W.T. Kim

Application of quasicrystalline particles as a strengthening phase in Mg-Zn-Y alloys

Abstract Fine-grained magnesium alloys reinforced by quasicrystalline particles were easily developed by thermomechanical processes for as-cast Mg-rich Mg–Zn–Y and Mg–Zn–Y–Zr alloys. The deformation behavior of the alloys at room and high temperatures was investigated and compared with that of commercial AZ31, AZ61 and AZ91 alloys. The yield strength of the Mg–Zn–Y alloys, increasing with an increase of the volume fraction of the quasicrystalline phase, is relatively high due to the strengthening effect of the quasicrystalline particles. At high temperatures, the level of flow stress of the Mg–Zn–Y alloys is lower than that of commercial magnesium alloys due to the softness of the eutectic region, but the alloys exhibit much higher elongation since the large number of quasicrystalline particles in the Mg–Zn–Y alloys can effectively prohibit microstructural evolution of the α-Mg matrix during deformation. Icosahedral particles in the alloy are also stable against coarsening during deformation near the melting temperature of the eutectic due to their low interfacial energy, thereby forming a stable quasicrystalline particle/matrix interface. The stability of both the quasicrystalline particles and the microstructure of the Mg–Zn–Y alloys provides a large elongation with no void opening at the interface between the quasicrystalline particle and the α-Mg matrix.

In situ formation of two amorphous phases by liquid phase separation in Y–Ti–Al–Co alloy

The Y28Ti28Al24Co20 alloy undergoes metastable liquid phase separation in the under-cooled liquid state and subsequently solidifies into two different Y-rich and Ti-rich amorphous phases. Secondary phase separation occurs due to the supersaturation of the primary separated liquids as the temperature decreases. Depending on the degree of local undercooling, a wide range of length scale of the microstructure is observed. The characteristic length scale of the two amorphous phases is ∼250nm near the air side of the ribbon, and ∼25nm near the wheel side of the ribbon.

Parameter for glass forming ability of ternary alloy systems

A parameter (σ parameter) for glass forming ability (GFA) of the ternary bulk metallic glasses is proposed, taking into consideration the relative thermodynamic stability and atomic configuration of the liquid phase. The σ parameter, defined by ΔT*×P′, combines the effects of melting temperature depression (ΔT*) and effective atomic size mismatch between constituting elements (P′). The parameter shows better correlation with GFA than previously proposed parameters. The relationship between σ and GFA can be expressed as Dmax=2.0×10−1exp(20.01σ), where Dmax is the maximum thickness for glass formation. Since σ can be calculated simply using data on melting temperature and atomic size, the GFA can be easily estimated using this parameter.

The effect of Bi2O3 addition on the microstructure and magnetic properties of YIG

Abstract The effect of Bi 2 O 3 addition on the microstructure and magnetic properties of yttrium iron garnet prepared by a conventional ceramic method was studied using scanning electron microscopy, X-ray diffractometry, magnetometry and ferromagnetic resonance experiments. The sintering temperature could be reduced by 50 K by adding 1.5 wt% Bi 2 O 3 without variation in magnetic properties of YIG. A YIG specimen sintered at 1623 K with 1.5 wt% Bi 2 O 3 showed ferromagnetic resonance linewidth ΔH of 62 Oe.

Comparative study of the tribological behavior of thermal sprayed quasicrystalline coating layers

To investigate the role of tribological reactions on the friction and wear of quasicrystalline materials, coatings with two alloy compositions have been prepared by plasma and HVOF spraying techniques. The tribolayers were characterized by the formation of a transfer film on the counterface and densification of the coating subsurfaces. It was observed that the thickness of the transfer film and pore-free region were dependent on the composition and process used for the deposition of the coatings as well as the sliding velocity. As the sliding velocity increased, the growth rate of the transfer film decreased, resulting in a decrease of the coefficient of friction. On the other hand, the wear rate appeared to be controlled by the thickness of the pore-free region formed within the coating surface zone.

Enhancement of the quasicrystal-forming ability in Al-based alloys by Be-addition

Abstract The influence of beryllium (Be) addition on the quasicrystal-forming ability (QFA) in Al–Cu–Fe and Al–Mn systems has been investigated using conventional solidification technique. For a series of as-cast (Al 62− x Be x )Cu 25.5 Fe 12.5 ( x =0,1,3,5,7) alloys, the Be addition modified the icosahedral phase formation mechanism from peritectic reaction to primary solidification and resulted in the increase in the volume fraction of the i-phase from 40% for x =0 to 90% for x =7. Such an enhancement of the QFA with the increase of Be content can be represented by using the reduced quasicrystal transition temperature proposed in the present paper. In Al–Mn alloy system, microstructural changes due to the addition of Be were examined in detail by comparing as-cast Al 90 Mn 2.5 Be 7.5 alloy with Al 97.5 Mn 2.5 bulk ingot. For this alloy system, the substitution of Al by Be was found to reduce substantially the critical Mn-content and cooling rate necessary for the formation of the i-phase.

Precipitation of the icosahedral phase in amorphous Zr65Cu17.5−xAl7.5Ni10Agx (x=0,2.5, 5, 7.5 and 10) alloys

An amorphous phase in Zr65Al7.5Cu17.5Ni10 crystallizes via co-precipitation of the I-phase and NiZr2 phase in the first crystallization step, followed by decomposition of the I-phase into the CuZr2 and NiZr2 phases. The NiZr2 phase transforms to a stable Zr6NiAl2 phase at a high temperature. The alloys containing Ag crystallize via a two-step process: firstly, the I-phase nucleates homogeneously and grows in an amorphous matrix; secondly, the quasicrystal and remaining amorphous phase transforms into the stable CuZr2 and Zr6NiAl2 phases. With increasing Ag, the I-phase becomes more thermally stable and the grain size in the I-phase decreases due to increased frequency of homogeneous nucleation. The quasi-lattice constant of the I-phase decreases with increasing Ag content.

Suppression of discontinuous precipitation in AZ91 by addition of Sn

The effect of Sn (5 wt%) addition on the aging behavior of the AZ91 alloy has been investigated in the present study. The addition of Sn effectively suppresses the discontinuous precipitation during aging treatment. The aging response of the Sn containing AZ91 alloy is far better than that of the AZ91 alloy due to much higher density of continuous precipitation in the matrix. The yield strength and total elongation to failure at the peak aged condition of the AZ91 and Sn containing AZ91 alloys are 119.4 and 161.9 MPa and 8.8 and 8.6%, respectively, indicating that 35.6% increase of yield strength can be obtained by the addition of Sn in the AZ91 alloy maintaining almost same level of ductility.

Sliding friction and wear behavior of Al-Ni-Co-Si quasicrystalline coatings deposited by the high-velocity oxy-fuel spraying technique

The sliding friction and wear performance of Al–Ni–Co–Si quasicrystalline coatingsdeposited by the high-velocity oxy-fuel technique were investigated under dry slidingconditions. This study indicated that changes in the imposed sliding test conditionsmodified the friction and wear behavior of quasicrystalline coatings. Qualitativeanalysis of the contact interface and wear debris were performed with the aim ofunderstanding the role of the third body on the friction and wear processes.The dependence of the coefficient of friction on the sliding velocity and counterpartmaterial was explained by the stick-slip behavior. It was also shown that testconditions favorable for the formation of thick intermediate layers and the densificationof the coating subsurface led to low wear rates. Large cylindrical particles, formed byagglomeration of small wear debris, were suggested as a beneficial factor for thereduction of the coefficient of friction.I. INTRODUCTIONFollowing the discovery of ordered crystals withquasi-periodicity by Shechtmanet al.

A novel parameter to describe the glass-forming ability of alloys

In this paper, we propose a new parameter for glass-forming ability (GFA) based on the combination of thermodynamic (stability of stable and metastable liquids by ΔTm = Tmmix − Tl and ΔTx = Tx − Tg, respectively) and kinetic (resistance to crystallization by Tx) aspects for glass formation. The parameter is defined as e = (ΔTm + ΔTx + Tx)/Tmmix without directly adding Tg while considering the whole temperature range for glass formation up to Tmmix, which reflects the relative position of crystallization curve in continuous cooling transformation diagram. The relationship between the e parameter and critical cooling rate (Rc) or maximum section thickness for glass formation (Zmax) clearly confirms that the e parameter exhibits a better correlation with GFA than other commonly used GFA parameters, such as ΔTx (=Tx − Tg), K (=[Tx − Tg]/[Tl − Tx]), ΔT*(=(Tmmix − Tl)/Tmmix), Trg (=Tg/Tl), and γ (=[Tx]/[Tl + Tg]). The relationship between the e parameter and Rc or Zmax is also formulated and evaluated in the st...