Effect of prior cold work on age hardening of Cu-4Ti-1Cd alloy

This research is part of a project whose scope was to develop high strength ternary alloys based on Cu-Ti system with the primary aim of substituting them for toxic and expensive Cu-Be alloys. In this pursuit, age hardening behaviour of Cu-4Ti-1Cd alloy has already been investigated and the present paper reports the investigations on the influence of prior cold work by rolling of 50, 75 and 90% on the age hardening of a Cu-4Ti-1Cd alloy using hardness and tensile tests and optical as well as transmission electron microscopy. As a result of cold work followed by aging, hardness of the alloy increased from 237 Hv in solution treated condition to 425 Hv on 90% cold work and peak aging. Similarly, yield and tensile strengths of the alloy reached maxima of 1037 and 1252 MPa respectively on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent β^l, Cu₄Ti phase in addition to high dislocation density and deformation twins. Both hardness and strength of the alloy decreased on overaging due to the formation of incoherent and equilibrium β, Cu₃Ti phase. However, the morphology of the discontinuous precipitation was changed to globular shape due to large deformations and overaging.     

Precipitation hardening and recrystallization in Cu-4% to 7% Ni-3% Al alloys

The effects of microstructure on mechanical properties in three cold-worked Cu-4% to 7% Ni-3% Al alloys have been investigated by changing ageing time at 500 °C. Hardness and strength in the Cu-7% Ni-3% Al and Cu-5.5% Ni-3% Al alloys increase with ageing time and have maximum values at an ageing time of 10³–10⁴s at 500 °C, then decrease. During ageing of Cu-7% Ni-3% Al at 500 °C, the coherent Ni₃Al phase was first precipitated out and later incoherent NiAl phase was formed. Ni₃Al formed during the initial stage of ageing is likely to be a transient phase. The increases in hardness and strength are due to the precipitation of coherent Ni₃Al phase. Coherent Ni₃Al particles are effective in increasing the strength and retarding the recrystallization process. On the other hand, the hardness and strength in the Cu-4% Ni-3% Al alloy gradually declined with ageing time. Only incoherent NiAl phase was formed during ageing at 500 °C. Decreases in hardness and strength in the Cu-4% Ni-3% Al alloy are attributed to softening during recovery and recrystallization, because incoherent NiAl particles have an insufficient effect to increase the strength.     

Precipitation hardening of cast Zr-containing A356 aluminium alloy

The effect of small additions of zirconium on the hardness, grain size, precipitate type and size of cast A356 aluminium alloy was investigated. The cast alloys were solution treated and then artificially aged for different periods of time. Hardness tests and scanning electron microscope (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) studies were carried out on the as-cast, as-solutionised and age-hardened specimens. Incoherent, coarse Al₃Zr particles formed in the microstructure during the solidification of the alloy and caused grain refinement in the as-cast structure. These particles dissolved and reprecipitated as smaller-size particles during the solution treatment, causing the hardness of the alloy to remain constant at high temperatures for long periods of time due to the slow diffusion of Zr in the α-Al.     

Infrared brazing Ti-6Al-4V and SP-700 alloys using the Ti-20Zr-20Cu-20Ni braze alloy

Great efforts have been made in brazing high-strength α-β titanium alloys below their beta-phase transformation temperature in order to obtain optimized mechanical properties. The brazing temperature of the cold roll-bonded Ti-20Zr-20Cu-20Ni foil is roughly 70 °C lower than that of Ti-15Cu-15Ni filler metal. Moreover, the detrimental Cu-Ni and Cu-Ni-Zr rich Ti phases can be greatly reduced or eliminated by properly choosing the brazing thermal cycle. This research demonstrates the potential application of Ti-20Zr-20Cu-20Ni foil in brazing titanium alloys.     

Effect of cobalt additions on the age hardening of Cu-4.5Ti alloy

The effects of 0.9 and 1.8 wt% cobalt additions on the age hardening behaviour of Cu-4.5Ti alloy have been investigated. It has been observed that though Co addition results in the refinement of grain size and the Cu-Ti-Co alloys exhibit age hardening (giving rise to peak hardness on aging at 400°C for 16 hours), the peak hardness as well as the corresponding yield and tensile strengths were found to decrease with increasing cobalt content. The electrical conductivities of 0.9 and 1.8 wt% Co alloys were found to be 6% and 10% International Annealed Copper Standard (IACS) and 7% and 14% IACS in solution treated and peak aged conditions, respectively. Like in the binary Cu-Ti alloys, precipitation of ordered, metastable and coherent Cu₄Ti(¹) precipitate was found to be responsible for maximum strengthening in these alloys. In addition, coarse intermetallic phases of Ti and Co, viz. Ti₂Co and TiCo particles have been observed in all the conditions studied. The inferior mechanical properties of Cu-Ti-Co alloys compared with those of the binary Cu-Ti alloys are attributed to the depletion of Ti from matrix, which is consumed to form Ti₂Co and TiCo phases.     

Precipitation hardening and hydrogen embrittlement of aluminum alloy AA7020

AA7020 Al–Mg–Zn, a medium strength aluminium alloy, is used in welded structures in military and aerospace applications. As it may be subjected to extremes of environmental exposures, including high pressure liquid hydrogen, it could suffer hydrogen embrittlement. Hydrogen susceptibility of alloy AA7020 was evaluated by slow strain-rate tensile testing, and delayed failure testing of hydrogen-charged specimens of air-cooled, duplexaged, and water-quenched duplex agedmaterials. The resistance to hydrogen embrittlement of the alloy was found to be in the order of air-cooled duplex aged alloy > as-received (T6 condition) > water quenched duplex aged material.     

Serrated yielding in Cu-1 wt-%Cd alloy

Abstract

The influence of grain size and temperature on the serration patterns of the Portevin-Le Chatelier (PLC) effect and on the yield and flow stresses in a Cu-1 wt-%Cd alloy was investigated in the temperature range 150 to 360 ° C. Two kinds of serration patterns were observed in this alloy. Type I occurred at lower temperatures and its yield points are moderately spaced. Type II consists of regular jerky flow observed athigher temperatures. The Hall-Petch equation is obeyed over the temperature range in which jerky flow occurs. The Hall-Petch parameter k (?) is observed to show a local maximum in the temperature range where serrated flow is first observed. The PLC effect is associated with the solute- dislocation interactions, implying that k (?) contains a component associated with grain size dependent dislocation storage.     

Cyclic hardening and softening of Al-2.9% Cu-2.2% Mg-0.6% Mn alloy

Electron microscopic research results are given for the Al-2.9% Cu-2.2% Mg-0.6% Mn alloy structure after different operating times corresponding to cyclic hardening or cyclic softening. It is shown that cyclic hardening results in precipitation of S particles and cyclic softening results in three processes: dissolution of S particles, precipitation of S particles instead of S ones and/or formation of regions free from precipitates. The main fatigue crack passes through cyclic-hardened material only and the hardening phase is merely the S particles. Relations between structure, cyclic hardening, cyclic softening and accumulated cyclic deformation are observed.     

Precipitation behavior and tensile property of the stress-aged Ti-10Mo-8V-1Fe-3.5Al alloy

A kind of metastable beta titanium alloy, i.e. Ti-10Mo-8V-1Fe-3.5Al, was aged under elastic compressive stress. Transmission electron microscopy (TEM) investigations revealed that, in the stress-aged samples, precipitated alpha plates were parallel to each other. However, in the conventionally aged samples, alpha plates intersected each other. In other words, variant of alpha plate was selected in the stress-aged samples. Tensile tests showed that, at equivalent tensile strength, the stress-aged samples exhibited higher ductility, compared to the conventionally aged ones.     

Cu—1C—1Cd粉末复合体致密化过程的研究

系统地研究了Ｃｕ－１Ｃ－１Ｃｄ粉末复合体制备过程中,原始粉末状态,压制压力,烧结温度等工艺参数对材料相对密度的影响。结果表明,原始粉末的差异对制备工艺的全过程均产生规律性的影响。压制压力对压坯密度的影响符合粉末冶金材料的一般压制规律,在烧过程中,镉和孔隙扩散对烧结密实化过程起着重要作用。     

Precipitation of iron-rich intermetallic phases in Al-4.6Cu-0.5Fe-0.5Mn cast alloy

The solidification of Al-4.6Cu-0.5Fe-0.5Mn (206 type) cast alloy has been studied using Thermal Analysis, Differential Scanning Calorimeter, Scanning Electron Microscopy, Electron Back-Scattered Diffraction (EBSD), and Transmission Electron Microscopy (TEM). It is interesting to note that an iron-rich intermetallic phase, Al₃(FeMn), is experimentally observed to be dominantly present in the fully solidified cast structure of the experimental 206 cast alloy, in addition to β-Fe (Al₇Cu₂(FeMn)/Al₇Cu₂Fe) phase. Al₃(FeMn) phase is formed through a eutectic reaction approximately at 640 °C during solidification, possibly resulting from the phase selection and segregation of Fe in liquid Al. The presence of the Al₃(FeMn) phase has been confirmed by both EBSD and TEM. It is also found that both β-Fe and Al₆(FeMn) are possible to nucleate on Al₃(FeMn), as confirmed by the calculated low planar disregistries. The possible solidification reactions have been established for 206-type cast alloy at 0.5%Fe.     

Laser surface melting of Ti-6Al-4V alloy

Surface hardening of Ti-6Al-4V alloy with laser surface melting (LSM) in a nitrogen atmosphere has been studied. In LSM, hardness increased almost three-fold in comparison to that of the substrate, the latter having a Vickers hardness of 350, by the formation of TiN in the range of 100m of melt depth. Hardness, then, decreased slowly and reached a minimum of 580 VHN at a maximum melt depth of 750m. -Ti was formed in the heat-affected zone (HAZ) with a VHN of 450. Ageing treatments were performed for all specimens at 450 °C and different ageing times (1–20h). Short ageing treatments increased the hardness in the melted zone as well as in the HAZ (1–3h). Long ageing treatments (7–20h) resulted in uniform hardness distribution in the melted zone.     

Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 m melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 m. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.     

Restoration mechanism during hot tension of an Al-4. 5Cu-1. 5Mg alloy

Specimens of an Al-4. 5Cu-1. 5Mg alloy are deformed in hot tension at temperatures of 20°C, 300°C, 400°C and 480°C at strain rates of 0.00083–0.083 S⁻¹. Immediately they are water quenched after the tension. The microstructural changes which occur during the hot tension have been investigated using true stress-true strain curves, X-ray photographs, optical and electron microscopy. It has been shown that the restoration mechanism consists of both dynamic recrystallization and dynamic recovery. The dynamic recrystallization is accelerated with the increase of deformation temperature and strain rate. The nucleation mechanism of dynamic recrystallization is discussed.     

Microstructural investigation of a rapidly solidified Ti-6Al-4V-1B-0.5Y alloy

A Ti-6Al-4V-1B-0.5Y (wt%) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibre and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distribution in the (+) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phase in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB with a B27 structure, while the yttrium oxide has a cuboidal morphology and has been identified as bcc Y₂O₃. Detailed TEM examination also revealed that yttrium addition has a strong influence on the TiB morphology by comparing the microstructures of Ti-6Al-4V-1B alloys with and without yttrium addition. Under similar processing conditions, the TiB phase in the consolidated alloys without yttrium addition mainly has a nearly equiaxed morphology with a finer particle size, while the TiB phase in the consolidated alloy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.     

Microstructural assessment of laser nitrided Ti-6Al-4V alloy

A microstructural study of the phases developed during the laser nitriding of a Ti-6Al-4V alloy by, using a CL5 continuous CO₂ laser with a spinning beam and concentration of 80% nitrogen, was undertaken. The vertical sections, perpendicular to the melt track were examined by optical microscopy and scanning electron microscopy (SEM), while specimens for X-ray diffractometry (XRD), X-ray photospectroscopy (XPS) and transmission electron microscopy/selected area electron diffraction (TEM/SAED), were taken parallel to the melt track. In this way the variation in microstructure as a function of depth from the laser treated surface, was studied. This supplemented XRD and XPS investigations undertaken previously. Two zones were identified. Zone 1, within 50 m of the surface, contained well defined dendrites of fcc TiN_0.8, plus hcp TiN_0.3 and hcp Ti. Zone 2, below 50 m, consisted of needles of hcp Ti. From a consideration of the hardness profiles in Zone 2, it is suggested that at the top of the zone, the phase is, in fact, a solid solution containing 3–4% N, which decreased to <1% N at the bottom of the zone. The TEM/SAED study permitted the three phases fcc TiN_0.8, hcp TiN_0.3 and hcp Ti to be identified through a combination of morphology and SAED patterns. This also showed that the fccTiN_0.8 contained fringes, which were considered to be stacking fault fringes and allowed this phase to be readily recognized in the TEM. The presence of stacking faults may be associated with the high nitrogen concentration of 80% used for the laser nitriding in this work.     

The high-cycle fatigue behavior of Ti-6Al-4V alloy

The cyclic stress-strain behavior and the S/N behavior of a “pseudo-elastic” alloy, Ti-6Al-4V of coarsened microstructure and of a “plastic” material, copper, are compared. In accord with views recently put forth by Freudenthal, important differences are noted between these two types of materials. These differences include the mode of crack initiation, the intensity of the Bauschinger effect, and the level below macroscopic yield at which long-life fatigue data lie.     

Precipitation hardening in Inconel* 625

Abstract

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni₃ (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.