Detection of shock-wave-induced internal stresses in Cu-Al-Ni shape memory alloy by means of acoustic technique

1. The present results indicate that the stress-induced β₁→γ₁′ martensitic transformation occurs for an impact duration of 2 × 10⁻⁶ s. This time interval appears to be sufficient also for the subsequent deformation of the γ₁′ martensitic phase to occur.2. A structure memory effect has been found: Cu-Al-Ni austenitic crystals, shock-loaded at room temperature to induce γ₁′–martensite, recall during subsequent temperature-induced martensitic transformation the martensitic variant structure (elastic properties) formed under the shock loading.3. Elastic properties of quenched β₁′ and γ₁′ crystals of the Cu-Al-Ni system are extremely sensitive to the shock-wave loading. Mechanisms of these effects, as well as of the structure memory effect, include the generation of internal stresses due to the high elastic anisotropy of the martensitic phases. These internal stresses either change the distribution of martensitic variants or govern the formation of the martensitic variant structure during the temperature-induced martensitic transformation. The generation of high internal stresses by impact loading of the β₁′ martensitic phase is also detected by several anelastic phenomena.4. In contrast to elastic and anelastic properties, transformation temperatures are insensitive to the impact loading, pointing to the difference of structural elements responsible for the anelastic effects and for the interval and hysteresis of the thermoelastic martensitic transformation.     

Microstructural changes produced by plastic deformation in the UNS S31803 duplex stainless steel

Plastic deformation by cold rolling produces important changes on microstructure of the duplex stainless steel UNS S31803. Structure refinement and martensitic transformation were detected and analyzed by microscopy, X-ray diffraction and magnetic measurements. True deformations in the range of 0.92–3.38 were applied. The maximum amount of ′ martensite was 30.2% obtained with the maximum deformation applied (3.38). The annealing at 400 °C promotes a further increase of ′ martensite content, as observed before in austenitic metastable steels. Hardness against deformation curves of AISI 304L and duplex steel were compared and analyzed. The stability of the martensite phase with temperature was investigated by magnetic measurements, and it is found that the reverse reaction ′ → γ starts between 500 and 520 °C.     

Influence of high-energy impact actions on the elastic and anelastic properties of martensitic Cu–Al–Ni crystals

The influence of high-energy impact shock-wave loading on the microplasticity and macroscopic performance of the Cu–Al–Ni crystals in the β₁′ martensitic phase has been studied. Elastic and anelastic properties of quenched and aged polyvariant single crystals before and after impact shock-wave loading were measured in the temperature range 80–300 K, at a frequency of about 100 kHz in the strain amplitude-independent and amplitude-dependent ranges by means of the composite oscillator technique, and in the MHz frequency range using the pulse–echo technique. High-velocity impact loading of the specimens was realised by plane shock-waves with stress pulses with a duration of 2·10⁻⁶ s and stress amplitudes up to 5 GPa. A pronounced influence of impact shock-wave loading on the elastic and anelastic properties of the β₁′ martensite has been observed. A strongly marked softening of the material and an enhancement of damping properties are revealed up to the highest stress pulse amplitudes. This behaviour differs fundamentally from the one observed in ‘ordinary’ fcc metals. Changes of the defect structure induced by shock-wave loading, which may be responsible for the observed phenomena, have been discussed.     

Surface mechanical attrition treatment induced phase transformation behavior in NiTi shape memory alloy

The phase constituents and transformation behavior of the martensite B19′ NiTi shape memory alloy after undergoing surface mechanical attrition treatment (SMAT) are investigated. SMAT is found to induce the formation of a parent B2 phase from the martensite B19′ in the top surface layer. By removing the surface layer-by-layer, X-ray diffraction reveals that the amount of the B2 phase decreases with depth. Differential scanning calorimetry (DSC) further indicates that the deformed martensite in the sub-surface layer up to 300 μm deep exhibits the martensite stabilization effect. The graded phase structure and transformation behavior in the SMATed NiTi specimen can be attributed to the gradient change in strain with depth.     

Composition-Dependent of 6 M Martensite Structure and Magnetism in Cu-Alloyed Ni-Mn-In-Co by First-Principles Calculations

The composition dependence of the crystal structure and magnetism of the 6 M martensite for the Cu-doped Ni_43.75Mn_37.5In_12.5Co_6.25 alloy at different site occupations (Cu substitution for Ni, Mn, In, and Co, respectively) is investigated in detail with the first-principles calculations. Results show that the austenite (A) phase exhibits a ferromagnetic (FM) state in all occupation manners, the 6 M martensite possesses an FM state except for the case of Cu substitution at the normal Mn (Mn1) site, and the non-modulated (NM) martensite displays a ferrimagnetic (FIM) state apart from the Cu substitution at the Ni, Mn1, or In sites. The Cu atom destabilizes the A, 6 M, and NM phases regardless of the occupation manner. The one-step martensitic transformation from the A to NM phase occurs in the case of Cu substituting for Mn1, excess Mn (Mn2), or Co; for Cu substituting Ni, a martensitic transformation including 6 M martensite happens, i.e., A → 6 M → NM; however, the martensitic transformation disappears when Cu replaces In site. From the equilibrium lattice constants, it can be speculated that the substitution of Cu for Ni can effectively reduce the thermal hysteresis (∆T_Hys). The magnetic properties are found to be greatly reduced by the substitution of the non-magnetic element Cu for the ferromagnetic Mn atom, whereas the effect is fewer in the remaining cases. It is predicted that the alloy has more favorable properties when Cu replaces Ni. The present results can lay a theoretical foundation for further development of multielement magnetic shape memory alloys.     

Microstructure evolution in CoNiGa shape memory alloys

Magnetic shape memory CoNiGa alloys hold great promise as new smart materials due to the good ductility and a wide range of martensitic transformation (MT) temperatures as well as magnetic transition points. This paper reports the results of investigations on the equilibrium phase constitution and microstructure evolution in quenched or aged CoNiGa alloys using the optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods. The dendritic γ phase decreases as lowering of Ga content in studied two series of samples (Co₅₀Ni_50 − xGa_x, x = 0–50 and Co_100 − 2yNi_yGa_y, y = 15–35). Some γ′ precipitates with different morphologies were found in given alloys conducted with water quenching (WQ) at 800 °C or long-time ageing at 300 °C. After 800 °C quenching, the γ′ phase has a rod-like shape for the Co₅₀Ni₃₀Ga₂₀ alloy but shows a Widmanstätten morphology as Ga increases to 25 at%, and trends to be block structure in further high Ga content alloy. In the case of 300 °C aged alloys, the γ′ particles prefer to nucleate in interior of γ phase or at the interface of β–γ. We also presented an illustrative vertical section phase diagram keeping 50 at% Co, and isothermal section phase diagram at 1150 and 800 °C of the CoNiGa system. Based on the schematic ternary phase diagram, the composition scope which potentially holds over the magnetic pure martensite phase structure at room temperature (RT) was pointed out. It is believed that this optimized range alloys would play an important role in the functional materials design for application.     

马氏体在位错偶上形核长大的分子动力学模拟研究

应用分子动力学方法和ＮｉＡｌ合金的嵌入原子势,模拟研究了位错偶上马氏体形核长大的过程和微观机理。计算结果表明,马氏体形核的位置与位错应力场的分布有关 。     

Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti₃₆Ni_49−xHf₁₅Cu_x (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)₂Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)₂Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)₂Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)₂Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.     

Effect of deformation by stress-induced martensitic transformation on the transformation behaviour of NiTi

It has been reported that martensite in near-equiatomic NiTi is thermally stabilised after a moderate deformation via martensite reorientation. This work continues the study by investigating the effect of deformation via stress-induced martensitic transformation on the transformation behaviour of the alloy. It was observed that the stress-induced martensite was also stabilised relative to the thermal martensite formed on cooling, as indicated by an increase in the critical temperature for the reverse transformation. Associated with the stabilisation, the heat effect, as determined by differential scanning calorimetry, and the temperature interval of the reverse transformation, were measured and found to decrease with increasing level of deformation. The experimental results also demonstrated that the stress-induced martensitic transformation was microscopically localised, as expected for a first-order phase transformation.     

Structural anelasticity of NiTi during two-stage martensitic transformation

The two-staged thermoelastic martensitic transformation (TMT) B2→R→B19′ in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young’s modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19′ martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19′ phase.     

Comparison of thermally induced and deformation induced martensite in Fe–29% Ni–2% Mn alloy

The morphology and crystallography of martensite either formed by cooling to subzero temperatures (thermal effect) or by compression deformation were compared for different austenite grain sizes of Fe–29% Ni–2% Mn alloy by transmission electron microscope (TEM). TEM observations revealed both and ′ martensite formation within large grained austenite phases by thermal effect whereas only ′ martensite formation was observed in small grained austenite phases. On the other hand, compression deformation effect caused only ′ martensite formation in both small and large grained austenite phases of Fe–29% Ni–2% Mn alloy. Thermally induced ′ martensite exhibited a lenticular morphology with partial twinnings that are peculiar to this kind of morphology. The crystallographic orientation relationship between austenite and thermally induced ′ lenticular martensites was found to be as Kurdjumov–Sachs (K–S) type relationship.     

Towards understanding anelasticity of the β1′ martensitic phase of Cu–Al–Ni

The present work continues the series of experimental investigations undertaken in order to elucidate the mechanisms controlling elastic and anelastic properties of the β₁′ martensitic phase of Cu-based shape memory alloys. The paper reports an attempt to distinguish between ‘dislocation’ and ‘interface’ mechanisms of the internal friction in the β₁′ martensitic phase of Cu–Al–Ni single crystals. Two types of experiments have been performed. First, the ultrasonic strain amplitude-independent and amplitude-dependent internal friction (ADIF) of a monovariant specimen for temperatures 90–300 K is carefully re-examined. Second, in situ measurements of the ADIF and of the influence of ultrasonic oscillations on the plastic deformation (acoustoplastic effect) were carried out during quasistatic deformation of a quenched polyvariant specimen. Experimental results support a dislocation rather than an interface mechanism of anelasticity, at least at ultrasonic frequencies and moderate strain amplitudes.     

On the two-way shape memory behavior in NiTi alloy—An experimental analysis

The present research aims to understand the mechanism of the two-way memory effect (TWME) in NiTi alloy through experimentally investigating shape recovery in the course of various thermomechanical processes. Particular attention was paid to the effect of martensitic strain, either due to pre-deformation or constrained forward transformation, on the TWME. Attempts were made to clarify the correlation of the stress-assisted two-way memory effect (SATWME) and the TWME to martensitic strain resulting from pre-straining and/or constrained cooling. When the martensitic strain resulting from constrained forward transformation exceeds the initial pre-strain, it directly influences the SATWME and TWME. Partial reverse transformation through thermal arrest during heating leads to accommodation of stress-assisted and detwinned martensite variants that result in formation of internal forward and back stresses. TWME is promoted through dominant internal forward stress formation, while the dominance of internal back stress reduces the TWME by reducing the martensitic strain.     

Eutectoid transformations accompanied by ordering

Eutectoid transformations accompanied by ordering, unlike ordinary ones, proceed through non-pearlitic modes of transformations. Eutectoid invariants are classified into two categories in binary systems. The eutectoid invariant of A3()→D0₁₉(₂) + L1₀(γ) in the Ti-Al binary system belongs to the first category, in which one product phase has an ordered structure of a parent phase. Its transformation product exhibits a γ/₂ lamellar structure consisting of nearly perfectly aligned alternate lamellae of γ and ₂, which is formed by precipitation of γ plates in either or ₂ matrix with the Blackburn orientation relationship. The eutectoid invariant of A1(γ)→D0₂₂(γ″) + L1₂(γ′) in the Ni₃V-Ni₃Al pseudo-binary system is an example of the second category, in which both product phases have different ordered structures of a parent phase. The transformation of a 75Ni-18V-7Al alloy results in a ‘checkerboard’ pattern consisting of a periodic array of columns of γ′ and two γ″ orientation variants, which are formed by phase separation simultaneous with ordering.     

Stress-induced martensitic transformation in Fe–Ni bicrystals

The effect of applied stress on martensitic transformation behaviour near the grain boundary was examined using Fe–32 at.% Ni bicrystals containing a 90°2 1 1 tilt or a 90°{2 1 1} twist grain boundary focusing on the martensite-start stress (σ_M), the morphology of martensites and the variant selection. The σ_M for single crystals and tilt boundary bicrystals increases almost linearly with increasing temperature (T), and the σ_M−T relation for tilt boundary bicrystals is situated in the higher temperature side than that for single crystals. In contrast, the σ_M of the twist boundary bicrystals exists between the correlated σ_M−T relations for single crystals and tilt boundary bicrystals. The variant selection near the tilt boundary is not sensitive to applied stress; some variants with the habit plane almost parallel to the boundary were symmetrically formed similar to those in the thermal transformation. The effect of loading axis to grain boundary on the martensitic transformation behaviour was also investigated.     

两种不同变形方式对NiTi合金回复特性的影响

研究了马氏体再取向(MR)和应力诱发马氏体转变(SIM)两种不同变形方式对Ni50.2Ti49.8合金拉伸性能及预应变后加热回复特性的影响.结果表明:拉伸变形中,MR变形方式的应力平台结束时的应变值较SIM变形方式大.在拉伸预应变处于应力平台阶段时,两种变形方式在相同预应变后加热回复有相同的记忆能力;在拉伸预应变大于应力平台时,两种变形方式在相同预应变后加热回复,SIM变形方式的逆相变温度和回复应变略高于MR变形方式.     

Influence of Inter-Pass Temperature on Residual Stress in Multi-Layer and Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel PipesEI北大核心CSCD

9% Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9% Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9% Cr heat-resistant steel. Neverthe-less, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9% Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9% Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (M-f), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (M-s).     

Effect of γ2-phase precipitates on the martensitic transformation of a β-CuAlBe shape memory alloy

The influence of γ₂ precipitates on the MT of a Cu–22.66Al–2.98Be (at%) polycrystalline alloy was analysed by compression test, differential scanning calorimetry and microscopy. As the precipitate content (or size) increases, the stress-induced martensitic transformation occurs at higher stress levels. The thermally induced martensitic transformation is only significantly affected when the γ₂ phase content exceeds 14%, the thermal peaks are shifted to higher temperatures and less β phase transforms to martensite. TEM observations suggest good coherency between both phases, and dislocation bundles remain in the β side of the γ₂/β interface after the compression-induced transformation to martensite. The evolution of σ_s with fvγ₂ is analysed considering a chemical and a mechanical contribution.     

Ratcheting Behavior of a Non-conventional Stainless Steel and Associated Microstructural Variations

Ratcheting fatigue behavior of a non-conventional stainless steel X12CrMnNiN17-7-5 has been investigated with varying combinations of mean stress (σ_m) and stress amplitude (σ_a) at room temperature using a servo-hydraulic universal testing machine. X-ray diffraction profile analysis has been carried out for assessing possible martensitic phase transformation in the steel subjected to ratcheting deformation. The results indicate that ratcheting strain as well as volume fraction of martensite increases with increasing σ_m and/or σ_a; the phenomenon of strain accumulation is considered to be governed by the associated mechanics of cyclic loading, increased plastic damage as well as martensitic transformation. A correlation between strain produced by ratcheting deformation and martensitic transformation has been established.     

Microstructure of martensitic phase in the Co39Ni33Al28 shape memory alloys revealed by transmission electron microscopy

In the Co₃₉Ni₃₃Al₂₈ alloy, the ferromagnetic shape memory effect was investigated. A martensitic transformation (MT) occurred in the Co₃₉Ni₃₃Al₂₈ alloy when the temperature was lower than the martensitic start transformation temperature, M_S = 233 K. The morphologies and microstructures of the martensitic phase characterized by transmission electron microscope (TEM) showed a new 28-layered (28M) modulated martensite consisting of pinstripes and co-existing with the non-modulated martensite. Anomalies in magnetic properties and strain emerging around the MT have been briefly discussed.