Materials Science Forum Vols. 783-786

Abstract: We investigate the non uniform plastic deformation of a TWIP FeMnC steel by diffraction of high energy synchrotron X-rays. In particular, we observe the propagation of bands of plastic strain localisation. Debye-Scherrer rings are recorded in situ during tensile tests at two different strain rates. Discontinuous initial rings characteristic of unstrained polycristals with no texture become rapidly continuous after several percents of plastic strain due to strain gradients within the grains and a strong texture develops. The crystallographic dependence of the Young’s modulus is estimated and is consistent with the elastic anisotropy of a cubic crystal. A delay between the serrations on the macroscopic tensile curve and the stepwise variations of the diffracted peak’s position and width are consistent with propagating bands nucleating outside the X-ray beam. Slower and thinner bands are observed at the lowest strain rate. A tensile test interrupted to perform a few minute relaxation leads to a displacement of the nucleation site of the bands from one end to the middle of the gauge part.

Abstract: The activated slip systems were analyzed in the cold rolling of a Ni₃Al single crystal with an initial orientation of ~[-112](512), which showed an irregular rolling deformation, i.e. widening, bending, and shear deformation. A phenomenological crystal plasticity model was applied using a spectral method. The boundary condition was optimized to reproduce the actual rolling deformation, as follows. That is, the orthogonal components of the deformation gradient were given from the measured widening and reduction, and the shear components were iteratively optimized as to that the final orientation was as close to the experimental one as possible. The calculated result showed that three slip systems, a3, b1, and d1 in the Bishop-Hill notation, were mainly activated in the irregular rolling deformation, which result was consistent to the previous observation of the slip traces [Kishida et al., Philos. Mag. 83 (2003) 3029]. The three activated systems were identical to those activated in the plane-strain condition. However, the quantitative comparison revealed that the activity of b1 was significantly reduced in the irregular rolling deformation, while the activity of d1 was enhanced instead. The less activity of b1 and the enhancement of d1 can be understood assuming a strong interaction between a3 and b1. The reaction of this pair has been reported to form the superlattice intrinsic stacking fault (SISF) in Ni₃Al [Chiba and Hanada, Philos. Mag. A. 69 (1994) 751]. It is likely that the formation of the SISF, which are considered immobile in Ni₃Al, restrained the activation of b1, leading to the irregular rolling deformation.

Abstract: Intermetallic light weight TiAl-alloys are expected to replace the heavy Ni-based super alloys in several high temperature applications. However until now they cannot be used at temperatures above 700°C for longer times due to their insufficient oxidation resistance. The high temperature oxidation behavior can be improved drastically for the use at temperatures up to at least 1050°C by small amounts of fluorine in the surface region of TiAl-components. A thin protective alumina layer is formed after an optimized fluorine treatment during exposure in oxidizing high temperature environments. Results of isothermal and thermocyclic high temperature oxidation tests of untreated and halogen treated TiAl-samples of new types of TiAl-alloys containing Mo, Cu and Si will be presented in this paper. These results will be compared and discussed considering the beneficial effect of fluorine for a later use as e.g. turbine blades in jet engines. Key words: Titanium aluminides, high temperature oxidation, halogen effect,

Abstract: NbSi₂/MoSi₂ duplex silicide crystals are potentially a new-class of ultra-high temperature structural materials. Improvement in the thermal stability of their lamellar microstructure was accomplished by the addition of a minute amount of either Cr or Zr. The mechanical properties of the duplex silicide, such as fracture toughness and high temperature strength, show strong orientation dependence, thereby suggesting the importance of the control of microstructure to improve their properties.

Abstract: High temperature compression properties of Al-, Cr-or Nb-added Ni₃(Si,Ti) based intermetallic compounds were investigated by uni-axial compression test and microstructural observation. The Al-or Cr-added Ni₃(Si,Ti) alloys after homogenization heat treatment exhibited a two-phase microstructure consisting of L1₂ and Ni-solid solution phases. The Nb-added Ni₃(Si,Ti) alloy after homogenization heat treatment exhibited a triple-phase microstructure consisting of G-phase with D8a structure and Ni-solid solution phase in the L1₂ matrix. The volume fraction of Ni-solid solution phase increased in order of Cr-, Nb-and Al-added Ni₃(Si,Ti) alloys. The Cr-added Ni₃(Si,Ti) alloy was deformable at high strain rate, while the Nb-added one was deformable at low strain rate. It can be considered that the deformability of Ni₃(Si,Ti) at high temperature is closely correlated with volume fraction of Ni-solid solution phase and recrystallization behavior.

Abstract: The mechanical properties of g-TiAl at elevated temperatures have been investigated extensively over the last 30 years. Designed alloys have been proposed from the first generation alloy (Ti-48Al-2Cr-2Nb) to the second, the third and the fourth generations. However, a decisive chemical composition of g-TiAl has not been agreed among researchers yet. The main reasons for this situation are difficulties in compositional control of Ti-Al-X-Y. In this paper, the high temperature tensile properties of g-TiAl alloy with lots of different composition have been examined from the room temperature to 1200C and the tensile strength data of those specimens have been summarized. It is clear that Ti/Al atomic ratio plays an important role on the behaviors of the high temperature strength since the Ti/Al atomic ratio is strongly related to the phase stabilities between g and a₂ phases in the binary Ti-Al phase diagram. A very narrow confine of a/a₂ atomic ratio exists in the specimens having the comparatively high tensile strength at the elevated temperatures. Moreover, additions of the third elements such as Cr, Nb, Ta and W to g-TiAl contribute on the increase of the tensile strength and the shift of the phase stability among a₂, b and g phases. In order to utilize g-TiAl alloys in the various machine components at high temperatures, the severe process controls of melting, casting, thermo-mechanical treatments and heat treatments are indispensable.

Abstract: Recycle-type Fe₃Al (hereinafter designated as Re-Fe₃Al) based alloys reinforced by the carbides of TiC or ZrC were processed by the high frequency induction melting method using a high-carbon Cr steel sludge, Al can scraps and the transition metals of Ti or Zr. The carbides were synthesized by in-situ reaction between the transition metal and carbon in the molten iron aluminum alloy. Vickers hardness values are 309HV0.5 for Re-Fe₃Al/TiC alloy, and 473HV0.5 for Re-Fe₃Al/ZrC alloy, which are higher than that of P-Fe₃Al (preprared from pure-Fe and-Al). The cutting performance of the Re-Fe₃Al baed alloys was compared with a High-Speed-Steel (HSS) by cutting tests for pure-Cu extruded bar (C1020) using a lathe under a dry condition. Tool life limit was estimated from frank wear length after the cutting tests of C1020 by finish-machining. Tool life limit of Re-Fe₃Al/TiC alloy is more than16 min; P-Fe₃Al was 12 min; HSS was 8 min, Re-Fe₃Al/ZrC alloy was 7 min at the cutting speed of 100m/min. Also, tool life limit of the Re-Fe₃Al/TiC alloy was more than twice times as long as that of the HSS at the cutting speed of 300/min. The relationship between cutting speed and tool life limit clearly indicated that the Re-Fe₃Al/TiC alloy was better than the HSS at a higher cutting speed. Therefore, it was concluded that Re-Fe₃Al/TiC alloy has excellent cutting tool performance.

Abstract: Directional solidification of binary peritectic TiAl alloys through preforms with straight channels with spacing on the order of the dendritic tip radius were simulated using a modified solid-liquid interface tracking model. Interruption of the steady-state growth of the primary β-phase by constraint of solutal diffusion within very thin sections of ceramic preforms can lead to solidification conditions favorable for the nucleation and continued growth of the peritectic α-phase even after the growth front has exited the preform.

Abstract: In this study, two heat treatment schemes were proposed to study the high temperature mechanical behavior of directionally solidified MAR-M247 superalloy. Two withdraw rates, namely, 60 and 180 mm/h were used to produce directionally solidified MAR-M247 specimens by the Bridgeman type furnace. Standard heat treatment (HT1) procedures are solution treatment at 1230°C for 2 h/ArC, then first aging at 980°C for 5 h/AC and followed by second aging at 870°C for 20 h/AC. Modified heat treatment (HT2) is solution treatment at 1260°C for 3 h/ArC and first aging at 980°C for 6 h/AC, then the same second aging procedure. Uneven size, shape and fusion-alike of gamma prime precipitates are observed after full HT1 scheme, whereas even size but fine gamma prime precipitates are observed in HT2 specimen. All three tensile properties (elongation to failure, ultimate tensile strength and yield strength) of HT2 specimens are higher than these of HT1 specimens either at room temperature 25°C or at high temperature 982°C for both withdraw rates. Uneven distribution of the γ′ precipitates attributes to the initial fracture of HT1 specimens.

Abstract: In 2006, J. Sato et al. discovered the L1₂ gamma-prime phase in the Co-Al-W alloy system. Since that time there has been a significant academic effort to characterize and understand the thermodynamics, the structure, and properties of alloys in this system. That work has shown that such alloys have promise as next generation high temperature materials due to the ability to engineer a high gamma-prime content alloy with a higher gamma prime solvus and higher melting point than many Ni-base gamma-prime strengthened alloys. Furthermore, a relatively narrow range of solidification temperature and large range of temperature between the gamma-prime solvus and the solidus are two characteristics that suggest cast and wrought versions of the alloys should be able to be manufactured routinely. However, to date all published research has been on small, laboratory scale samples typically less than about 2 kg, cast and hot rolled or cast as single crystals. This paper describes ATI’s experience in assessing the feasibility of manufacturing a cast and wrought billet product in the Co-Al-W-X alloy system. Three 22 kg heats were produced to examine a small range of alloy compositions of potential commercial interest: Co-9Al-9W, Co-9Al-9W-2Ti, and Co-9Al-9W-2Ti-0.02B, respectively. Each heat was vacuum induction melted and vacuum arc remelted. The as-cast microstructure will be presented. Ingots were homogenized and hot worked. The microstructure at various stages of production are shown and hot working behavior is described.