High temperature microhardness study of cavitation in commercial superplastic Supral 150 (Al–6Cu–0·5Zr) alloy and certain steels

Abstract

A high temperature microhardness tester was used to measure the in situ hardness, at temperatures between 20 and 500°C, of aluminium matrix in the coarsened microstructure of a commercially available superplastic Supral 150 alloy. At superplastic temperature, the hardness of Supral 150 was found to be much lower than the hardness of its aluminium matrix and CuAl₂ particles. Activation energy values for deformation of the alloy and its phases were determined from hot hardness data. These data were used to explain the occurrence of cavitation in this superplastic alloy. Similar results are reported for (α+Fe₃C)- and (α+γ)-steels.

MST/758     

Effect of γ precipitates on the cavitation behavior of wrought AZ31 magnesium alloy

In the present work, the influence of γ eutectic phase on the cavitation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate hot tensile tests. The tensile tests were executed in two temperature range of 300–425 °C and 450–500 °C; i.e. somewhat below and higher eutectic melting temperature of γ precipitates (∼437 °C). The hot compression characteristics of the experimental alloy were also considered to assist explaining the related deformation mechanisms. The results indicated that a large amount of cavities originates from the γ second phases, specially located on grain boundaries. A sharp transition was realized from higher (>3%) to lower cavity area fraction (<0.02%) between 450 °C and 500 °C. The latter was attributed to the effect of γ liquid phase on stress relaxation through accommodating the grain boundary sliding phenomena. In addition, the current work explores the room temperature mechanical properties of tensile deformed specimens using shear punch testing method.     

Effects of ageing treatments on transformation temperatures and precipitation kinetics in a Cu-Zn-Al shape-memory alloy

The effects of ageing treatments on transformation temperatures, hardness, and precipitation kinetics in a Cu-14.2Zn-8.5Al (wt%) shape-memory alloy were investigated. Quench-ageing treatment temperatures varied from 100 to 500° C with times up to 200 h after the solution treatment. The martensitic transformation temperature, M _s, of the hot-rolled material was decreased from 55 to 51 °C by the solution treatment. The temperature hysteresis (A _f-M _f) was 50° C for the hot-rolled condition, but was reduced to 30° C after the solution treatment. The maximum hardness for material aged at 500° C was lower than that for that aged at 300 or 400° C. The apparent activation energy for hardness increase in this alloy was 110 kJ mol^–1, compared with 72 kJ mol^–1 for the similar copper-based shape-memory alloy Cu-21.2 Zn- 6.0 Al. The ordering temperatures for B₂ and DO₃ superlattices were in the neighbourhood of 480 and 260° C, respectively. The tensile ductility and yield strength of this alloy were significantly reduced by the ageing treatment at 400° C.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 m) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures 500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.     

Strain-softening behavior of an Fe-6.5 wt%Si alloy during warm deformation and its applications

An Fe-6.5 wt%Si alloy with columnar grains was compressed at a temperature below its recrystallization temperature. The Vickers hardness and structure of the alloy before and after deformation were investigated. The results showed that with an increase in the degree of deformation, Vickers hardness of the alloy initially increased rapidly and then decreased slowly, indicating that the alloy had a strain-softening behavior after a large deformation. Meanwhile, the work-hardening exponent of the alloy decreased significantly. Transmission electron microscopy confirmed that the decrease of the order degree was responsible for the strain-softening behavior of the deformed alloy. Applying its softening behavior, the Fe-6.5 wt%Si alloy with columnar grains was rolled at 400 °C and then at room temperature. An Fe-6.5 wt%Si thin strip with thickness of 0.20 mm was fabricated. The surface of the strip was bright and had no obvious edge cracks.     

Hardening and phase stability in rapidly solidified Al–Fe–Ce alloys

Rapidly solidified (RS) Al–Fe–Ce alloys were prepared by melt spinning. The phases present and the thermal stability, at temperatures up to 500 °C, were then followed by X-ray analysis, chemistry, hardness and thermal analysis techniques. The results obtained indicated that the alloys studied have enhanced mechanical properties compared to commercial aluminium alloys and castings of the same alloy compositions, and the RS alloy also exhibit good stability up to about 300 °C; a result of stable second phase particles. It is suggested that these results indicate that there are two mechanisms responsible for the hardening and stability of the RS alloys: solid solution strengthening at lower temperatures, and semicoherent particles formed from supersaturated solid solution at higher temperature. The maximum hardness, after 2 h ageing occurred at about 300 °C. At higher temperatures the dispersed phase became incoherent with a dramatic loss in hardness.     

Effects of Pre-Ageing Treatment on Subsequent Artificial Ageing Characteristics of an Al-1.01Mg-0.68Si-1.78Cu Alloy

The influences of pre-ageing temperature and natural ageing time on subsequent artificial age hardening behavior and precipitation sequence of new type Al-1.01Mg-0.68Si-1.78Cu alloy were investigated by hardness test, differential scanning calorimetry (DSC) test and transmission electronic microscopy (TEM) observations. When pre-ageing temperature is 20℃ (natural ageing), the peak hardness of subsequent artificial aged alloy is lower than that of T6 treated alloy (negative effect), while a positive effect a...     

High Strength AA7050 Al alloy processed by ECAP: Microstructure and mechanical properties

Commercial AA7050 aluminium alloy in the solution heat-treated condition was processed by ECAP through routes A and B_C. Samples were processed in both room temperature and 150 °C, with 1, 3, and 6 passes. The resulting microstructure was evaluated by optical microscopy (OM) and transmission electron microscopy (TEM). Only one pass was possible at room temperature due to the low ductility of the alloy under this condition. In all cases, the microstructure was refined by the formation of deformation bands, with dislocation cells and subgrains inside these bands. The increase of the ECAP temperature led to the formation of more defined subgrain boundaries and intense precipitation of spherical-like particles, identified as η′ and η phases. After the first pass, an increase in the hardness was observed, when compared with the initial condition. After 3 passes the hardness reached a maximum value, higher than the values typically observed for this alloy in the overaged condition. The samples processed by route B_C evolved to a more refined microstructure. ECAP also resulted in significant strength improvement, compared to the alloy in the commercial overaged condition.     

Superplastic deformation of Al-Li-Cu-Mg alloy sheet

Al-2.5 Li-1.2 Cu-0.6 Mg-0.12 Zr (wt%) alloy sheet was cold-rolled, solution heat-treated for 20 min at 510° C, prestrained by 3% and superplastically deformed at 450 to 540° C at strain rates between 1×10^–4 and 2.8×10^–1 sec^–1. The maximum elongation obtained was 300%. Significant cavitation occurred above about 0.5 strain at a rate (void volume/unit strain) of 4% at 540° C and 6% at 500° C. The onset of cavitation coincided with a reduction in the room-temperature tensile properties after reheat-treatment. During annealing at 500 to 540° C, grain coarsening near the sheet surface was associated with magnesium and lithium depletion. Superplastic deformation produced a fine equiaxed microstructure by dynamic recrystallization.     

Superplasticity of AlMgSi alloys

Commercial AlMgSi alloy sheets produced by thermomechanical treatment are found to be superplastic between 500 and 570°C at strain rates of 10^–5–10^{–3 –1} The strain rate sensitivity,m, is about 0.4. It was found that the highly alloyed sample contains pre-existing cavities in higher volume fraction than the alloy of lower concentration. An exponential growth of cavity volume fraction was found during superplastic deformation which is characteristic of plasticity controlled cavitation. The growth rate of the cavity volume fraction can be decreased by applying back pressure.     

Analysis of high temperature deformation behavior of a high Nb containing TiAl based alloy

The effects of deformation temperature and strain rate on the hot deformation behaviors of as-cast Ti-45Al-8.5Nb-(W,B,Y) alloy were investigated. The results indicated that when deformation temperature is below 1250 °C, the flow stress decreases with the increase of deformation temperature and decrease of strain rate, once deformation temperature reaches 1250 °C, the flow stress is not sensitive to strain rate any more. A neural network model was established to predict the flow stress of this high Nb containing TiAl based alloy during hot deformation. The predicted flow stress curves are in good agreement with experimental results.     

The annealing phenomena and thermal stability of severely deformed steel sheet

However, there are many works on annealing process of SPDed non-ferrous metals, there are limit works on annealing process of SPDed low carbon steel. Therefore, in this study the annealing responses after constrained groove pressing (CGP) of low carbon steel sheets have been investigated. The sheets are subjected to severe plastic deformation at room temperature by CGP method up to three passes. Nano-structured low carbon steel sheets produced by severe plastic deformation are annealed at temperature range of 100-600 °C for 20 min. The changes of their microstructures after deformation and annealing are studied by optical microscopy. The effects of large strain and annealing temperature on microstructure, strength and hardness evolutions of the nano-scale grained low carbon steel are examined. The results show that annealing phenomena can effectively improve the elongation of SPDed sheets with preserving the hardness and mechanical strength. Also, the thermal stability of microstructure and mechanical properties can be observed through annealing temperatures up to 400 °C and temperature of 400 °C is achieved as an optimum annealing temperature in which both strength and elongation are increased and hardness inhomogeneity of the sheet is minimum. Annealing at temperatures of higher than 400 °C leads to abnormal grain growth.     

Mechanical properties of a cold-rolled annealed HSLA steel

Hot rolled strips of an HSLA steel containing niobium and vanadium were cold rolled in the range 15 to 80% and annealed for 1 h at various temperatures from 400 to 700 ° C. The response to the amount of deformation and annealing temperature were studied in terms of changes in its hardness, tensile strength and ductility. Hardness and strength were observed to increase while ductility decreased with the amount of deformation. Although annealing of the steel up to 500 ° C did not show significant changes in its mechanical properties, a sharp decrease in hardness and strength and improvement in ductility were observed on annealing the steel at temperatures greater than or equal to 600 ° C. Microstructural studies showed complete recrystallization in the samples subjected to 60% deformation followed by annealing at 600 ° C.     

Dynamic ageing and the mechanical response of Al–Mg–Si alloy through equal channel angular pressing

In this paper, dynamic ageing characteristics associated with the application of equal channel angular pressing (ECAP) to Al6061 alloy at elevated temperatures was investigated. Followed by ECAP, Vickers microhardness measurement on the cross-sectional planes and microstructural observations were undertaken using transmission electron microscopy. The combination of the ECAP process with dynamic ageing at both 100 °C and 150 °C resulted in a significant increase in hardness. The grain size was measured as ∼160 nm after four passes. A comparison with the published data on the same alloy processed by ECAP at room temperature and statically aged, suggests several advantages in incorporating dynamic ageing with ECAP. These advantages consist of the ability to attain better grain refinement, increased hardness and the potential for saving time and energy.     

High-temperature strength of bulk single crystals of III-V nitrides

A Vickers indentation method was used to determine the hardness of AlN and GaN, grown by the hydride vapor phase epitaxy technique, in the temperature range 20–1400 °C. At room temperature, the hardnesses of GaN and AlN are 10.2 and 17.7 GPa, respectively. The hardness of GaN and AlN shows a gradual decrease from RT and then a steep decrease from around 1000 °C. AlN is harder than GaN but softer than SiC. The steep decrease of the hardness means the beginning of macroscopic dislocation motion and plastic deformation. The mechanical strength of bulk single-crystal GaN is investigated at elevated temperatures directly by means of compressive deformation. The yield stress of GaN in the temperature range 900–1000 °C is around 100–200 MPa, i.e., similar to that of 6H-SiC and much higher than those of Si, Ge, GaAs.     

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s⁻¹. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.     

Precipitation behavior of Ag–Pd–In dental alloys

Age-hardening characteristics and precipitation behavior of Ag–25%Pd–3%In–1%Zn–0.5%Ir alloy were investigated in detail by means of hardness testing, X-ray diffraction, electron microscopy and resistivity measurement. The solution treating could be accomplished at 980 °C and the aging in the temperature range from 950 to 850 °C occurred by continuous precipitation. The aging in the temperature range from 850 to 450 °C occurred first, forming GP-zones with a hardness increase and then in overaging stage by forming discontinuous precipitation, which consisted of lamellae of solute (Pd, In, Zn) depleted Ag-rich phase and (Pd,Ag)₃(In,Zn) intermetallic phase. The hardness increased very fast to its peak in 10 min during aging at temperatures between 450 and 550 °C.     

Annealing behaviour of amorphous Fe80B20 on continuous heating

Resistance measurements during direct heating of Fe₈₀B₂₀ amorphous alloys indicate phase changes occur at 395, 500, 720 and 840° C. Samples heated to these temperatures, and maintained for five minutes in a neutral atmosphere, show that a hardness maximum occurs at the crystallization temperature of 395° C and that annealing at 500° C produces a material with the same hardness. Above 500° C the microhardness is seen to drop below that of the amorphous alloy. Saturation magnetization measurements show a steady increase following each anneal, up to a temperature of 720° C, and the rate of increase is seen to drop in the range of 720 to 840° C. X-ray diffraction studies show that only a small fraction of the matrix is crystallized following the anneal at 395° C and the transformed phases are -Fe and Fe₃B. Following annealing at 500° C, an increased proportion of -Fe and Fe₃B are observed with complete crystallinity while samples heattreated at 720° C are seen to consist of a three-phase mixture of -Fe, Fe₂₃B₆ and Fe₂B. Annealing at 840° C is seen to produce an equilibrium phase mixture of -Fe and Fe₂B phases. Only in the sample annealed at 395° C is a fraction of the amorphous phase seen to persist, indicating that a 5 min anneal is not sufficient, at this temperature, to induce complete crystallization. These structural features are corroborated by field ion microscope analyses, made at liquid nitrogen temperature in a medium of pure neon, and scanning electron microscopy, and are also consistent with our earlier study involving the isothermal annealing, for various times, of Fe₈₀B₂₀ alloy at 780° C.     

Characteristics of the continuous coarsening and discontinuous coarsening of spinodally decomposed Cu-Ni-Fe alloy

The spinodal decomposition and coarsening reaction of a 45Cu-30Ni-25Fe alloy aged with and without prior 75% cold rolling have been studied by Vickers hardness tester, X-ray diffraction, optical microscope, and scanning and transmission electron microscopes. Spinodal decomposition took place in the alloys aged in the temperature range of 600–950°C without prior deformation, and then spinodal structure in these alloys would coarsen only continuously. Spinodal decomposition and continuous coarsening reaction of spinodal structure took place in the alloys aged at lower temperature with prior deformation. This process was corresponding to recovery of spinodal structure in the deformed alloys. Discontinuous coarsening reaction of spinodal structure would also take place in the alloys aged at higher temperature with prior deformation. This process was corresponding to recrystallization of spinodal structure in the deformed alloys.