ni, Co, Fe) followed by a ceramic top coat of RE2Zr2O7 (RE - rare earth element). In this paper we present the possibility of the electron backscatter diffraction (EBSD) method for characterisation of the microstructure and crystallographic orientation of a new TBC consisting of a Gd2Zr2O7 top coat and a NiFeCrAlY bond coat after long thermal exposure (1100 °C, 500 h). During thermal exposure, a thermally grown oxide (TGO) layer forms at the bond coat/top coat interface. The TGO is mainly composed of Al2O3. But, there is a possible reaction between Gd2Zr2O7 and Al2O3, leading to Gd-Al-O phases. Phase composition plays an important role in controlling the stress evolution, TGO deformation and crack propagation. Application of SEM-EDS-EBSD techniques allows direct characterisation of the chemical composition, phase composition and crystallographic orientation in the ceramic top coat and TGO layers. This paper presents the possibilities of using the EBSD method for phase identification (Gd2Zr2O7, spinel Ni(Al,Cr)2O4, GdAlO3 and other phases) and orientation analysis of grains in the TGO layer.

Non-Metallic Inclusions in Castings Made of Nickel-Base Superalloys

The paper presents investigations concerning causes of non-metallic inclusions forming in castings made of nickel-base superalloys. The most common reason of inclusions forming is erosion and thermal decomposition of crucibles and molds by the action of liquid alloy. In case of superalloys containing reactive elements (Hf), many reactions at the alloy-crucible and alloy-mold interfaces take place.

Investigation of Stress Corrosion Cracking in Magnesium Alloys

The paper organises the current state of knowledge concerning the effect of hydrogen on stress corrosion of magnesium alloys. This review describes phenomena and mechanisms connected with stress-corrosion cracking (SCC) in commonly used magnesium alloys from Mg-Al-Zn system. In addition, some information about SCC in alloys from Mg-Y-RE-Zr and Mg-Al-RE systems is described. It seems that microstructural factors (e.g., matrix α-Mg and intermetallic phases) related to the presence of Y, Zr and rare earth elements (RE) plays an essential role in hydrogen-induced cracking (HIC).

Analysis of high angle boundaries in directionally solidified turbine blade made of CMSX-4® superalloy

High angle boundary (HAB) and low angle boundary (LAB) are casting defects of single crystal (SX) and directionally solidified (DS) turbine blades and decrease the lifetime of these blades during service. During directional solidification, primary dendrite arms grow in the opposite direction of the thermal gradient direction and perpendicular to the mushy zone interface. When this interface is not flat, primary dendrite arms growing in various areas of the mushy zone are characterized by different growth directions. Then, after the primary dendrite tips contact each other, LABs or HABs form (depending on the angle between the directions of the primary dendrite arms). This paper presents characterization studies of HABs in a DS turbine blade made of CMSX-4? superalloy. The blade was characterized by three columnar grains with HABs. Qualitative and quantitative analyses of the HABs using electron backscatter diffraction in a scanning electron microscope (SEM-EBSD) were carried out. The EBSD technique helped to determine the crystallographic orientation of the grains near the HAB, misorientation angles between grains (and inside each grain), and the angle of deviation between the [001] direction and the blade axis.

Properties of WE43 Metal Matrix Composites Reinforced with SiC Particles

It is well known that the properties of metal matrix composites depend upon the properties of the reinforcement phase, on the matrix and on the interface. A strong interface bonding without any degradation of the reinforcing phase is one of the prime objectives in the development of metal matrix composites. The objective of this work is to characterize the interface structure of WE43/SiC particles composite and their mechanical properties at ambient and elevated temperatures. Magnesium alloys containing yttrium and neodymium are known to have high specific strength, good creep and corrosion resistance up to 523 K. The addition of SiC ceramic particles strengthens the metal matrix composite resulting in better wear and creep resistance while maintaining good machinability. In the present study, WE43 magnesium matrix composite reinforced with SiC particulates was fabricated by stir casting. The SiC particles with 15, 45 and 250 µm diameter were added to the WE43 alloy. Microstructure characterization of WE43 MMC with 45 µm showed a relatively uniform reinforcement distribution and presence of inconsiderable porosity. Moreover, the Zr-rich particles at the particle/matrix interface were visible. The presence of SiC particles assisted in improving hardness and decreasing the tensile strength creep resistance.