Effect of Aging on Microstructure and Shape Memory Properties of a Ni-48Ti-25Pd (At. Pct) Alloy

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.     

The R-phase transition and associated shape memory mechanism in Ti-Ni single crystals

The shape memory mechanism associated with the R-phase transition was investigated using age-treated Ti-Ni single crystals. Tensile tests and in situ optical microscopic observations were carried out. Four distinct variants of the R-phase were found, and these could be correlated with each other by twinning relationships. Six twinning planes in each variant were determined using two-surface analysis and were found to consist of three {110} and three {100} planes. A specific morphology consisting of the four types R-phase variants was formed upon cooling below the T_R point. The combination of the four variants produces a zero macroscopic strain, indicating that the specific morphology is the result of self-accommodation. By stressing, the most favorable R-phase variant was formed from the remaining variants by twinning deformation. Upon heating after unloading, the specimen showed a shape recovery which is a function of temperature or rhombohedral angle of the R-phase. Most of the shape recovery was brought about by a change in the lattice parameter of the R-phase, but not by the reverse-transition. The manner of the shape recovery is quite different from that associated with the reverse martensitic transformation. On the basis of the results obtained, the mechanism of the shape memory associated with the R-phase transition in alloys of the Ti-Ni type has been clarified.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

Microstructural stability on aging of an α + β titanium alloy: ti- 6ai- 1.6zr-3.3mo- 0.30si

The development of the microstructure on aging of an (α + β) type titanium alloy containing 6A1-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The β-transus temperature of this alloy is approximately 1243 K. Solution treatment in the β-phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite struc-ture. The martensitic structure was confirmed to be orthorhombic (α″) using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation ofs ₂ silicide particles and thin films of β sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters α= 0.70(1) nm andc = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation ofs ₂ silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of β and acicular martensite. The relative sizes of silicide precipitates and width of thin films of β phase increase with increasing aging time. The sites for silicide precipitation are mainly at α′-α′ boundaries, α-β interfaces, and sometimes within regions of transformed β. The kinetics ofs ₂ silicide precipi-tation in this alloy is faster than in commercial near-α titanium alloys. This is attributed to the presence of Mo, a strong β stabilizer. Formerly Reader, Department of Metallurgical Engineering, Centre of Advanced Study, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India     

An Investigation on Phase Formations and Microstructures of Ni-rich Ni-Ti Shape Memory Alloy Thin Films

Ni-rich Ni-Ti alloy thin films were fabricated by radio frequency (RF)-direct-current (DC) magnetron sputtering using elemental Ni and Ti as sputter targets. Si (100) was chosen as a substrate that was either held at room temperature or at 573 K (300 °C) during the depositions. The 380-nm-thick films were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy, grazing incidence X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy, and Vickers microhardness tester. The results suggests that because of the lack of surface mobility of the adatoms at the room temperature, the deposited films were smooth and amorphous, with a crystallite size of 15 nm accompanied with a porous morphology. At higher substrate temperatures, an increase in the surface diffusion leads to the formation of partially crystalline, rougher films with a denser, compact, fibrous grain microstructure. The formation of Ni-rich precipitates such as Ni₄Ti₃, Ni₂Ti, and Ni₃Ti along with small amount of the NiTi phase were attributed to the localized heating and cooling within the grains. Few grains exhibited band structures that are believed to be a result of <110> type II twins.     

Ni55Mn25Ga18Ti2高温形状记忆合金的热循环稳定性

选择双相韧化的Ni?Mn?Ga?Ti高温形状记忆合金为研究对象。制备了淬火态Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金,并对其在室温至480 ℃之间进行高达500次的相变热循环,获得了5, 10, 50, 100和500次热循环态样品。采用X射线衍射、扫描电镜、能谱仪、同步热分析仪及室温压缩等实验方法,研究了淬火态和热循环态合金样品的微观组织、相变行为、力学及记忆性能,进而分析其热循环稳定性。研究结果表明:经500次循环后,Ni₅₅Mn₂₅Ga₁₈Ti₂合金相结构和显微组织未发生明显变化,均为由非调制四方结构的板条马氏体相和面心立方富Ni的γ相组成的双相结构;随着循环次数增加,马氏体相变温度几乎不变,逆马氏体相变温度和相变滞后在循环5次后趋于稳定;抗压强度及压缩变形率波动幅度较小;形状记忆性能下降,但形状记忆应变仍保持在1.4%以上;Ni₅₅Mn₂₅Ga₁₈Ti₂高温形状记忆合金显示出良好的热循环稳定性。      

Layer growth in Au-Pb/In solder joints

The solid state reaction between a Pb-In solder alloy and thin film Au has been investigated at ten aging temperatures ranging from 70 to 170°C. Also, bulk Au-solder samples were aged at 150°C for metallographic analysis. No significant difference was found between the aging behavior of thin and bulk Au specimens. A thin single phase layer of Au₉In₄ was found adjacent to Au while a thick two-phase layer of AuIn₂ and Pb was found between Au₉In₄ and solder. The Pb phase was shown to have considerable mobility and able to ripen at room temperature. Peculiar planar interface instabilities and voids in the Au-Au₉In₄ interface were found. Although the total layer thicknessvs aging time data could be closely fitted with a power law relationship, it was shown that a linear relationship also fits well and is consistent with accepted metallurgical concepts. An activation energy of 0.61 eV was found by regression analysis of the intermetallic growth kinetics.     

Layer growth in Au-Pb/In solder joints

The solid state reaction between a Pb-In solder alloy and thin film Au has been investigated at ten aging temperatures ranging from 70 to 170°C. Also, bulk Au-solder samples were aged at 150°C for metallographic analysis. No significant difference was found between the aging behavior of thin and bulk Au specimens. A thin single phase layer of Au₉In₄ was found adjacent to Au while a thick two-phase layer of AuIn₂ and Pb was found between Au₉In₄ and solder. The Pb phase was shown to have considerable mobility and able to ripen at room temperature. Peculiar planar interface instabilities and voids in the Au-Au₉In₄ interface were found. Although the total layer thicknessvs aging time data could be closely fitted with a power law relationship, it was shown that a linear relationship also fits well and is consistent with accepted metallurgical concepts. An activation energy of 0.61 eV was found by regression analysis of the intermetallic growth kinetics.     

Microstructure and Shape Memory Characteristics of Powder-Metallurgical-Processed Ti-Ni-Cu Alloys

Even though Ti-Ni-Cu alloys have attracted a lot of attention because of their high performance in shape memory effect and decrease in thermal and stress hysteresis compared with Ti-Ni binary alloys, their poor workability restrains the practical applications of Ti-Ni-Cu shape memory alloys. Consolidation of Ti-Ni-Cu alloy powders is useful for the fabrication of bulk near-net-shape shape memory alloy. Ti₅₀Ni₃₀Cu₂₀ shape memory alloy powders were prepared by gas atomization, and the sieved powders with the specific size range of 25 to 150???m were chosen for this study. The evaluation of powder microstructures was based on a scanning electron microscope (SEM) examination of the surface and the polished and etched powder cross sections. The typical images showed cellular/dendrite morphology and high population of small shrinkage cavities at intercellular regions. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis showed that a B2-B19 one-step martensitic transformation occurred in the as-atomized powders. The martensitic transformation start temperature (M_s) of powders ranging between 25 and 50???m was 304.5?K (31.5?°C). The M_s increased with increasing powder size. However, the difference of M_s in the as-atomized powders ranging between 25 and 150???m was only 274?K (1?°C). A dense cylindrical specimen of 10?mm diameter and 15?mm length were fabricated by spark plasma sintering (SPS) at 1073?K (800?°C) and 10?MPa for 20?minutes. Then, this bulk specimen was heat treated for 60?minutes at 1123?K (850?°C) and quenched in ice water. The M_s of the SPS specimen was 310.5?K (37.5?°C) whereas the M_s of conventionally cast ingot is found to be as high as 352.7?K (79.7?°C). It is considered that the depression of the M_s in rapidly solidified powders is ascribed to the density of dislocations and the stored energy produced by rapid solidification.     

Internal oxidation and mechanical properties of TZM-Mo alloy

The internal oxidation behavior of the bcc alloy TZM-Mo (Mo-0.5 wt pct Ti-0.08 wt pct Zr-0.02 wt pet C) was investigated in low-pressure O₂, CO, and H₂O environments at 1098 and 1273 K. The results indicate that a diffusion process controls the kinetics of the oxygen absorption at 1098 K, while bulk diffusion and gas-metal interaction at the specimen surface both affect the rate at 1273 K. The carbon content of TZM in these experiments increased initially and then decreased. Decarburization became significant only after extended exposure at 1273 K. The deformation and fracture behavior of both oxidized and heat-treated TZM specimens were studied at temperatures to 1589 K. TZM specimens showed an increase in strength and a linear decrease in ductility with oxygen content. Oxidized TZM lost its ductility completely at an oxygen level of 300 ppm at room temperature, 1366, and 1589 K, but 500 ppm was required at 1098 K. The ductility of embrittled TZM was increased significantly with heat treatment at high temperatures and was almost completely restored after annealing at 1973 K. The change in mechanical properties is discussed in terms of internal oxidation and precipitation of oxides.     

Influence of Aging Treatment on In-Situ Electrical Resistance Variation During Aging of Nickel-Rich NiTi Shape Memory Wires

Electrical resistance variations of Ni_50.9Ti_49.1 shape memory wires were studied during aging treatment at different temperatures via in-situ electrical resistance measurement. The results showed that during aging treatment, a cyclic behavior was observed in the electrical resistance variations, which could be related to the precipitation process. The evaluation of transition temperatures was conducted, after aging, using differential scanning calorimetry (DSC) analysis. The precipitation process is found to occur in four different stages. The results show that depending on the stress level around precipitates, two-, three-, or four-step martensitic transformation could be observed in DSC curves. In the points with maximum stress level (during precipitation process), four-step martensitic transformation is observed.     

The effect of aging temperature on transformation parameters of porous NiTi shape memory alloy fabricated by SHS

This study reports some different and efficient approaches for the aging temperatures which cause changes in the transformation parameters of porous NiTi (Ti-50.5 at % Ni) shape memory alloy (SMA) with porosity of 55.3% fabricated by self propagating high temperature synthesis (SHS). Changes which occurred in phase transformations due to the effects of various thermal procedures have been investigated by using X-ray diffraction. The micro structure was investigated using a scattering electron microscope and a light optical microscope. The shape memory behavior and transformation temperatures were studied through the differential scanning calorimetry (DSC). During the compression tests, it was observed that the compressive strength of the specimens was higher than the compact human bone, and that it increased with increasing aging temperature.     

Size Dependence of Microstructure Characteristics for Chromium Disilicide Thin Films

We have used computer analysis to study linear and bulk characteristics of the microstructure of CrSi₂ thin films consisting of crystalline and amorphous phases; we used transmission electron microscopy to determine the microstructure of the films. As the film thickness increases, the linear dimensions of the crystallites and their volume fraction increase, while the average interparticle distance and the specific number of particles decrease. In this case, the microstructure evolves from a matrix type to a matrix-statistical type. It is due to phenomena including recrystallization during film deposition, coalescence of discrete islands of crystallites, and «sintering» of contacting particles.     

Aging effects in a Cu-12Al-5Ni-2Mn-1Ti shape memory alloy

The isothermal aging effects in an as-quenched Cu-11.88Al-5.06Ni-1.65Mn-0.96Ti (wt pct) shape memory alloy at temperatures in the range 250 °C to 400 °C were investigated. The changes in the state of atomic order and microstructural evolutions were traced by means of in situ X-ray diffraction and electrical resistivity measurements, as well as transmission electron microscopy (TEM) and optical observations. The kinetics of the aging process, i.e., the temperature and time dependence of the properties including hardness, resistivity, martensitic transformation temperatures, and shape memory capacity were characterized, and at least three temperature-dependent aging stages were distinguished: (1) D0₃ or L2₁ atomic reordering, which causes the martensitic transformation temperatures to shift upward and leads the M18R martensite to tend to be a N18R type structure; (2) formation of solute-depleted bainite which results in a drastic depression in martensitic transformation temperatures and loss of the shape memory capacity, accompanied by the atomic disordering in both the remaining parent phase and bainite; and (3) precipitation of the equilibrium α and γ ₂ phases and destruction of the shape memory capacity.     

Environment-Assisted Cracking in Custom 465 Stainless Steel

The influence of cold work and aging on the environment-assisted cracking (EAC) behavior and mechanical properties of Custom 465 stainless steel (SS) was studied. Four sets of specimens were made and tested. All specimens were initially solution annealed, rapidly cooled, and refrigerated (SAR condition). The first specimen set was steel in the SAR condition. The second specimen set was aged to the H1000 condition. The third specimen set was 60 pct cold worked, and the fourth specimen set was 60 pct cold worked and aged at temperatures ranging from 755 K to 825 K (482 °C to 552 °C) for 4 hours in air. The specimens were subsequently subjected to EAC and mechanical testing. The EAC testing was conducted, using the rising step load (RSL) technique, in aqueous solutions of NaCl of pH 7.3 with concentrations ranging from 0.0035 to 3.5 pct at room temperature. The microstructure, dislocation substructure, and crack paths, resulting from the cold work, aging, or subsequent EAC testing, were examined by optical microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The aging of the cold-worked specimens induced carbide precipitation within the martensite lath, but not at the lath or packet boundaries. In the aged specimens, as aging temperature rose, the threshold stress intensity for EAC (K_IEAC)_, elongation, and fracture toughness increased, but the strength and hardness decreased. The K_IEAC also decreased with increasing yield strength and NaCl concentration. In the SAR and H1000 specimens, the EAC propagated along the prior austenite grain boundary, while in the cold-worked and cold-worked and aged specimens, the EAC propagated along the martensite lath, and its packet and prior austenite grain boundaries. The controlling mechanism for the observed EAC was identified to be hydrogen embrittlement.     

Grain-size effect on shape-memory behavior of Ti35.0Ni49.7Zr15.4 thin films

The grain-size effect on shape-memory behavior of fine-grained Ti_35.0Ni_49.7Zr_15.4 thin films was investigated by transmission electron microscopy. The films, with various grain sizes ranging from about 150 to about 400 nm, were prepared by heat treatment of amorphous films at the three different annealing temperatures of 773, 873, and 973 K, for three different annealing times of 5 minutes, 1 hour, and 10 hours. The film annealed at 773 K for 5 minutes showed a nearly single phase of (Ti,Zr)Ni, while the films annealed at high annealing temperatures and/or long annealing times showed λ ₁ precipitates. For a high annealing temperature of 973 K, the critical yield stress (σ _c) was dominantly dependent on the grain size of the matrix, obeying the Hall-Petch equation. On the other hand, for a low annealing temperature of 773 K, σ _c was dominantly dependent on the amount of λ ₁ precipitates. The M _S temperature decreases almost linearly with increasing σ _c. The films showed sufficient fracture toughness, probably due to the nanometer scale of the grain size and the agglomerated shape of λ ₁ particles.     

Effect of Cooling Rate on Microstructure and Mechanical Properties of K465 Superalloy

 K465 superalloy, as a material for production of turbine nozzle, shows high mechanical properties as well as microstructure stability in critical and severe service conditions. The alloy gains its appropriate microstructure and strength through solid solution strengthening mechanism. Heat treatment parameters such as: time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from solid solution affect the microstructure of the alloy. Among these parameters cooling rate from solid solution is the most effective. Therefore, in this study the effect of cooling rate on microstructure and mechanical properties (tensile and stress properties) were investigated. For this purpose, three different cooling rates were applied on the cast K465 specimens after solution treatment at 1210℃ for 4 h. Microstructures of the specimens then were studied using optical and electron microscopy. Also, tensile tests were performed at room temperature and stress rupture tests were performed under the condition of 975℃ and 230 MPa. It was found out that with increasing cooling rate the size of the γ' precipitates decreases and the mechanical properties of specimens increases. Also, it was shown that the shape and volume fraction of primary γ' particles are largely influenced by the cooling rate following solution treatment at 1210℃ for 4 h.     

Microstructural Response During Isothermal and Isobaric Loading of a Precipitation-Strengthened Ni-29.7Ti-20Hf High-Temperature Shape Memory Alloy

A stable Ni-rich Ni-29.7Ti-20Hf (at. pct) shape memory alloy, with relatively high transformation temperatures, was shown to exhibit promising properties at lower raw material cost when compared to typical NiTi-X (X = Pt, Pd, Au) high-temperature shape memory alloys (HTSMAs). The excellent dimensional stability and high work output for this alloy were attributed to a coherent, nanometer size precipitate phase observed using transmission electron microscopy. To establish an understanding of the role of these precipitates on the microstructure and ensuing stability of the NiTiHf alloy, a detailed study of the micromechanical and microstructural behaviors was performed. In-situ neutron diffraction at stress and temperature was used to obtain quantitative information on phase-specific internal strain, texture, and phase volume fractions during both isothermal and isobaric testing of the alloy. During isothermal testing, the alloy exhibited low isothermal strains due to limited detwinning, consistent with direct measurements of the bulk texture through neutron diffraction. This limited detwinning was attributed to the pinning of twin and variant boundaries by the dispersion of fine precipitates. During isobaric thermal cycling at 400?MPa, the high work output and near-perfect dimensional stability was attributed to the presence of the precipitates that act as homogeneous sources for the nucleation of martensite throughout the material, while providing resistance to irrecoverable processes such as plastic deformation.     

Two-way shape memory effect of NiTi alloy induced by constraint aging treatment at room temperature

A new optimal training procedure to induce a two-way shape-memory effect (TWSME) in bending mode has been investigated in a Ni-rich NiTi alloy that had underwent a preaging heat treatment followed by constraint aging at room temperature. The aim of the present investigation is to develop a new training procedure to induce TWSME as well as to examine the relationship between the obtained properties and microstructures of specimens prepared by various preaging temperatures. It is suggested that the R-phase plays an important role in the establishment of two-way shape-memory behavior on cooling.     

Kinetics and Equilibrium of Age-Induced Precipitation in Cu-4 At. Pct Ti Binary Alloy

Transformation kinetics and phase equilibrium of metastable and stable precipitates in age-hardenable Cu-4 at. pct Ti binary alloy have been investigated by monitoring the microstructural evolution during isothermal aging at temperatures between 693 K (420 °C) and 973 K (700 °C). The microstructure of the supersaturated solid solution evolves in four stages: compositional modulation due to spinodal decomposition, continuous precipitation of the needle-shaped metastable β′-Cu₄Ti with a tetragonal structure, discontinuous precipitation of cellular components containing stable β-Cu₄Ti lamellae with an orthorhombic structure, and eventually precipitation saturation at equilibrium. In specimens aged below 923 K (650 °C), the stable β-Cu₄Ti phase is produced only due to the cellular reaction, whereas it can be also directly obtained from the intergranular needle-shaped β′-Cu₄Ti precipitates in specimens aged at 973 K (700 °C). The precipitation kinetics and phase equilibrium observed for the specimens aged between 693 K (420 °C) and 973 K (700 °C) were characterized in accordance with a time–temperature–transformation (TTT) diagram and a Cu-Ti partial phase diagram, which were utilized to determine the alloy microstructure, strength, and electrical conductivity.