Materials Science Forum Vols. 738-739

Abstract: Low cost Cu-Zn-Al foams present, in specific composition ranges, good pseudoelastic properties associated with interesting damping capacities, which establish these materials as attractive from the point of view of structural applications. These foams are manufactured by infiltration of small SiO2 spheres into the molten alloys. After alloy solidification the SiO2 spheres are removed by immersing the material in a solution of aqueous hydrofluoric acid. In this work, we have investigated the effect of two different foam preparation techniques: inductive and resistive melting. The paper details both the physico-chemical and mechanical characterization of the foams synthesized with the two different methods. The purpose of this study is to determine the quality of each synthesis method in order to identify the best alternative.

Abstract: The effect of martensitic transformation on the transport and optical properties of Cu-4.9Mn-23.4Al (at.%) alloy is reported. The resistivity measurements revealed the transformation temperatures Ms =284K and Af=303K. The optical properties of Cu-4.9Mn-23.4Al (at.%) alloy have been investigated at several temperatures near the region of austenite-martensite transformation. It was shown that intraband absorption is dominates in alloy for ħω < 2.5 eV energy range. Plasma Ωp and relaxation γ frequencies of free charge carriers for Cu-4.9Mn-23.4Al (at.%) alloy were evaluated at several temperatures on the basis of Drude analysis of the alloy optical properties in the intraband absorption region. The growth of electrical resistance induced by martensitic transformation upon alloy cooling obtained directly from the resistivity measurements correlates nicely with the results derived from the optical measurements and can be explained by the reduction of the plasma frequency Ωp as well as increase in relaxation frequency γ of free charge carriers.

Abstract: The temperature and amplitude dependence of internal friction fcc Mn45Cu55 alloy aged at 400 °C were studied. Two low-temperature internal friction peak observed in the quenched state. Physical mechanism of the peaks was determined by the effect of frequency and strain amplitude on the temperature dependence of internal friction. The influence of the heat treatment to the internal friction of the investigated alloy was shown.

Abstract: The influence of thermomechanical processing on the Ti-21.8Nb-6Zr (TNZ) and Ti-19.7Nb-5.8Ta (TNT) (at%) alloys’ structure, phase composition, mechanical and functional properties is studied. Both alloys possess polygonized dislocation substructure (average subgrain size  100 nm), and manifest superelastic behavior at room temperature and recovery stress generation during constant-strain temperature scanning experiments. After aging treatment, both alloys were -phase precipitation hardened, but their mechanical behavior was impacted differently -- it was detrimental for TNZ and beneficial for TNT. The different impact of aging heat treatment on the mechanical behavior of these alloys is explained by the differences in the -phase nucleation rate, precipitates’ size, shape, volume fraction and distribution, and by their effect on the alloys’ critical stresses and transformation temperatures.

Abstract: Among the different systems for high temperature shape memory alloys (SMA’s), equiatomic RuNb and RuTa alloys demonstrate both shape memory effect (SME) and MT temperatures above 800°C. Equiatomic compounds undergo two successive martensitic transformations, β (B2) → β’ (tetragonal) → β’’ (monoclinic), whereas out of stoechiometry alloys exhibit a single transition from cubic to tetragonal. In the case of two successive martensitic transformations, we expect to have a finer microstructure of the second martensite because it is supposed to develop inside the smallest twin elements of the former one. In equiatomic Ru-based alloys, if the first martensitic transformation is “normal”, the second one gives different unexpected microstructures with, for instance, twins with a thickness which is larger than the smallest spacing between twin variants of the first martensite. In fact, the reason for this unexpected hierarchy of the twins size is that the second martensitic transformation takes place in special conditions: geometrically, elastically and crystallographically constrained.

Abstract: The structure of the metastable austenite Fe–32%Ni alloy quenched for martensite and subjected to the α→γ transformation at slow heating (to 593–773 K) with the formation of a nanocrystalline austenite of different orientation was investigated. Electron diffraction analysis has revealed that nickel-enriched nanocrystalline γ-phase obtain ordered L10 superstructure. Together with the γ-phase, there was detected the disperse ε-martensite with a hexagonal close-packed (hcp) lattice. Along with the realization of ordinary orientation relationships (ORs) of Kurdjumov-Sachs, between a matrix α-phase and an ordered γ-phase in the sample of Fe-32%Ni alloy after slow heating to 773 K there have been established Headley-Brooks Ors. Cyclic transformation promotes the improvement of many functional properties of steels (strength, coercive force, controlling of the thermal expansion coefficient and others).

Abstract: Welding of low carbon martensitic steels with yield strengths above 690 MPa requires careful attention to the welding procedure to avoid hydrogen assisted cold cracking (HACC) and to minimise degradation of the mechanical properties of the weldment. Investigations of the microstructural and hardness gradients in the heat affected zone (HAZ) of these types of steels revealed that the peak hardness does not occur in the grain coarsened heat affected zone (GCHAZ) adjacent to the fusion boundary, as normally observed for ferritic steels, but is displaced towards the grain refined region (GRHAZ). This phenomenon, referred to as the displaced hardness peak (DHP) effect, is considered to arise when the hardenability of the steel is sufficiently high to produce the same microstructure in the both the GC and GR heat affected zones, but the enhanced structural refinement of the GRHAZ increases the hardness and strength above that of the GCHAZ. Implications relative to the susceptibility of the weldments to HACC are discussed.

Abstract: Advances in the development of new high strength steels have resulted in microstructures containing significant volume fractions of retained austenite. The transformation of retained austenite to martensite upon straining contributes towards improving the ductility. However, in order to gain from the above beneficial effect, the volume fraction, size, morphology and distribution of the retained austenite need to be controlled. In this regard, it is well known that carbon concentration in the retained austenite is responsible for its chemical stability, whereas its size and morphology determines its mechanical stability. Thus, to achieve the required mechanical properties, control of the processing parameters affecting the microstructure development is essential.

Abstract: The deformation induced α´-martensite formation in the metastable austenitic steels AISI 304, AISI 321 and AISI 348 was investigated in tensile and low cycle fatigue tests at ambient and low temperature. By means of stress strain and magnetic measurements, the mechanical behavior and phase transformation were characterized. The susceptibility of deformation induced α´-martensite formation depends on the chemical composition, the temperature and the degree of cumulated plastic strain. On the basis of comprehensive experimental data a mathematical model was developed to describe and predict the α´-martensite formation under cyclic loading in the temperature range -60 °C to 25 °C. The influence of test temperature and austenite stability of the model parameters was studied.

Abstract: Japanese sword has finer grain size and lath martensite in the microstructure of sharp edge amazingly. Nowadays these structures are considered to be one of the ideal structure at which are greatly aimed to strengthen or improve toughness of steels. Though the carbon content of its sharp edge is 0.70 mass %, there are no lenticular martensite and no micocracking in that area. As a result of bending test by actual sword specimen, one sword was finally bent, the other sword was broken. However it is found the sharp edge in Japanese sword has such a large bending strength 2500, 4600MPa respectively as modern, high performance tool steels and the difference of crack propagation under bending depends on the microstructure distribution and the grain size in cross section of Japanese sword.