Microstructure and properties of hot isostatically pressed powder and extruded Ti25V15Cr2Al0·2C

Abstract

The influence of process route on the microstructure and tensile behaviour of specimens prepared from hot isostatically pressed powders and extruded ingot of the burn resistant alloy, Ti–25V–15Cr–2Al–0·2C (wt-%), has been investigated. Samples based on gas atomised (GA) and plasma rotating electrode process (PREP) powders have been studied. Microstructural examination shows that many PREP powder particles are single crystals, whereas GA particles are polycrystalline. The mechanical properties of hot isostatically pressed specimens have been assessed using tensile testing monitored by acoustic emission, while microstructures have been characterised by synchrotron X-ray microtomography and optical and analytical scanning electron microscopy. Tomographic examination revealed a small fraction (<0·002 vol.-%) of pores in samples made from hot isostatically pressed GA powders, but no porosity was detected in samples made from hot isostatically pressed PREP powder. In view of their similar tensile behaviour, it is concluded therefore that the porosity does not contribute to the scatter and poor ductility in these hot isostatically pressed samples. These pores increased in size and volume fraction after heat treatment above the hot isostatic press temperature. The large scatter in tensile properties of both hot isostatically pressed GA and PREP samples was correlated with the presence of large (100–400 μm) circular crack initiation sites on the fracture surfaces, but the origin of these initiation sites has not been identified.     

Hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy superalloy

The hot compressive deformation behavior of a new hot isostatically pressed Ni–Cr–Co based powder metallurgy (P/M) superalloy was studied in the temperature range of 950–1150 °C and strain rate range of 0.0003–1 s⁻¹ using Gleeble-1500 thermal simulator. The dynamic recrystallization-time–temperature (RTT) curve was developed and the constitutive equation of flow stress during hot deformation was established. The results show that the flow stress decreases with increasing deformation temperature and decreasing strain rate. The flow stress represents as the characteristic of dynamic crystallization with the increasing of strain at the deformation temperatures lower than 1100 °C and strain rates higher than 0.0003 s⁻¹. The beginning time of dynamic crystallization has no linear relationship with deformation temperature in the condition of strain rate lower than 0.01 s⁻¹. Besides, the experiments verify that the hyperbolic sine model including the variable of strain reflects the changing law of flow stress during the hot deformation process.     

Reverse cyclic fatigue of a hot isostatically pressed silicon nitride at 1370 °C

The uniaxial, reverse cyclic fatigue performance of a commercially available hot isostatically pressed silicon nitride was examined at 1370 °C in air and with a 1 Hz sinusoidal waveform using button-head tensile specimens. Specimens did not fail in less than 10⁶ cycles when the applied stress amplitude was less than 280 MPa. Slow crack growth occurred at stress amplitudes 280 MPa and failure always occurred during the tensile stroke of the waveform. Multi-grain junction cavities resulted (i.e., the accumulation of net tensile creep strain) as a consequence of the reverse cyclic loading even though the specimens endured half their life under tensile stresses and the other half under compressive stresses. The presence of multi-grain junction cavities was a consequence of the stress exponent of tensile creep strain being greater than the stress exponent of compressive creep strain. Lastly, it was observed that the static creep resistance of this material improved when it was first subjected to reverse cyclic loading at 1370°C for at least 10⁶ cycles at 1 Hz. Silicon nitride grain coarsening (which was a consequence of the completion of the to silicon nitride solution/reprecipitation process that occurred during the history of the reverse cyclic loading) lessened the capacity for grain boundary sliding resulting in an improved static creep resistance.     

Thermal properties of Mg–Li and Mg–Li–Al alloys

Abstract

The present work is a study of the thermal properties of Mg–xLi–y Al with x= 4, 8 and 12 wt-% and y= 0, 3 and 5 wt-% as a function of temperature in the range 20–375°C. The thermal diffusivity and coefficient of thermal expansion (CTE) have been measured and the thermal conductivity calculated. The thermal diffusivity of all alloys decreases with an increasing content of lithium. The CTE of the single phase alloys Mg–4Li and Mg–12Li has a linear character, and the CTE of Mg–12Li is higher than that of Mg–4Li. The influence of thermal stresses in the two phase alloy Mg–8Li is perceptible in terms of temperature dependence of the CTE. In Mg–4Li–3Al and Mg–4Li–5Al, an influence of the solution of AlLi phase on all the studied thermal properties has been found.     

Fatigue behavior of friction stir welded Al–Mg–Sc alloy

The effect of Friction Stir Welding on the fatigue behavior of Al–Mg–Sc alloy has been studied. To reveal the influence of the welding parameters, different travel speeds of the welding tool have been used to provide weld seams with varying microstructural features. Crack initiation as well as crack propagation behavior under fatigue loading has been investigated with respect to the local microstructure at the crack initiation sites and along the crack path. Fatigue cracks were mostly initiated around the stir zone and the adjacent thermo-mechanical affected zone independent from hardness distributions in the weld seams. In some specimens, defect-like feature was observed at the crack origins, which shortened the fatigue lives. It has been found that while the effect of the tool travel speed on the fatigue lifetime seems to be little, the varying and complex local microstructure in the weld seam basically affects both the crack initiation sites and the crack propagation paths.     

Microstructure and mechanical properties of hot pressed Al2O3–ZrO2 ceramics prepared from ultrafine powders

Abstract

The microstructure and mechanical properties of hot pressed Al₂O₃–ZrO₂ ceramics prepared from ultrafine powders were studied by means of hardness and fracture toughness tests, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Experimental results show that Al₂O₃–ZrO₂ ceramics combine the high modulus of Al₂O₃ and phase transformation toughening of ZrO₂ and give good mechanical properties. The fracture toughness of the samples increases monotonically with increasing ZrO₂ content. When the content of ZrO₂ is low, the ZrO₂ particles are surrounded by Al₂O₃ grains and the matrix is thus strengthened, but when the content of ZrO₂ is high, the microcracks resulting from the tetragonal monoclinic phase transformation decrease the strength of the material. The relative density of the samples also increases with increasing ZrO₂ content, which is beneficial to both strength and toughness. Banded abnormal growth of the Al₂O₃ grains parallel to the hot pressing plane was found in the samples. This phenomenon is considered to be a result of the priority of Al₂O₃ grain growth and aggregation of ZrO₂ along the pressing direction in the later stages of hot pressing.

MST/1359     

Effect of hot isostatic pressing on mechanical properties and microstructure of 70–30 cupronickel castings

Abstract

The present work is part of an investigation into the use of hot isostatic pressing to recover 70–30 cupronickel castings. These alloys have particularly good corrosion resistance and, when strengthened with silicon and chromium, produce a material capable of use in very severe conditions of stress and massive corrosion. However, it is not possible to recover such castings by the application of repair welding, because of the possibility of reduced corrosion resistance in the vicinity of the weld. Hot isostatic pressing represents an alternative method of casting recovery. The results reported in the present work refer to the effect of hot isostatic pressing on mechanical properties, microstructure, and the level of segregation in the alloys. Hot isostatic pressing may be used to remove casting defects in the form of fine pores up to total porosity of 5%. However, in cases where porosity takes the form of very large defects, the mechanical properties of the recovered region are inferior to those of the originally sound material. This effect is probably associated with the presence of very finely distributed oxide particles in the originally defective parts of the casting. The optimum hot isostatic pressing temperature for the best overall combination of properties was 950°C.

MST/1732     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

Investigation of mechanical properties of advanced Al–Zn–Mg–Cu alloy

Abstract

The mechanical properties of the rapidly solidified 7000 series powder alloy CW 67 were investigated for various extrusion and heat treatment conditions. The principal aim of the work was to ascertain the optimum processing route for peak aged (T6) material. The highest proof stress in the T6 condition was found to be 572 MN m^?2 for material extruded at 325°C and aged for 13·5 h at 120°C after solutionising. The ductility of this material was found to be 13·5%. The fracture toughness was measured in two orientations and found to be approximately 21 MN m^?3/2 in the short transverse direction and 44 MN m^?3/2 in the longitudinal direction. Degassing and hot compaction was found to improve the fracture toughness of the material substantially.

MST/1504     

Mechanical properties of Al–Li alloys Part 2 Fatigue crack propagation

Abstract

Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 2091, 8090, and 8091 are examined, with specific emphasis on the role of microstructure. In Part 2, results on fatigue crack propagation behaviour are presented for both ‘long’ (≥ 5 mm) and ‘microstructurally small’ (~1–1000 μm) cracks and compared with behaviour in traditional high strength aluminium alloys. In general, it is found that the growth rates of long fatigue cracks in Al–Li alloys are up to two to three orders of magnitude lower than in traditional 7000 and 2000 series alloys, when compared at an equivalent stress intensity range ?K. By contrast, corresponding growth rates of microstructurally small fatigue cracks were up to two to three orders of magnitude higher than the long crack results. Such observations are attributed to the prominent role of crack tip shielding in Al–Li alloys resulting from the tortuous and deflected nature of the crack paths which results in a reduced crack tip ‘driving force’ from crack deflection and, more importantly, from the consequent crack closure induced by the wedging of fracture surface asperities. Since microstructurally small cracks are unable to develop the same level of shielding from crack closure by virtue of their limited wake, small crack growth rates are significantly accelerated. Unlike fracture toughness behaviour, artificial aging of commercial Al–Li alloys to peak strength has a mixed influence on the (long crack) resistance. Although behaviour at higher growth rates is relatively unaffected, in 2091 nominal threshold ?K_TH values are increased by 17%, whereas in 8090 and 8091 they are decreased by 16–17%. However, all alloys show reduced effective fatigue thresholds at peak strength after correcting for crack closure.

MST/926b     

Microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy produced by rapid solidification powder metallurgy

The microstructure and mechanical properties of hot extruded Mg–Al–Mn–Ca alloy was investigated. Both rapid solidified powders and cast billets were extruded at 573, 623 and 673 K to optimize the processing conditions for obtaining better mechanical response. Powder was consolidated to prepare the extrusion billets using both cold compaction and Spark Plasma Sintering at 473 K. The tensile properties of the extruded alloy were then evaluated and correlated to the observed microstructure. The results show that the use of rapid solidified powder could lead to effective grain refinement, which in turn resulted in the improved mechanical response, especially compared to the extruded conventional cast material.     

Mechanical properties and corrosion resistance of Ti–6Al–7Nb alloy dental castings

With the aim of applying a novel titanium alloy, Ti–6Al–7Nb, to a dental casting material, a comprehensive research work was carried out on its characteristics, such as castability, mechanical properties and corrosion resistance in the present study. As a result, Ti–6Al–7Nb alloy exhibited sufficient castability by a dental casting method for titanium alloys and enough mechanical properties for dental application. It is also showed excellent corrosion resistance through an immersion test in 1.0% lactic acid and an anodic polarization test in 0.9% NaCl solution. From these results, it is concluded that this Ti–6Al–7Nb alloy is applicable as a dental material in place of Ti–6Al–4V alloy, which includes cytotoxic vanadium.     

Study of hot forging behavior of as-cast Mg–3Al–1Zn–2Ca alloy towards optimization of its hot workability

Mg–3Al–1Zn–2Ca (AZX312) alloy has been forged in the temperature range of 350–500 °C and at speeds in the range of 0.01–10 mm s⁻¹ to produce a rib-web shape with a view to validate the processing map and study the microstructural development. The process was simulated through finite-element method to estimate the local and average strain rate ranges in the forging envelope. The processing map exhibited two domains in the following ranges: (1) 350–450 °C/0.0003–0.05 s⁻¹ and (2) 450–500 °C/0.03–0.7 s⁻¹ and these represent dynamic recrystallization (DRX) and intercrystalline cracking, respectively. The optimal workability condition according to the processing map is 425–450 °C/0.001–0.01 s⁻¹. A wide flow instability regime occurred at higher strain rates diagonally across the map, which caused flow localization that should be avoided in forming this alloy. The experimental load–stroke curves correlated well with the simulated ones and the observed microstructural features in the forged components matched with the ones predicted by the processing map.     

Effect of Equivalent Stress Amplitude on Multiaxial Fretting Fatigue Behavior of Al–Zn–Mg Alloy

Strength of Materials - The fretting fatigue behavior in Al–Zn–Mg alloy is investigated under conditions of various equivalent stress amplitudes of cyclic multiaxial loadings and the...     

Strengthening behavior of Al–Cu–Mg alloy sheet in hot forming–quenching integrated process with cold–hot dies

The novel technology combining hot forming and quenching together has been developed to improve formability and avoid thermal distortion for heat-treatable aluminum alloy forming. In this paper, cold–hot composite dies are proposed to use in hot forming–quenching integrated process. Heated lower dies are used to avoid rapid temperature decrease of heated sheet. Water-cooled upper dies are then used to accomplish quenching and reduce thermal distortion. The effects of temperature of lower dies, quenching condition and precipitate distribution on strengthening behavior in this process were investigated systematically. The strengths were measured by Vickers hardness and uniaxial tensile tests. It was found that the upper cold dies could ensure effective quenching. Lower dies could be heated to avoid rapid temperature decrease of heated sheet resulting in good strength of parts. The corresponding hardness, yield and tensile strengths were 140.7 HV, 295.7 and 469.2 MPa, respectively. For comparison, the process with both hot dies at temperatures ranging from 100 to 350 °C was investigated. The temperature of both hot dies could only be improved to 250 °C, otherwise the strength decreased. The strengthening phase was dispersed lath-shaped S phase with an average cross-section of approximately 50 × 100 nm, which was observed with TEM and SEM methods.     

An analysis of hot deformation of an Al–Cu–Mg alloy produced by powder metallurgy

The high-temperature plasticity of a 2014 aluminium alloy produced by powder metallurgy was investigated in a wide range of temperatures and strain rates. When the strain rate was plotted as a function of stress (either peak flow stress in torsion, or applied stress in tensile creep), the alloy exhibited the same threshold-like behaviour observed in similar materials. The microstructure of representative torsioned samples was analysed in a transmission electron microscope (TEM) and the characteristics of particles and precipitate distribution were estimated. The dependence on stress and temperature was analysed by means of the conventional constitutive equations used for describing the hot-working behaviour and by means of a modified form of the sinh-equation, where the stress was substituted by an effective stress i.e. by the difference between the actual stress and a threshold stress. This temperature-dependent threshold stress was found to be a constant fraction (15%) of the Orowan stress generated by the dispersion of alumina particles and of precipitated intermetallic phases.     

Changes of flow stress and microstructure during hot deformation of Al–1Mg–1Mn

Abstract

Plane strain compression tests have been carried out at strain rates between 0·5 and 10 s^?1 and temperatures in the range 275–510°C, both under nominally isothermal conditions and with temperature decreasing. Also, temperature or strain rate have been changed in the interval between two deformations. In all cases, the stress–strain curves obeyed a mechanical equation of state, described by constitutive relationships in terms of strain and instantaneous value of Zener–Hollomon parameter Z. When the value of Z varies slowly during deformation, flow stress is uniquely related to subgrain size and to dislocation density within subgrains, but these relationships break down in transition structures developed after a change of Z between two deformations. The existence of an equation of state for mechanical behaviour, but not for microstructure, is considered to result from important contributions of both dislocation velocity and density to hot strength.

MST/1066