Thermodynamics of the massive,bainitic and martensitic transformations in FeC,FeNi,FeCr and FeCu alloys

The γ → α transformation in pure iron and iron alloys with dilute alloying elements is a five-staged process, each stage has its own transformation-start temperature and transformation product: the first and second stages are a massive-type and a bainitic-type transformation respectively, while the subsequent stages (3, 4 and 5) are martensitic transformations. Thermodynamic calculation reveals that the driving force varies linearly with the transformation-start temperature for each stage of the transformation in FeC, FeNi, FeCr and FeCu alloys. Based on the linear relations, the calculated M_a, B_s and M_s are in good agreement with experimental points.     

A study of phase decomposition in CuNiFe alloys

A study of phase decomposition in two CuNiFe alloys was realized by AP-FIM. It was possible to confirm that phase decomposition takes place via spinodal decomposition in this alloy system, as the amplitude of the composition modulation increased with ageing time without practically any change in the wavelength of the modulation. It was also observed that the morphology of the decomposed phases is related to the coherency-strain energy as predicted by Cahn's theory of spinodal decomposition. By analysis of the data of the modulation wavelength, it was possible to estimate the coherent spinodal temperature to be 800 ± 25 and 900 ± 25 K in Cu46 at.% Ni4 at.% Fe and Cu48 at.% Ni8 at.% Fe alloys, respectively. In the early stage of decomposition, the change in modulation wavelength showed a time exponent as small as about 0.07. On the other hand, in the coarsening stage the change in the modulation wavelength agreed well with the LSW theory of thermally activated growth. The activation energy for this coarsening process was determined to be 216 ± 10 and 232 ± 10 kJ/mol in the Cu46 at.% Ni4 at.% Fe and Cu8 at.% Fe alloys, respectively. The compositions of the decomposed phases are consistent with the miscibility gap in the calculatedequilibrium CuNiFe phase diagram.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

Kinetics of leaching of a low-grade FeNiCuCo matte in ferric sulphate solution

A study of the dissolution of a low-grade FeNiCuCo matte in acid ferric sulphate solution is reported. This matte contained a number of different phases, and a detailed quantitative mineralogical analysis of the matte was used as a basis for mathematical modelling of the leaching process. The mathematical model assumed that the matte particles were leached by a shrinking-particle mechanism, and that the surface reaction was rate-limiting and electrochemical in nature. The experimental results verified these assumptions, and showed that the redox potential of the solution best approximated the potential of a matte particle during leaching.     

A study of work hardening in austenitic FeMnC and FeMnAlC alloys

Two austenitic FeMnAlC alloys with aluminium contents of 0 and 2.7 wt% were strained in tension between 193 and 823 K. Serrated stress-strain curves, inverse strain-rate dependence of flow stress, and high work hardening exhibited in particular temperature ranges for both alloys were characteristic of dynamic strain aging. The apparent activation energy for the onset of serration increased from 14.4 to 22.3 kcal/mol due to the addition of 2.7 wt% Al. It was found that the high work-hardening rate cannot be attributed to strain-induced deformation twinning when serrated stress-strain curves occurred. From the evidence of the present study and the known effect of aluminium on the diffusivity and activity of carbon in austenitic high-manganese steel, it is suggested that dynamic strain aging is the major cause of work hardening within the intermediate temperature range from 298 to 493 K for 0 wt% Al and 393 to 593 K for 2.7 wt% Al in the present austenitic FeMnAlC alloys.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

Homogeneous martensitic nucleation in FeCo precipitates formed in a Cu matrix

The martensitic transformation on subambient cooling has been monitored in defect-free nanocrystalline f.c.c. FeCo particles that have been coherently precipitated in a Cu matrix; such a CuFeCo system was chosen for study because the f.c.c. → b.c.c. transformational driving force in FeCo alloys is exceptionally large. Transformation is found to occur at a driving force of ∼ 10kJ/mol, a factor of 7 higher than the known critical driving forces for heterogeneous nucleation in bulk alloys. The observed transformation characteristics are entirely consistent with classical homogeneous coherent nucleation, whereas heterogeneous nucleation and homogeneous semicoherent nucleation can be ruled out in this case. An observed variation in transformation temperatures is explained by the experimentally-determined differences in Co content (and, hence, in transformational driving force) among the FeCo precipitates in relation to their distribution of particle sizes. The role of thermal activation in the homogeneous nucleation process is demonstrated by applying a high magnetic field to impose an increment of driving force at low temperatures.     

Structure-property relationships in AlLiCuMgAgZr alloy X2095

Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al₃Li) particles and reduced subgrain boundary T₁ precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength.     

Fatigue in CuZnAl single crystals

Fatigue properties associated with the β-18R martensitic transformation are analyzed in CuZnAl single crystals. The fatigue induced changes are studied by optical, scanning and electron microscopy. It is found that in the interior of the crystals dislocation defect arrays are created which act as local obstacles for the martensitic transformation and lead to the formation of extrusions and holes at the surface, which with increasing numbers of cycles join to form continuous cracks of about 1 μm width. Crack propagation seems to be little affected by composition or orientation.     

The formation of polytwinned structures in FePt and FePd alloys

• 1.1. The polytwinned structures in the FePt and FePd alloys derived from a nucleation and growth process in which the coherent ordered regions form aligned particle arrangements under the influence of the elastic strain energy. The impingement and coalescence of the ordered particles to form twin related structural domains give rise to a high density of APB's within the twin plates.
• 2.2. The nuclei form as disks along the {110} planes of the cubic matrix and these nuclei may merge with an order parameter or c/a ratio less than the equilibrium value but as the strain energy of the system relaxes the c/a ratio approaches equilibrium.
• 3.3. The polytwinned structure undergo a coarsening process under the mutual influence of the strain and surface energies analogous to “discontinuous coarsening” of lamellar two-phase aggregates resulting from cellular phase separation.

     

Calculation of the equilibrium phase diagrams and the spinodally decomposed structures of the FeCuNi system

Thermodynamic stability of ternary systems with miscibility gaps is discussed and applied to the f.c.c. phase of FeCuNi alloys. The stability of the system with respect to infinitesimal composition fluctuation is described. The direction most likely for initial composition fluctuation is defined as the one with the largest negative curvature on the Gibbs energy surface. Along this direction, zero-time wavelength at initial stage of spinodally decomposed alloy structure may be calculated. The thermodynamic values of the f.c.c. FeCuNi alloys are obtained from limited amount of thermochemical and phase equilibrium data. It is found that the ternary interaction parameters for both the f.c.c. and liquid phases of FeCuNi may be approximated from the binary interaction parameters of the constituent binaries.     

The martensite transformation in FeNiC alloys

The effect of austenite defect structure upon the sub-zero martensite burst transformation temperature in FeNiC has been investigated using a combination of optical and electron microscopy, differential scanning calorimetry and microhardness testing. In the absence of a change in composition or dislocation density, the martensite start transformation temperature (M_s) was found to be determined by the grain size of the austenite. Above a grain size of 150 μm, M_s was found to be independent of grain size, but below 150 μm, the transformation temperature was strongly depressed by up to approximately 50 K at a grain size of 10 μm. For any given grain size, an increase in the dislocation density from that typical of a fully recrystallised specimen, i.e. approximately 10¹⁰ lines m⁻², to that of approximately 10¹⁵ lines m⁻² raised M_s by approximately 15 K. The depression of M_s and reduction in the initial burst size of the transformation with decreasing grain size was found to be related to the observation that a fine grain size results in a heterogeneous transformation restricted to a few small pockets of grains. The depression of M_s in the fine grained alloy is consistent with a segregation of active martensite nuclei into a few small grains, a suppression of the autocatalytic stimulation of martensite plates between adjacent grains, and a possible reduction in the number of martensite nuclei.     

Hydride formation in an AlLiCu alloy

A hydride phase, LiAlH₄, has been identified in Al-2.0%Li-2.2%Cu alloy electrochemically charged with hydrogen. This is a hydride having the composition of LiAlH₄. The orientation relationship of the hydride and the matrix has been determined and rationalized with the O lattice theory. The thermodynamic stability of the hydride is discussed and possible formation mechanisms explained. The hydride forms from the grain boundary phase, AlLi, as the maximum amount of available Li is already present in the AlLi (δ) phase.     

The thermodynamics of NiCuH solid solutions

Solubility isobars at a pressure of 1.01 × 10⁵ N/m² have been measured in the temperature range of 714–1430 K for H in NiCu solid solution matrix alloys in the entire range of compositions from NiH to CuH. The solubility data provide values for the partial molar enthalpy and excess entropy of the dissolved H-atoms. The thermodynamic parameters have been shown to be consistent with a statistical model in which the energy levels of the interstitial atoms are a function of the local atomic configuration of the sites in which they are located.