D. X. Li

The influence of alloying on the α2/(α2+γ)/γ phase boundaries in TiAl based systems

By employing both experimental and theoretical approaches, a comparative study of the alpha(2)/(alpha(2) + gamma)/gamma phase boundaries in some Ti-Al-X (X = Nb, Ta, V, Cr, Mn, Fe or Ga) systems has been carried our. The phase constitution of Ti-(33-51 at.%)Al-(w1-3 at.%)X alloys with a composition step of 1 at.% Al was determined experimentally by using the X-ray diffraction method on bulk samples equilibrated at 1173 K. The alpha(2)/(alpha(2) + gamma) and (alpha(2) + gamma)/gamma phase boundaries were calculated using a model that describes the phase boundaries in terms of sublattice site occupancies of alloying species in the two ordered phases. The differences between the predicted and experimentally determined phase boundaries were found to be less than about 1 at.% in most cases. The predicted volume fractions of constituent phases were also compared with experimental measurements by a combination of metallography and transmission electron microscopy. Agreement between the two is good for 14 ternary and two quaternary alloys containing 46 at.% Al. The present work suggests that, in the order of increasing strength of stabilization at 1173 K, V, Nb and Ta stabilize the alpha(2) phase, whereas Cr, Mn, Fe and Ga stabilize the gamma phase

Preliminary investigation of directionally solidified NiAl–28Cr–5.5Mo–0.5Hf composite

An in-situ NiAl/Cr(Mo)-Hf eutectic composite has been successfully fabricated using the Bridgman technique. The microstructure was analyzed using scanning electron microscopy (SEM) with energy disperse spectroscopy and transmission electron microscopy (TEM). The mechanical behavior of the composite, including compressive and tensile properties at room and elevated temperature, was studied. It was found that the strengths of the composite are higher than that of NiAl alloys. Subsequently, the mechanism of improving the strength was analyzed

Interfacial reactions and mechanical properties of 6061Al matrix composites reinforced with alumina-coated Al18B4O33 whiskers

Abstract Sol–gel γ-Al 2 O 3 and α-Al 2 O 3 ceramic coatings were deposited onto surfaces of aluminum borate whiskers to control the spinel reactions between whiskers and matrix alloys. Interface observations, tensile experiments and hardness measurements of the composites were conducted to evaluate the coating effect. By comparison with γ-Al 2 O 3 coating, the α-Al 2 O 3 coating was found to be more effective in controlling the interfacial reactions, and it helped to enhance the elastic modulus of the composite. The introduction of alumina coatings to the composite postponed the peak-aging process, due to a smaller depletion of magnesium in the matrix of the coated composites.

Formation of nanoscale alpha-Al in a superdralumin under high current density electropulsing

Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, Shenyang 110016, Peoples R China.;Zhang, W (reprint author), Chinese Acad Sci, Shenyang Natl Lab Mat Sci, Inst Met Res, 72 Wenhua Rd, Shenyang 110016, Peoples R China

Bonding characteristics of micro-alloyed B2 NiAl in relation to site occupancies and phase stability

A recently developed mean-field model has been combined with first principles calculations of binding energy to investigate the site occupancies of micro-alloying elements and vacancies in NiAl as well as the stability of the micro-alloyed B2 phase with respect to disordering and second-phase formation. The theoretical results suggest that the transition metal elements in the same row of the periodic table increasingly tend to the Ni sublattice with increasing atomic number. Micro-alloying addition tends to decrease the vacancy concentration of NiAl alloys. Alloying with X that substitutes for Ni is predicted to have the sides of its solubility lobe parallel to the Ni-X side of the isotherm, but parallel to the Al-X side if X substitutes for Al. Micro-alloying was shown to raise the ordering temperature of the B2 phase over the corresponding binary alloy, in contrast with the effect of vacancies. Alloying effects on ordering temperature and the formation of point defects appear independent of the site substitution behaviour, and are less significant for 3d than for 4d and 5d transition metal elements

Microstructure and Phase Stability Studies on Heusler Phase Ni2AlHf and G-phase Ni16Hf6Si7 in Directionally Solidified NiAl–Cr(Mo) Eutectic Alloyed with Hf

Small additions of Hf to directionally solidified NiAl-Cr(Mo) eutectic resulted in precipitation of a high density of Heusler phase Ni2AlHf along with fine G-phase Ni16Hf6Si7. The Heusler phase was mainly located on the grain boundary region. The fine G-phase formed in the presence of Si, which was a contamination resulting from contact with ceramic shell molds during directional solidification of the alloy. These fine G-phases were cuboidal in shape and coherent with the NiAl matrix. After hot isostatic pressing and aging treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitated preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl matrix, and small amounts of them precipitated within the Cr(Mo) phase. The structures of the NiAl/Ni2AlHf and NiAl/Ni16Hf6Si7 interfaces were investigated by high-resolution electron microscopy. The habit plane of the fine G-phase was {001}(NiAl). This result was in good agreement with calculation based on the linear elastic theory. The misfit dislocation network on the NiAl/Ni2AlHf ((1) over bar 10) interface was calculated from the O-lattice model and compared with the observation, which showed good agreement.

Solidification and interfacial structure of in situ Al-4.5Cu/TiB2 composite

AbstractIn situ particle reinforced Al-4.5Cu/TiB2 composite was fabricated with TiO2, H3BO3, Na3AlF6 powders and Al-4.5Cu alloy by reaction in melt. The composite can be directly casted into moulds to make composite parts. TiB2 particles distribute uniformly in the matrix. The average size of TiB2 particles is 0.93 μm. At the atomic scale, TiB2 is hexagonal, and exhibits hexagon or quadrilateral shape. The orientation relationships exist in the interfaces between TiB2 particle and α-Al, and between the reinforced small Al2Cu phase and α-Al in the composite. They are

Theoretical and experimental studies of the dislocation structure at the NiAl-Cr(Mo) interfaces

\begin{gathered} \left[ {0\bar 332} \right]_{TiB_2 } //\left[ {110} \right]_{\alpha - AI} \left\langle {01\bar 11} \right\rangle _{TiB_2 } //\left\langle {002} \right\rangle _{\alpha - AI} and\left[ {\bar 1\bar 32} \right]_{\alpha - AI} //\left[ {123} \right]_{AI_2 Cu} , \hfill \\ \left\langle {211} \right\rangle _{\alpha - AI} //\left\langle {111} \right\rangle _{AI_2 Cu} \hfill \\ \end{gathered}