Evidence for direct liquid to alpha transformation in zirconium alloys

The possibility of solidification of unalloyed zirconium and zirconium–niobium alloys from the liquid phase directly into the low temperature α phase is examined and the role played by solidification microstructure in modifying the β to α′ martensitic transformation in rapidly solidified zirconium and zirconium–niobium alloys is discussed.     

Using the modified embedded-atom method to calculate the properties of Pu-Ga alloys

In this article, the semi-empirical modified embedded atom method is used to develop a model of Pu-Ga alloys. Employing classical calculations, the model is used to predict thermodynamic properties of these alloys as well as the complex Pu-Ga phase diagram. For more information contact M.I. Baskes, Los Alamos National Laboratory, P.O. Box 1663, MS-G755, Los Alamos, NM 87545, USA; (505) 667-1238; fax (505) 667-8021; baskes@lanl.gov.     

The transformation substructure of quenched iron-copper alloys

Specimens of three Fe-Cu alloys have been austenitized and quenched in gas helium blown at high speed and cooled in a coil submerged in liquid nitrogen. A few specimens were subsequently tempered. Electronmicroscopy shows that a lath microstructure—in some cases with fine precipitates—is present. Composite selected-area diffraction patterns from areas enclosing two or more laths show that their misorientations are consistent with rotations about [011], that equal those dictated to convert one Kurdjumov-Sachs variant into another. In most cases, laths adopt few orientations. This selectivity respect to the type and number of orientations may contribute to the high thermal stability exhibited by the lath microstructures.     

The thermomechanical processing of alpha/beta titanium alloys

In this article, the thermomechanical processing of ingot-metallurgy alpha/beta titanium alloys is summarized, with special emphasis on microstructure evolution and workability considerations. Primary hot working dealing with the conversion of ingot structures to fine-equiaxed wrought structures is addressed. In this regard, the breakdown of lamellar microstructures, the occurrence of cavitation/wedge cracking, and the development of crystallographic texture are described. Secondary processes such as sheet rolling, superplastic forming, and closed-die forging are also reviewed.     

Strain-induced martensitic transformation in biomedical Co-Cr-W-Ni alloys

The nucleation,variant selection,and orientation dependence of the strain-induced martensitic transformation(SIMT) process in biomedical Co-Cr-W-Ni alloys were investigated.The experimental results show that theε-hexagonal-close-packed phase was preferentially formed at the Σ3 twin boundaries and high-angle grain boundaries during the tensile process.The theoretical analysis shows that the variant selection of SIMT is governed by Schmid’s law.However,the SIMTed ε-phase did not form equally on th...     

The effect of boron and alpha grain size on the massive transformation in Ti–46Al–8Nb–xB alloys

Samples of Ti–46Al–8Nb containing up to 1 at.% B have been examined using optical microscopy after cooling over a wide range of cooling rates from the α phase field in order to understand the influence of boron and grain size on the massive transformation. The grain size of the samples was controlled either by varying the boron level or by appropriate processing of B-free and B-containing alloys. The results show that the addition of boron suppresses the feathery and the Widmanstätten transformation. The massive transformation and the lamellar transformation are strongly influenced by prior α grain size independent of whether the grain size was achieved by heat treatment or by addition of boron. In fine-grained samples the range of cooling rates over which the massive transformation occurs is restricted by formation of the lamellar microstructure at high cooling rates. These observations are discussed in terms of the factors controlling the nucleation and the progression of these transformations.     

The effect of austenite ordering on the martensitic transformation in Fe-Ni alloys

Experimental results show that, in Fe-Ni alloys with a constant austenite grain size, values of Ms (or Mb) decrease in two steps with a decrease of quenching temperature below 950°C. Ms values of specimens quenched from above 700°C decrease with a decrease in quenching temperature much more profoundly than those quenched from below 700°C. There exists an “Ms valley” corresponding to a strength peak of the parent phase from quenching around 800°C, the order-disorder transition temperature of Fe₃Ni. The burst size corresponding to quenching from below 700°C is smaller than that from quenching above 950°C for the same burst temperature. A structural inhomogeneity in bulk specimens resulting from ordering in austenite quenched around 800°C was found by TEM. According to the relationships between strengthening and order parameters, and also the values of Ms and the strength of the parent phase, it is suggested that the decrease in Ms in the first step is associated with the effects of ordering domain size and strengthening, and that the decrease in Ms in the second step arises from the decrease in driving force for martensitic transformation resulting from the increase in degree of long-range order. It is confirmed that ordering does occur in the Fe-Ni alloys studied and that ordering of the austenite markedly affects Ms (Mb).     

Phase transformation behavior in Al-Zn-Cu alloys by hammering

The 5Cu40Zn55Al and 15Cu20Zn65Al alloys were prepared in the AI-Zn-Cu system, There exist the metastable phases ε and θ in the two alloys after homogenization treatment and furnace cooling, respectively. It is shown that the particles are refined from 3 mm to less than 10 μm after hammering the two alloys but there are still metastable phases. This means that the phase constituents of the two alloys have no changes by the deformation, which is different from that by balling. The phase constituents are not changed at room temperature by hammering, which is dependent on the deformation mechanism of hammering.     

Polymorphic transformation: The basis of thermocycling treatment of titanium alloys

Conclusions  

Age-induced four-stage transformation in Ni-rich NiTi shape memory alloys

A four-stage transformation has been observed in an aged Ti–50.7 at.%Ni shape memory alloy by differential scanning calorimetry (DSC). Such phenomenon is able to occur in the alloys with compositions around 50.7 at.%Ni by aging at 673 K. The multi-stage transformation behavior observed in this study is attributed to the complex microstructural evolution, i.e., formation of large-scale and small-scale heterogeneities which induced various stress fields and affected the phase transformation sequence.     

Thermoelastic transformation in Cu - Al - Mn alloys

Heat treatment of Cu - 10%Al - Mn alloys with a low concentration of manganese can be accompanied by a transformation. The effect of manganese on the temperature range of this transformation and its kinetics is investigated.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 5 – 6, August, 1996.     

The influence of partial cycling on the martensitic transformation kinetics in shape memory alloys

The martensitic transformation kinetics during partial cycling and the so-called “hammer” effect has been carefully characterized in Cu–Al–Ni shape memory alloys. These temperature memory effects were measured by adiabatic calorimetry and analyzed within the frame of a new thermodynamic model. A straightforward study of the nucleation processes explains on quantitative grounds the shift of the reverse transformation to higher temperatures and the presence of a secondary C_p peak associated to these phenomena. The optical observations support the calorimetric results. Finally, the release of the elastic energy in the martensitic state due to the different thermal cycles has been determined.     

The effect of austenite ordering on the mode of reverse transformation in Fe-Pt alloys

Three types of martensite-to-parent reversal were observed depending upon the amount of thermal hysteresis. One is the successive nucleation of fine austenite plates which are nonparallel to the martensite-parent interface, commonly observed in specimens with large thermal hysteresis. The second is the nucleation of austenite plates parallel to the martensite-parent interface and is observed in specimens with medium thermal hysteresis. The third is the backwards movement of the martensite-parent interface and is observed in specimens with small thermal hysteresis.     

Thermoelastic phase transformation in TiNi alloys under cyclic instrumented indentation

Austenitic and martensitic TiNi shape memory alloys were studied by means of cyclic instrumented indentation with the purpose of documenting the phase transformation. Recovery indexes were determined from the energy absorbed during loading, unloading and reloading and their relation with the microstructure and shape memory properties established. Significant changes on the load–displacement curves, associated to the transformation phenomenon, were exhibited by both materials. Under spherical cyclic nanoindentation, a classic pseudoelastic behavior was observed in the austenitic alloy, which was characterized by high recovery indexes and constitutes the first experimental evidence of this property under such testing conditions. On the other hand, the martensitic alloy showed lower recovery indexes, as a consequence of a faster stabilization process of the martensite plates.