Irreversibility and transformation randomness of thermoelastic martensitic transformation in Ni–Ti alloys

In this work,the in situ optical observation was carried out in complete and incomplete transformation cycles of Ni-Ti alloys.In complete transformation cycles,initial martensite plates nucleate randomly in austenite.However,in a partial transformation cycle,the existing martensite plates have an influence on guiding the formation of subsequent martensite plates.And the randomness decreases with the decrease in transformation volume involved in the partial cycle.It is suggested that the randomness of transformations contributes to the introduction of defects,and the irreversibility associates with transformation randomness of martensite plates.For instance,a higher randomness in transformations could introduce more defects and more obvious irreversibility.On the other hand,defects generated in thermoelastic martensitic transformation are responsible for the hysteresis of transformations.Therefore,the randomness of transformations also contributes to the transformation hysteresis.These results could help further understanding on some martensitic transformation phenomena of shape memory alloys,such as the nonlinear and history-dependent characteristic.     

Application of an analytical phase transformation model

Employing isothermal and isochronal differential scanning calorimetry, an analytical phase transformation model was used to study the kinetics of crystallization of amorphous Mg82.3Cu17.7 and Pd40Cu30P20Ni10 alloys. The analytical model comprised different combinations of various nucleation and growth mechanisms for a single transformation. Applying different combinations of nucleation and growth mechanisms, the nucleation and growth modes and the corresponding kinetic and thermodynamic parameters, have been determined. The influence of isothermal pre-annealing on subsequent isochronal crystallization kinetics with the increase of pre-annealing can be analyzed. The results show that the changes of the growth exponent, n, and the effective overall activation energy Q, occurring as function of the degree of transformation, do not necessarily imply a change of nucleation and growth mechanisms, i.e. such changes can occur while the transformation is isokinetic.     

R-phase transformation of aged Ti-Ni shape memory alloy

Ti-50.6Ni（molar fraction, %） shape memory alloy solution treated at 850 ℃ for lh followed by ageing treatment at 450 ℃ for 3 h was studied with differential scanning calorimetry（DSC）, X-ray diffractometry（XRD） and transmission electron microscopy（TEM）. DSC measurement reveals two separate transformation peaks. XRD and TEM demonstrate that a three-stage transformation occurs. The Ti3 Ni4 precipitates are coherent with the R- phase. The crystal structure of R-phase was analyzed by two diffraction patterns method. The diffraction patterns of R-phase were obtained in detail from the same region.     

Phase transformation behavior in Al-Zn-Cu alloys by hammering

The 5Cu40Zn55Al and 15Cu20Zn65Al alloys were prepared in the AI-Zn-Cu system, There exist the metastable phases ε and θ in the two alloys after homogenization treatment and furnace cooling, respectively. It is shown that the particles are refined from 3 mm to less than 10 μm after hammering the two alloys but there are still metastable phases. This means that the phase constituents of the two alloys have no changes by the deformation, which is different from that by balling. The phase constituents are not changed at room temperature by hammering, which is dependent on the deformation mechanism of hammering.     

Incomplete transformation of Ti-Ni-X ternary shape memory alloy

If the reverse transformation of a shape memory alloy is arrested, a kinetic stop will appear in the next complete transformation. The kinetic stop temperature has a close relation with the previous arrest temperature. This kinetic stop can be regarded as a ＂memory＂ of the previous arrest temperature. This phenomenon is named temperature memory effect（TME）. The TME induced by incomplete cycling in Ti-Ni-Nb and Ti-Ni-Cu alloys was systematically investigated by performing either a single incomplete cycle, or a sequence of incomplete cycles with different arrested temperatures. The results indicate that TME only exists in the heating process, and TME can occur both in B 19＇→B2 and B 19→B2 reverse transformation during heating process. But, there is no evidence of TME during cooling in the Ti-Ni-X ternary alloys. And the reverse transformation temperature interval （At-As） of the Ti44-Ni47-Nb9 alloy induced by TME can be significantly enlarged compared with that of the Ti-Ni-Cu alloy by performing multi-times incomplete transformation cycling with a decreasing order arrested temperature.     

Continuous cooling transformation curve of a novel Al-Cu-Li alloy

1 Introduction Al-Li alloy is perspective for wide applications in aircraft industry. It has an attractive combination of medium strength with good plasticity, fracture toughness, corrosion resistance and weldability[1- 3]. Continuous cooling transformati…     

Calculation of AF transformation kinetics in HSLA steel weld

It is of great importance for obtaining the perfect welding properties to control the acicular ferrite (AF)transformation behavior reasonably in steel weld. AF continuous transformation kinetics in the HSLA steel weld was calculated and modeled based on the direct growth on the inclusions inert interface. The simulation results are coincident with the experimental value well.     

Consolidation of nanostructured SiC with disorder–order transformation

Sintering of nanostructured cubic SiC to 98% relative density is accomplished without the use of additives or very high pressures. Evidence demonstrating the role of a disorder–order transformation in the densification process is provided.     

Aging transformation in Cu-3.2Ni-0.75Si alloy

1　INTRODUCTIONHavinghighstrengthand goodelectricalandthermalconductivities,copperalloysareusedinanumberofengineeringapplications ,suchaselectricresistanceweldingelectrodes ,thelinertubeofcon tinuouscastingcrystallizer ,integratedcircuitleadframe ,aerialconductorofelectriclocomotive .Therepresentativeprecipitation hardenablecopper baseal loysfortheapplicationsareCu Cr[1] ,Cu Cr Zr[2 ] ,Cu Fe Cr[3] ,Cu Ni Si[4 6 ] etc .PrecipitationinCu Cr ,Cu Cr Zr ,Cu Fe Palloysusuallystartsfromth…     

An extended analytical model for solid-state phase transformation upon continuous heating and cooling processes: Application in γ/α transformation

An extended analytical model is derived for non-isothermal solid-state phase transformation assuming interface-controlled growth mode, e.g. polymorphic or allotropic transformation. In the modeling, incorporation of thermodynamic factor into kinetics of nucleation and growth is performed, so that the model can be used to describe the transformation occurring either near or far from the equilibrium state. Furthermore, the effect of the initial transformation temperature is included through a special treatment for the “temperature integral”, so that the model can be used to depict the transformation during either continuous heating or continuous cooling. Numerical calculations demonstrate that the extended analytical model is accurate enough for practical use. On this basis, a general rate equation for non-isothermal (isochronal heating and cooling) transformation is derived. Applying the present model, the overall kinetic behavior of γ/α transformation in binary substitutional Fe-based alloys (e.g. Fe–Mn and Fe–Cu) upon cooling, measured by dilatometry, is described successfully. Compared with previous work, where a site saturation assumption is generally made, the prevalence of continuous nucleation deduced using the present model prediction seems more reasonable.     

Two-stage reverse transformation in hyperstabilized β1′ martensite

A Cu–Al–Be alloy is hyperstabilized by quenching into β₁^′ martensite: only a fraction of martensite is retransformable to bulk β-phase at temperatures approximately 100 K above the nominal transformation temperatures. The rest of the hyperstabilized martensite undergoes the second stage of reverse transformation at much higher temperatures by means of the nucleation of lamellar β-phase.     

Long-range order relaxation and phase transformation in γ-TiAl alloys

Residual electrical resistivity measurements were used for characterizing γ-TiAl intermetallic compounds and for investigating the kinetics of long-range order (LRO) relaxation. In annealed and slowly cooled materials, a linear relationship between residual resistivity and aluminium content was observed in the composition range from 50 to 56.1 at.% Al. The isothermal relaxation of LRO after small temperature changes was investigated in γ-TiAl compounds (with 49.8, 50.7, 52.6 and 54.1 at.% Al). The approach to equilibrium could be satisfactorily represented by a sum of two exponential relaxation processes. The slower process was assigned to LRO relaxation. From the investigation of the temperature dependence of relaxation times, an activation enthalpy of 2.97±0.15 eV was determined for the stoichiometric alloy. This value is close to the activation enthalpy for titanium self-diffusion in TiAl. For Al contents higher than 50.7 at.%, the LRO changes were found to be superimposed on a supplementary phenomenon, which produces a continuous resistivity decrease. A TEM investigation showed that this drift was associated with the precipitation of Ti₃Al₅, which is a superstructure of the L1₀ structure.     

Energy transformation analysis during friction welding of superalloy lnconel 718

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Abnormal austenite–ferrite transformation behaviour in substitutional Fe-based alloys

The γ→α phase transformation behaviours of Fe-Co and Fe-Mn alloys were systematically investigated by dilatometry and Differential Thermal Analysis (DTA). Two kinds of transformation kinetics, called normal and abnormal, were recognized for the first time and classified according to the variation of the ferrite formation rate. These transformation characteristics were observed for both isothermally and isochronally conducted annealing experiments. A transition, from abnormal to normal transformation kinetics, occurs for Fe-1.79at.%Co when successive heat treatment cycles are executed, which contrasts with Fe-2.26at.%Mn for which only normal transformation kinetics occurs after each of all successive heat treatment cycles. A possible mechanism for the appearance of abnormal transformation kinetics is given, which is based on the austenite grain size. Light microscopical analysis indicates a repeated nucleation of ferrite in front of the migrating γ/α interface.     

Phase transformation and properties of quasicrystal particles/Al matrix composites

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Phase transformation of fully lamellar γ-TiAl alloys in α γ field

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｍｅｃｈａｎｉｃａｌｐｒｏｐｅｒｔｉｅｓｏｆｔｗｏ ｐｈａｓｅγ ＴｉＡｌｉｎｔｅｒｍｅｔａｌｌｉｃａｌｌｏｙｓａｒｅｓｔｒｏｎｇｌｙｄｅｐｅｎｄｅｎｔｏｎｍｉ ｃｒｏｓｔｒｕｃｔｕｒｅｓ[1] .Ｔｈｅｒｅａｒｅｆｏｕｒｋｉｎｄｓｏｆｍｉｃｒｏｓｔｒｕｃ ｔｕｒｅｓ :ｎｅａｒ ｇａｍｍａ ,ｄｕｐｌｅｘ ,ｎｅａｒｌａｍｅｌｌａｅａｎｄｆｕｌｌｙｌａｍｅｌｌａｒ .Ｔｈｅｂｅｓｔｔｅｎｓｉｌｅｐｒｏｐｅｒｔｉｅｓｏｆγ ＴｉＡｌｉｎｔｅｒ ｍｅｔａｌｌｉｃａｌｌｏｙｓｒｅｓｕｌ…     

Welding temperature field analysis for featheredged cylinder based upon conformal transformation

The accurate calculation and measurement of welding temperature field is an important precondition for welding metallurgical analysis and welding process controlling. In this paper, the conformal transformation is firstly used to analyze the welding temperature field of featheredged cylinder. The center of the cylinder is chosen as the origin of column coordinate system, and every point may be expressed as complex field vector. The branch isogonality counterchanges the line parallel with the fusion line in half-infinite z-plane to the circle concentric with the fusion line in infinite cylinder. The Laplace equation and Poisson＇s equation still keep validity, so the temperature field equation can be solved. The conformal transformation and equation solution is processed by Matlab program language. It shows that the obtained analytical modeling of temperature field for featheredged cylinder based on conformal transformation is effective and accurate.     

Lüders-like deformation associated with stress-induced martensitic transformation in NiTi

Stress-induced martensitic transformation in NiTi may proceed in a Lüders-type manner. This paper presents thermal analysis and TEM evidences of the presence of austenite after deformation to beyond the end of the Lüders-type stress plateau, suggesting that the end of the stress plateau is not the end of the phase transformation.     

Phase transformation in an yttrium–hydrogen system studied by TEM

Phase transformations in Pd-capped epitaxial yttrium films grown on (0 0 0 1) sapphire substrates covered with a Ti buffer layer and hydrogenated for different times were studied using transmission electron microscopy (TEM). For short hydrogen charging times, the phase transformation from α-Y to β-YH₂ is associated with the nucleation and growth of two orientational variants, which after coalescence form twin-related lamellae of the β-YH₂ phase with twin interfaces parallel to the substrate plane. Shockley partial dislocations are present at the twin boundaries; their glides during phase transformation are responsible for the formation of the twin lamellae. Superlattice reflections were observed for β-YH₂, and the existence of a new long-range ordered superstoichiometric YH_2+x phase was suggested. A structural model of the ordering based on the occupation of octahedral interstitial sites by H in a doubled cell of Y-face-centered cubic was offered. For samples hydrogenated for longer times, β-YH₂-to-γ-YH₃ phase transformation was accompanied by cracking along the twin boundaries, which eventually developed into a network of pores and caused significant swelling of the films. No γ-YH₃ phase was observed directly in TEM because of its unstable nature under the high vacuum of a microscope column. The fully transformed YH₃ films have over a 60% increase in its thickness, which is mostly accounted for by the high volume fraction of pores.