P. Gargarella

Correlation between glass-forming ability, thermal stability, and crystallization kinetics of Cu-Zr-Ag metallic glasses

The glass-forming ability (GFA) of the Cu-Zr-Ag system is evaluated based on the large amount of literature data available and discussed in the frame of a predictive amorphization criterion which combines topological instability and electronic criteria. The correlation between GFA, thermal stability, and crystallization kinetics of (Cu0.5Zr0.5)100−xAgx (x = 0, 2, 6, and 10) metallic glasses is further investigated. The enhancement of the GFA of the alloys and the thermal stability/fragility of the supercooled liquid can be traced back to a large size effect/volume mismatch and electronic effects. However, the apparent activation energy of crystallization decreases with increasing Ag content in the alloys which may be due to a nanoscale microstructural heterogeneity induced by the Ag addition. At a certain Ag content, a small amount of AgZr crystals precipitate together with Cu10Zr7 and CuZr2 and the crystallization mechanism changes from interface-controlled one-dimensional growth to three-dimensional gro...

Atomization and Selective Laser Melting of a Cu-Al-Ni-Mn Shape Memory Alloy

Shape memory alloys (SMAs) are a class of material that undergoes a reversible shape change after a plastic deformation. The recovery of the original shape is possible due to a structural transformation upon heating to a critical temperature. The shape memory effect is related to a martensitic-austenitic transformation from a phase with a low symmetry (martensite) to a high-temperature phase (parent phase) [1]. Cu-based shape memory alloys have the advantage of large thermal and electrical conductivities and the system Cu-Al-Ni alloys are quite attractive due to better stabilisation against aging phenomena [2].

Phase Formation, Thermal Stability and Mechanical Properties of a Cu-Al-Ni-Mn Shape Memory Alloy Prepared by Selective Laser Melting

Selective laser melting (SLM) is an additive manufacturing process used to produce parts with complex geometries layer by layer. This rapid solidification method allows fabricating samples in a non-equilibrium state and with refined microstructure. In this work, this method is used to fabricate 3 mm diameter rods of a Cu-based shape memory alloy. The phase formation, thermal stability and mechanical properties were investigated and correlated. Samples with a relative density higher than 92% and without cracks were obtained. A single monoclinic martensitic phase was formed with average grain size ranging between 28 to 36 μm. The samples exhibit a reverse martensitic transformation temperature around 106 ± 2 °C and a large plasticity in compression (around 15±1%) with a typical “double-yielding” behaviour.

Prediction of good glass formers in the Al-Ni-La and Al-Ni-Gd systems using topological instability and electronegativity

A new criterion has been recently proposed combining the topological instability (λ criterion) and the average electronegativity difference (Δe) among the elements of an alloy to predict and select new glass-forming compositions. In the present work, this criterion (λ.Δe) is applied to the Al-Ni-La and Al-Ni-Gd ternary systems and its predictability is validated using literature data for both systems and additionally, using own experimental data for the Al-La-Ni system. The compositions with a high λ.Δe value found in each ternary system exhibit a very good correlation with the glass-forming ability of different alloys as indicated by their supercooled liquid regions (ΔTx) and their critical casting thicknesses. In the case of the Al-La-Ni system, the alloy with the largest λ.Δe value, La56Al26.5Ni17.5, exhibits the highest glass-forming ability verified for this system. Therefore, the combined λ.Δe criterion is a simple and efficient tool to select new glass-forming compositions in Al-Ni-RE systems.

Predicted glass-forming ability of Cu-Zr-Co alloys and their crystallization behavior

The phase formation and the glass-forming ability (GFA) of the Cu-Zr-Co system were studied. For the latter, predictions from a modified topological instability criterion were used to select six alloys. Five new glassy compositions were found and a good correlation was observed between the predicted GFA and characteristic thermal indicators. The addition of Co stabilizes a B2-type phase at room temperature in these alloys, which could be useful for the development of shape memory bulk metallic glass composites.

Structural evolution in Ti-Cu-Ni metallic glasses during heating

The structural evolution of Ti50Cu43Ni7 and Ti55Cu35Ni10 metallic glasses during heating was investigated by in-situ synchrotron X-ray diffraction. The width of the most intense diffraction maximum of the glassy phase decreases slightly during relaxation below the glass transition temperature. Significant structural changes only occur above the glass transition manifesting in a change in the respective peak positions. At even higher temperatures, nanocrystals of the shape memory B2-Ti(Cu,Ni) phase precipitate, and their small size hampers the occurrence of a martensitic transformation.

Phase Separation in Rapid Solidified Ag-rich Ag-Cu-Zr Alloys

The microstructure and phase formation of rapid solidified Ag-rich Ag-Cu-Zr alloys were investigated. Two types of structure; interconnected- and droplet-type structures, were obtained due to phase separation mechanisms. The former was spinodal decomposition and the later was nucleation and growth mechanism. Depending on the alloy compositions, three crystalline phases; FCC-Ag, AgZr and Cu10Zr7 phases were observed along with an in-situ nanocrystalline/amorphous composite. Vickers hardness testing indicated a significant increase of hardness in the nanocrystalline/amorphous-composite alloy.

Phase formation in rapid solidified Ag–Y alloys

The phase formation in a rapidly solidified Ag–Y system was evaluated by means of two predictive amorphization criteria: the thermodynamic γ* parameter and the new λ + Δh1/2 criterion. The former considers only the thermodynamic contribution, while the latter combines the effect of an atomic size mismatch between elements and their electronic interaction. The results showed a marginal glass-forming ability (GFA) with the best composition region for glass formation located near the Y-rich eutectic region. The melt-spun ribbon contained an amorphous matrix with a distribution of Y solid solution nanocrystals. Two new metastable phases found in the study were identified. One with an orthorhombic structure was precipitated during crystallization of the highest GFA alloy. The other having a hexagonal structure was formed under rapid solidification of the near-eutectic alloy lying between the Ag51Y14 and Ag2Y phases. The results also showed no improvement in the predictability of the γ* and λ + Δh1/2 amorphizat...