Effect of nanocrystallization on the mechanical behavior of Fe-Ni-based amorphous alloys

Melt-quenched amorphous alloys Fe₅₈Ni₂₅B₁₇, Fe₅₀Ni₃₃B₁₇, and Ni₄₄Fe₂₉Co₁₅B₁₀Si₂ are used to study the effect of the structural parameters of the nanocrystalline phases precipitating at the first stage of crystallization on the strength characteristics of the alloys in an amorphous-nanocrystalline state. It is shown that, at a constant nanoparticle size, the dependence of microhardness H _V on the volume fraction (V _V ) or the volume density (N _V ) of these nanoparticles can be described by the relation HV = K(V _V ) ⁿ , where n = 1/3. A relation HV = f(d) that is analogous to the Hall-Petch relation is detected at a fixed volume density of nanoparticles and their average size d > 80–100 nm. At d < 70–80 nm, HV decreases anomalously with decreasing d.     

Martensitic transformation in Zr−Ti alloys

Martensitic transformation in a series of Zr−Ti alloys has been studied by transmission electron microscopy. A transition in morphology and substructure was observed with increasing additions of titanium. The 5 wt pct Ti alloy was found to be predominantly dislocated lath martensite while the 10 wt pct Ti alloy showed mainly a twinned plate morphology. Twins within the martensite plates could be classified into two categories, namely, thick twins and thin twins mainly on planes and, to a lesser extent, on and planes. In the case of the thick twins, the specific variant of the twin plane was always found to correspond to a plane of the {110}_bcc type, which is a mirror plane in the parent bcc crystal. When the specific variant of the composition plane was not parallel to a mirror plane, the twins were observed to be very thin. Evidence of slip within individual twin bands pointed to the operation of a multiple shear inhomogeneous deformation. Observations concerning deformation twins due to impingement effect are also discussed in this paper.     

The forming of Zr3Al-base alloys

Tubes have been formed from the intermediate phase Zr₃Al (L1₂ type) which is of potential use as a structural element in thermal nuclear power reactors. The procedure is to extrude Zr/Zr₂Al two-phase ingots at temperatures above 1270 K in theβ Zr + Zr₂Al two-phase field and then transform the extruded product at lower temperatures to Zr₃Al via the peritectoid transformation Zr + Zr₂Al → Zr₃Al. For small tubes (⪝ 3.2 cm OD) the extrusion constants at 1375 and 1425 K, respectively, are ≃ 400 and ≃ 300 MN/m² for the conditions chosen. The experimental extrusions indicate no fundamental barriers to forming pressure tubes from Zr-Al alloys containing 7.6 to 9.0 wt pct Al.     

The athermal omega transformation in Zr−Nb alloys

The athermal ω reaction in Zr?Nb alloys has been investigated by transmission electron microscopy. In a 12 wt pct Nb alloy, ω forms periodic arrays of plates on {112} β planes. These plates are 10 to 40Å thick and 150 to 200Å in diam. The ω electron diffraction reflections are relatively sharp in the 12 pct Nb alloy but become diffuse as the solute content is increased. The diffuse intensity exists as sheets parallel to {111}β reciprocal lattice planes. It is proposed that this diffuse intensity results from a large amplitude lattice vibration rather than from small particle size effects,i.e. ω forms when the bee lattice becomes unstable to a transdiffraction effects identical to those in the Zr?Nb alloys.     

Pipe formation in Pb-Sn alloys

It has been shown that upward flowing pipes can be formed in the solid-liquid region of a Pb-Sn alloy when a density inversion exists in the liquid. Pipes do not form when there is no density inversion. These findings support proposed mechanisms for freckling and A segregate formation, based on observations of transparent ammonium chloride-water models.     

Banded microstructure formation in off-eutectic alloys

The transition from a eutectic to a single phase has been examined through critical experimental studies in the vicinity of the transition conditions. The actual transition is shown to depend on the dynamics of the two competing phases. Close to the transition, an oscillating mode was found in which the system oscillated between the two steady-state morphologies. The transition from a eutectic to a single phase was observed not to be sharp, but to occur over a finite range of velocities in which an oscillating or a banded structure was formed, in which bands of primary phase and eutectic grew alternately in the growth direction. The effects of the imposed velocity and composition on this oscillatory mode were examined. It was found that, when the conditions were farther from the transition point, the oscillations damped out and the system selected one of the stable morphologies. The alloy composition had a significant influence on the oscillation behavior, and the oscillations were found to increase as the compositions deviated from the eutectic composition. The regime of banded structures was established for the carbon tetrabromide-hexachloroethane system, and a conceptual model is presented for the formation of bands. The results are generalized to show that the transition from one phase to the other is accompanied by a transition zone, in which the dynamic processes give rise to an oscillating microstructure between the two phases. Such a transition zone occurs in eutectic, peritectic, and monotectic systems and also during the cellular-to-planar transition of single-phase microstructures.     

Convection and channel formation in solidifying Pb-Sn alloys

A suite of experiments on the dendritic solidification of Pb-Sn melts has been carried out. The first goal has been to quantify the longitudinal macrosegregation, and hence the convective vigor through the dendritic (“mushy”) zone during solidification, as a function of the mushy zone Rayleigh number. The mushy zone Rayleigh number Ra _m is a ratio of the driving compositional buoyancy force to the retarding Darcy frictional force. The second goal has been to characterize the formation of convection channels as a function of Ra _m . In a fixed furnace, the melts were program cooled and solidified from beneath, at various cooling rates. Two different temperature gradients were examined. Each pairing of cooling rate and temperature gradient results in a different Ra _m . As expected, the measured longitudinal macrosegregation increased with Ra _m . The vestiges of convection channels on a solidified ingot surface (which we call “freckle trails”) were observed for all conditions except for the most rapid cooling rate with the smaller temperature gradient (i.e., the smallest Ra _m ) and for the slowest cooling rate with the larger temperature gradient (i.e., the largest Ra _m ). Under the latter solidification conditions, the vestiges of convection channels in an ingot interior (which we call “chimneys”) were observed. Chimneys were not observed in other ingots. When present, the number of freckle trails decreased and the width of the trails increased with increasing Ra _m . The trails became more diffuse as well. It appears that Ra _m may control channel characteristics as well as convection and the resulting macrosegregation. There appear to be two critical values, a lower one for surface freckle trails and a higher one for interior chimneys. Conditions at the Earth’s inner-outer core boundary (ICB) may be those exhibiting high Ra _m convection, so that convection channels, if they exist, could be as large as several hundred meters in width.     

Icosahedral and decagonal phase formation in Al-Mn alloys

The solidification conditions leading to the formation of the icosahedral phase in Al-Mn alloys have been investigated, using samples prepared by melt spinning and electron beam surface melting. It is found that the icosahedral phase can grow with a range of compositions, but that it grows in competition with another metastable phase which is decagonal. Both of these phases can displace the equilibrium intermetallic phases by nucleating ahead of them in the melt when the solidification velocity is greater than a few centimeters per second. The relative abundance of the icosahedral and decagonal phases varies with composition and solidification rate. Icosahedral crystals in electron beam melt trails are often about 25 μm in diameter, and they grow dendritically along a preferred crystallographic direction. A Guest Scientist at the National Bureau of Standards. A Guest Scientist at the National Bureau of Standards.     

Microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr alloys

Age hardening,microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr (x=0,2,4,6 wt.%) alloys (MM represents Ce-based misch-metal) were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the formed precipitates being responsible for age hardening changed from fine hexagonal-shaped equilibrium Mg12MM phase to metastable β’ phase with bco crystal structure when Y was added into Mg-1.5MM-0.4Zr alloy,and the volume fraction of precipitate phases also increased. With the increase of Y content in Mg-Y-1.5MM-0.4Zr alloys,it was found that the age hardening was enhanced,the grain sizes became finer and the tensile strength was improved. The cubic-shaped β-Mg24Y5 precipitate phases were observed at grain boundaries in Mg-6Y-1.5MM-0.4Zr alloy. It was suggested that the distribution of prismatic shaped β’ phases and cubic shaped β-Mg24Y5 precipitate phases in Mg matrix might account for the remarkable enhancement of tensile strength of Mg-Y-MM-Zr alloy. It was shown that the Mg-6Y-1.5MM-0.4Zr alloy was with maximum tensile strength at aged-peak hardness,UTS of 280 MPa at room temperature and 223 MPa at 250 oC,respectively.     

Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys

The thermodynamics of titanium in Fe-Cr alloys and of inclusion formation in Fe-Cr-N-Ti alloys was investigated. A metal-nitride-gas equilibration technique was used to measure the activity of titanium. The equilibrium titanium content of the metal that is in equilibrium with pure solid titanium nitride and nitrogen gas at 1 atm was determined. The activity coefficients of titanium it(f_Ti) relative to 1 wt pct standard state in Fe were calculated for Fe-Cr alloys from the experimental results. The first-order interaction coefficient between titanium and chromium, e _Ti ^Cr , was determined to be 0.024 at 1873 K. The solubility of nitrogen in Fe-Cr alloys was measured and was found to increase with chromium content, which is in agreement with previous work. Thermodynamic calculations were made in order to predict under what conditions titanium nitride will form in 409 stainless steel and was compared with inclusions found in plant samples. The inclusion stability diagrams for 304 stainless steel and Fe-18 pct Cr and Fe-9 pct Cr alloys were computed.     

Heats of formation in transition intermetallic alloys

The heats of formation in intermetallic alloys are calculated within a tight-binding scheme for the d band. We show that the difference in bandwidth between the metals and the difference between their energy levels are two dominant effects in the determination of the formation energy of these alloys. The influence of charge transfer, determined by a self consistent way, on alloy formation is studied.     

Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

A series of aluminum-10 wt pct silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple “isolation” model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.     

Thermodynamics of inclusion formation in Fe-Ti-C-N alloys

The thermodynamics of the formation of titanium carbonitride in liquid iron-titanium-carbon-nitrogen alloys were investigated in order to predict under what conditions it will form in liquid steel. A metal-carbonitride equilibration technique was used. Titanium carbonitride of a desired composition was made by mixing and high-temperature sintering of very fine powders of titanium nitride and carbide. The formation of titanium carbonitride was confirmed by lattice parameter measurements on the samples before and after the experiments. The equilibrium concentrations of titanium, carbon, and nitrogen in equilibrium with a specific titanium carbonitride were obtained at 1873 K. Activities of titanium carbide and nitride relative to pure solid titanium carbide and nitride were calculated. It was found that titanium carbonitride solid solution is almost ideal. From the results, calculations were performed to predict at which composition various carbonitrides will form.     

Nitrogen solubility and nitride formation in austenitic Fe-Ti alloys

The equilibrium nitrogen solubility and nitride formation in austenitic Fe and Fe-Ti alloys were measured in the temperature range from 1273 to 1563 K. Specimens 0.5 mm thick were equilibrated with four different nitrogen-argon gas mixtures containing 1 pct hydrogen. The nitrogen solubility in austenitic iron obeys Sieverts' law. The equilibrium nitrogen content was determined to be log (wt pct N)_γ-Fe, P_{N2=1 atm} = (539 ± 17)/T − (2.00 ± 0.01). The precipitated titanium nitride was identified as cubic TiN, and the solubility product was determined to be log(wt pct Ti) (wt pct N) = −14,400/T + 4.94.